The surface rupture of the 1957 Gobi-Altay, Mongolia, earthquake

Geological Society of America Special Paper 320 1997

The Surface Rupture of the 1957 Gobi-Altay, Mongolia, Earthquake

ABSTRACT

The Gobi-Altay earthquake on December 4, 1957, produced not only what remains in 1995 the world's best preserved surface rupture of a great earthquake, but also a spectrum of deformation that provides a microcosm of active intracontinental mountain building. Left-lateral strike slip and north-south shortening dominated deformation along the easterly trending Gobi-Altay mountain range, but normal faulting, east-west shortening, long wavelength folding of the basement, and rotation about vertical axes also occurred. Building on a joint Soviet-Mongolian expedition in 1958, we quantify slip on the various ruptures and interpret them in terms of earthquake recurrence and intracontinental tectonics. The principal, “Bogd,” rupture trends east-southeast along the northern margin of the Gobi-Altay. Left-lateral strike-slip offsets of 3–4 m characterize much of its 260km length, but in one ~40-km portion, offsets reach 5–7 m. The rupture is neither straight nor everywhere simple; jogs and steps with multiple, subparallel strands are common. Vertical components vary with no obvious pattern except immediately north and northeast of the two principal mountain massifs of the region, Ih Bogd (3,957 m) and Baga Bogd (3,590 m), where the consistently uplifted south side suggests that repetitions of such components of oblique reverse slip have elevated these massifs. Moreover, near both massifs, ruptures splay from the steeply dipping Bogd rupture into gently, southward dipping faults on which thin slices of uppermost crust have been thrust onto the basins to the north to form rows of low hills, the Dalan Türüü and Hetsüü forebergs. Thrust or reverse faulting characterizes two ruptures south of the Ih Bogd massif, each ~ 15–25 km south of the Bogd rupture. Vertical components average 2-3 m, with a maximum of 5 m, along the Gurvan bulag (spring) rupture, the western of these which is directly south of the Ih Bogd summit plateau. Along the Tsagaan Ovoo-Tevsh uul (peak) zone, south of the lower eastern end of the Ih Bogd massif (Dulaan Bogd, 2,565 m), slip reached a maximum of only ~2 m. In one place, the surface rupture reflects localized folding of bedrock, suggesting that the causative fault has not yet broken through to the Earth's surface. The pattern of slip and its variation along both ruptures support the contention that repeated slip during earthquakes similar to that in 1957 has built the high terrain. A third important rupture, the “Toromhon Overthrust,” trends roughly north and lies south of the Bogd rupture between the Ih Bogd and Baga Bogd massifs. Although only ~21 km long, slip was very large with vertical components ranging from 2 to 6 m and strike-slip components from nil to 2–3 m. In places, reverse slip seems to

Kurushin, R. A., Bayasgalan, A., Ölziybat, M., Enhtuvshin, B., Molnar, P., Bayarsayhan, Ch., Hudnut, K. W., and Lin, J., 1997, The Surface Rupture of the 1957 Gobi-Altay, Mongolia, Earthquake: Boulder, Colorado, Geological Society of America Special Paper 320.

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R. A. Kurushin and Others have occurred, but along much of the rupture, the scalloped shape of the surface trace attests to a gentle westerly dip. Although evidence of a previous Quaternary rupture is clear at several localities along the Bogd and Gurvan bulag ruptures, we saw virtually no such evidence along the Toromhon Overthrust. Finally, in addition to superficial deformation within the Ih Bogd massif, deepseated faulting across the summit plateau of Ih Bogd, and therefore between the Bogd and Gurvan bulag ruptures, attests to internal deformation of the massif. Components of left-lateral strike slip reaching 1.5 m seem to absorb some of the large strike slip (5–7 m) that occurred farther west along the Bogd rupture. A component of approximately north-south extension may be a manifestation of stretching across a large-scale fold (wavelength ~25 km) as the Ih Bogd massif rises. The “Bitüüt collapse wedge structure” of Solonenko, however, appears to be a large landslide within the massif. The distribution of slip in 1957 concurs with the general impression of individual massifs within the Gobi-Altay being caught within an obliquely convergent, leftlateral shear zone. Deformable blocks of crust within the zone slide past neighboring zones on left-lateral faults, like the Bogd rupture, and are thrust onto neighboring basins, as shown by the forebergs and by the Gurvan bulag and Tsagaan Ovoo-Tevsh uul zones. Rates of slip vary along faults and imply that blocks rotate with respect to adjacent regions about nearby vertical axes. Moreover, the intervening crustal blocks have not behaved rigidly; rather they seem both to have undergone gentle folding, as shown by the smoothly varying regional elevations of the summit plateaus, and to have deformed internally, as seen by components of strike slip and extension across the summit plateau of Ih Bogd. Separate blocks appear to move somewhat independently of one another, as required by slip on the Toromhon Overthrust, which shows convergence between the Ih Bogd and Baga Bogd massifs. Thus, the rupture in 1957 provides a snapshot of intracontinental mountain building in action, replete with laterally varying styles and amounts of permanent deformation. Localities where scarps show more slip than occurred in the 1957 earthquake commonly indicate approximately twice as much slip and therefore suggest that the previous earthquake was associated with a similar amount of slip. Whether all such scarps ruptured simultaneously, as in 1957, or in separate smaller earthquakes remains an open question. Nevertheless, the relative sizes of the offsets permit the occurrence of repeated, or “characteristic,” earthquakes, for which amounts of slip, however, vary along the ruptures. Differences in amounts of slip along the rupture do not seem to indicate gaps that will be filled by future events, but rather long-term variations that contribute to the overall regional strain of the region. Simple calculations of changes in elastic stresses show that slip on each of the Bogd strike-slip rupture and on the Gurvan bulag thrust rupture should increase the Coulomb stress on the other rupture. Therefore, slip on one is likely to have triggered slip on the other. The relative location of the epicenter to the Ih Bogd region suggests that slip on the Bogd rupture triggered the thrust faulting, but either could occur in other earthquakes or other settings. The distribution of slip in 1957 resembles the pattern that would occur if a repeat of the 1857 Fort Tejon earthquake occurred on the San Andreas fault in southern California simultaneously with a rupture of the Sierra Madre–Cucamonga fault along the base of the San Gabriel Mountains. Not only do the relationships of strikeslip and thrust faulting correspond to one another, but also the distribution of strike slip along the Bogd rupture reveals similarities to that of the 1857 earthquake along the San Andreas fault. The 1957 Gobi-Altay earthquake may serve as a prototype for a more disastrous earthquake in southern California than is commonly imagined.

Surface rupture of the 1957 Gobi-Altay earthquake INTRODUCTION Following the recognition of plate tectonics and the consequent appreciation for large horizontal movements of the earth’s crust, earth scientists have increasingly realized that most large structures involving the upper crust have grown by repeated slip on major faults during earthquakes. Each major earthquake in an accessible region is now studied with a thoroughness that characterized few earthquakes before 1960. Beginning with the 1906 San Francisco earthquake, studies of earthquake ruptures have provided not only fundamental data for understanding earthquakes and their hazards, but also the basis for much of what is known of continental tectonics. Ironically, however, this growing appreciation for earthquakes has developed largely in a period when great earthquakes have been rare. Although 15 “Great Shallow earthquakes,” with M ≥ 7.9 according to Richter (1958, Tables XIV-1 and 2), occurred in central and eastern Asia between 1897 and 1956, the most recent such earthquake anywhere within an intracontinental setting took place on December 4, 1957, in the GobiAltay of southern Mongolia (Figs. 1 and 2, Table 1). One could argue that all earthquakes can be treated as the sum of smaller sub-earthquakes, and therefore little is to be learned from examining a major earthquake. Conversely, the surface rupture of a great earthquake provides the best test of such an argument. The arid climate of the Gobi-Altay has preserved the surface rupture of the 1957 earthquake so well that in the 1990s sharply defined scarps remain throughout the length of the rupture. An investigation of its surface rupture could provide basic information useful for understanding how great earthquakes might differ from smaller ones. In particular, its surface rupture bears both qualitative and quantitative similarities to the surface ruptures along both the San Andreas fault and along the main thrust faults bounding the Los Angeles Basin in southern California and therefore provides a prototype for the most destructive earthquake likely to occur in the Los Angeles region (Bayarsayhan et al., 1996). The faulting associated with the 1957 earthquake offers an opportunity to examine some of the details of active intracontinental tectonics on a mountain-range scale. The range of mountains comprising Noyon uul, Ih Bogd, Dulaan Bogd, and Baga Bogd (Fig. 2) is typical of ranges not only in the Gobi-Altay, but also of intracontinental ranges in general. High topography overlooks basins of different depths and different accumulations of sediment, as do such ranges in the Tien Shan, the Nan Shan, or southern California. Covered only by thin layers of sedimentary rock, largely terrigenous in origin, metamorphic basement rock cores the ranges, which have been warped and thrust onto adjacent basins. Seen on a regional scale, these ranges typify what Argand (1924, p. 215–222; Argand and Carozzi, 1977, p. 36–42) called “basement folds” (Fig. 3). Thus, faulting in 1957 provides an instantaneous look at how the ranges on the northern side of

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the Gobi-Altay form and deform, and therefore inadvertently created a laboratory experiment of intracontinental mountain building in action. With these aspects of the earthquake in mind, we carried out a study of the surface rupture of the 1957 earthquake. Our immediate goal was to quantify as comprehensively as possible displacements, which comprise the full gamut of possible deformation. Such quantification allows comparisons with larger, finite, deformation both in the Gobi-Altay and with other regions of similar tectonic style. Moreover, such a quantification is necessary to test ideas of “characteristic earthquakes,” repetitions of earthquakes with similar distributions of slip, and of segmentation of faults into portions that rupture separately from one another. BACKGROUND In 1956, prior to the earthquake, N. A. Florensov, V. P. Solonenko, and A. A. Treskov of the Institute of the Earth’s Crust in Irkutsk, Russia, launched a program to evaluate earthquake hazards in hitherto relatively aseismic regions of northeastern Asia. Precociously calling the approach “paleoseismogeological” (Florensov, 1960; Florensov and Solonenko, 1963; Solonenko, 1966), later translated as “paleoseismological” (Florensov and Solonenko, 1965, p. 1), they used geological, and especially geomorphological, data to recognize and evaluate earthquake hazards in this area. The earthquake in 1957 not only confirmed their evaluation, but also justified the approach that they had taken. In January 1958, one month after the earthquake, Florensov and Solonenko visited the epicentral region briefly and flew over the entire region to estimate the dimensions and scale of surface faulting (Florensov, 1958; Solonenko, 1959; Solonenko et al., 1960). Although only a cursory examination of the faulting was possible, this expedition was important because when they returned later, spring and summer flooding had already destroyed some of the clear surface faulting (see Figs. 67–69 of Florensov and Solonenko, 1963, 1965). The main results of this preliminary expedition, however, were the recognition of extensive deformation and the obtaining of sufficient evidence to justify a much more extensive study, ultimately presented in the classic monograph edited by Florensov and Solonenko (1963, 1965). In the autumn of 1958, N. A. Florensov and V. P. Solonenko returned with a team of Mongolian and Russian geologists1 and additional participants to provide technical and logistical assistance. R. A. Kurushin was the youngest among the geologists and at present (1996) is one of a few still alive. This group divided into 1 J. Dügersüren, Ch. Düvshir, Minsel, L. Natsag-Yum (leader), and Sh. Tseveg from the Mongolian side, and A. P. Bulmasov, A. S. Eskin, S. D. Khil’ko, R. A. Kurushin, N. A. Logatchev, A. V. Luk’yanov, A. P. Shmotov, and M. A. Solonenko from the Russian side.

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R. A. Kurushin and Others

Figure 1. Map of Asia showing its major topographic and tectonic features, the location of the 1957 Gobi-Altay earthquake, and epicenters of other major earthquakes in Asia since 1897, indicated by dates within ellipses (Table 1). Dotted areas distinguish regions higher than 1,000 m, 3,000 m, and 5,000 m, with the densest dots indicating areas higher than 5,000 m, least dense dots higher than 1,000 m, and white areas between 0 and 1,000 m. Dashes indicate oceans and large lakes.

seven parties that covered separate regions. None of the group visited the entire rupture, which perhaps accounts for the absence of a coherent description of the surface faulting in Florensov and Solonenko’s (1963, 1965) monograph, a deficiency we try to remedy. In the summer of 1958, prior to the detailed field investigation, complete aerial photograph coverage2 of the region was made at a scale of approximately 1:25,000. In addition, photographs were taken at a scale of 1:10,000 along a single line over parts of the main, east-west–trending Bogd rupture. In addition to Florensov and Solonenko’s (1963, 1965) monograph, Luk’yanov (1965) presented extensive additional data, some of which have proven to be very important to our work. In many regions, the minor surface ruptures that he reported are no longer sufficiently clear for offsets to be measured reliably. In subsequent years, several reconnaissance investigations to the Gobi-Altay region, including one that three of us made in 1990 (Baljinnyam et al., 1993), yielded short reports of new observations and opinions about the surface rupture of the Gobi-Altay earthquake (e.g., Khil’ko et al., 1985; Trifonov, 1985, Trifonov and Makarov, 1988). To the best of our knowledge, however, our study was the first since 1958 to

attempt a comprehensive investigation of the surface faulting, and specifically to quantify slip along the principal ruptures. Toward this end, we took advantage of the data and results of Florensov and Solonenko’s (1963, 1965) and Luk’yanov’s (1965) monographs, and of course Kurushin’s memory. We had complete access to the aerial photographs taken in 1958 for our field work in 1993 and 1994. In addition, Kurushin found and transcribed all field notebooks of the Russian geologists3 from 1958. The examination of the field notebooks followed our field work and, as a result, left open some questions, but it also provided numerous quantitative observations not available in Florensov and Solonenko (1963, 1965). With the advantage of hindsight and with the aerial photographs in the field, we focused attention on localities where

2Negatives of the aerial photographs are stored at the Institute of the Earth’s Crust in Irkutsk. 3 This transcription, in Russian, GSA Data Repository Item 9726, is available from Document Secretary, Geological Society of America, P.O. Box 9140, Boulder, Colorado 80301.

Surface rupture of the 1957 Gobi-Altay earthquake the rupture was especially clear and where displacements could be quantified. We used a “total station,” a combined theodolite and laser ranging device, to make topographic maps of regions tens of meters in dimension and/or profiles where 1957 offsets are unambiguous and quantifiable (Appendix A). We tried to avoid areas of complicated deformation. As should be clear in the detailed description below, our work turned up many features that seem to have gone unnoticed by previous investigators, including what we consider to be the most spectacular surface faulting associated with the earthquake (see Figs. 26–30 following later in this study). We found nearly all localities photographed by Florensov and Solonenko (1963, 1965) and thereby confirmed their observations. We also found the rare locality where we disagree with what they wrote. Most important, however, our goal was to provide objective information that will allow others to evaluate the magnitude of surface faulting at many localities in the area. Thus, our work is not merely a polishing of what was for the most part a thorough study. Despite our attempt to carry out a comprehensive investigation, it was clear throughout our work that we too could not have seen all significant surface rupturing. Too often we stumbled onto surprises not reported, and apparently not seen, by Florensov and Solonenko (1963, 1965) or others, which continually reminded us that other surprises surely remain for future investigators. The variety of analyses reported here have required some division of labor to prepare this monograph. Kurushin analyzed aerial photographs and coordinated the field work carried out with Bayarsayhan, Bayasgalan, Enhtuvshin, Molnar, and Ölziybat, who shared the task of surveying the sites. Molnar analyzed the survey data and prepared the maps and profiles shown in Appendix A. Kurushin built the mosaics of aerial photographs, analyzed the Bitüüt landslide, and synthesized information from the field notebooks of Russian geologists in 1958. With minor help from Molnar, he also traced the topographic maps and consolidated the results presented in Plate 1. Bayasgalan and Enhtuvshin processed the Landsat imagery. Hudnut and Lin calculated the strain fields discussed in the last section of the book. Molnar took principal responsibility for preparing the text. The bulk of the following text addresses details of the sur-

Figure 2. Simplified summary map of surface faulting associated with the 1957 Gobi-Altay earthquake. Details are shown on Plate 1. Selected contours in meters are shown with the summit plateau of Ih Bogd defined by hatching and the lake Orog Nuur surrounded by dots. Dark black lines denote the major strands of the surface ruptures of the 1957 earthquake. Arrows show the sense of strike slip, with numbers indicating the offset in meters. Where a vertical component was measured, “h ~” gives the amount in meters. Teeth indicate reverse or thrust faulting, and tics indicate normal slip, with teeth or tics on the down-dip side. Large letters, A, B, C, etc., divide the Bogd rupture into segments discussed in the text.

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R. A. Kurushin and Others TABLE 1. GREAT SHALLOW EARTHQUAKES IN ASIA* Year

Name

Place

1897 1902 1905 1905 1905 1906 1911 1912 1920 1927 1931 1934 1947 1950 1951 1957

Assam Atushi Kangra Tsetserleg Bulnay Manas Chon-Kemin

Shillong Plateau, India Tien Shan, China Himalaya, India Northern Mongolia Northern Mongolia Tien Shan, China Tien Shan, Kyrgyzstan Burma China China Altay, China Himalaya, Nepal Himalaya, Bhutan Himalaya, India Tibet Mongolia

Haiyuan Gulan Fu-yun Bihar-Nepal Assam Damshung Gobi-Altay

Latitude (°N) 26 40 33 49.5 49.2 43.5 42.8 21 36.62 38.05 46.89 27.55 28.63 28.38 30.98 45.31

Longitide (°E)

Magnitude (M)

91 77 76 97 96 85 77.3 97 105.40 102.37 90.06 87.09 93.73 96.76 91.49 99.21

8.7 8.6 8.6 8.4 8.7 8.3 8.7 7.9 8.6 8.3 7.9 8.4 7.9 8.7 7.9 8.3

*According to Richter, 1958, Tables XIV-1 and 2.

face rupture, beginning at the west end of the Bogd rupture, the main rupture, some 260 km long and characterized throughout most of its length by left-lateral strike-slip faulting (Fig. 2). This discussion is keyed with Plate 1, a map drawn at a scale of 1:100,000, showing measured offsets, localities where we made detailed topographic maps, areas covered by aerial photographs shown in following sections, and selected sites where Florensov and Solonenko’s team measured offsets. After discussing the entire Bogd rupture, we describe the Toromhon Overthrust, the north-south zone of thrust faulting between Ih Bogd and Baga Bogd. We then present observations of two zones of reverse faulting along the southern margin of the Ih Bogd massif (Fig. 2), the Tsagaan Ovoo-Tevsh uul and the Gurvan bulag rupture systems. This section concludes with a discussion of deformation on the Ih Bogd summit plateau and the Bitüüt landslide. As some readers may find this discussion too detailed, following it, we provide a short summary of these ruptures, with discussions of peculiarities that make them interesting. We conclude with interpretations of the significance of the Gobi-Altay earthquake rupture for earthquakes elsewhere and for intracontinental mountain building. In preparing Plate 1, we have redrawn topography from topographic maps produced in the Soviet Union at a scale of 1:100,000, using names transliterated from the Cyrillic on Mongolian versions of these maps. In the text, however, we use shorter forms for a few features known better by abbreviated names. For instance, the ridge capped by the high peak, Ih Bogd (“Great Saint” or “Great Elevated One”), becomes, literally, Ih Bogdiyn nuruu, and the peak itself is listed as Tergüün Bogd. We have not translated Mongolian geographic terms, like nuruu which means ridge, but we include as Table 2 a short glossary of such terms. Baljinnyam et al. (1993) gave a longer glossary, plus

a brief discussion of pronunciation and grammar, for the linguistically curious reader. SECTIONS OF THE RUPTURE The following discussion summarizes in some detail what we know about the surface rupture of the 1957 Gobi-Altay earthquake. We subdivide the ruptures by subregion, and in discussions of each, the first paragraph gives a brief summary of the deformation within the subregion. The most quantitatively accurate information is derived from maps and cross sections that we made in 1993 and 1994 and that are presented in Appendix A. All reference to such sites are referred by site numbers in Plate 1, Table 3, and Appendix A. To help readers interested in yet more detail, we also refer to site numbers, using the symbol “#” to TABLE 2. GLOSSARY OF MONGOLIAN GEOGRAPHIC TERMS Am Baga Bogd Bulag Gol Hayrhan

= = = = = =

Höndiy

=

Ih Nuur Nuruu Sayr Uul Zereglee

= = = = = =

Gorge Small, minor Saint, elevated one Spring River, not major, but with water at all times Mountain, with the connotation of being sacred in some respect Wide, flat valley, presumably with a seasonal stream Great Lake Ridge, in the sense of a small range of mountains Dry gully or valley, arroyo Peak, mountain Mirage (literally, but used to mean a low row of hills, a foreberg)

Surface rupture of the 1957 Gobi-Altay earthquake

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TABLE 3. SUMMARY OF LOCATIONS AND MEASURED OFFSETS AT SITES SURVEYED

1 2 3 4 5 6† 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Lat. (N)

Long. (E)

45°10.18' 45°09.00' 45°09.00' 45°08.50' 45°07.67' 45°06.30' 45°06.13' 45°06.17' 45°06.33' 45°06.08' 45°05.83' 45°05.42' 45°03.83' 45°02.42' 45°03.75' 45°03.17' 45°02.50' 45°01.83' 45°00.00' 44°59.50' 44°59.33' 44°58.50' 44°57.17' 44°56.50'

99°16.08' 99°31.75' 99°31.83' 99°39.00' 99°51.58' 99°56.60' 99°59.50' 99°59.58' 100°00.33' 100°04.75' 100°05.00' 100°08.58' 100°16.17' 100°20.83' 100°27.92' 100°28.83' 100°31.17' 100°33.00' 100°35.50' 100°36.00' 100°38.33' 100°43.83' 100°53.00' 100°56.50'

Elevation Strike (m) (°) 2,170 1,710 1,705 1,690 1,790 1,760 1,680 1,740 1,700 1,680 1,690 1,910 1,870 1,965 1,380 1,400 1,365 1,345 1,480 1,500 1,555 1,565 1,770 1,650

∆uh* (m)

102 3.2 ± 0.8 103 5.1 ± 0.5 90 5.0 ± 0.5 105 4.0 ± 1.5 103 5.8 ± 1.5 115 10.4 ± 1.5 111 5.5 ± 0.5 87 7.0 ± 1.4 80 3.9 ± 0.5 101 3.0 ± 0.8 97 3.5 ± 1.0 95 3.5 ± 0.5 115 3.0 ± 1.0 110 3.9 ± 1.0 285 … 308 … 301 … 350 … 315 … 335 … 100 3.2 ± 0.5 115 1.3 ± 0.3 107.5 2.9 ± 0.5 103 3.6 ± 1.0

∆uv* (m) 0.4 ± 0.2 0.4 ± 0.2 0.7 ± 0.3 0.5 ± 0.3 1.0 ± 0.5 0.0 ± 2.0 -5.1 ± 1.1 -5.0 ± 1.0 0.0 ± 0.5 0.1 ± 0.3 0.4 ± 0.2 -1.3 ± 0.4 -2.0 ± 1.0 1.6 ± 0.5 1.0 ± 0.2 2.2 ± 0.5 2.0 ± 0.5 2.0 ± 0.5 2.0 ± 1.0 2.7 ± 0.5 0.2 ± 0.2 -0.3 ± 0.2 -1.0 ± 0.5 0.4 ± 0.2

25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

Lat. (N)

Long. (E)

44°56.50' 44°56.33' 44°56.17' 44°56.08' 45°00.89' 44°59.61' 44°59.40' 44°57.97' 44°54.37' 44°52.74' 44°49.43' 44°55.67' 44°55.17' 44°52.71' 44°52.60' 44°50.01' 44°49.09' 44°49.90' 44°49.80' 44°50.10' 44°50.30' 44°54.90' 44°57.40'

101°01.33' 101°02.45' 101°03.58' 101°04.75' 101°29.65' 101°32.23' 101°32.20' 101°31.39' 101°41.89' 101°48.65' 101°58.02' 101°03.33' 101°02.83' 101°00.75' 101°00.75' 101°00.10' 100°48.04' 100°26.90' 100°22.90' 100°19.90' 100°18.80' 100°06.10' 100°07.20'

Elevation Strike (m) (°) 1,600 1,607 1,560 1,580 1,591 1,725 1,749 1,874 2,124 1,716 1,725 1,595 1,620 1,666 1,666 1,905 1,914 2,100 2,060 2,060 1,940 1,880 2,040

89 103 102 99 278 0 350 292 112 106 325 45 5 38 10 135 115 90 85 115 120 100 353

∆uh* (m)

∆uv* (m)

4.5 ± 1.5 4.3 ± 0.5 3.5 ± 1.5 4.1 ± 1.0 … -3.3 ± 0.6 -3.0 ± 1.0 … 1.8 ± 0.4 4.8 ± 1.1 … -0.6 ± 1.0 … -4.0 ± 2.0 -2.8 ± 0.5 3.0 ± 0.8 … 0.0 ± 1.0 0.0 ± 1.0 0.0 ± 2.0 0.0 ± 2.0 0.0 ± 2.0 -0.1 ± 0.4

0.3 ± 0.1 0.0 ± 0.2 -2.0 ± 1.0 -0.7 ± 0.3 1.8 ± 0.3 0.7 ± 0.3 0.8 ± 0.3 1.9 ± 0.3 -1.2 ± 0.4 -2.6 ± 0.4 2.6 ± 0.4 1.6 ± 0.5 3.0 ± 0.5 4.0 ± 1.0 5 to 6 -5.0 ± 0.4 2.5 ± 0.5 0.7 ± 0.2 1.5 ± 0.2 4.0 ± 0.3 5.2 ± 0.5 1.9 ± 0.3 0.7 ± 0.2

*For ∆uh, positive numbers quantify left-lateral components of strike slip, and for ∆uv, positive numbers indicate vertical components of slip, with the uplifted side to the left, looking in the direction of the strike. † The offset at Site 6 may represent slip in two events.

denote them, used by the Russian investigators in 1958. The original descriptions can be found on the Russian transcript of the field notes available as GSA Data Repository Item 9726. Their locations are best found by correlating the discussions in the text with quantities shown on Plate 1. Some features of the surface deformation are sufficiently common that a brief introduction should reduce redundant discussion. Rows of low hills are found separated from the main massifs; we follow Florensov and Solonenko (1963, 1965) in calling them “forebergs.” The hills have formed by thrust slip on faults that dip, apparently at gentle angles, toward the neighboring high massifs, but that steepen beneath the hills. Seen from the lower regions, the forebergs manifest themselves as hills, but when seen from the massifs, they appear even lower, as eroded steps down from the wide alluvial fans that emanate from the high massifs. Where rock crops out within the forebergs, it is sedimentary in origin, commonly Cenozoic in age, and, in general, dipping toward the massifs (Florensov and Solonenko, 1963, 1965). In many sections of the rupture, slip included a significant thrust or reverse component. Surface deformation associated with such components commonly is more complex than that where either nearly purely strike slip or large normal components occurred. We observed this complexity both at the scale of the surface rupture itself and at larger scales with deformation spread over distances of kilometers. We relate most of this com-

plexity to variations in the dips of faults with depth, such that a component of reverse slip requires internal strain of either the upper or lower block (Fig. 3). Because it is so much thinner, the upper block presumably deforms more readily than the lower block, although we have no way to demonstrate that the lower block has not deformed. One can distinguish two simple situations: the fault either flattens or steepens as it approaches the surface. Where a thrust rupture reaches the surface, it creates an overhanging wall that collapses quickly. Thus, in effect the fault flattens to a horizontal surface. The collapse of the hanging wall requires that it stretch, creating tension cracks and superficial grabens at the front of the scarp (Fig. 3a), a phenomenon that Luk’yanov (1965, Fig. 20) recognized and used to argue for a component of thrust or reverse slip. If a change in dip occurs at depth within the earth, two pairs of possibilities can occur. If the fault steepens with depth (Fig. 3b), slip can occur on a normal fault dipping opposite to the underlying thrust fault, or the thrust fault can splay into two stands that dip in the same direction but by different amounts. If the main fault flattens with depth (Fig. 3c), two opposite-dipping reverse or thrust faults can form, or a strand with normal faulting can form in the hanging wall so as to isolate a small wedge. We observed normal components of both types, dipping in either direction, along much of the front of the Baga Bogd massif. The presence of such superficial deformation can make the faulting at

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R. A. Kurushin and Others the front of the range misleading and, if overlooked, can lead to a large error in inferred displacements. We exploited aerial photographs to recognize such superficial faulting, but examination of some scarps in the field left no doubt about the sense of slip. Section A–B: Bogd rupture from the Bayan Tsagaan nuruu to the Ulaan bulag höndiy The magnitude of slip grows from a negligible amount at the western end of this subregion to as much as 5–7 m at its eastern end. Rupturing occurred in a broad zone, as wide as 0.5–1 km, with separate splays and complexity that makes measuring the total offset difficult except in selected localities. The western end of the rupture has not been defined clearly. Florensov and Solonenko (1963, 1965, Plate 2) tentatively sketched a rupture along the base of hills (an unnamed foreberg) northeast of Bayan Tsagaan nuruu (Plate 1, Figs. 2 and 4). Although they indicated a low thrust or reverse scarp, ≤ 1 m high and 0.9 m wide, neither in their monograph, nor in any of the field notes, was definitive evidence for a rupture in this area given. A. S. Eskin (#2228)4 traversed the western part in 1958 but did not note evidence of deformation. A. P. Shmotov (#2691) crossed the eastern part and noted a series discontinuous cracks with openings of 0.2–0.3 m over a zone 1.5 km long. We simply adopt Florensov and Solonenko’s (1963, 1965) indication of a rupture and assume minor thrust faulting. At the westernmost portion where the 1957 rupture is clear, A. V. Luk’yanov (#1929) described a low thrust scarp striking N10°E with the west side uplifted 0.4 m (Plate 1). The zone of deformation, consisting of tension cracks and mole tracks, curves southward and then southeastward where, in places, it is 4–5 m wide but with a negligible vertical component. Luk’yanov reported a small left-lateral component of 0.2 m. Two to three kilometers farther east, N. A. Logatchev (#1215) noted a 0.3-mhigh scarp facing south, which was clear in 1994. A south-facing scarp with an increasing amount of leftlateral slip characterizes the rupture for approximately 6 km east (Plate 1). The scarp is not straight, and several en echelon splays,

4 These numbers, prefaced by the symbol “#,” refer to entries in field notebooks of Soviet geologists in 1958, which have been transcribed and are available as GSA Data Repository Item 9726.

Figure 3. Drawings showing simple effects of surface deformation above dip-slip faults that are not planar. (a) Pattern commonly observed where a rupture associated with a thrust or reverse fault reaches the surface. The overhanging part of the hanging wall collapses, stretching the surface and leaving tension cracks and grabens on the front of the scarp. (b) Deformation of the hanging wall above a region where the fault steepens at depth. Either a normal fault dipping in the direction opposite to that of the main fault, or a reverse fault dipping in the same direction as the main fault, forms to accommodate strain in the hanging wall. (c) Deformation of the hanging wall above a region where the fault flattens at depth. Here a normal fault dipping in the same direction as the main rupture, or a reverse fault dipping opposite to it, can form to accommodate strain in the hanging wall.

Surface rupture of the 1957 Gobi-Altay earthquake

9

Figure 4. Landsat Thematic Mapper image of the western end of the rupture zone, showing the locations of the Bayan Tsagaan nuruu, a foreberg just northeast of it, the Bahar uul, and other surroundings. North is toward the top. Note how the topographic expression of the foreberg, marked by white arrows, resembles those of the Dalan Türüü (Fig. 42) and Hetsüü (Fig. 64) forebergs. Clouds over the Bayan Tsagaan nuruu obscure some of the topography at its eastern end.

apparently with normal components of slip, lie to the north. Near the eastern part of this section, the scarp is relatively simple where it crosses gentle terrain south of a low hill (Fig. 5). Logatchev (#1243) reported 2.5 m of left-lateral slip and a height of 1 m in this general area (Plate 1). In 1994, we estimated ~3 m of left-lateral offset of the thalweg of a gully (Fig. 6), approximately where Luk’yanov had also measured 3 m. The most convincing measure of displacement in this area was V. P. Solonenko’s observation on January 3, 1958, of vehicle tracks offset 3.4 m left laterally, and vertically approximately 1 m with the south side down (Fig. 7, Plate 1). No more than 200 m to the east, Solonenko also found a horse trail offset left laterally 3.5 m with a vertical component of 0.8 m. East of this road, two parallel scarps mark a narrow graben, called the Bahar graben by Florensov and Solonenko (1963, 1965), approximately 13 km long and 0.3 to 0.6 km wide along the southern foot of Bahar uul (Fig. 8, Plate 1). In the middle of the graben, the northern trace vanishes, and only one trace can be seen (Fig. 8). We surveyed the topography of an offset gully (Site 1) where the vertical offset is small (Figs. 9 and 10)5 and estimated 3.2 ± 0.8 m of left-lateral slip. Farther east, again the rupture splits into parallel splays that define the eastern part of the Bahar graben (Fig. 11). Measured strike-slip offsets do not exceed 3 m, but because the rupture comprises more than one branch, bounding that offset is difficult. For instance, in one area N. A. Florensov (#169) measured 1.5 m of left-lateral slip on the northern branch but did not examine the southern branch in

5 For nearly all sites surveyed, we constructed maps like that in Figure 9, and from the maps we constructed profiles like those in Figure 10. Appendix A contains all such maps and profiles plus, for many sites, block diagrams of the topography also constructed from the topographic maps. We present Figures 9 and 10 here as illustrations of the type of detailed data that we gathered.

detail. In another, Luk’yanov (#1789) measured 2.5 m of leftlateral slip on the southern strand, but on the northern strand he estimated it to be only 0.15–0.2 m. From the observations of these researchers the maximum values of subsidence of the graben reach 2 m along the northern strand and about 1 m along the southern strand. In much of the area between the Bahar graben and the wide alluviated valley herein called Ulaan bulag höndiy (Plate 1), only one strand could be recognized. Approximately 2 km east of the eastern end of the Bahar graben, Florensov (#176, 177) inferred left-lateral offsets of 2.5–3 m. Farther east, however, Baljinnyam et al. (1993, Fig. 38c) reported a left-lateral offset of 7 ± 2 m of a ridge ~2 km west of Ulaan bulag höndiy (Plate 1). In this same

Figure 5. Aerial photograph M-649-28/VI/58-2 showing the relatively simple trace of the 1957 rupture across gentle terrain near the western end of the rupture (see Plate 1 for its location). A white dot shows the location of the photograph in Figure 6. North is toward the top. (Scale is approximate.)

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Figure 6. Photograph looking north showing a south-facing scarp near the western end of the rupture, at the white dot in Figure 5. We estimated ~3 m of left-lateral slip, marked by R. A. Kurushin (left) standing on the northern uplifted side and A. Bayasgalan on the southern side (diamonds surround both). Note also that the heights of the scarp on the right and left of the gully differ slightly, suggesting left-lateral slip. A jeep on the left side of the photograph provides an additional scale. Photograph by P. Molnar, August 24, 1994.

area, on November 8, 1958, Florensov noted that “amplitudes of horizontal displacement vary from 5 to 7 m,” but we cannot pinpoint better where the change from 3–4 m to 5–7 m lies. Just west of Ulaan bulag höndiy, the deformation is complex with horsts and grabens between two parallel traces tens of meters apart (Baljinnyam et al., 1993, Fig. 38). V. P. Solonenko (#582) had inferred a rapid variation in offset from 2.15 m to 7.75 m along only 185 m of the scarp (but shown as 350 m in Florensov and Solonenko, (1963, 1965, Fig. 157). We suspect that Solonenko’s estimated variation in offset does not reflect large deep-seated strain, but merely the transfer of slip from one trace to another. Where the scarp crosses the Ulaan bulag höndiy (Plate 1), no trace of it remains, but in January 1958, a northfacing scarp 1.5–2 m high marked the rupture (Florensov and Solonenko, 1963, 1965, Fig. 68).

5–7 m. Over the next 5 km eastward and near the main trace, recent erosion has exposed a syncline formed in Early Cretaceous sedimentary and volcanic rock, clearly seen on aerial photos (Fig. 12). The rupture follows the southern edge of the syncline, which may have formed during Cenozoic strike slip along the main east-west fault separating the Nuuryn Höndiy (“Valley of Lakes”) (Fig. 2) and the mountains to the south6. The rupture is not straight, and vertical components vary along the trace with larger components where the strike is more easterly than its typical east-southeast orientation. We mapped topography at two nearby sites (2 and 3) where the scarps are sharp (Figs. 13 and 14) and left-lateral strike-slip components are approximately 5 m. The local strike of the smooth, sharply defined trace varies

Section B–C: Bogd rupture from Ulaan bulag höndiy to Öndgön Hayrhan (“Egg Mountain”)

6 Florensov and Solonenko (1963, 1965) refer to this as the Dolino-ozersky fault, literally “Valley of Lakes” fault in Russian, but we will simply use “Bogd fault,” recognizing that the rupture in 1957 did not exactly follow the older structure everywhere.

The maximum magnitude of slip (5–7 m) and the most spectacular scarps of the Bogd rupture characterize this portion of the rupture (Plate 1). In some parts, however, the rupture splays into separate traces with oblique orientations and large vertical components, making for a complex zone of deformation where total offsets cannot be measured reliably. From Ulaan bulag höndiy eastward for approximately 15 km (Plate 1), the rupture is very sharp. Throughout most of this area, slip seems to be confined to only one strand. Only along a short rupture, hundreds of meters long and roughly 2 km north of the main trace, has subsidiary deformation been noted. A. V. Luk’yanov (#1959) and N. A. Florensov (#194) reported a low reverse scarp facing northeast, striking N60°–70°W and as much as 0.2 m high (Plate 1). Just east of Ulaan bulag höndiy, the trace faces north for ~1 km, and several gullies in the late Quaternary cover are offset

Figure 7. Photograph, taken by V. P. Solonenko on January 3, 1958, showing offset vehicle tracks. View south-southeast across the scarp. The truck is parked on the tracks, which can be seen in the foreground, offset left-laterally 3.4 m.

Surface rupture of the 1957 Gobi-Altay earthquake

Figure 8. Aerial photograph M-649-9/VIII/58-2977 showing the ruptures that make the western part of what Florensov and Solonenko (1963, 1965) called the Bahar graben, south of the Bahar uul. Black arrows denote scarps forming two traces. The southern trace faces north and casts a shadow. The northern trace faces south and is illuminated by the sun. The two traces can be seen in the western part of the photograph (see Plate 1 for its location), but the northern trace dies out eastward. Site 1 (Fig. 9) is shown by the white box. North is toward the top. (Scale is approximate.)

Figure 9. Detailed topographic map of Site 1, where Bogd Rupture along the southern foot of the Bahar uul consists of only one strand (Fig. 7). A relatively wide gully with a deep young channel crosses the scarp between adjacent ridges. Deflections of contours along y ≈ 11 m, particularly those for 2.5 m < z < 4.5 m in the western part of the area, can be explained either by left-lateral slip or by a vertical component with the south side up. The closer spacing of contours parallel to the fault trace on the left side of the map, than above or below the trace, however, imply that any vertical component must have involved uplift of the north, not south, side. Thus, these deflections imply left-lateral slip of 3–5 m. The relatively deeply and freshly incised main channel of the gully shows, at most, only a slight vertical offset. The deflections of contours south of the trace near x = 45 m show that the channel once lay east of its present course on the south side of the fault. Farther east, the trace follows a steep southwest slope, and slumping of material has modified the topography there. Dark lines along y = 10 m and y = 13 m mark the profiles shown in Figure 10.

11

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R. A. Kurushin and Others

Figure 10. Topographic profiles parallel to the rupture at Site 1, showing the vertical and horizontal components of slip. The dark solid line and the dashed line show topographic profiles along the lines y = 10 m and y = 13 m in Figure 9. The thin solid line shows the profile along y = 13 m displaced to be aligned with that along y = 10 m. It matches the western slopes of the gully and the present channel on the north side to the abandoned channel on the south side, we estimate a horizontal separation of the profiles of 4.0 ± 0.8 m. The relatively large uncertainty includes the difficulty of matching the profiles on the east side of the gully and uncertainties in the position of the channel before the earthquake. The trend of the gully is oblique to the fault trace (Fig. 9). Correcting for an obliquity of 75° between structures and the fault trace and a distance of 3 m between profiles reduces the horizontal separation to 3.2 m. The profiles are separated vertically by 0.9 m, but the southward slope of the regional topography accounts for roughly 0.5 m of the measured vertical separation of 0.9 m, leaving a vertical component of slip of only 0.4 ± 0.2 m. (Readers should note that we include this and Figure 9 as examples of the kind of data we gathered, all of which are presented in Appendix A, but unfortunately this example is not the clearest.)

Figure 11. Mosaic of aerial photographs M-649-28/VI/58-252, -253, and -254 showing the Bahar graben of Florensov and Solonenko (1963, 1965). Note the two roughly equidistant strands of the rupture, denoted by black arrows. As in Figure 8, the southern rupture faces north and casts a shadow, but in most places the northern trace faces south and is brightly illuminated. North is toward the top. (Scale is approximate.)

Surface rupture of the 1957 Gobi-Altay earthquake

13

Figure 12. Mosaic of aerial photographs M-649-28/VI/58-227 and -226 showing the trace of the 1957 rupture along the southern margin of a syncline in Mesozoic sedimentary rock and basalt. This syncline has been exposed by late Quaternary erosion of unconsolidated sediment that covers this area. Black arrows denote the sharply defined trace, which clearly is not straight. White boxes surround Sites 2 and 3. Black arrow on the right indicates north. (Scale is approximate.)

between N100–105°E to N80–85°E as it continues eastward for another 12 km across both deformed Mesozoic sedimentary rock and flat pediments on which a thin cover of Quaternary sediment masks the deeper structure (Figs. 12 and 15). Just east of Site 3 (Fig. 12), where the trace steps north 200–300 m and a component of extension is required, the rupture is marked by several splays with south-facing scarps. Then ~500 m east of Site 3, slip is localized on a single trace. Again, clear left-lateral offsets can be found where small gullies have been displaced, and values of 5–7 m characterize clear offsets (Fig. 16). Where

Figure 13. Photograph taken by V. P. Solonenko on November 10, 1958, looking east-northeast across the sharply defined scarp at Site 2, where we measured 5.1 ± 0.5 m of left-lateral slip and a vertical component of 0.4 ± 0.2 m (Florensov and Solonenko, 1963, 1965, Fig. 112).

the scarp trends east-southeasterly, it faces south and in places has dammed drainage and ponded water emanating from springs along the fault zone (Fig. 15). At Site 4, approximately 0.5 km east of Sevsüüliyn bulag (Plate 1). Deformation was not localized on a narrow scarp, but a left-lateral offset of 4.0 ± 1.5 m is apparent. For 12 km east of Sevsüüliyn bulag (Plate 1), the rupture is much less simple than to the west. In an area 3 to 5 km east of Sevsüüliyn bulag, we found reliably measured offsets to be sparse. N. A. Logatchev and A. S. Eskin recognized two obliquely oriented scarps with different amounts and senses of slip. Loga-

Figure 14. Photograph, taken by Molnar on August 18, 1993, looking west-northwest along the scarp at Site 3, where we measured 5.0 ± 0.5 m of left-lateral slip and a vertical component of 0.7 ± 0.3 m.

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R. A. Kurushin and Others

tchev (#1262) reported a strike-slip component of only 3.5 m along the main trace, with a south-facing reverse scarp 0.5 m high, and 0.7 m of left-lateral slip on a north-facing, obliquely striking splay, also with a reverse component roughly 0.3 m high (#1265). Eskin (#2203, 2206) described comparable heights of scarps, but east of where Logatchev worked, the sense of vertical slip on the more important scarp changes to face north. Farther east, Eskin

Figure 15. Aerial photograph M-649-10/VIII 58-3352 of Sevsüüliyn bulag region. In the western part of the photograph, the trace, denoted by black arrows, crosses relatively steep terrain dissected by ephemeral streams, where sharply defined ridges and gullies are offset 5–6 m. The white dot shows the location from which the photograph in Figure 16 was taken. Farther east, the trace follows relatively smooth terrain; the south-facing scarp has blocked drainage and provides a ground water barrier that manifests itself as a line of springs. The main spring, Sevsüüliyn bulag, is marked by the dark area between the right-most arrow and the white box, which shows Site 4. North is toward the top. (Scale is approximate.)

Figure 16. Photograph looking south-southwest (from spot shown by white dot in Fig. 15) across a fresh scarp west of Sevsüüliyn bulag. R. A. Kurushin (surrounded by diamond) stands in the downstream thalweg. A triangle surrounds his hat, which rests on the ridge at the end of the upstream thalweg, 6 m from him. Photograph by P. Molnar, August 19, 1993.

(#2205) reported a height of 2–2.3 m with only 1.5 m of leftlateral slip and inferred a component of reverse faulting and noted a southerly dip of 75°–85°. Between the localities described by Logatchev and Eskin along the northern edge of these scarps, we saw evidence of two events, with comparable amounts of slip. Where the scarp crosses a gully and faces south on the east side of the gully, the fault cuts bedrock and reveals a lower surface of fresh, lightcolored, only mildly weathered gray conglomerate offset 3–4 m horizontally (Fig. 17). Above the bedrock exposed in 1957 is a comparable area of pitted conglomerate, weathered brown by desert varnish. Apparently a preceding earthquake exposed the upper section of more weathered conglomerate. About 2 km east of this area, again the trace is complex, consisting of several obliquely trending strands (Plate 1). As can be seen in an aerial photograph of the area (Fig. 18), the main scarp from the west bifurcates. A low scarp continues east and faces north. A more prominent scarp curves toward the southeast for approximately 1.5 km. We estimated left-lateral slip of 3–4 m along this scarp, where it strikes N120°E. Approximately 500 m farther southeast, it disappears in an area of young alluvial deposits. Among the hills 100–200 m farther east, the 1957 rupture reappears as a complex zone of faulting with many short separate strands. Where this deformation coalesces into sharply defined traces, the zone strikes N50°E. Individual traces are not straight, and strike-slip offsets were not clear to us. Vertical components as large as 2 m appear to reflect normal faulting (Fig. 19). Approximately 2 km farther east, this northeasterly trending, normal scarp curves east-southeast, where we estimated

Figure 17. Photograph, looking north, at the scarp that cuts conglomerate a few kilometers east of Site 4, showing evidence of ruptures at two different times. At the base of the scarp, the conglomerate is not significantly weathered, but its upper half is deeply pitted and darkened by desert varnish. B. Enhtuvshin (surrounded by a diamond) and R. A. Kurushin (surrounded by a square) provide scales for the apparent vertical components. Nearby we estimated a vertical component of 0.7 m; thus slip seems to have been largely strike-slip, ~3–4 m. The relative heights of the fresh, white scarp and the darker upper scarp suggest that comparable amounts of slip occurred in 1957 and the preceding event. Photograph by P. Molnar, August 20, 1993.

Surface rupture of the 1957 Gobi-Altay earthquake

Figure 18. Aerial photograph M-10/VIII 58-3383 showing a complex set of faults (see Plate 1 for the location of photograph). Note that a trace, marked by a black arrow, enters the photograph on the left, and splits into two traces, with the more prominent curving southeastward (also denoted by black arrows). Near the eastern (right) side of the photograph, the most prominent scarps trend northeast. They face northwest and cast shadows that define them on the photograph. The white dot near the right shows the location from which the photograph in Figure 19 was taken. North is toward the top. (Scale is approximate.)

a left-lateral offset of ~4 m, and merges with the poorly defined scarp north of the hills along the northwestern margin of the Noyon nuruu to form a single, simple trace farther to the east. The trace is particularly sharp, although not everywhere straight, over most of its length along the northern margin of the Noyon nuruu (Plate 1). Near the foot of the highest part, offsets are especially clear both on aerial photos (Fig. 20) and on the ground. At the mouth of the Nurgiyn am, A. V. Luk’yanov (#1764) reported a prominent south-facing scarp as much as 2 m high on the west side of a low ridge and a comparably high, but north-facing, scarp on its east side, indicative of left-lateral slip. He estimated 7.5–8 m of left-lateral offsets of numerous gullies in this region. We mapped Site 5 only 100–200 m west of the Nurgiyn am and measured 5.8 ± 1.5 m of left-lateral strike-slip displacement of small gullies and a vertical component of 1 ± 0.5 m with the north side up, opposite in sense to that of the regional topography. In this area, we paced a series of 14 offset gullies and intervening ridges and found nearly all to be separated by 5 to 6 m, with an average left-lateral separation of 5.7 m, only two apparently as large as 7 m, and one as small as 3–4 m. Along part of the rupture shown in Fig. 20, offsets about twice as large (10–12 m) suggest that two events occurred since the gullies formed. Approximately 5 km east of this area where strike-slip offsets are especially clear, the main trace splits into two branches that surround hilly terrain in red and gray early Cretaceous sedi-

15

Figure 19. Photograph, viewed south-southwest, showing a surface break striking roughly N55°E in the zone of complex deformation west-northwest of Noyon uul (Fig. 18). B. Enhtuvshin (above the black triangle) standing near the base provides a scale. The scarp here shows a large component of normal faulting. Photograph by P. Molnar, August 20, 1993.

mentary rock (Plate 1, Fig. 21). The scarp of the northern branch faces north, suggesting reverse faulting, but the height is not large (~0.5–1 m). Logatchev (#1176) noted left-lateral slip of only 1.5 m, and we observed similar, relatively small amounts (~1 m). The amount of strike-slip displacement seems to decrease toward the east, but we found vertical components to be significant (~1–1.5 m) near where the Ulaan Shandiyn am crosses the scarp (Plate 1). In contrast, the western end of the southern branch is poorly defined, and displacements seem to be small (~1 m). Farther east, however, beginning just north of a fresh landslide marked on Plate 1, we found clear left-lateral offsets of ~5 m (Figs. 22 and 23). At Site 6, ~500 m east of the strike-slip offset in Fig. 23, we mapped a stream channel and neighboring terrace offset left laterally 10.4 ± 1.5 m (Fig. 22), suggesting that the displacement results from more than one event. Corroboration of this inference lies ~100–150 m east, on the east bank of the adjacent valley, where left-lateral slip has created a sharply defined south-facing scarp (Fig. 24). The lower part of the scarp presents a fresh surface. The upper part exposes gray, pitted, early Cretaceous coarse conglomerate. Slip in an earthquake before 1957 apparently exposed this part of the conglomerate. This southern strand continues east as the principal rupture (Fig. 22) and becomes simple and particularly impressive. As is clear on aerial photos (Fig. 25), the scarp wraps around the southern margin of the prominent light-colored hill, Öndgön Hayrhan, and is not straight. Because of large vertical components with the south side up as much as 5 m (Figs. 26–30), the scarps cast shadows to the north. At two localities (Sites 7 and 8), we measured 5.5 ± 0.5 m and 7.0 ± 1.4 m of left-lateral slip and corresponding vertical components of 5.1 ± 1.1 m and 5.0 ± 1.0 m. The strike-slip faulting continues around to the eastern end of the Öndgön Hayrhan, where deformation becomes dis-

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Figure 20. Mosaic of aerial photographs M-649-28/VI/58-192 and -191, showing clear left-lateral strike-slip offsets along the Bogd rupture north of Noyon uul and near the Nurgiyn am (Plate 1). Black arrows mark the scarp. The white box shows the location of Site 5. The Nurgiyn am is the valley on the eastern edge of the photograph. Near Site 5, we measured offsets of 5–6 m, but notice that in the center of the mosaic, offsets roughly twice as large are present. In this area, Ritz et al. (1995) estimated a late Quaternary slip rate of ~1.2 mm/yr. Arrow on the right points north. (Scale is approximate.)

tributed on several short, normal-fault splays, each tens of meters long (Fig. 31). Just east of Öndgön Hayrhan, the main strand strikes N75–80°E. We surveyed Site 9 in this area (Fig. 32) and measured only 3.9 ± 0.5 m of left-lateral slip. Slip here, however, is not localized on only one strand. This east-northeast striking trace intercepts another strand striking east-southeast (N105°E) ~100 m east of Site 9 (Figs. 25 and 31). West of where they meet, the east-southeast–striking strand continues toward Öndgön Hayrhan and reveals itself as a minor scarp facing south on the north side of Öndgön Hayrhan. We could not trace this scarp, or others, to the west end of Öndgön Hayrhan. To the east, this eastsoutheast–striking strand is the main, and only, strand. As discussed below, east of the Hüühniy höndiy, left-lateral offsets are consistently 3–3.5 m. Thus, in the area of Öndgön Hayrhan, the main trace from the east splits with nearly all displacement being absorbed by slip on the trace that curves around the south side of Öndgön Hayrhan. Section C–D: Bogd rupture from the Hüühniy höndiy to the Dalan Türüü foreberg The magnitude of left-lateral slip across the northern margin of Ih Bogd, 3 to 3.5 m, is consistently smaller than that farther west. Moreover, along much of the rupture north of Ih Bogd, the south side has moved up relative to the north, consistent with the high regional topography. For a distance of ~2 km east of the Hüühniy höndiy, the rupture consists of two parallel traces, ~0.3 km apart, with a graben

Figure 21. M-10/VIII 58-3376 showing the divergence into two strands (denoted by black arrows) of the rupture west of the Shandiyn Am, the valley near the eastern edge of the photograph (See Plate 1 for location). The northern strand is the clearer in the west, although the displacements that we measured along it are approximately half as large as those where only one strand is present. The southern strand is more important toward the east, where strike-slip offsets of 5–6 m are clear on the ground (Fig. 23). North is toward the top. (Scale is approximate.)

Surface rupture of the 1957 Gobi-Altay earthquake

17

Figure 22. Aerial photograph M-649-28/VI/58-179 showing the trace of the southern branch (denoted by black arrows) near the Uhaagiyn am (See Plate 1 for location). In the western part, left-lateral offsets are clear; the photograph in Figure 23 was taken looking south, down the second valley east of the left-most black arrow. The left-lateral offset at Site 6, enclosed in the white box, is easily recognized, and the area shown in Fig. 25 lies just east of the box. Farther east, the trace curves across gentler topography, but remains clear. North is toward the top. (Scale is approximate.)

between them (Fig. 33). Farther east, where only a single trace is clear, we measured 3–3.5-m offsets of gullies at several localities. From detailed mapping of two adjacent sites (10 and 11), we estimate left-lateral offsets of 3.0 ± 0.8 m and 3.5 ± 1.0 m (Fig. 34), with small vertical components (< 0.5 m). East of Sites 10 and 11, the rupture is complex where it crosses a landslide that predates the 1957 earthquake (Plate 1), but again 3–3.5-m offsets characterize slip east of the landslide (Fig. 35). We mapped the topography at Site 12 (Fig. 36) and measured 3.5 ± 0.5 m of left-lateral slip. Whereas west of Site 12, vertical components are small, here there is a significant vertical component of 1.3 ± 0.4 m, with the south side up. The vertical component is clear ~1 km east of Site 12, where the scarp crosses a wide valley (Fig. 37). The higher older terrace on the south side of the scarp stands 2.5–3 m above its northward continuation, but the lower terrace lies only 1–1.5 m above its continuation. If the vertical component measured at Site 12 applies to this area, it implies that the higher (2.5–3 m) scarp developed with two earthquakes. The abundance of boulders exposed in the scarp prohibited quantitative analysis of the scarp using the diffusion equation (e.g., Hanks et al., 1984) to test this idea. Approximately 0.5 km farther east, the scarp climbs over topography that slopes steeply northward. For a short portion just west of the Tsagaan Burgasniy sayr (Plate 1), where the local strike of the trace becomes east-northeast (Fig. 35), the vertical component is ~2 m. High scarps on west sides of gullies, and low, or nonexistent, scarps on east sides attest to oblique left-lateral slip (Fig. 38). A few hundred meters east of the Tsagaan Burgasniy sayr, Eskin (#2056) reported left-lateral slip of 3 m and a vertical component of 2.5 m with the south side up, and 3 km farther east he (#2049) reported only 2.5 m of

strike slip, but as much as 3 m of vertical slip, with the south side up. Deformation within the mountains south of this area may have absorbed some of the 3–3.5 m of slip observed east and west of this area. Approximately 12 km farther east, to the Urd Burgasny am, evidence of strike slip is sparse and less convincing than to the west, but vertical components commonly are large. We searched for definitive evidence for strike-slip offsets and at Site 13 mapped the least unconvincing example that we saw (Fig. 39). A hint of an offset gully on the front of a high scarp is consistent

Figure 23. Photograph, looking south, at a scarp crossing a gully approximately 500 m west of Site 6. R. A. Kurushin stands in the valley in front of the scarp. Photograph by P. Molnar, August 23, 1993.

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Figure 24. Mosaic of photographs looking north at the eastern edge of the gully that lies immediately east of Site 6 (Fig. 22). On the right, opposing black triangles show the top and bottom of the scarp, defining a small vertical component (<0.5 m) at this locality. In the valley, black triangles show the top of the fresh scarp. R. A. Kurushin provides a scale in two localities. Photographs by P. Molnar on August 23, 1993.

Figure 25. Mosaic of aerial photographs M-649-28/VI/58-174 and -173 showing scarps (denoted by black arrows) on the south side of Öndgön Hayrhan (Egg Mountain), the light-colored hill in the center. Note the segmented nature of the scarps and the overall curved trace. White boxes surround Sites 7, 8, and 9, from west to east. At the eastern end of Öndgön Hayrhan, just west of Site 9, the trace splays into a series of normal scarps, which may indicate superficial deformation (Fig. 31). The trace continues to the east, beyond Site 9, where it intercepts another scarp that strikes east-southeast. Also on the north slope of Öndgön Hayrhan, superficial deformation can be seen in places. White dot indicates the spot from which photograph in Figures 29 was taken. Arrow on the right points north. (Scale is approximate.)

Surface rupture of the 1957 Gobi-Altay earthquake

19

Figure 26. Photograph, by P. Molnar on August 22, 1994, looking west-southwest across the basin formed by 5.5 ± 0.5 m of left-lateral slip and a vertical component of 5.1 ± 1.1 m at Site 7. B. Enhtuvshin on the top of the scarp and A. Bayasgalan at its base define the scale.

Figure 27. Photograph, by P. Molnar on August 22, 1994, looking southeast at the same scarp as in Figure 26.

Figure 28. Photograph looking east-southeast along the rupture between Sites 7 and 8. A. Bayasgalan at the base of the scarp in the left center and B. Enhtuvshin on the south side in the distance provide scales for vertical components. Photograph by P. Molnar, August 22, 1994.

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Figure 29. View west along the rupture approximately 300 m east of Site 8. Circles surround people on or next to the scarp. Photograph, using a 135-mm lens, by P. Molnar on August 25, 1995.

with 3.0 ± 1.0 m of left-lateral slip, with a vertical component of 2.0 ± 1.0 m. Because of large boulders, meters in dimension, within the fan, strike-slip offsets are poorly defined. A long profile across the scarp shows a 6.8-m cumulative offset of the fan and, therefore, that vertical components of slip with the south side up must have prevailed in late Quaternary time. This style of deformation with large vertical components characterizes faulting along the northern edge of Ih Bogd just north of its high summit plateau (Plate 1). We suspect that the 3–3.5-m leftlateral offsets, typical of regions to the west, and to the east of the Dalan Türüü foreberg discussed in the next section, pass across the northern margin of Ih Bogd but are obscured by the thrust components that cut fans with boulders. A zone with complicated deformation surrounds a hill between the Urd Burgasny am (“South Bush gorge”) and the Bitüütiyn am (literally “Closed” or “Dead-end gorge”) (Fig. 40, Plate 1). Deformation on the north side of the hill is complex. Although we did not examine it thoroughly, our observations south of the hill agree with Luk’yanov’s (1965). West of the Urd Burgasny am, the rupture is diffuse and not sharply defined, but the scarp is prominent on its east side. A. V. Luk’yanov (#1597) measured a vertical component ~2.5 m, with the north side up, and 3–4 m of left-lateral offset where the scarp cuts a horse trail (Fig. 41). In the same general area, we too saw a south-facing

scarp approaching 2 m in height. The sense of the vertical component with the north side up, opposite to that farther west or east and in a region where reverse components occurred, surely is a manifestation of there being at least two active strands. Post1957 erosion of the area that we mapped (Site 14) made inferring both the strike-slip offset of 3.9 ± 1.0 m and a substantial vertical component of 1.6 ± 0.5 m more uncertain than elsewhere. Near the Bitüütiyn am, ~3 km east of Site 14, the scarp curves to strike east-northeast, and both the vertical and the strike-slip components decrease. Luk’yanov (#1595) reported only 1.2–1.3 m of left-lateral slip and a height of the scarp of only 1–1.2 m, but nearby, approximately 800 m west of Bitüütiyn am, we estimated a strike-slip offset of 3 to 4 m. Where the scarp crosses the Bitüütiyn am, N. A. Florensov (#52) reported a 1–1.5-m high south-facing scarp in the valley floor, but in his notebook he showed a sketch of a graben 5 m wide with the southern edge defined by a scarp 0.5 m high. Evidence of disruption among the boulders in the valley floor persists, but we could not measure an offset. East of the Bitüütiyn am, a zone of recent surface disruption, which includes mole tracks, tension cracks, and scarps with vertical components of slip, can be traced for several kilometers, but in no place between the Bitüütiyn am and the Dalan Türüü foreberg could we find examples of strike-slip dis-

Surface rupture of the 1957 Gobi-Altay earthquake

Figure 30. Photograph looking south across the rupture approximately 500 m east of Site 8. The large apparent vertical component is due mostly to left-lateral slip. Photograph by P. Molnar, August 25, 1995.

placement that we could measure reliably. The rupture zone is tens of meters wide (Florensov and Solonenko, 1963, 1965, Fig. 162), which makes quantifying the amount of strike slip difficult. This zone of disruption curves into a northeasterly trend, along which deformation remains evident but not obviously large, and continues northeastward toward the west end of the Dalan Türüü foreberg. Section D–E: Dalan Türüü foreberg Northeast of Ih Bogd, the average strike of the break in slope at the base of the mountainous terrain trends more eastsoutheasterly than farther east or west where they strike nearly east-west. Where the base of the Ih Bogd massif strikes most southeasterly, a prominent row of hills called Ar Zeregleeniy Aarag (“Behind the Mirage’s Pelvis”), 9–10 km long and with relatively steep northeast slopes, has formed roughly 3–5 km northeast of the northeastern edge of the massif (Fig. 42). Florensov and Solonenko (1963, 1965) referred to this row of hills as the Dalan Türüü (“Dolon Turu”) foreberg. On their southwest sides, the hills grade gently into alluvial fans onto which debris from the Ih Bogd massif has been deposited. Within the foreberg, Cenozoic sedimentary rock commonly dips southwest but forms an anticline near its northern edge (C. Prentice and D.

21

Schwartz, 1995, personal communication). Thus, the foreberg seems to have formed by thrusting and folding of this sedimentary rock and its Quaternary cover of fan debris northeastward onto the lower terrain of the Nuuryn Höndiy to the north. A fresh scarp bounds the northeastern margin of the foreberg (Fig. 43). We mapped four sites (15–18) where scarps show vertical components that increase from 1.0 ± 0.2 m in the northwest to 2.2 ± 0.5 m and 2.0 ± 0.5 m in the center and 2.0 ± 0.5 m in the southeast. At the southeast end of the hills, the scarp, which follows the foot of the hills, curves southward making almost a right angle (Plate 1). In this general area, N. A. Florensov (#38) measured a height of 1–1.5 m. Near the base of the Ih Bogd massif, south of the east end of the foreberg, another trace of surface ruptures can be seen (Plate 1). We estimated a height of 0.8 m at one locality, and Florensov (#36) measured 0.4 m at another on an alluvial fan emanating from the mountains immediately to the south. Southeast of the foreberg, flow in the Huustiyn am seems to have destroyed any hills that may have formed and to have deposited a large alluvial fan (Plate 1). This fan, however, has been cut by a high scarp (Fig. 44), which may be a continuation of the thrust fault that bounds the foreberg where hills are clear. Florensov (#31) examined the western part of this scarp, where its height reaches 6–8 m. He described evidence from the base of the scarp for ~1 m of uplift in 1957. Farther southeast we constructed a long profile across the scarp cutting the fan (Site 19), which shows a vertical component of slip of 15.8 m since deposition on the fan ceased, presumably in late Quaternary time, with a suggestion of 2 ± 1 m in 1957. At Site 20, only 100 m from the southeast end of this rupture, where it intercepts the main strike-slip continuation of the Bogd rupture, the vertical component is 2.7 ± 0.5 m. Little evidence allows a quantitative estimate of the dip of the thrust fault underlying the foreberg. The curved and segmented shape of the scarp across the landscape, however, suggests a gentle southwest dip. Surface deformation was not confined to the edge of the foreberg. Large, steep scarps formed near the crests of the hills (Figs. 45 and 46; Florensov and Solonenko, 1963, 1965, Figs. 14 and 103). Moreover, Florensov (#26) reported minor cracks on the alluvial fan between the foreberg and the Ih Bogd massif. These scarps are short, less than 1 km in length. They curve along strike, and they face both north and south, forming small grabens and horsts within the hills (Fig. 46). Strike-slip components were not obvious to us on most scarps, except a small left-lateral component (±0.5 m) on one. The northeast orientation of these normal-fault scarps, oblique to the front of the foreberg, might reflect a component of left-lateral shear parallel to the foreberg. Thus, these scarps, some of which exceed 1 m in height, appear to represent superficial deformation of the hanging wall of the underlying thrust fault; they result from a combination of stretching of the earth’s surface above where the dip in the thrust fault changes (Fig. 3, a or b) and left-lateral shear. Insofar as they represent stretching of the surface over a growing anticline, they are the brittle manifestations of faultbend folding (Suppe, 1983, 1985, p. 343–348), and hence are sim-

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Figure 31. Photograph, using a 20-mm lens, looking south-southwest of Site 9 and the east end of Öndgön Hayrhan, where two branches, indicated by black arrows, intersect. White rock on the right marks the east end of Öndgön Hayrhan. On the east end of the hill, a series of scarps, between black arrows, define a wide pull-apart (see Fig. 25). In the foreground, the scarp entering from the left near the base of the hill can be seen to diverge into two scarps, one of which continues straight, below the hill, and the other curves toward the upper right of the photograph. Photograph by P. Molnar, August 17, 1993.

ilar to the normal faults and grabens caused by the El Asnam, Algeria, earthquake of 1979 (King and Vita-Finzi, 1981; Philip and Megraoui, 1983). Hence, they imply a steeper fault beneath the crest of the foreberg than beneath its northern margin. Although A. V. Luk’yanov (#1592, 1594) reported some deformation within the Ih Bogd massif south of the Dalan Türüü foreberg, major deformation at the foot of the massif appears to be absent. Florensov and Solonenko (1963, 1965) reported little evidence of such deformation, and we too saw none. The clearest faulting between the foreberg and the massif lies near the intersection of the Dalan Türüü thrust system and the strike-slip zone farther east (Plate 1). Florensov (#32, 36) reported no evidence of strike slip along the westward continuation of the strike-slip trace, but he did note that the vertical component decreased westward from nearly 1 m near the junction to only 0.4 m 1 km farther west. Whereas strike slip characterizes the Bogd rupture both east and west of the foreberg, where the rupture follows the east-southeast trend of the base of the Ih Bogd massif, immediately northeast of the Ih Bogd summit plateau slip includes a component of reverse faulting between the two east-southeast ruptures and where the trend is more southeast. The Dalan Türüü foreberg presumably results from an augmented convergent component in this area. Section E–F: Bogd rupture from the Dalan Türüü foreberg to the Toromhon sayr East of the Dalan Türüü foreberg, the rupture consists of relatively straight scarps of nearly pure strike slip separated by

regions where the rupture splays into more than one strand. In some areas, two or more prominent ruptures bound basins or hills, making the most splayed section of the Bogd rupture. The thrust system bounding the Dalan Türüü foreberg and the large alluvial fan farther east (Fig. 42) terminates at the east-

Figure 32. Photograph looking southeast at Site 9. Note the clear scarp to the right of the gully in the center, on which M. Ölziybat stands, and the near absence of a scarp left of it. The large vertical face is due to left-lateral slip of 3.9 ± 0.5 m of topography that slopes eastward, and a negligible vertical component (0.0 ± 0.5 m). Photograph by P. Molnar, August 17, 1993.

Surface rupture of the 1957 Gobi-Altay earthquake

Figure 33. Aerial photograph M-649-10/VIII/58-2736 of the area just east of the Hüühniy höndiy, which includes Sites 10 and 11 (surrounded by white boxes) and the graben west of them. Two traces (marked by arrows) define the graben. Farther east, they join to form a single scarp. North is toward the top. (Scale is approximate.)

west–trending Bogd strike-slip rupture. For most of the 10 km east of the thrust system, deformation is confined to one strand of nearly pure strike slip (Figs. 47 and 48). We measured 3.2 ± 0.5 m of left-lateral slip at Site 21. The eastern end of this 10-kmlong zone trace includes a significant vertical component, ~3 m

23

high, where it truncates a large alluvial fan (Fig. 49). Deformation in 1957 seems to be responsible for half of that height. Between 5 and 7 km east of Site 21, the rupture diverges into at least three splays (Plate 1). Strike slip on the southern splay, which locally trends southeast (N115°E), is modest (Fig. 50) and seems to die out eastward. With a tape measure, we measured 1.5 m at two localities, and surveying of Site 22 ~100–200 m east of them yields 1.3 ± 0.3 m. N. A. Florensov (#41) described the northern branch, bordering the north side of Ulaan Huts uul, as a system of cracks, dying out to the east, with the south side uplifted 0.3–0.5 m. The middle trace bends around the southern foot of Ulaan Huts uul, where it strikes N75°E; we estimated a vertical component of ~1 (±0.3) m, with the north side up. East of Ulaan Huts uul, the scarp curves eastward to strike N110°E, and Florensov (#55) reported a north-facing scarp as much as 2 m high and an opening of 0.5 m. In both regions none of us saw a strike-slip component, and we infer normal faulting. About 2 km farther east, Florensov (#57) again reported a north-facing scarp as much as 1.5 m high, which apparently results from oblique normal faulting. Eastward, this middle trace emerges as the main trace. The apparent deficit in strike slip, between 3 and 4 m observed west and east of this locality and between 1.3 and 1.5 m along the southern trace (Site 22), presumably results from a transfer to the northern trace west of where we measured only ~1.5 m and/or from counterclockwise rotation of the block between these two traces.

Figure 34. Photograph of Site 11, viewed south, shows B. Enhtuvshin standing on the ridge between gullies in the foreground, and M. Ölziybat (circled) on the south side of the trace next to the eastern edge of the two gullies. We measured 3.5 ± 1.0 m of left-lateral slip and a vertical component of 0.4 ± 0.2 m. Photograph by P. Molnar, August 16, 1993.

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This style of deformation, in which a predominantly strikeslip rupture diverges into splays that include large vertical components, is particularly well displayed where the trace crosses high terrain east of the Züün huuray, northwest of Dulaan Bogd uul, and south of Shanagan Shuvuun Baast uul (Plate 1, Fig. 51). Florensov (#8–10) measured small vertical components of 0.2 to 0.5 m along a thrust or reverse splay trending east-northeast and following the foot of the Shanagan Shuvuun Baast uul (Plate 1). A component of thrust or reverse slip there is consistent with the presence of elevated terrain north of the main rupture. The principal, apparently narrow, strike-slip rupture, however, passes through the western side of the high terrain, where it divides into two (or more) splays that bound a basin with gentle topography (Fig. 51) (Florensov and Solonenko, 1963, 1965, Fig. 165; Luk’yanov, 1965, Fig. 14). At the eastern end of the basin, scarps with large vertical components again join in a single strike-slip trace with only a small vertical component (Fig. 52). In a brief visit, we saw no evidence of strike slip along scarps within the basin. Instead normal slip apparently characterizes a fresh scarp along the eastern end off the southern margin of the basin (Fig. 53). Just east of this basin, surveying of Site 23 reveals left-lateral slip of 2.9 ± 0.5 m with the south side up 1.0 ± 0.5 m. Larger strike-slip offsets of adjacent gullies (9 m on the left gully and 6 m on the right of the mapped area) may indicate slip before 1957. Strike-slip faulting prevails eastward from Site 23, east of the high terrain in that area. At Site 24 (Fig. 54), ~4.5 km east of Site 23 and 3 km west of the Shavinahyn sayr, we measured offsets of 3.6 ± 1.0 m for a series of gullies. A scarp seen on aerial photographs approaches the main Bogd rupture <100 m east of Site 24 and strikes northwest for 1–2 km. In its middle part, where it crosses an alluvial fan, the height approaches 0.4 m, with the north side up. Thus, there might be reason to expect a different amount of displacement farther east. We found the trace for 1–2 km east of Site 24 to be obscure

Figure 35. Aerial photograph M-649-10/VIII/58-2733 showing the portion of the Bogd rupture east of where it crosses a pre-existing landslide (on the left edge). Note that the trace (marked by black arrows) is not straight, and that the prominent shadows cast by the scarp attest to significant vertical components. The white box surrounds Site 12, and two white dots show the places from which photographs in Figures 37 and 38 were taken. North is toward the top. (Scale is approximate.)

until reaching a major valley, where a sharply defined scarp reveals ~3–3.5 m of left-lateral slip. This contrasts, however, with observations reported by V. P. Solonenko, who (#520) noted 6.15 m of left-lateral slip (and a vertical component of 1.5–2 m with the south side uplifted) on the west side of the Shavinahyn sayr (Florensov and Solonenko, 1963, 1965, Figs. 131 and 152). Solonenko (#521–523) described complex deformation along a 3-km rupture to the west, with slip decreasing to 5.5 m and then 4.05 m (Florensov and Solonenko, 1963, 1965; see Fig. 117). East of the Shavinahyn sayr, evidence of strike slip is also abundant (Fig. 55). Florensov (#120) reported only 4–4.5 m of left-lateral slip (and a vertical component of 0.6–1.0 m with the north side up) on the east bank of the Shavinahyn sayr, but

Figure 36. Photograph, by P. Molnar on August 15, 1993, looking south at the western of two gullies at Site 12, where we measured 3.5 ± 0.5 m of left lateral slip and a vertical component of 1.3 ± 0.4 m.

Figure 37. Photograph looking west along the Bogd rupture toward Site 12 and toward the ancient landslide in Figure 35. Note the vertical component with the south (left) side up. This area lies west of the left white dot in Figure 35. Photograph by P. Molnar, August 15, 1993.

Surface rupture of the 1957 Gobi-Altay earthquake

25

Figure 38. Photograph looking south-southeast at the area just west of the Tsagaan Burgasny sayr (south of the right white dot in Fig. 35). The prominent scarp on the west (right) side of the gully and the low scarp on the east side indicates left-lateral slip. M. Ölziybat (circled) stands on the scarp to the right of the gully. Photograph, with 135-mm lens, by P. Molnar on August 15, 1993.

5–6 m of left-lateral slip on its west bank (#116). A drop from 5–6 m (or 6.15 m, according to Solonenko) to 4–4.5 m of leftlateral slip across the valley roughly 300 m wide requires large strain. The different reported senses of vertical components on the two sides of the valley suggest that vertical and horizontal separations may have been conflated with offsets. The different directions in which scarps face are consistent with pure strikeslip displacement, and both the vertical and the horizontal components may reflect apparent offsets that conspire to overestimate the left-lateral component. Florensov (#120–121) also measured amounts of strike slip at four localities farther east of the Shavinahyn sayr to the 101°E meridian, and nowhere did the amount exceed 5 m. His report of 4–4.5-m strike-slip and 1-m vertical components 100 m east of the east bank of the Shavinahyn sayr offers some corroboration for what he saw at the east bank. At the next valley, he inferred 5 m of left-lateral slip, apparently without a vertical component. East of 101°E (Plate 1), he estimated left-lateral offsets of 3.0 to ~5 m (#67–68) along a zone ~2.5 km long. We too found possible offsets between 3 and 5 m but could not find a consistent, reliable amount. This variability is troubling; such differences in offset cannot reflect deepseated slip without enormous local strains. We surveyed Site 25 along a scarp striking N89°E to quantify slip where Baljinnyam et al. (1993, Fig. 36) had reported 5 m. Unfortunately, objective interpretation of the topography allows for either 3 m of offset of a low ridge

Figure 39. Photograph looking south at Site 13. Note the hint of a gully on the front of a scarp behind M. Ölziybat on the left. B. Enhtuvshin stands on the right, at the head of its apparent former continuation. The scarp cuts an alluvial fan containing large boulders. We estimated a left-lateral component of slip of 3.0 ± 1.0 m and a vertical component of 2.0 ± 1.0 m. Over a longer profile, we measured a cumulative vertical offset of 6.8 m. Photograph by P. Molnar, August 14, 1993.

between gullies, or 5 m between the upstream and downstream segments of the gully on the left side of the area. Surveying of Site 26, ~250–300 m east of Site 25, yields 4.3 ± 0.5 m of left-lateral slip (Fig. 56). Although a short zone of additional deformation lies only tens of meters north of this

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Figure 40. Mosaic of aerial photographs M-649-10/VIII/58-2685 and -2684 showing the complex deformation just west of the Bitüütiyn am. Whereas farther west, slip is localized on one trace, the rupture in this area surrounds hills between the Urd Burgasny am (~400 m to the left of the white dot) and the Bitüütiyn am (the easternmost valley). The scarp on the south side faces south, unlike those farther west or east. The white box shows Site 14. The white dot shows where the photograph in Figure 41 was taken. The trace curves northeastward east of the Bitüütiyn am. North is toward the top. (Scale is approximate.)

trace, we infer this deformation to be largely tension cracking near a steep slope and not deep-seated faulting, as H. Philip, T. Rockwell, and E. Zilberman (1995, personal communication) also inferred. Perhaps, the most definitive measurement of slip was made by N. A. Florensov and V. P. Solonenko on January 5, 1958, when they measured left-lateral offsets of 2.8 m at two localities: (1) a trail cut by the fault roughly 200 m west of the Toromhon sayr (Solonenko et al., 1960, Fig. 16), and (2) tire tracks along the Toromhon sayr at the prominent scarp (Fig. 57; Solonenko et al., 1960, Fig. 13). In 1995, we saw that the rupture where Florensov photographed the western 2.8-m offset included splays that defined a zone ~5 m in width. Yet, we saw little reason to suspect that more than 0.5 m of offset could be hidden by slip on the minor splays. Moreover, only parts of this rupture, ~200 m in length, were characterized by a wide zone. Thus, we have no reason to doubt these measurements of 2.8 m. Yet, the freshness of the features with measured offsets of 3–4 m imply that they too originated in 1957. Perhaps fault creep increased the amount from 2.8 m in this area to greater than 3 m by the 1990s, as has been observed elsewhere, for instance on the San Andreas fault in California following the 1966 Parkfield earthquake (Smith and Wyss, 1968). Assuming that differences in strike-slip offsets as large as 2 m cannot be common occurrences at localities separated by only a few kilometers, we conclude that an offset of roughly 3–4 m formed in 1957 along the entire rupture between Site 24 and the Toromhon sayr (Fig. 2). Our difficulties in quantifying offsets between the Shavinahyn and Toromhon sayrs illustrate how one locality is insufficient to define a reliable offset. Nevertheless, our inability to estimate slip reliably in the field prevents us from categorically dismissing Solonenko’s inferences of

6.15 m, 5.5 m, and 4.05 m west of the Shavinahyn sayr and Florensov’s of 4–4.5 m and 5 m east of it (Plate 1). Section F–G: Bogd rupture from the Toromhon sayr to Baga Bogd The Bogd rupture can be traced east of the Toromhon sayr for at least 5 km as a clearly defined strike-slip rupture (Plate 1). For ~20 km farther east, between the Taany am and the Baruun hanh sayr (Fig. 2, Plate 1), the trace is poorly defined, but we suspect that left-lateral strike slip along a zone trending east-northeast characterizes the regional deformation. North of Baga Bogd, the rupture steps north to a set of hills,

Figure 41. Photograph, by R. A. Kurushin on September 25, 1958, looking north and showing a south-facing scarp 2–2.5 m high near Site 14. A. V. Luk’yanov stands on the scarp at the end of a trail. A hammer, standing on its head at the base of the scarp, marks its southward continuation and shows 3–4 m of left-lateral displacement. (A new trail climbs the scarp to the right of hammer.) The white dot in Figure 40 shows where this photograph was taken.

Surface rupture of the 1957 Gobi-Altay earthquake

27

Figure 42. Landsat Thematic Mapper image of the region near the Dalan Türüü foreberg. White arrows point toward the scarp along the northern margin of the foreberg (upper center) and also toward scarps on abandoned alluvial fans adjacent to the Bitüüt am and the Hustiyn am. Note also the absence of a sharp break in slope at the base of the mountains just southwest of the foreberg. North is toward the top.

herein called the Hetsüü foreberg, analogous to the Dalan Türüü foreberg. The Bogd rupture trends east-southeast across the Toromhon and Builsny Zadgay sayrs. The Toromhon Overthrust (discussed in a following section) approaches this area from the south but does not seem to have affected slip on the Bogd rupture east and

Figure 43. Photograph looking south-southwest at the scarp along the base of Dalan Türüü foreberg from 1 km southwest of the southwest corner of Orog Nuur. Note the scarp across the base of the hills at the top of the alluvial fan. Photograph by P. Molnar, August 11, 1993.

Figure 44. Aerial photograph M-649-28/VI/58-131, showing the alluvial fan cut by a high scarp, east of the Dalan Türüü foreberg, and the junction of that scarp with the Bogd rupture to its south. Black arrows mark the scarps. The high scarp on the alluvial fan enters in the upper left. Two white boxes show Sites 19 and 20. The main strike-slip rupture trends roughly east-west and continues ~2 km west of its truncation of the thrust scarp. North is toward the top. (Scale is approximate.)

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Figure 45. Photograph looking west-southwest along a prominent scarp within the Dalan Türüü foreberg, southwest of Site 17. Photograph by P. Molnar, August 8, 1993.

west of it (Fig. 58). We mapped two areas (Sites 27 and 28) in order to determine the amount of strike slip. Site 27 was chosen because V. P. Solonenko (#511), in Florensov and Solonenko (1963, 1965), had reported a maximum of 8.85 m of left-lateral slip at this locality. Because a neighboring gully appeared offset only 3–4 m (Baljinnyam et al., 1993, Fig. 34), we mapped both gullies together to quantify the 1957 offset (Fig. 59). Indeed, the offsets of both the smaller, western gully and the southern part of the larger gully are only 3.5 ± 1.5 m, with the north side down 2.0 ± 1.0 m. Similarly, ~1.5 km farther east at Site 28, 4.1 ± 1.0 m of left-lateral slip occurred (Fig. 60). Thus, if amounts of strike slip east and west of the Toromhon overthrust differ, the difference must be small, less than 1 m.

Near Site 27, Florensov and Solonenko (1963, 1965, Fig. 187) reported a northward dip of 65°, which is what one sees on the face of the scarp at the surface. The curvature of the trace across deep gullies indicates a comparable northward dip (Fig. 61), and hence a component of normal slip in this area. This nearly linear scarp can be traced for at least 1 km eastward from Site 28, along which we saw numerous offset gullies of 3–4 m. Farther east, the trace splits into at least two branches with one trending northeast and curving eastward. This trace seems to bound a small graben, which is clear on the satellite imagery (Fig. 62). H. Philip (1995, personal communication) measured a vertical component of 0.5 m, with the south side down, on the northern trace near its western end, but noted that this trace seems to become poorly defined a few kilometers farther east. We visited the eastward continuations of these branches, along the Taany am. Evidence of disruption along the southern trace, including strike slip, was clear, but we could not find a convincing marker demonstrating how much slip had occurred. We saw a scarp north of this area, as high as 1 m, but whose fresh offset did not exceed 0.5 m (Plate 1). Moreover, it faced north not south. Thus, if it is the continuation of the trace studied by H. Philip, the sense of slip must vary along it. In any case, it appears that the Bogd rupture becomes diffuse and poorly developed ~2 km east of Site 28. In the 15 km between the Taany am and Ar huuray sayr, evidence of fresh surface faulting is sparse (Plate 1). We spent little time in this area and rely on observations made in 1958. East of Huts uul, N. A. Florensov (#86–88) mapped two northeast-trending subparallel ruptures, ~1 km apart, with evidence of extension across them. The southern one dips northwest at 85–90°, with the vertical component less than 1 m. He inferred

Figure 46. Photograph looking southwest across a graben within the Dalan Türüü foreberg. The scarp in Figure 45, which faces north, can be seen near the top of the ridge in the upper left. In the lower right, adjacent to A. Bayasgalan (surrounded by a diamond), a scarp faces south. A third, smaller scarp in the middle of the photograph, near R. A. Kurushin (circled) faces north. Photograph by P. Molnar, August 8, 1993.

Surface rupture of the 1957 Gobi-Altay earthquake left-lateral strike slip but apparently could not quantify it. Along the northern scarp, where the southern side had dropped 0.5 m, Florensov saw no evidence of strike slip but noted that the Mongolian geographer, J. Dügersüren, did suggest such slip farther west. Although linear features can be seen, a clear trace is not apparent on the Landsat imagery of this area (Fig. 62). On the alluvial fan northwest of Baga Bogd and west of the Baruun Hahn sayr, a fresh rupture can be traced on aerial photos with the aid of stereo viewing. On the ground we found a continuous arcuate rupture from near the Ar huuray sayr northeast for at least 7 km to the eastern side of the alluvial fan of the Baruun hanh (Plate 1). Mole tracks and tension gashes demonstrate left-lateral strike slip. At one locality near the west bank of the Baruun hanh sayr we estimated 3 m of left-lateral slip (Plate 1). On the east bank, deformation is spectacular; scarps 1–2 m high bound a graben ~40 m wide, with smaller scarps within it (Fig. 63). The graben curves from a northeast to a north-northeast trend, and the orientations of the scarps with respect to that of the overall structure suggest that left-lateral slip occurred along the regional trend of the graben. Toward the north-northeast, the graben becomes diffuse and not easy to trace. It heads toward the west end of the row of hills that we call the Hetsüü foreberg, analogous to the Dalan Türüü foreberg northeast of Ih Bogd. Florensov and Solonenko (1963, p. 167–169; 1965, p. 176–179) referred to this foreberg as the “Goshu dzergele,” where zereglee is the Mongolian word for mirage but was understood to mean low hills (a foreberg). We use the name Hetsüü, given to the hills on more recent maps, to identify the foreberg.

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side down as much as 0.5 m. This deformation is similar to that observed within the Dalan Türüü foreberg and likewise suggests a stretching of the upper block over a curved thrust fault that flattens near the surface (Fig. 3). The row of hills forming the Hetsüü foreberg continues eastward reaching its maximum elevation on the flat summit of Ih (“Great”) Hetsüü (Fig. 64, Plate 1). Just west of Ih Hetsüü, however, the 1957 surface rupture abruptly curves southward into a northerly striking zone of right-lateral strike slip. The height of the scarp exceeds 2 m at the corner (Fig. 67). Florensov and Solonenko (1963, p. 169; 1965, p. 177) recognized the abrupt change in orientation. N. A. Logatchev (#1058) reported 2–3 m of strike slip along an apparently north-south zone, but he did not note a sense of slip. At

Section G–H: Hetsüü foreberg A rupture can be traced along virtually the entire length of the Hetsüü foreberg (Fig. 64), with the vertical component increasing from a small value (0.1 m) in the northwest to 2 m in the southeast. At the southeast end, the rupture abruptly turns south, along which right-lateral slip occurred. In the area directly south of the foreberg, deformation appears to have been minor. A curving scarp follows the northern foot of the foreberg, whose average trend is approximately N115°E. At the northwest end, the scarp formed in 1957 is barely discernible, with vertical components as small as 0.1 m. In the eastern half, where the height exceeds 1 m, two or more subparallel scarps formed in many places (Fig. 65). Nowhere is the scarp straight for distances greater than tens of meters; instead, it wraps around the bases of hills. Between the pair of splays in Fig. 65, we found a cross-trending rupture (N10–20°W) with evidence of at least 1 m of right-lateral slip (Plate 1). We mapped Site 29 where the scarp was relatively simple (Fig. 66), and a vertical component of 1.8 ± 0.3 m formed. At one section of the frontal scarp, N. A. Logatchev (#1060) found exposures of the fault surface and measured an average southward dip of 30–40°. Logatchev (#1059) also found a zone of tension cracks, 50–100 m wide south of the line of ruptures. Openings reached 0.2 m, and some cracks showed vertical offsets with the south

Figure 47. Aerial photograph M-649-28/VI/58-128, showing the portion of the Bogd rupture (denoted by black arrows) that includes Site 21 (shown by the white box) and along which strike-slip offsets are particularly clear. North is toward the top. (Scale is approximate.)

Figure 48. Photograph looking south at a left-lateral offset of a gully at Site 21, where we measured 3.2 ± 0.5 m of left-lateral slip and a vertical component of 0.2 ± 0.2 m. R. A. Kurushin provides a scale. Photograph by P. Molnar, August 9, 1993.

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Figure 49. Photograph looking south across the main strike-slip strand approximately 7 km east of Site 21. A clear vertical component has displaced the alluvial fan. R. A. Kurushin (left) and M. Ölziybat (right), surrounded by diamonds, stand in the upstream and downstream thalwegs of a small gully. The vertical component in 1957 appears to account for about half the height of the scarp. Photograph by P. Molnar, August 8, 1993.

Sites 30 and 31, we measured 3.3 ± 0.6 m and 3.0 ± 1.0 m of right-lateral slip. The small vertical components (0.7 ± 0.4 m and 0.8 ± 0.3 m) with the west side up are consistent with scissors faulting associated with the growth of the foreberg. This right-lateral rupture can be traced southward across most of the smooth surface of the alluvial fan between the foreberg and the base of the Baga Bogd massif. Where the trace splays into more than one strand, it is hard to recognize, but at one locality, we estimated a right-lateral offset of 3 m (Fig. 68). As for the area south of the Dalan Türüü foreberg, slip along the front of the Baga Bogd massif south of the Hetsüü foreberg was minor. In one locality, northeast of Baga Bogd near the base of the mountain, we observed a clear fresh scarp with mole tracks and tension cracks consistent with a small component of left-lateral slip and with a height of approximately 0.3 m. Near 101°30′E, S. D. Khil’ko (#3000) noted a system of easterly trending ruptures with heights from nil to 0.5–0.6 m, cutting ridges between gullies. These observations indicate relatively modest deformation compared with what we observed both along much of the foreberg to the north and farther east on the northeast flank of the Baga Bogd massif. Apparently the large displacements west and east of the foreberg were transferred northward to the foreberg by strike-slip faulting at its west and east ends and, as discussed below, by the rotation of a block of upper crust about a vertical axis.

Section H–I: Bogd rupture northeast of Baga Bogd The deformation along the base of the Baga Bogd massif is impressive from where the right-lateral zone reaches the massif

Figure 50. Photograph looking northeast across the southern, strikeslip, strand at Site 22, ~8 km east of Site 21. R. A. Kurushin (right) stands at the base of the scarp at its intersection with the upstream thalweg and M. Ölziybat (left) at the head of the downstream thalweg. We measured only 1.3 ± 0.3 m of left-lateral slip and a vertical, apparently reverse, component of -0.3 ± 0.2 m at Site 22. Arrows on the hill in the background above R. A. Kurushin point at the trace of the middle strand, an east-northeast–trending normal-fault scarp. Photograph by P. Molnar, August 8, 1993.

Surface rupture of the 1957 Gobi-Altay earthquake

Figure 51. Landsat Thematic Mapper image showing a right step in the main strike-slip fault responsible for the Bogd rupture north of Dulaan Bogd, with a basin (near the center of the image) within the step and hills (Shanagan Shuvuun Baast uul) north of that basin. The main trace of the Bogd rupture, marked by white arrows, crosses the image at the foot of the mountains east and west of the basin. An aerial photograph (Fig. 52) shows the eastern end of the basin in the center of the image. Paleozoic rock crops out in the Shanagan Shuvuun Baast uul. The arcuate band of low hills farther north (upper right) suggests a minor foreberg; we saw no evidence of faulting in 1957 within these hills. The white arrow at the far left of the image points to the normal-fault scarp in the background of Figure 50. North is toward the top.

Figure 52. Mosaic of aerial photographs M-649-28/VI/58-100, -99, and -98 showing the eastern end of the basin in the center of Figure 51 and the eastward continuation of the Bogd rupture. Black arrows mark scarps. One scarp follows the southern margin of the basin. East of it, left-lateral offsets are clear along the main trace. The white box marks Site 23, and a white dot shows the location from which the photograph in Figure 53 was taken. North is toward the top. (Scale is approximate.)

31

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Figure 53. Photograph looking south across the scarp on the southeast edge of the basin in Figures 51 and 52. A large component of normal faulting can be inferred from the steep free faces. A circle surrounds S. Neustadtl. Photograph by P. Molnar, August 6, 1993.

to the eastern end of the rupture. In many places the scarp is high (>2 m), and where it is simple, left-lateral offsets of ~3 m can be recognized. The rupture in this area, however, commonly is complex, with strands both along the edge of the massif and within it. Just east of where the north-south right-lateral zone approaches the base of Baga Bogd, we estimated left-lateral offsets of 1.5 m to 3 m, with the smaller the more precisely measured (Plate 1). We mapped one area where the vertical component reaches 1.9 ± 0.3 m. This locality (Site 32), however, proved unsuitable for determining the strike-slip component. Along much of the zone farther east to the Züün hanh sayr (Plate 1), we found indications of left-lateral slip to be prevalent but only few features that were defined sharply enough to allow an estimate of the amount of slip. At Site 33 on the east margin of the Züün Hanh sayr (Fig. 69), we measured 1.8 ± 0.4 m of left-lateral slip that occurred with the south side up 1.2 ± 0.4 m, presumably by a component of thrust faulting. This component of strike slip, smaller than those farther west or east, may underestimate the total left-lateral slip in 1957. Marked deformation within the Baga Bogd massif farther south is apparent on aerial photographs just east of the Züün hanh sayr (Fig. 70) and farther east (Fig. 71). In particular, orientations of tension cracks along the scarp south of Site 33 imply a component of left-lateral slip (Fig. 70). We crossed a small south-facing scarp, 0.2 m high, in the upper left of Figure 71, en route to the more significant scarp where we observed a vertical component of 0.7 m with the south side down (Fig. 72). The trace of the fault across the sides of the valley yields an estimate of the southward dip of ~35°, and hence

the unambiguous inference of normal faulting, with the majority of the Baga Bogd massif dropping down with respect to its northern flank. Such normal faulting in a setting with a large component of regional convergence is not unprecedented. The large grabens that formed in the El Asnam earthquake in Algeria (King and VitaFinzi, 1981; Philip and Megraouhi, 1983) provide a prototype for this style of deformation. Yet unlike the case in Algeria, the southward dip of the normal fault in Figures 71 and 72 is most easily understood as resulting from slip on a deeper thrust fault whose dip near the front of the massif is steeper, not gentler, than that beneath the interior of the massif (Fig. 3c).

Figure 54. Photograph looking west-southwest along the rupture at Site 24. Note the small vertical component with the north side up (0.4 ± 0.2 m) and the left-lateral strike-slip component of 3.6 ± 1.0 m. Photograph by P. Molnar, August 5, 1993.

Surface rupture of the 1957 Gobi-Altay earthquake

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Figure 55. Mosaic of aerial photographs M-649-28/VI/58-85 and -84, showing the area east of the Shavinahyn sayr, the locations of Site 25 and Site 26 (white boxes), and the Toromhon sayr (on the east edge of the photograph). Note the strike-slip offsets along much of this portion (denoted by black arrows). The white dot shows where N. A. Florensov took the photograph in Figure 57. North is toward the top. (Scale is approximate.)

Where the Böhiyn am crosses the front of the range, ~5 km east of the Züün hanh sayr, S. D. Khil’ko (#3045) observed scarps due to thrust faulting of the mountains over the lower area to the north. On the west bank, the height of the scarp reached 3–3.5 m, but on the east bank, he reported only 0.5 m, perhaps

Figure 56. Photograph looking southwest at Site 26 where a meander in the gully is offset by the fault. The gully in the foreground curves around the hill in the lower right corner of the photograph. The steep sloping area in the left-center of the photograph next to Ch. Bayarsayhan (indicated by a triangle over him) marks the previous southern edge of the meander in the stream. It has been displaced to the left (east) along the rupture 4.3 ± 0.5 m (0.0 ± 0.2 m vertically). Because of the left-lateral offset, it has been abandoned by the currently active gully. A. Bayasgalan stands on the south flank of the rupture to the right of the meander and above a steep face where left-lateral slip of a westerly sloping surface created the high scarp. Arrows on the edges of the photograph denote the scarp. Photograph by P. Molnar, August 14, 1994.

the result of treating apparent slip as offset. When we crossed this area in 1994, much of the scarp had been eroded, but an “island” of uplifted terrain persisted within the recently alluviated valley. The scarp at its base remained ~3 m high (Fig. 73). Approximately 10 km east of the Züün hanh sayr, we mapped a very impressive scarp at Site 34 (Fig. 74) across the Tsagaan Burgasny am, whose bed consists of huge boulders.

Figure 57. Photograph by N. A. Florensov on January 5, 1958, looking east along the Bogd rupture, across the Toromhon sayr (white dots in Figs. 55 and 58). V. P. Solonenko stands on the scarp, where they measured a vertical component of 1.7 m and a left-lateral offset of 2.8 m.

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Figure 58. Mosaic of aerial photographs M-649-28/VI/58-83 and -82, showing Sites 27 and 28 (white boxes), as well as the site of N. A. Florensov’s photograph in Figure 57 (white dot) where he measured 2.8 m of strike slip. East of the Toromhon (lower left) and Builsny Zadgay sayrs, the scarp crosses the south side of a hill and faces north. North is toward the top. (Scale is approximate.) Black arrows point to the scarp.

Figure 59. Photograph, using a 20-mm lens, looking north, downhill across the gullies at Site 27. R. A. Kurushin (circled) stands next to the scarp and provides a scale. M. Ölziybat stands closer, near the crest of the ridge between the gullies in the center of the photograph. Note the wide gully with an apparently large offset on the left and the smaller gully, with a smaller offset on the right. We measured 3.5 ± 1.5 m of left lateral slip and a vertical component of 2.0 ± 1.0 m. Photograph by P. Molnar, August 4, 1993.

Surface rupture of the 1957 Gobi-Altay earthquake

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Figure 60. Photograph looking south at Site 28 showing a strike-slip offset of 4.1 ± 1.0 m with a vertical component of 0.7 ± 0.3 m. A. Bayasgalan and E. T. Brown on the left stand in former segments of the same gully, and G. Gibson and D. Schwartz near the center stand on an offset ridge. Photograph by P. Molnar, August 22, 1995.

The displacements that we measured at this locality were the largest that we saw along the front of Baga Bogd: 4.8 ± 1.1 m of left-lateral slip, and 2.6 ± 0.4 m of vertical component with the south side up. Among us, there is some doubt about whether this offset includes slip before 1957, for the magnitude of the slip is large. The senior author thinks that the vertical offset of the younger, eastern part of the valley does not exceed half the amount measured at Site 34, similar to the difference reported by Khil’ko at the east bank of the Böhiyn am. The entire scarp at Site 34, however, seemed fresh to others of us. The small gullies cut by the rupture appeared to be abandoned only recently, not hundreds to thousands of years ago as would be likely if the scarp were the result of two major earthquakes. The hanging wall is sharply dissected by the present stream, as if it was only recently elevated above the downstream flank. The sizes of the boulders that crop out at the scarp make it unlikely that methods for dating scarps based on diffusion of material from the hanging wall to the foot wall will be useful. Finally, we estimated similar 3–4 m left-lateral offsets of gullies approximately 1 km farther east (44° 52.0′N, 101° 50.0′E), where the scarp crosses hilly terrain. About 6 km east-southeast of Site 34, the mountain front forms an embayment, concave to the northeast. Steep slopes of the mountains give way to large alluvial fans that begin at relatively high altitudes. Surface ruptures can be found throughout the region from the base of the mountains across the fans to the lower, flatter areas northeast of the mountain front (Plate 1). The many scarps show a variety of styles of deformation (Luk’yanov, 1965, Fig. 18), including reverse, normal, and strike-slip faulting (Plate 1). Conjugate strike-slip faulting occurred in the flat area north of the high terrain. A. V. Luk’yanov measured one pair of short ruptures with 0.25 m of right-lateral on a plane striking N15°E (#1912) and 0.4 m of left-lateral on a plane striking N110°E (#1913). Directly east ~2–3 km, a scarp bounds low hills growing out

Figure 61. Photograph looking east along the Bogd rupture across a relatively deep gully approximately 100 m east of Site 27, showing the clear dip to the north of the trace at this locality. Note four people for scale along the scarp: A. Bayasgalan at the bottom of the nearest valley floor, north of the scarp, a second on the first hill in the distance on the south side, and two on the most distant ridge, one on each side of the fault. Photograph by P. Molnar, August 22, 1995.

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Figure 62. Landsat Thematic Mapper image showing the region near Sites 27 and 28, where the Bogd rupture splays into a small graben and crosses rugged terrain apparently along northeasterly trending scarps. White arrows mark the Bogd rupture west and east of the Toromhon sayr, the scarp forming the northern and southern edges of the graben, and east-northeast continuations of the Bogd rupture toward and through rugged terrain. Notice also the escarpments farther north that seem to bound other minor forebergs. The left side of the image overlaps with that in Figure 51. North is toward the top.

Figure 63. Photograph looking northeast along the graben just east of the Baruun hanh sayr and south-southwest of the Hetsüü foreberg. Heights of scarps on the southeast (right) side and casting shadows reach 2 m, and those on the northwest side, exposed to direct sun, appearing bright, exceed 1 m. Circle surrounds two people in the distance. Photograph by P. Molnar, August 15, 1994.

Surface rupture of the 1957 Gobi-Altay earthquake

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Figure 64. Landsat Thematic Mapper image showing most of the Baga Bogd massif and its surroundings. The low row of hills trending obliquely across the upper part of the image, through the large cross marking 45°N, 101.5°E, defines the Hetsüü foreberg. The flat hill southeast of the cross is Ih Hetsüü, after which the foreberg is named. Sharp breaks in slope (indicated by white arrows) follow both the northern side of Baga Bogd, which ruptured in 1957, and the southern side, which apparently did not rupture. The more arcuate traces around Baga Bogd than north of Ih Bogd suggest that the main faults are not vertical but dip south. North is toward the top.

the base of the alluvial fan, suggestive of a miniature foreberg. We recognized a fresh scarp, as much as 0.5 m high, at the base of the higher scarp defining this foreberg, near the Bayan sayr (Plate 1). At the break in slope between the high terrain to the south and the alluvial fans emanating from the mountains (~4 km southwest of the foreberg), vertical components with the northern side down characterize most scarps. Both the sharp expression of the scarp, with free faces and little debris at their bases (Fig. 75), and the curvature of the trace of the scarp across deep valleys implies a dip to the north and, hence, normal faulting. Again, evidence of strikeslip faulting is sparse. Luk’yanov (#1843) reported a scarp ~1 m high along the base of the high terrain, reaching 1.1 m (#1844) in the middle of the normal scarp that is convex to the north. In one locality along this zone (#1843, delineated by <0.5 m on Plate 1), he suggested that “possibly there was a small amount of left-lateral

slip (~0.5 m).” He also wrote that farther southeast (#1844) where a gully crosses the scarp, either there was no strike slip, or the amount was less than 0.5 m. Still farther southeast, he reported a height of only 0.3–0.4 m, which we confirmed near the bend in the trace on Plate 1. Approximately 1–1.5 km north of this scarp, another, lower scarp, approximately 0.3–0.5 m high, also appears to indicate normal faulting (Luk’yanov’s #1905). Thus, we consider that these scarps formed by the same general process that created the scarps within the Baga Bogd massif south of Site 33, but with opposite dips of the normal faults. Hence the main thrust fault beneath this area appears to steepen with depth (Fig. 3b). Section I–J: Bogd rupture, north and east of Bulgan uul The main surface rupture appears to follow the base of the hills north of Bulgan uul and eastward (Fig. 2, Plate 1). The

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Figure 65. Mosaic of aerial photographs M-649-10/VIII/58-1349 and -1348 of Hetsüü foreberg. Note both the ruptures on the front, where thrust faulting occurred, and along the eastern part, where rightlateral strike-slip faulting occurred. Black arrows point to the scarps. White boxes surround Sites 29, 30, and 31, left to right on photograph. The white triangle shows where the photograph in Figure 67 was taken. North is toward the top, parallel to the sides. (Scale is approximate.)

Figure 66. Photograph looking southwest at Site 29, on the Hetsüü foreberg, with A. Bayasgalan (circled) at the scarp and B. Enhtuvshin bending over the total station near the jeep. We measured a vertical component of 1.8 ± 0.3 m. The Baga Bogd massif, behind the foreberg, forms the horizon. Photograph by P. Molnar, August 13, 1994.

Surface rupture of the 1957 Gobi-Altay earthquake smooth form of the scarp, without a free face, but with abundant cracking of material on the face of the scarp, suggests thrust or reverse faulting (Fig. 76). North of the main scarp, conjugate ruptures indicate largely strike-slip deformation, with left-lateral slip on east-northeast–trending scarps being more significant than the conjugate right-lateral slip on shorter north-northeast–trending ruptures. At its eastern end, the main scarp ends with several approximately parallel, northerly trending thrust or reverse scarps that face both east and west. North of the easternmost 5 km of the main rupture, a network of short ruptures (from 200 m to 2 km in length) attest to distributed deformation. A. V. Luk’yanov (1965) mapped short

Figure 67. Photograph, looking south-southeast of the corner in the rupture, delineated by black arrows, where the zone of thrust faulting along the Hetsüü foreberg to the west transforms into right-lateral strike-slip faulting along a north-south zone. Photograph, with a 135-mm lens, by P. Molnar on August 11, 1994.

Figure 68. Photograph looking east at a right-lateral offset in the flat area between the Hetsüü foreberg and Baga Bogd to its south. Ch. Bayarsayhan (left) and R. A. Kurushin (middle) stand on the north bank of a channel offset ~3 m, and A. Bayasgalan (right) stands in front of the scarp, on its west side. This place lies just south of the area shown in the aerial photograph in Figure 65. Photograph by P. Molnar, August 11, 1994.

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strands with en echelon mole tracks and open tension gashes providing clear evidence of strike-slip faulting (Plate 1). Many of these small-scale features can still be recognized in the topography, but in 1994 we could see no clear offset features that allowed us to quantify the slip. Thus, most estimates shown on Plate 1 were taken from Luk’yanov’s notes, and only the longer ruptures are plotted on Plate 1. Most ruptures strike eastnortheast and show left-lateral slip (Fig. 77). The main rupture is several meters wide in some places, but in others the scarp remains a sharp break, and its height exceeds

Figure 69. Photograph looking west across Site 33 in the foreground just beyond the jeep and near the edge of the stream valley, across the Züün hanh sayr, and toward the scarp as it crosses the high terrain farther west at the foot of Baga Bogd. We measured 1.8 ± 0.4 m of leftlateral slip and a vertical component of 1.2 ± 0.4 m at Site 33. Photograph by R. A. Kurushin, August 8, 1994.

Figure 70. Aerial photograph M-649-28/VI/58-52, showing a portion of the Züün hanh sayr, on the left, with poplar trees growing in it and a curved scarp (indicated by arrows), apparently with a normal component, crossing high terrain in the center. A series of tension gashes suggest a component of left-lateral slip along this fault just west of the easternmost black arrow. This scarp lies ~1 km south of the main rupture at the foot of Baga Bogd. Sides of the photograph are oriented N15°W. (Scale is approximate.)

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Figure 71. Aerial photograph M-649-28/VI/58-40 of a normal fault scarp (denoted by black arrows) in the mountains south of where the Böhiyn am debouches onto alluvial fans. The normal fault dips south; its trace is curved because it crosses hills and valleys. See Figure 72 for a view east (from white dot) at the scarp where it crosses a valley. North is toward the top. (Scale is approximate.)

Figure 73. Photograph looking south at the Bogd rupture where it crosses the Böhiyn am. An “island” of river bed remains after erosion has removed most of the uplifted hanging wall and destroyed the scarp. A. Bayasgalan below the scarp on the left and three mounted horseman provide a scale for the height by the scarp. Photograph by P. Molnar, August 9, 1994.

Figure 74. Photograph looking southwest at Site 34 where a small gully has been cut by the rupture. A. Bayasgalan, holding the reflector on a pole, stands in the upstream thalweg of the deepest channel on the scarp, and R. A. Kurushin stands in the downstream thalweg in front of the scarp. We measured a left-lateral component of 4.8 ± 1.1 m and a vertical component of 2.6 ± 0.4 m at Site 34. Photograph by P. Molnar, August 9, 1994.

Figure 72. Photograph looking east, through a 20-mm lens, across a normal fault in the mountains (see Fig. 71) south of where the Böhiyn am debouches onto alluvial fans. A. Bayasgalan (circled) stands in the foreground, where the scarp crosses the valley floor. The dip of the fault can be inferred from the trace up the east side of the valley. (Black arrows point at the scarp on the hillside.) Photograph by P. Molnar, August 9, 1994.

2 m. We mapped one locality (Site 35, Fig. 76) along the main rupture, where a vertical component of 2.6 ± 0.4 m can be seen. Approximately 2 km east of Site 35, Luk’yanov (#1900) reported a north-facing scarp, 1.5 m high, crossing a deep gully, and dipping 80° to the south. Although this steep dip may not extend to great depth, there seems little doubt that the large vertical components in this area result from a component of reverse faulting. Evidence of folding and stretching of the hanging wall can be seen in several localities. Luk’yanov (#1883) described a thrust rupture with a vertical component of 1.5–2 m in the region of Figure 78, ~5 km east of Site 35. He reported a gently sloping layer of bedrock (siltstone) and clumps of sod, 6–8 m wide and 0.7 m in thickness, overlying the undeformed lower block at the

Surface rupture of the 1957 Gobi-Altay earthquake

Figure 75. Photograph looking east-southeast at a scarp ~1 m high, formed by normal faulting, ~10 km southeast of Site 34 and ~5 km southwest of Site 35 (see Plate 1). A. Bayasgalan (left) and Ch. Bayarsayhan provide a scale. Photograph by P. Molnar, August 7, 1994.

foot of the rupture. The surface of the layer was cut by a complicated network of tension cracks with negligible opening and, at the “brow” of the scarp, by a sharply defined zone, 1–1.5 m wide, with two to three subparallel branching cracks. In cross section, the thrust plane dipped southwest at 46° from just below this zone of extension at the brow of the scarp into the earth along a

41

tectonized zone ~1 m wide. At shallower depths, the ~4-m-wide wedge of bedrock, sliced with tension cracks, lay directly on what was the surface of the earth before the earthquake. Thus, the cross section described by Luk’yanov in 1958 illustrates the simple pattern in Fig. 3a. Not far from this locality, Luk’yanov (1965; #1886) observed a trail cut by a thrust fault at an angle of 30°. He found the trail on the down-thrown block to lie 7 m northwest of its continuation, suggesting a large apparent left-lateral component. When he extrapolated the trail under the thin, collapsed upper block, however, he found that it reached the thrust plane at a point where no strike-slip displacement would have occurred. A simple calculation shows that the horizontal component consists of 3–4 m of thrust slip without strike slip (Luk’yanov, 1965, p. 39–40). We also saw high scarps, attesting to significant displacement (2.2 m in Fig. 79), but with little evidence of significant strike-slip faulting. At one locality, only a few hundred meters west of the area in Figure 79, we estimated a left-lateral offset of only 0.5 ± 0.5 m for a shallow gully. At the eastern end of the low foothills of Bulgan uul, just southeast of the main rupture, Luk’yanov mapped several northerly striking ruptures that remained clear in 1994. An old man living in the yurt on the uplifted east side of one rupture told us that before the earthquake in 1957, the surface was smooth and dipped gently east. Like Luk’yanov, we infer thrust, or perhaps reverse, faulting, from the scalloped traces across the topography and from the rounded hanging walls over scarps, which

Figure 76. Photograph looking southwest at Site 35, where we measured a vertical component of 2.6 ± 0.4 m. The rupture in the eastern (left) part consists of two splays, which join near the central part of the photograph, above the jeep. Circles surround P. Molnar (left) and Ch. Bayarsayhan on the scarp. Photograph by R. A. Kurushin, August 7, 1994.

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R. A. Kurushin and Others superficial and reflects a deeper thrust or reverse faulting of the eastern end of the Ölziyt uul massif onto the area east of it (Plate 1). This process apparently is analogous to that responsible for the Toromhon Overthrust, discussed in the next section. Luk’yanov (#1691) reported a short (~125 m) isolated rupture following the crest of a ridge in the Unegt nuruu, 7 km south of the system described above (Plate 1), with a vertical component of 0.15 m, and opening of a crack to 0.1 m. Luk’yanov (#1808) also mapped a short (1.1 km) rupture striking N70–75°W in the Arts Bogd uul, 6 km farther south. The displacements include 10–30 mm of opening at tension gashes and vertical components of 30–50 mm. We suspect that deformation at both localities was superficial, probably caused by shaking. Unfortunately, neither the members of the 1958 expedition nor we had time to examine the area farther east, where, judging from the aerial photos, additional minor surface ruptures seem possible, if not likely. Toromhon Overthrust

Figure 77. Photograph viewed east-northeast along a scarp 0.2 m high (Plate 1), where evidence of left-lateral strike slip is clear. Tension cracks have opened with northeast-southwest extension in the foreground, and mole tracks in the background show components of compression. Photograph by A. V. Luk’yanov, October 16, 1958. Russian words in the foreground and background translate as extension and compression, respectively.

lacked fresh free faces. Luk’yanov (#1858) also reported small right-lateral components in the southern part of this system of north-south ruptures, north of Huts uul (Plate 1). As scarps face both east and west, however, not all of the causative faults can extend deep into the crust. We suspect that this deformation is

Solonenko et al. (1960) named a short north-northeasterly trending zone of prominent surface faulting between Ih Bogd and Baga Bogd the “Toromhon Overthrust,” for in the north it follows the Toromhon sayr where Florensov and Solonenko first examined it in January 1958. Along much of its 21-km length, the trace is marked by a high scarp, with vertical components exceeding 1 m and reaching a maximum of 5–6 m. Strike-slip components also can be recognized, with right-lateral slip where the rupture strikes north or northeast and left-lateral where it strikes northwest. The connection between the linear trace of the Bogd rupture east of the Toromhon sayr and the Toromhon Overthrust, approximately 1 km east of the Toromhon sayr, is not clear (Plate 1). The trace of the rupture across the Builsny Zadgay sayr is clear on aerial photographs taken in June 1958 (Fig. 80), but subsequent flow in the valley has destroyed it there. Florensov and Solonenko (1963, 1965, Fig. 129) reported “compressional cracks” defining a continuation of the overthrust on the northeast bank of the Buil-

Figure 78. Photograph looking west along the rupture that lies at the base of the hills east of Bulgan uul (Plate 1). The rupture is >8 m wide. Note the tension crack across the brow of the scarp, similar to that in Figure 3a. Photograph by A. V. Luk’yanov, October 15, 1958.

Surface rupture of the 1957 Gobi-Altay earthquake

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Figure 79. Photograph using a 135-mm lens viewed southwest at the scarp, ~2 m high, east of Bulgan uul. R. A. Kurushin (circled) provides a scale. Luk’yanov took the photograph in Figure 78 just east of the hill in the upper right. Photograph by P. Molnar, August 6, 1994.

sny Zadgay sayrs. In 1976, S. D. Khil’ko and R. A. Kurushin paid special attention to this area but could not recognize such compression. In badlands topography, they observed only superficial, isolated short tension cracks, subparallel to the Bogd rupture and suggestive of land sliding. In the hills southwest of the Builsny Zadgay sayr, the trace is defined by a scarp facing east to southeast. Baljinnyam et al. (1993) disagreed among themselves on the magnitude of strikeslip faulting, with some inferring a large right-lateral component (~4 m) at one locality ~1 km southwest of the Builsny Zadgay sayr. The rupture crosses gullies and ridges, but is most clearly seen where it cuts a northeast-sloping surface (Fig. 81). In 1993, we mapped this area to evaluate this inference. Indeed, a rightlateral offset of 4 to 6 m would allow a match of this displaced surface, but this appears to be largely an apparent offset. A ridge at the northeast end of the mapped area shows very little right-lateral offset, only 0.6 ± 1.0 m. A vertical component of 1.6 ± 0.5 m accounts for most of the apparent lateral offset of the surface. About 300 m farther south, the trace crosses a pass and curves into a nearly north-south trend (N5°E), where it cuts a gently southwest-sloping surface (Fig. 82). We measured a vertical component of 3.0 ± 0.5 m (Site 37). No strike-slip component could be measured, but if a right-lateral component did occur in 1957, then the vertical component must have been larger than 3 m. In the same area, Solonenko (#515) inferred 3–3.2 m of vertical slip, but for reasons not clear to us, he also reported left-lateral slip of 4 m (Florensov and Solonenko, 1963, 1965, Fig. 153). Although this might appear to be a typographical error or an error in transcription, Solonenko’s field notes show the same illustration as his Figure 153 and make the same claim.

Figure 80. Aerial photograph M-649-28/VI/58-401 showing the Toromhon Overthrust crossing the Builsny Zadgay sayr and approaching the Bogd rupture, which is sharply defined in the upper right corner of the photograph. Black arrows denote the obliquely trending Toromhon Overthrust across the middle. The white box shows Site 36. North is toward the top. (Scale is approximate.)

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R. A. Kurushin and Others

Figure 81. Photograph looking northwest at a part of the surface rupture mapped at Site 36. M. Ölziybat (right) stands on the northwest side, and A. Bayasgalan on the southeast side of the fault. The height of the scarp could be explained by either 2 m of vertical slip (northwest side up) or 6 m of right-lateral slip. Mapping the area northeast and southwest, however, revealed only 0.6 ± 1.0 m of right-lateral slip and a vertical component of 1.6 ± 0.5 m. Photograph by P. Molnar, August 3, 1993.

Early Cretaceous basalt (Tevsh Formation) on the northwest and Late Cretaceous sedimentary rock (Builsny Formation) to the east. The height of the scarp ~1 km south of Site 37 does not exceed 4 m and decreases to the south (Figs. 83 and 84). Approximately 0.5 km north of where the scarp dies out, we observed shallow gullies offset right laterally ~1 m (Fig. 84). Throughout its length of ~7 km, this trace curves smoothly with local strikes of N5°E to N45°E about a mean orientation of N15°E. Despite evidence of strike slip, the predominant sense of slip appeared to be reverse, or thrust, slip, but nowhere could we measure the dip reliably. Approximately 1 km south of Site 37, a second trace intercepts this N15°E-trending scarp at a sharp angle. This southwest-trending trace is no longer easily recognized within the basalt of the Tevsh Formation. In 1958, Florensov (#113) noted that where this N65°E-trending rupture intercepted the N15°Etrending trace, 7–9 subparallel tension cracks (with openings of as much as 0.1 m) crossed an alluvial fan on the western, hanging wall of the latter scarp. The nature of this junction suggests that the more northeasterly trending zone formed later than the more northerly zone. In contrast to the more northerly trending zone, the northeasterly trending trace apparently includes straight portions, but overall it smoothly changes strike from N60–70°E to N30°E, between the Builsny Zadgay basin and the pass between it and the Toromhon sayr. As noted above, the northeasterly trending trace through the basalt is not well preserved. At its southwest end, where it curves

Figure 82. Photograph looking west at Site 37, with A. Bayasgalan (circled) standing near the base of scarp, and M. Ölziybat at the total station in the foreground. We measured a vertical component of 3.0 ± 0.5 m at Site 37. Photograph by P. Molnar, August 3, 1993.

M. A. and V. P. Solonenko (#780) estimated the largest vertical component along the Toromhon Overthrust of 9.2 m ~300 m south of Site 37 (Florensov and Solonenko, 1963, 1965, Fig. 123). Visiting and photographing this locality two days later, Florensov (#112) deduced a vertical component of 5–6 m (Florensov and Solonenko, 1963, 1965, Fig. 110). Benefiting from repeated visits to this region, we agree more with Florensov. We suspect that Solonenko’s inferred offset includes pre-existing topography, due to erosion, so that the height he measured did not form in 1957 alone. No more than 100 m south of the left edge of the photo in Florensov and Solonenko’s Figure 110, we observed a scarp no higher than 5 m that offsets a gentle surface formed by the confluence of shallow gullies. A sharply defined trace continues south of Site 37 for approximately 3.5–4 km, where it follows the contact between

Figure 83. Photograph looking south at the scarp that forms the southern end of north-south branch of the Toromhon Overthrust, the scarp that includes Sites 36 and 37. G. Gibson stands on the hanging wall ~1.5 m above D. Schwartz on the footwall to the left. Notice that the scarp crosses the saddle on the horizon also. Photograph by P. Molnar, August 23, 1995.

Surface rupture of the 1957 Gobi-Altay earthquake into a more northerly trend to form a second important strand of the Toromhon Overthrust, it follows the western edge of outcropping basalt of the Tevsh Formation. Approached from the south, the northern end of this north-northeasterly trending rupture curves to trend N65°E, as Florensov had noted, but diverges into a zone of scarps ~1 m in height that strike roughly N25°W and face southwest. We infer that they mark an en echelon zone of tension cracks within a right-lateral zone that links the two main north-northeasterly trending traces. A single trace of the western trace of the Toromhon Overthrust is clear farther south, where it strikes N65°E for ~1 km. The maximum height of the southeast-facing scarp is 1.5–2 m with a right-lateral component commonly of 1.5 m. Farther south, the trace follows hills along the east bank of the Toromhon sayr for

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~5 km, and offsets of gullies draining into the sayr are spectacular. The overall trend is approximately north-northeast, but the trace is by no means straight. Solonenko et al. (1960, Fig. 26) noted three parallel traces in the area of Site 38. They reported only negligible slip along the western trace and only a small vertical offset (0.5–0.8 m) along the middle trace. The eastern trace was unquestionably major and continuous with the main scarps to the north and south. Only this eastern trace remained clear and was surveyed in 1994. Just north of Site 38, the scarp is relatively simple, and we observed gullies with small vertical components of ~1 m (west side up) and consistent right-lateral offsets of 1.5 m (Fig. 85). Southward, the amplitude of the vertical component grows to 4 m and to 5–6 m in 300–400 m (Figs. 86 and 87).

Figure 84. Photograph looking southwest and downhill, obliquely across the southern end of the north-south branch of the Toromhon Overthrust, ~300 m south of that in Figure 83. G. Gibson stands on the ~1-m high scarp, which faces east. Gullies have been offset right-laterally ~1 m. Photograph by P. Molnar, August 23, 1995.

Figure 85. Photograph viewed west-northwest of a right-lateral offset, ~200 m north of the northern edge of Site 38 (Fig. 86). The local strike is N20°E. From left to right, R. S. Yeats and H. Philip stand on a small ridge and D. Schwartz and T. Rockwell stand in a gully offset ~1.5 m right-laterally, with a vertical component of about1 m. Photograph by P. Molnar, August 23, 1995.

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Figure 86. Photograph, looking west-northwest by N. A. Florensov on January 5, 1958, shows the scarp, ~4 m high, across the gully in the northern part of Site 38. Note the warping of the hanging wall south (left) of the gully in the foreground and the right-lateral offset.

We mapped a relatively large area at Site 38, in order to place precise bounds on the estimated amounts of slip, but the nature of the trace and subsequent erosion and deposition made our results less definitive than we had hoped. Faulting in 1957 dammed gullies, whose thalwegs were well defined in 1958 (Figs. 86 and 87) but are now buried by sediment trapped in ponded basins. Thus, defining the pre-earthquake topography east of the scarp was not easy. Moreover, where the scarp crosses hills between the gullies it splays into branches with scalloped planforms. These scalloped traces define thrust ruptures that extend to the east over the crests of east-west ridges (Fig. 88). They provide unambiguous evidence of thrust, not reverse (and certainly not normal) faulting. The large

vertical offsets, which are localized where the scarp crosses gullies, are distributed over an area >10 m wide. Moreover, the landscape varies along strike. Thus, matching profiles parallel to the scarp in order to measure the horizontal components proved to be difficult. We conclude that right-lateral slip of 2–3 m occurred, and that vertical components reached 4–6 m at Site 38, but clearly these large amounts are not representative of the entire Toromhon Overthrust. The difference of 2 m in vertical components over a distance of only 100 m (compare photographs in Figs. 86 and 87) can be explained in part by a difference in local strike; the slip vector is oriented more parallel to the northern scarp of Site 38, and hence more convergence occurred in the southern part. Yet, this may be only a partial explanation. Because the scarp crosses relatively steep topography, warping of the earth’s surface due to internal deformation of the hanging wall could easily go undetected; as noted previously, Solonenko et al. (1960, Fig. 26) reported deformation of the hanging wall west of the main scarp at lower elevations. Perhaps, additional deformation, obscured by the relief, also occurred. In any case, the marked lateral variations in amounts of dip slip at the surface ruptures cannot be reliably extrapolated to depth along a steep fault. These variations must reflect superficial variations in dip or local strength of the rock. The scalloped traces of the ruptures over hills between the gullies (Figs. 88 and 89) suggest to us that the surfaces of these ruptures must dip gently west or west-northwest. These arcuate traces intersect the more prominent and straighter traces where the latter cross the gullies. Florensov examined one such straight

Figure 87. Photograph by N. A. Florensov, on January 5, 1958 (Florensov and Solonenko, 1963, 1965, Fig. 69), looking west-northwest at the scarp across the next gully to the south, in the southern part of the area mapped as Site 38. Note the person (surrounded by the diamond) at the base of the scarp where it cuts the gully in the foreground. Here a right-lateral offset can also be inferred, and the vertical component reaches 6 m.

Surface rupture of the 1957 Gobi-Altay earthquake

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Figure 88. Photograph, taken by P. Molnar on August 23, 1995, looking south-southwest along the scarp shown in Figure 86 showing both the steep scarp, in the foreground and right-center, and the thrust splay, to its left.

trace within the area of Site 38 and measured a steep southeastward dip of 83°, which he inferred to be the dip of the main rupture. This would suggest normal rather than thrust slip. We suspect that he did not measure the plane for the main rupture, but rather a break within the hanging wall separating the main part of the hanging wall to the west and a sliver of material thrust over the ridge to form the scalloped trace (Fig. 3a). The scalloped trace requires a west-northwestward dip of < 30°. The scarp of the Toromhon Overthrust continues for 3 km south of Site 38 as a thrust zone with similar topographic expression; ~1 km south of Site 38, Florensov (#114) noted a scarp height of 3–3.5 m. The scarp faces east and follows the western side of hilly terrain. Slip has elevated the lower western hanging wall and obviously is not responsible for the local relief. At the southern end of this portion, deformation is especially complex. From this region, a zone of relatively minor deformation continues southwest (the Tsagaan Ovoo-Tevsh uul zone, discussed in the next section), and a second zone of major deformation can be traced east-southeast from the west side of the Toromhon sayr. The deformation within this knot of complexity seems to mark the transition from a north-northeasterly overall trend to a northwesterly trend of the Toromhon Overthrust. Within the knot are short ruptures with different orientations and different senses of vertical slip. V. P. Solonenko (#518) inferred left-lateral slip of 1.2 m on one short, southwesterly

striking trace. As he noted (#519), traces with vertical components face both north and south with more than 1 m of displacement (Fig. 90) in some places. In one area, as many as four parallel southwest-facing scarps strike northwest (N60°W), each with a vertical component as large as 0.5 m. From this localized area, only ~1 km in dimension, a single, southeasterly striking trace emerges, with a northeasterly facing scarp (Florensov and Solonenko, 1963, 1965, Fig. 9). Tension cracks within the hanging wall follow much of the scarp (Fig. 91) and provide additional evidence of a gentle westward dip (Fig. 3a). At Site 39 the vertical component is large (5.0 ± 0.4 m), and there appears to be a substantial left-lateral strike-slip component: 3.0 ± 0.8 m (Fig. 92). Again, the inference of overthrust faulting seems inescapable; the surface of the hanging wall at its brow shows numerous small, superficial grabens and normal faults suggestive of its being bent and stretched. Although the strike-slip component might appear to be more uncertain because of the poorly defined thalweg on the northeastern side, we observed comparable vertical and strike-slip offsets farther northwest along this trace. About 1 km southeastward from Site 39, the trace crosses a wide valley and abruptly becomes very difficult to follow (Plate 1). We found a low west-facing scarp (height, ~0.2 m) striking north-south ~6 km south of Site 39, but this trace seemed to die out farther south. Solonenko (#794), however, reported an east-facing thrust scarp 0.05-0.2 m high, ~200 m long, and trend-

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Figure 89. Photograph looking southwest shows the trace of the Toromhon Overthrust south of Site 38 and demonstrating a thrust component of slip. White arrows point at portions of the scalloped planform. Scarps follow the west sides of hills; if the faults were even approximately planar, their intersections with the topography would require a very gentle west-northwestward dip. Note also multiple scarps in the middle ground. Photograph by P. Molnar, August 23, 1995.

ing N20°W about 3 km southeast of this locality, near the watershed of the Nuuryn Höndiy and the basins to the south. These ruptures merge into a single trace farther north. Moreover, in the same area, Solonenko (#796, 797) reported yet two more small, opposite facing thrust ruptures striking N30–40°W and 0.2–0.3 m high. Although the extent and relationship of these structures to one another, or to the Toromhon Overthrust, remains unclear, they almost surely reflect deformation within the southeastern end of the thrust system between Ih Bogd and Baga Bogd. Whereas Florensov and Solonenko (1963, 1965) reported the Toromhon Overthrust to be 30–32 km long, we found it to extend no more than 21 km south of the Bogd rupture. Because neither N. A. Florensov nor V. P. Solonenko described the southern end of the zone in their notebooks, we do not understand the reasons for inferring such a long zone. Unlike the Bogd rupture, evidence for an earlier, late Quaternary, earthquake, in the form of a pre-existing scarp, was virtually nonexistent along the Toromhon Overthrust. The only example (Fig. 90) of a possible earlier scarp is in the knot where the north-northeasterly and southeasterly scarps join. Given the complexity of deformation in that area, this one example does not provide very convincing evidence.

Figure 90. Photographs looking southeast at a short section of rupture near the corner (or knot) of the Toromhon Overthrust, ~4 km south of Site 38 (see Plate 1). Note that here the northeast side has been uplifted 1–2 m by the recent rupture, seen in the foreground, and overall (adjacent to the jeep) as much as 4–5 m, presumably in part by previous ruptures. Note also the folding and stretching of the surface of the hanging wall, indicating thrust faulting.

Surface rupture of the 1957 Gobi-Altay earthquake

Figure 91. Photograph looking southeast along the northwest-southeast zone of the Toromhon Overthrust, southeast of the knot where two segments intersect. The most prominent disruption is a trough, apparently a graben. To its left (northeast), the surface dips gently northeast, suggesting that the graben formed by the stretching of the surface when it was thrust onto the area to the left (see Fig. 3a). Arrows delimit the northeast (left) edge of the thrust sheet. B. Enhtuvshin (right) and R. A. Kurushin stand at the base of the hill. Photograph by P. Molnar on August 16, 1994.

Tsagaan Ovoo-Tevsh uul rupture Beginning approximately 2–3 km southwest of the corner in the Toromhon Overthrust, scattered traces of surface deformation define an arcuate zone of deformation, ~25 km in length, southeast of the Jaran Bogd massif. This zone was first recognized and sketched from aerial reconnaissance in January 1958, when Solonenko et al. (1960, p. 36, Fig. 9) considered it to be one of three main ruptures making up the Toromhon Overthrust. Yet, it received little attention by participants of the expedition in 1958. Although N. A. Logatchev noted ruptures at five localities, the complete rupture is not found on Florensov and Solonenko’s (1963, 1965, Plate 2) summary map, with the exception of a few zones of eastwest disruption along the southernmost part of the trace. Our representation of this zone on Plate 1 is based on Logatchev’s observations, our brief visits in 1993 and 1994, and its continuity on aerial photographs. (Markings on these aerial photographs made by Logatchev and others were still clear in 1993–1994, when we took them to the field.) Standing on Tsagaan Ovoo (Plate 1), Logatchev (#1136) recognized a thrust rupture that followed the base of the hills to the southeast with an uplifted northwest flank. Within the hills north of the scarp, Logatchev also recognized minor cracking and exten-

49

sion, which may reflect deformation of the hanging wall above a change in dip of an underlying thrust fault, or merely superficial slumping. We followed the rupture at the base for about 4 km, where the overall strike is N60–65°E and the north side was uplifted ~0.4 m above the south side. In one place, we also saw evidence for minor left-lateral slip along this zone (<1 m). Farther southwest, the deformation is especially clear both on the ground and on aerial photographs (Figs. 93 and 94). Logatchev (#1141) mapped one scarp that faces north and dies out near a dry valley shown in Figure 93. He noted a second, higher, south-facing scarp farther south (#1140), which we too examined. A profile across this scarp (Site 40) indicates a vertical component of 2.5 m, but further examination revealed that the height decreases to only ~1 m ~500 m to the west (Fig. 94) and disappears completely 1 km farther west. In the area of Figure 94, the scarp formed in bedrock composed of fissile siltstone and sandstone. The deformation appears to be localized folding, rather than faulting. Beds south of the scarp dip gently (10°) eastward. At the scarp, they bend abruptly, although continuously, to dip steeply 50–55°) southward (Fig. 94). This same pattern is apparent at a larger scale (Fig. 93). West of where the scarp dies, basalt and sedimentary rock of the Tevsh Formation (Florensov and Solonenko, 1963, 1965) also dip gently eastward, but north of the scarp, these units strike eastward and dip steeply south. The transition occurs by folding with the nose of a syncline plunging southeast. Thus, we conclude that the syncline seen on the aerial photograph is actively forming, perhaps as a laterally propagating fault-propagation fold (Suppe, 1985, p. 348–352). North of this fold, we recognized two fresh scarps on aerial photographs (Fig. 93) that face north. Neither is high: 0.3 m for the northern and 0.5 m for the southern scarp. We found suggestions of left-lateral slip, perhaps as large as ~1 m along both. This zone of disruption can be traced farther west-northwest on aerial photographs to Tevsh uul (Plate 1). We did not visit this zone farther west, but Logatchev (#1155, 1158) reported complicated deformation near Tevsh uul. In different places scarps face north or south with vertical separations of 0.4 m. At the northwest end, Logatchev (#1029) found an east-west rupture along the north slope of Tevsh uul, with the south side thrust over the north a small amount; the height of the scarp was only 0.1 m. A continuation of this zone northwest of Tevsh uul is uncertain. Along a traverse of the rupture toward the summit plateau of Ih Bogd, Logatchev (#1030, 1035) noted a 1.5-km long system of east-west tension cracks, each hundreds of meters in length. At the southernmost crack, openings varied from 0.2 to 0.7 m, and vertical components were typically 0.2–0.3 m high, and reached 0.5–0.6 m in one place, with the south side down. At the other ruptures, heights did not exceed 0.1 m, with no preferred sense of vertical component. It is not clear to us whether this is purely superficial deformation or a manifestation of deeper tectonic movement. In any case, these areas may mark a continuation of the Tsagaan OvooTevsh uul rupture zone and may be part of a zone where minor convergence and left-lateral slip (< 1 m) occurred in 1957.

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Figure 92. Photograph looking southwest, showing a high scarp at Site 39; the gully in the center is offset left laterally. We measured a left-lateral component of 3.0 ± 0.8 m and a vertical, reverse, component of 5.0 ± 0.4 m. Photograph by R. A. Kurushin, August 16, 1994.

Figure 93. Aerial photograph M-649-10/VIII/58-1978 of a portion of the Tsagaan Ovoo–Tevsh uul rupture, showing traces of scarps that include both reverse and strike-slip components. Two pairs of scarps, marked by arrows, can be seen. The highest is the southernmost (See also Figure 94). The white box denotes Site 40. North of this is scarp, but east of Site 40, a second scarp mapped by Logatchev (#1131) emanates from near the dry valley in the center of the photograph (near the black arrow in the valley). Both of these die out westward, where the outcrop pattern of the bedrock defines the nose of a southeasterly plunging syncline. North of the same rock units, two parallel, north-facing scarps continue westward across the photograph. North is toward the top. (Scale is approximate.)

Surface rupture of the 1957 Gobi-Altay earthquake

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Figure 94. Photograph looking northeast at the main scarp along the Tsagaan Ovoo–Tevsh uul zone. Arrows to the left of R. A. Kurushin mark the scarp, and diamonds delineate continuity of a lightcolored rock unit, structurally beneath a darker unit. These rock units are folded, but not offset by the fault. In the foreground they dip 10° east, but within the scarp they dip 50–55° south and strike ~10° more northeasterly than the scarp itself. Photograph by P. Molnar, August 17, 1994.

Gurvan bulag rupture and its continuations A series of south-facing scarps can be traced for 23 km along what Florensov and Solonenko (1963, 1965) called the Gurvan bulag zone, on the southern margin of Ih Bogd (Fig. 95), and farther west, beyond a gap with no obvious deformation, for another 15 km south of the Ölziyt uul. Most of these scarps face southward and seem to mark the surface expression of thrust or reverse faults that dip beneath Ih Bogd. The only exceptions to the pattern that scarps of the Gurvan bulag zone face south are provided by notes taken by V. P. Solonenko (#532). He wrote that at the eastern end of the zone the north flank lies 0.8–1.2 m below the south flank and inferred that the southern side had overthrust the northern (Plate 1). Nevertheless, he also wrote that 1.5 km farther east (#533), the sense of thrusting reverses, and the trace splits into two branches with the south sides lower by 0.5–0.7 m and by 0.25–0.4 m, respectively. We did not visit this area. At the easternmost locality that we studied (Site 41), the scarp faces south, and the vertical component of displacement is small: 0.7 ± 0.2 m. Farther west, the vertical component increases; at Site 42, 5.3 km west of Site 41, we measured a vertical component of 1.5 ± 0.2 m. The scarp height reaches its maximum offset near the center of the rupture (Figs. 95–97). We mapped two sites (43 and 44) where

vertical components reach 4.0 m (Fig. 98) and 5.2 m (Fig. 99). Just west of the areas we mapped, Solonenko (Sites 527–530) reported his largest estimates of scarp heights, “possibly 7–8 m.” He described deformation that includes both a steep scarp and extensive disruption of the surface of the hanging wall near the scarp. This is best shown by his cross section (Florensov and Solonenko, 1963, 1965, Fig. 148), where the scarp stands 4 m high, and the curvature of the surface of the hanging wall, if caused by warping, might suggest a vertical component greater by as much as 5 m. Some curvature might be the result of degradation of an older scarp. Solonenko, however, did note tension cracks within the hanging wall. It was also in this general area, 0.5–2 km west of Site 44, that Solonenko (#530) observed a cross section through the scarp (Florensov and Solonenko, 1963, 1965, Fig. 130); the fault dips north at 40° near the surface and decreases to 30° near the base of the cross section, where overturned Oligocene sedimentary rock of the Orog Nuur series has been thrust onto unconsolidated Quaternary sediment. N. A. Florensov (#134–136, 142–144) examined the western portion of the Gurvan bulag rupture and noted deformation similar to what Solonenko saw. The main rupture was not straight, but curved, with a more complicated structure on the surface between valleys than in the valley bottoms themselves.

52

R. A. Kurushin and Others

Figure 95. Landsat Thematic Mapper image of the Gurvan bulag rupture and the topography associated with thrust faulting of Ih Bogd over the basin to the south. Note the scalloped trace along the base of uplifted alluvial fans south of the main Ih Bogd massif. North is toward the top.

Figure 96. Mosaic of aerial photographs M-649-10/VIII/58-3233 and -3234 showing the central part of the Gurvan bulag rupture. The eastern and western white boxes surround Sites 43 and 44, respectively. Note that the rupture splits into two and then into more strands in the western part of the area shown. Maximum offsets were noted from east of this area, where a single trace can be seen. White blotches are flaws in the photograph. North is toward the top. (Scale is approximate.)

The highest scarp (#135) that he saw reached 3.8 m. Baljinnyam et al. (1993) measured a vertical component of 3 ± 1 m in this general area. To the west-northwest, scarp heights gradually decreased to 1–1.5 m. Where the rupture dies out, it splays into separate thrust branches spread over a region 200–250 m wide to form an asymmetric horst. The southern scarps faced south and stood 0.25–0.3 m high, but the northern faced north, and stood only 0.15 m high. On the hanging wall, Florensov found

abundant tension cracks with different orientations: parallel and perpendicular to the main rupture and diagonal, forming intersecting angles of 35–40° and 70–75°. Superficial grabens were abundant, also suggestive of extension of the upper block of a thrust fault. We suspect that the underlying thrust rupture continues westward, but that its surface expression is masked by irregular topography above it. West of a 4-km gap in clearly expressed scarps, and south

Surface rupture of the 1957 Gobi-Altay earthquake

53

Figure 97. Photograph looking north across the Gurvan bulag ruptures and the hills south of them toward Ih Bogd. The shadows at the base of the low hills define the rupture. The Ih Bogd summit plateau forms the skyline. Photograph by P. Molnar, August 19, 1994.

of the low range of mountains, the Ölziyt uul, a south facing scarp again can be recognized (Fig. 100). Like Florensov and Solonenko (1963, 1965), we traced this scarp westward for ~15 km. The largest displacements that we saw were in the eastern part; at Site 45 we measured a vertical component of 1.9 ± 0.3 m. In this same area, Florensov (#145) noted a south-facing thrust scarp 1.5–2 m high, again with tension cracks on the upper block, over a zone 25–30 m wide. At the eastern end of this rupture, deformation is spread over a wider zone with separate scarps, each with heights of ~0.8–1 m, separating thrust slices and giving way to a region where springs are common. To the west of Site 45, Solonenko (#538)

reported scarps as high as 2 to 5 m. We, however, saw no scarps with heights exceeding 2 m. Instead, the height seemed to decrease westward. South of the main rupture, we did see short zones of subparallel south-facing thrust scarps 0.3–0.7 m high, roughly parallel to the main trace, but nowhere with a combined offset >2 m. West of Site 45, the trace curves into a west-northwest trend with three roughly parallel splays. The southwestern trace lies at the base of a rounded, south-facing scarp 10–20 m high. The fresh part of the scarp commonly is low (≤0.3 m), but reaches 0.5 m in places. The middle trace faces north and is difficult to follow, for the scarp clearly is lower. The main

Figure 98. Photograph looking north at Site 43, an offset fan, along the Gurvan bulag rupture. A. Bayasgalan standing on the scarp provides a scale for the vertical component of 4.0 ± 0.3 m. Photograph by P. Molnar, August 19, 1994.

Figure 99. Photograph looking north at Site 44, along the Gurvan bulag rupture. Arrows note top and bottom of scarp, where we measured a vertical component of 5.2 ± 0.5 m. Photograph by P. Molnar, August 19, 1994.

54

R. A. Kurushin and Others

Figure 100. Photograph looking north across the Ölziyt rupture at Site 45, with A. Bayasgalan on the crest of the scarp and B. Enhtuvshin in front of it. Black arrows show top and bottom of scarp, and vertical component of 1.9 ± 0.3 m. Photograph by P. Molnar, August 20, 1994.

trace seems to follow the break in slope at the base of the Ölziyt uul, for heights along it reach 1–1.5 m. Florensov and Solonenko (1963, 1965, Plate 2) showed high scarps on east-west ruptures northeast of the Ölziyt uul almost to the western end of the Ih Bogd massif, but we observed only minor ruptures. On the northeast edge of the Ölziyt uul, a low scarp only 0.3–0.4 m high, trending N60°W, and with the southwest side up can be traced for ≤1.5 km along the base of the hills, but it may reflect only superficial deformation. In this area, Florensov also noted small tension cracks parallel to this scarp. This minor deformation might be superficial. Farther north, Florensov (#148) mapped a second, longer low trace that faces north (Fig. 101). The front is curved and in places splays into 2–3 traces. The height of the scarp is typically 0.2–0.3 m, but locally 0.5 m. As we walked along it, from end to end (~3 km), we persuaded ourselves that a small (~1 m) rightlateral component had developed during the 1957 earthquake, and we mapped Site 46 to quantify the strike-slip offset. A profile across the scarp demonstrates a vertical component of 0.7 ± 0.2 m, but profiles parallel to it indicate a negligible strike-slip component. In the 0.5–0.8 km northeast of this rupture, a series of hills on the large alluvial fans from Ih Bogd delineate what Florensov and Solonenko (1963, p. 338; 1965, p. 360) called an “embryonic foreberg.” Along the southwest slopes of these hills, ~3 km northeast of Site 46, we saw a southwest-facing scarp 0.1–0.2 m high for a distance of ~1.5 km. Within the Bulagtay Valley (literally “valley with springs”) yet farther northwest, Florensov and Solonenko (1963, 1965) described large vertical components and openings of tension cracks, which, from their map (their plate 2), seemed to suggest

Figure 101. Photograph looking southwest across the rupture at Site 46, with Ch. Bayarsayhan (left) standing on the scarp and A. Bayasgalan below it. We measured a negligible right-lateral component of 0.1 ± 0.4 m and a vertical component of 0.7 ± 0.2 m. Photograph by P. Molnar, August 21, 1994.

important thrust faulting. We saw little evidence of clear surface faulting, but only indications of slumping. Logatchev (#1163) described curved, discontinuous tension cracks as wide as 3–4 m, along some of which the northeast side rose at most 1.5 m with respect to the southwest sides. Most lacked a vertical component and were associated with northeast-southwest extension. Approximately 50–70 m southwest of the main rupture zone and roughly parallel to it, curved discontinuous compressional ridges formed in lake and swamp deposits. This system of ruptures apparently follows the northeast boundary of intrabasin subsidence, associated with the expulsion of ground water fed from Ih Bogd. Therefore, the ruptures seem to be connected with liquefaction and sliding on a sloping surface. Florensov and Solonenko (1963, p. 351–353; 1965, p. 373–375, Fig. 193) noted that similar styles of deformation, including deep troughs, formed near Orog Nuur. At the same time, it is impossible to exclude the possibility that slip on deeper faults, along the extension of the foreberg farther southeast, was associated with this deformation. We conclude that important thrust faulting occurred along the southern margin of Ih Bogd, but probably not in the valley west of it. Deformation on Ih Bogd and its surrounding high terrain As Florensov and Solonenko (1963, p. 262; 1965, p. 275) reported, numerous surface ruptures cross the summit plateau of Ih Bogd and the other high terrain between the Bogd rupture and the Gurvan bulag zone (Plate 1). Much of this deformation may be superficial, associated with landslides, rockfalls, and tension cracking on steep slopes. Ruptures throughout this area are not simple, and in some areas this complexity resulted from the relative movement of blocks of frozen ground. In northern and western Mongolia, we have observed similarly complicated deformation that appears to be the result of relative displace-

Surface rupture of the 1957 Gobi-Altay earthquake ments of blocks of permafrost (Baljinnyam et al., 1993). Nevertheless, some of the disruption of Earth’s surface appears to result from slip on faults that penetrate the crust and reflect tectonic deformation. The least ambiguous zone of tectonic deformation crosses the eastern part of the summit plateau from the Bitüütiyn am to the Icheetiyn gol where a series of anastomosing traces seem to define northwesterly trending zones of left-lateral strike slip (Plate 1). In numerous localities along the southwest zone, oriented N50°W, systematically oriented tension cracks and mole tracks attest to left-lateral slip (Figs. 102–107). Tension cracks can be discerned on aerial photographs, particularly where a huge gash, >1 m wide and nearly 2 m deep, formed (Fig. 102). In 1958, A. V. Luk’yanov (#1535) and R. A. Kurushin observed one such rupture, dipping southwest at 70°, with a vertical component of 2.5 m (southwest side down) with an opening across the tension gash of 2–2.5 m (Fig. 103). Although deep tension cracks no longer can be found, vertical components of slip remained clear in 1993 (Figs. 104–106). Near the southeast end of this zone, on the left bank of the Icheetiyn gol, a vertical component of the opposite sense, northeast side down a maximum of 0.9 m, could be seen (Fig. 106). Although evidence of strike-slip faulting is clear in some places, the flatness of the plateau provides few offset features that

55

Figure 103. Photograph looking east along the eastern end of the rupture in Figure 102 and showing a huge tension crack. The vertical offset is 2.2 m, and the opening reaches 2.5 m. Photograph by R. A. Kurushin, September 17, 1958. (See also Fig. 104 of Florensov and Solonenko, 1963, 1965.)

Figure 104. Photograph near that in Figure 103, but taken more recently and from the southeast. R. A. Kurushin stands on the northeast side, elevated above the southwest side ~2 m in this area. This scarp can be traced for more than 1 km on the flat surface. Note also the flat summit plateau of Ih Bogd on the horizon. Photograph by P. Molnar, August 24, 1993.

Figure 102. Aerial photograph M-649-2/VIII/58-2558 showing a rupture on the summit plateau of Ih Bogd where tectonic deformation is clear. The white dot shows the point from which the photograph in Figure 103 was taken. North is toward the top. (Scale is approximate.)

Figure 105. Photograph of a strand just above the headwaters of the Icheetiyn gol and parallel to the southeast continuation of the rupture shown in Figures 103 and 104. View is northwest along the northeast facing scarp and indicated by “h—0.9 m” on Plate 1. Photograph by R. A. Kurushin, September 15, 1958.

56

R. A. Kurushin and Others mit plateau, the system of ruptures passes through and beyond the area of the Bitüüt landslide, a huge landslide that formed in 1957, which is discussed in the next section. The Bitüüt landslide is not the only example of superficial deformation. Huge cracks can be seen on aerial photographs of much of this region (Figs. 108 and 109). Although most of this deformation appears to be superficial, some such scarps might reflect deep-seated deformation. Bitüüt landslide

Figure 106. Photograph looking southeast toward the Icheetiyn gol, along a trace parallel to that in Figure 105 and a few hundred meters northeast of it. Horses on the left provide a scale. Photograph by P. Molnar, August 24, 1993.

Deformation of the Earth’s surface in the upper reaches of the Bitüütiyn am, within the interior of the Ih Bogd massif, occurred at a scale that challenged the imaginations of the first investigators of the epicentral region (e.g., Solonenko et al., 1960, p. 41–42). A block of rock 138 × 106 m3 separated from the ridge on the north bank of the Bitüüt am and slid into the valley (Fig. 110). From both detailed field observations and photogrammetric analysis of the aerial photographs, however, V. P. Solonenko (#504) con-

Figure 107. Photograph looking north at blocks of sod that collectively form mole tracks along a strike-slip rupture parallel to those in Figures 102–106. Typical heights of the mole tracks reached 0.5 m, as in the photograph, by R. A. Kurushin, September 15, 1958. (Fig. 143 of Florensov and Solonenko, 1963, 1965.)

could be measured. Thus, quantifying horizontal displacements was difficult in 1958, and has become yet more so with time. Nevertheless, Luk’yanov (#1537) noted one locality with two nearby ruptures, where a gully was offset 0.5 m by one strand and at least 1 m by an adjacent strand. Approximately parallel to this zone and northeast of it, Luk’yanov (1965, Figs. 24 and 27) traced another zone of left-lateral slip, trending N40°W, and estimated a displacement of 0.5 m. Along this zone, mole tracks attest to strike slip (Fig. 107). Moreover, several fractures contribute to the overall deformation, some spread over zones 50–100 m in width (Luk’yanov, 1965, Fig. 28). These zones of strike-slip faulting continue both southeast and northwest from the summit plateau, but we are not aware of unequivocal evidence of deep-seated faulting along the continuations. Farther southeast, Luk’yanov (#1507, 1725, 1726) mapped possible continuations of these zones along both banks of the Icheetiyn gol as a series of southeast-striking scarps with heights up to 0.7 m, together with tension cracks (Florensov and Solonenko, 1963, 1965, Fig. 192). To the northwest of the sum-

Figure 108. Aerial photograph M-649-2/VIII/58-2424 of the southern flank of Ih Bogd showing (apparently) superficial deformation of the surface. Note prominent scarps across the flat ridge in the upper part of the photograph (trending east-southeast) and other scattered scarps farther south (delineated by arrows). North is toward the top. (See Plate 1 for location. Scale is approximate.)

Surface rupture of the 1957 Gobi-Altay earthquake

57

Figure 109. Mosaic of aerial photographs M-649-28/VI/58-360 and -361 of part of the southwest flank of Ih Bogd showing (apparently) superficial deformation. Florensov and Solonenko (1963, 1965), using photogrammetry, estimated a height of 10.7 m for the highest part of the prominent scarp in a zone of extension. North is toward the top. (See Plate 1 for the locations. Scale is approximate.)

cluded that in this area a hitherto unknown “gravitational-seismotectonic” structure, combining tectonic subsidence and superficial collapse of the side of the mountain, had formed. Calling it “the Bitüüt collapsed wedge structure” (Florensov and Solonenko, 1963, p. 310; 1965, p. 328), he inferred that the earthquake rejuvenated two faults that dip toward one another, one of which followed the east-west–trending mountain spur along the left bank of the Bitüüt valley, and the other lay near the base of the spur. The block confined between them, with dimensions of 3 km × 1.1 km, dropped as a wedge into the earth, with the amplitude of vertical slip on the north side reaching a maximum of 328 m (Florensov and Solonenko, 1963, p. 310–318; 1965, p. 328–337). Not all members of the Gobi-Altay expedition in 1958 shared Solonenko’s interpretation. Based on his own field observations (#1555–1556), Luk’yanov (1965, p. 44–45) suggested that it was primarily a seismically induced “gravitational” landslide that accompanied the earthquake, and therefore had little or no tectonic significance (Fig. 110). In contrast, S. D. Khil’ko and R. A. Kurushin also thought that it was a landslide but conceded that, at least in part, it was provoked by slip with a normal component on a fault following the crest of the spur. This, in fact, was the point of view expressed initially by participants of the first reconnaissance expedition of the earthquake in January 1958 (Solonenko et al., 1960, p. 41–42), for indeed a scarp seen clearly northwest of the landslide suggests that surface rupturing, not just shaking, was responsible for the landslide. For Solonenko, the basic evidence against the Bitüüt structure being a simple landslide and requiring an important tectonic element, the collapsed wedge, was his inference that much of the rock mass displaced from the ridge had disappeared without a trace. By constructing a topographic profile across the central part of the structure before and after the earthquake, he calculated that

Figure 110. Photograph showing the huge landslide in the upper Bitüüt valley. View is west-northwest. Photograph by Luk’yanov (1965, Fig. 26).

58

R. A. Kurushin and Others

a cross-sectional area of 107,000 m2 of rock was missing from the ridge, but the cross-sectional area of fragments deposited in the valley was only 55,000 m2. From this, Solonenko concluded that nearly half of the displaced rock had vanished. Then, allowing for an increased volume of the dismembered fragments and the creation of porosity, he deduced that the deficit must be 2.5 times (Florensov and Solonenko, 1963, p. 318; 1965, p. 337). Recognizing that one profile is insufficient for such a startling conclusion, one of us (Kurushin) carried out a more complete analysis. From photogrammetric analysis of aerial photographs at a scale of 1:50,000 taken in 1949 and at 1:25,000 in 1958, topographic maps of the Bitüüt structure before and after the earthquake were constructed for him (Fig. 111). From these maps, he constructed meridional profiles (perpendicular to the Bitüüt valley) and 250 m apart, at equal vertical and horizontal scales of 1:25,000, to define the relief before and after the earthquake (Fig. 112). These profiles were drawn to include the entire region where rock was removed from the ridge (Profiles IV–X) and deposited (II–XV). Profiles before the earthquake show that the crest of the ridge forming the northern side of the Bitüüt valley (oriented 290°) towered 500–650 m over the valley bottom. Not only was the entire south slope removed from the ridge for a distance of about 1.5 km along it, but so also were the crest of the ridge and 250 m of its north slope (VIII, Fig. 112). The maximum width of the displaced part of the ridge reaches 1 km, where the height decreased by 340 m. Vertical displacements vary from 350 m (IV) in the western part of the structure to 150 m (X) in the east. The mass of rock deposited in the valley occupies a much larger area than that removed. Its length parallel to the valley exceeds 3 km (Fig. 111), and its width across the valley reaches a maximum of 1.2 km (VI–IX, Fig. 112). Thin talus, rockfalls, and landslides, not easily recognized on the profiles, account for this extent of accumulation along the valley, but its total extent on the bottom of the valley is clearly exposed, for instance on profiles II and XV (Fig. 112). The displaced mass not only covers the valley floor with a thickness as much as 300 m, but also was carried onto the opposite side to a height reaching 130 m above the old valley floor (VI). This spreading out of material over the valley floor, if only for small distances up the valley (III), not less than 750 m down the valley (XI–XIV) attests to a large horizontal component of displacement. We estimated the difference in volumes of rock mass torn from the ridge and transferred to the valley by two analyses of the cross-sectional areas in Fig. 112. For both we used pairs of adjacent profiles and estimated volumes of material between them (Table 4). First, we calculated cross-sectional areas before (S49) and after (S58) the earthquake above the 2250 m contour and then estimated volumes for each (V49 and V58). Second, like Solonenko, who used only one profile, however, probably somewhere between profiles IV and VIII (Fig. 112), we calculated the crosssectional area of rock missing from the ridge (Amis) and that accumulated in the valley (Aacc) for each profile. Then from them, we calculated the volumes (Vmis and Vacc). We estimate that errors in heights could reach 20 m and in horizontal positions 25, which

would call for errors in volume of only 104 m3. Allowing for other sources of error, such as in estimating the landslide boundaries, in interpolating between profiles, or in including small rockfalls and talus in the overall volume of the landslide, this estimate could be doubled or even tripled. In any case, we consider the volume to be uncertain by only a few percent. Not only is there no deficit of mass after the earthquake, but, on the contrary, the differences of 68 × 106 m3 and 66 × 106 m3 calculated from each approach indicate larger volumes of material deposited than removed from the spur. An ~50% greater porosity of the accumulated than original material accounts for the difference between amounts. This comparison rules out Solonenko’s inference of a huge tectonic wedge falling into part of the Bitüüt valley. More likely the earthquake in 1957 merely provoked a very large landslide, an inference supported by other observations. Geologic mapping by Shmotov, Luk’yanov, and Solonenko showed the rock on the ridge to consist of gneissic granite with layers and packets of quartz- and biotite-bearing, intensely metamorphosed schist and black phyllitic slate with layers of marble. These layers dip southwest (toward 225–240°) at 45° to 60° and are cut by a multitude of similarly oriented ancient fractures with fault gouge, slickensides, and unconsolidated breccia. They form the northern limb of a large-scale asymmetric syncline with analogous schist on the right (south) side of the Bitüüt valley dipping 70°–80° northeast (toward 120° to 140°). Thus, the geologic and geomorphic conditions seem to have weakened the rock before the earthquake occurred. The surface along which rock separated from the ridge dips as steeply as 70° in its very upper part (Florensov and Solonenko, 1963, 1965, Fig. 171). Commonly, however, dips vary between 25° and 45°, with an average dip ~32°. The agreement of this dip with the average slope of the surface before the earthquake supports the inference that the material slid down slope. According to observations of Solonenko and Luk’yanov, the main part of the displaced rock mass consists of four large layered blocks of Paleozoic rock that underwent vertical displacement, and possibly rotation about horizontal axes, relative to one another, so that on the contemporary valley floor they form surfaces of different widths that slope gently to the south, separated by north-facing steps. A very wide (350–500 m) step follows a scarp and is separated from it by a deep, wide gully. Judging from the profiles (Fig. 112), the northern slope of the ridge moved largely horizontally and spread apart as it flowed over the valley floor. Other narrower (from 30–40 m to 100–120 m) steps at the front of the structure, too small to be seen on the scale of the profiles, characterize blocks that contain thick lenses of granitegneiss and granitized schist covered by fragments of soil layers and plants. They represent material from the crest and south slope of the ridge, originally cut by extension cracks and ancient tectonic fracturing. According to Solonenko, the 1957 earthquake reactivated the fractures with the formation of a graben and subsidence (as a wedge) of the blocks described above (Florensov

Surface rupture of the 1957 Gobi-Altay earthquake

Figure 111. Topographic maps of part of the Bitüüt valley, constructed from photogrammetric analysis of aerial photographs taken before the Gobi-Altay earthquake in 1949, at a scale of 1:50,000 (top) and after it in 1958, at a scale of 1:25,000 (bottom). Contours are spaced at 25 m, with dashed contours at 5 m. In both the identical rectangular areas surround a region higher than 2,250 m. Roman numerals identify profiles in Figure 112 and Table 4.

59

60

R. A. Kurushin and Others

Figure 112. Topographic profiles drawn across the region affected by the Bitüüt landslide (see Fig. 111). Dashed regions show area where material was removed, and dotted areas show accumulated material. Table 4 tabulates areas of these cross sections.

and Solonenko, 1963, p. 315; 1965, p. 329, 336, Fig. 174), but clearly a large landslide makes a simpler interpretation. We infer that rock broke away at a tectonic scarp, where it crossed a steep, south-sloping surface, but that most of the rock slid down the slope as a superficial landslide. SUMMARY OF SURFACE FAULTING IN 1957 We give here a brief summary of the surface faulting, focusing both on generalities and on peculiarities along the rupture that may make it different from other ruptures, but that also make it interesting. We begin with a discussion of the Bogd rupture, then consider the two forebergs within it, the Toromhon Overthrust, the Tsagaan Ovoo-Tevsh uul and Gurvan bulag thrust ruptures along the southern margin of Ih Bogd, and finally the deformation within the Ih Bogd massif itself. Bogd rupture, exclusive of the forebergs Left-lateral strike slip is the dominant style of deformation throughout most of the Bogd rupture, 260 km in length from the

Bayan Tsagaan nuruu in the west to the east end of the rupture, east of Bulgan uul (Plate 1, Fig. 2). The magnitude of slip grows rapidly from nil at the west end to 3–3.5 m (Figs. 6, 7, and 9), which characterizes most of the western 35 km of the rupture. The amount abruptly increases a few kilometers west of Ulaan bulag höndiy, and for approximately 40 km eastward, displacements of 5–7 m characterize the rupture (Figs. 13, 14, 16, 23, 24, and 26–30). The amount of displacement drops abruptly to 3–3.5 m just east of the low, white-colored hill, Öndgön Hayrhan (“Egg Mountain”) near the Hüühniy höndiy. The locus of this change lies north of the western terminus of thrust faulting along the southern margin of Ih Bogd and close to the projection of where the strike-slip deformation on the Ih Bogd massif should intercept the Bogd rupture (Plate 1, Fig. 2). The logical inference is that part of the 5–7 m of left-lateral slip to the west is transferred both into the Ih Bogd massif and south of it. Left-lateral slip of 3–3.5 m typifies the magnitude of offsets throughout most of the rest of the Bogd rupture, but with numerous complexities and variations. Although strike-slip faulting characterizes the average faulting throughout most of the Bogd rupture, and in particular west

Surface rupture of the 1957 Gobi-Altay earthquake

61

TABLE 4. BUDGET OF MATERIAL INVOLVED IN THE BITÜÜT LANDSLIDE Profile

I

S49 V49 S58 V58 Smis Vmis Sacc Vacc (103 × m2) (106 × m3) (103 × m2) (106 × m3) (103 × m2) (106 × m3) (103 × m2) (106 × m3) 1,431

1,431 353

II

1,393

III

1,368

IV

1,404

V

1,394

VI

1,348

VII

1,256

VIII

1,181

IX

1,065

X

960

XI

874

XII

818

XIII

750

XIV

628

XV

656

1,417 345

355

6 18

339

326

25

319

305

27

297

281

26

280

253

19

261

229

13

245

212

6

232

196

0

211

172

0

181

160

0

166

of Baga Bogd, the rupture is neither straight nor characterized by a single trace for long distances. In the westernmost 35 km of the rupture, long narrow graben-like structures are common. Some are as narrow as only many tens of meters, as illustrated well by Florensov and Solonenko (1963, 1965, Fig. 157; Baljinnyam et al., 1993, Fig. 38). In portions of the Bahar graben, which lies below and south of the Bahar uul (Plate 1), the northern and southern margins are hundreds of meters to 1 km apart (Figs. 8 and 10). The vertical components of slip have been preserved better than the strike-slip components, but the strike-slip offsets can be determined where the graben structure is poorly defined, and one trace is clearly the major one (Figs. 5–7 and 9). This wide rupture zone may reflect complex deformation at depth, but because of its narrowness at the surface and the consistent leftlateral offsets along it, we suspect that deformation at depth consists of nearly pure left-lateral strike slip. We observed the sharpest traces where offsets were large, ±5 m. In nearly every place where we saw clear offsets of this amount, only a single trace could be seen. These traces, however, are nowhere straight for distances longer than approximately 2 km, as best seen on aerial photographs (Figs. 12 and 15). Variations in strike are associated with differing vertical components,

9 30

0 0

3,939

15 41

0

666

20 78

0

658

21 82

0

791

21 85

0

896

18 80

46

956

17 66

61

1,001

20 70

92

1,085

20 86

112

1,152

14 75

105

1,224

11 38

98

1,330

10 53

48 346

343

3,871

0

351

1,330

3 24

0

1,386 350

0 0

0

1,421 346

Totals

0 356

5 10

138

204

with neither side consistently uplifted. At Ulaan bulag höndiy, the south side is uplifted, but farther east, at Sites 3 and 4, the north side is up (Figs. 14–16). Variations in strike reach 20° (Fig. 12 and 15), and projections onto linear continuations imply steps of 400 m from one another, or 200 m from the average trace of the fault. Such variations are comparable in scale to jogs on other faults commonly used to define segments with uniform characteristics that differ from adjacent segments. Thus, if the surface trace accurately mapped the fault surface at depth, such variations would suggest that the fault should be divided into numerous segments. A surface trace following geologic weaknesses at the surface above a straighter, more planer fault surface at depth seems less astonishing (to us). Between many straight traces, the surface rupture splays into more than one rupture, obliquely oriented by tens of degrees to the average strike of the fault zone. Some such splays surround depressions (Figs. 18, 33, 51, 52, and 62), presumably small grabens, that are bounded by faults with large normal components (Figs. 19, 50, and 53). Other splays wrap around hills (Figs. 21, 22, and 40) that apparently have been uplifted by vertical components (Fig. 39). Ignoring, for the moment, the largest splays and their associ-

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R. A. Kurushin and Others

ated uplifted blocks, the forebergs, the surface trace shows a complexity that the deeper fault surface need not, and almost surely does not, mimic. Suppose these splays continued to depths of 10–20 km before uniting in a single main fault surface. The dimensions of the depressions and hills transverse to the overall trend of the rupture are only ~1–3 km. The difference in dips of these splays and the main strands of the strike-slip fault that ruptured in 1957 would be no more than ~5–10° (= arctan 0.1–0.2). If these splays reflected deep-seated faulting, they too must dip very steeply. For pure strike slip along the main rupture, divergence or convergence must occur on planes striking differently from the main rupture. For a difference in strike of ∆θ, the component of divergence or convergence should be ∆u · sin∆θ. Such splays commonly intercept the main trace at angles of ∆θ = 30°; for ∆u = 3 to 6 m, convergent or divergent components should be 1.5 m or 3.0 m. Yet, with dips of the splays of δ = 80-85°, the vertical component should be ∆u · sin∆θ · tanδ, which, for the range of values given above for ∆u and δ, should be at least 7.5 m, and perhaps as large as 30 m. Nowhere were vertical components of this magnitude measured for such splays. We conclude that these splays can reflect at most only subtle variations in orientations of the main fault at depth. More likely, they constitute superficial deformation extending only 1–2 km below the surface above a more nearly planar strike-slip rupture at depth. With two notable exceptions, discussed below, vertical components of displacement along the relatively simple zones of the Bogd rupture show no predominant sense. Uplift of the northern side (Figs. 6, 7, 9, 14, 16, and 54) seems to be as common as that of the southern (Figs. 26–32, 48, 49, 57, and 59–61) in the portions north and west of Noyon uul and between the Dalan Türüü foreberg and the area just east of the Toromhon Overthrust (Fig. 2, Plate 1). Moreover, there is little correlation with local topography (Baljinnyam et al, 1993); in many areas the lower, downhill flank of the scarp was uplifted relative to the higher, uphill flank (e.g., Figs. 14, 26–30, 59, and 61). This lack of a consistent sense of the vertical component (exclusive of the prominent exceptions to be discussed) suggests that, in general, pure strike slip occurred, with vertical components resulting from local, superficial variations in strike and the corresponding components of extension or compression across obliquely oriented traces (Baljinnyam et al., 1993). The south side was consistently uplifted with respect to the north side where the trace follows the northern flank of Ih Bogd and the northeastern flank of Baga Bogd, the two highest mountains in the region. At nearly all sites between the Hüühniy höndiy and the Dalan Türüü foreberg (Fig. 2, Plate 1) and where the rupture is simple, the southern side rose between 1 and 3 m with respect to the northern side (Figs. 36–39), with the typical value being 2 m and decreasing westward. Only at the western end of this area, near the Hüühniy höndiy, did we see negligible north-facing scarps. Similarly, sites on the northeast side of Baga Bogd, particularly where the rupture appears to be simple, show vertical components of approximately 2 m (Figs. 69, 72, 74, 76, 78, and 79). This obviously

implies these two massifs have risen, at least in part, by oblique reverse slip. The average strike of N106°E of the rupture along the northern edge of Ih Bogd differs by only 6–9° from that west of Sites 10 and 11 or east of the Dalan Türüü foreberg (Fig. 2, Plate 1, Table 5). Let us apply the logic used above to address the dip of the main fault beneath the Ih Bogd massif and assume 3–3.5 m of slip of the southern side in the direction N98°E. Along the northern edge of Ih Bogd, there should be a component of convergence on a fault striking N106°E equal to ∆u · sin∆θ = 0.4–0.5 m (for ∆θ = 8° and ∆u = 3–3.5 m). Hence, with tanδ = ∆uv /∆u · sin∆θ a vertical component of ∆uv = 2 m implies a southward dip of δ = 75–80° for this portion. The largest sources of uncertainty in this estimated dip are in the 8° difference in strike and the vertical component of 2 m, each uncertain by about 30%. Hence, the dip could be as steep as 85° or as gentle as 70°. We may apply the same logic to the scarp northeast of Baga Bogd, but here the orientation of the slip vector along the Bogd rupture is less certain than in the Ih Bogd region. Let us consider two possibilities: (1) the slip vector again is parallel to N98°E, the orientation near Ih Bogd, and (2) that it is parallel to the trace northwest of Baga Bogd, N75°E, which seems to show essentially pure strike slip. The local strike of the rupture northeast of Baga Bogd of N110°E differs by 12° from the first and by 35° from the second. Again for displacement of ∆u = 3–3.5, we estimate 0.6–0.7 m and 1.7–2.0 m of convergence, respectively. With a vertical component of 2 m, the corresponding dips would be 77–80° or 45–50°. Because of uncertainties in both the horizontal component of slip and the orientation of the slip vector in this area, the inferred dip is much more uncertain than near Ih Bogd. Nevertheless, it appears that the fault dips more gently beneath Baga Bogd than beneath Ih Bogd, but not as gently as 30°. The deformation surrounding the northern margin of Baga Bogd reveals more complexity than that north of Ih Bogd. First, evidence of a major rupture northwest of Baga Bogd (Plate 1) is sparse. Strike-slip faulting is poorly developed. Although present, the rupture was difficult for us to recognize and trace in a single brief visit. Where we and elsewhere where Florensov saw it, the orientation of nearly pure strike-slip faulting was east-northeast, and hence not parallel to that farther west, where the strike is eastsoutheast. In contrast, on the northeast margin of Baga Bogd, deformation is impressive and distributed over a broad area, with significant ruptures both at the foot of the massif and within basement rock cropping out on its steep northeast flank (Figs. 69–76). Vertical components along most of the margin of the range appear to reflect thrust slip; cracking of the hanging wall near the scarp suggests that it has been stretched across this area as illustrated schematically in Figure 3a. Several examples of faulting within the mountains reveal normal faulting on planes dipping south (Figs. 71 and 72), as would be expected if the dip of the underlying thrust plane steepens as it approaches the surface farther north (Fig. 3c). Normal faulting also seems to occur near the base of the high terrain farther east, in an embayment northwest of Bulgan

Surface rupture of the 1957 Gobi-Altay earthquake uul (Figs. 2 and 75, Plate 1); here the planes dip north, and the occurrence of normal faulting suggests that the dip of the underlying thrust plane flattens as it approaches the surface farther north (Fig. 3b). North of the main outcrop of thrust faulting and east of it, deformation is distributed over a wide zone. A sharply defined scarp follows the northern margin of Bulgan uul (Figs. 76, 78, and 79). Presumably, the plane dips southward, and slip includes a substantial reverse component. Farther north, many short, obliquely oriented ruptures show conjugate strike-slip faulting (Fig. 77, Plate 1). Much of this deformation can still be seen, but measuring offsets is no longer easy. We rely on Luk’yanov’s (1965) mapping and descriptions of it. This widespread distribution of relatively minor faulting indicates superficial deformation of, presumably, sedimentary rock stressed by the overthrusting of it on its south and, therefore, is analogous to the deformation reported by Goldfinger et al. (1992) in the forearc of the Cascade subduction zone off the coast of Oregon. East of Bulgan uul, the zone of deformation ends with numerous north-south–trending ruptures showing vertical components, apparently of a thrust sense. Because scarps face both east and west, the surface faulting may reflect superficial

63

deformation of a thin thrust sheet that dips west, beneath the eastern end of the Baga Bogd massif. Overall, the deformation surrounding Baga Bogd is consistent with large-scale regional left-lateral shear along a zone oriented approximately east-west, with large thrust components on the eastern flanks, but with sufficient complexity that makes quantifying the amount of slip difficult. We rely on the largest amount that we measured, from Site 34 (Fig. 74, Plate 1), to give an estimate of the total strike slip (~4.8 m) and vertical components (~2.6 m). Yet, because we have only one such locality, plus some qualitative estimates from nearby, we must emphasize that this estimate could be unrepresentative of the slip during the earthquake. Dalan Türüü and Hetsüü forebergs The most prominent interruptions of the Bogd rupture lie just north of Ih Bogd (Fig. 42) and Baga Bogd (Figs. 64 and 65), where the rupture steps northward and bounds rows of low hills, called forebergs by Florensov and Solonenko (1963, 1965). At both forebergs, scarps that grew in 1957 bound the hills on their north sides. Sedimentary rock is clearly deformed, for it tilts, commonly southward, at angles of tens of degrees within the fore-

TABLE 5. SUMMARY OF DISPLACEMENTS ALONG VARIOUS 1957 SURFACE RUPTURES Lat. (°N)

Long. (°E)

Strike (°)

Dip (°)

Rake (°)

Length Displacement* (km) (m)

45.17 45.12

99.25 99.71

100 99

90 90

0 0

37 45

3.2 ± 0.8 6.0 ± 1.0

45.08 100.06 45.04 100.36 44.96 100.90

97 106 100

90 77 90

0 30 0

9 41 44

3.5 ± 0.5 3.5 ± 0.5 3.5 ± 0.5

44.95 101.38 44.92 101.73 44.75 102.02

75 115 100

90 60 45

0 33 90

30 40 11

3.5 ± 0.5 5.0 ± 1.0 3.0 ± 1.0

44.84 102.04

75

90

0

7

0.5 ± 0.1

44.78 102.09 44.80 101.03

0 216

45 45

90 110

8 12

2.0 ± 1.0 5.0 ± 1.0

44.85 100.99

150

60

62

4

5.0 ± 1.0

44.82 101.01

180

45

90

5

0.3 ± 0.1

44.85 100.92

240

45

90

7

1.5 ± 0.5

44.83 100.85

273

45

74

9

3.0 ± 1.0

44.86 100.73

300

45

90

10

1.0 ± 0.5

44.82 100.33 44.93 100.08 44.93 100.45

278 286 134

45 45 90

90 90 0

25 18 30

4.0 ± 1.0 3.0 ± 1.0 2.0 ± 0.5

Rupture

Bogd, west end. Bogd, Ulaan bulag to Öndgön Hayrhan. Bogd, near the Hüühniy höndiy Bogd, north-northeast of Ih Bogd. Bogd, from Dulaan Türüü to east of the Toromhon Overthrust. Bogd, northwest of Baga Bogd. Bogd, northeast of Baga Bogd. Bogd, north of Bulgan uul (thrust slip). Bogd, north of Bulgan uul (strike slip). Bogd, east end. Toromhon Overthrust, northern portion. Toromhon Overthrust, central portion. Toromhon Overthrust, southern portion. Tsagaan Ovoo-Tevsh uul, eastern portion. Tsagaan Ovoo-Tevsh uul, central portion. Tsagaan Ovoo-Tevsh uul, western portion. Gurvan Bulag. Ölziyt uul. Ih Bogd summit plateau.

*Orientations are defined by standard seismological conventions (e.g., Aki and Richards, 1980, p. 106), and displacements are parallel to the slip vectors.

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R. A. Kurushin and Others

bergs. Thus, it appears that both forebergs formed by thrust slip on gently southward dipping planes that must intersect the main, steeper ruptures at depth, beneath the Ih Bogd and Baga Bogd massifs. Moreover, these thrust faults appear to dip more steeply beneath the forebergs than south of them, where alluvial fans shape the topography, or at their northern edges, where the underlying ramps must flatten. The Hetsüü foreberg (Figs. 2, 64, and 65) appears to be a manifestation of counterclockwise rotation about a vertical axis of a thin slice of uppermost crust, presumably detached sedimentary rock. The rupture follows the northern edge of the foreberg with the vertical component increasing eastward. At its eastern end, the rupture abruptly curves southward (Fig. 67) and transforms into a rightlateral strike-slip zone with 3 m of offset (Fig. 69). This strike-slip rupture can be traced across the alluvial fan almost to the foot of Baga Bogd, with the slip vector essentially perpendicular to that of the main Bogd rupture and intersecting the foreberg at a high angle (Fig. 2, Plate 1). The height of the thrust scarp at the foot of the foreberg decreases monotonically from a maximum of about 2 m at its interception with the strike-slip rupture (Fig. 67) to only 0.1 m, or less near the western end of the foreberg, suggesting that the slice of upper crust forming the foreberg rotates counterclockwise about a vertical axis not far from the northwest end of the foreberg. The height of the foreberg above the fan also decreases westward, suggesting that the slip in 1957 is typical of that responsible for the foreberg. Finally, faulting on the southwest corner of such a crustal block, or slice, is also consistent with a counterclockwise rotation of the uppermost crustal slice about an axis near the northwest end, where slip is small. The rupture in the southwest corner of this slice reveals a large component of east-west to northwest-southeast extension; a zone roughly 50 m in width with steep scarps on its margins reveals small grabens on the alluvial fan (Fig. 63). Such extension is called for by counterclockwise rotation of the slice about an axis to its north, or northeast, near the northwest end of the foreberg. The slice of crust involved in creating the foreberg seems to be thin. Within the foreberg, a vertical component with the west side up along the north-south–trending, largely strike-slip portion attests to continued uplift of the foreberg with respect to its eastern surroundings. Only 1 km south, however, where the strikeslip fault crosses the alluvial fan emanating from Baga Bogd, the vertical component becomes negligible, suggesting that the foreberg grows by slip on a nearly flat fault that steepens near the foreberg itself. Thus, the counterclockwise rotation of this slice and the creation of the foreberg appear to be superficial deformation associated with left-lateral shear of the region north of Baga Bogd, and therefore accommodate the left-lateral slip of the Baga Bogd massif past the area to its north, as seen farther east or west. The deformation of the Dalan Türüü foreberg bears similarities to that of the Hetsüü foreberg, but differences are noteworthy. We saw no clear, simple strike-slip fault bounding the Dalan Türüü foreberg, although left-lateral faulting may connect the west edge of the foreberg to the main Bogd rupture. The vertical component along the Dalan Türüü foreberg increased from 1 m in

the west to 2 m along much of it (Figs. 43 and 44). The markedly scalloped shape of the scarp (Plate 1) over the gentle topography requires that the underlying fault not be planer and strongly suggests a gentle dip. We saw no clear evidence of right-lateral slip on a north-south–striking plane, as we did for the Hetsüü foreberg, but we cannot be certain of its absence. H. Philip (1995, personal communication), in fact, reported that he had seen evidence for such slip at one locality. In any case, the role of rotation may be less important in the construction of the Dalan Türüü than the Hetsüü foreberg. The Dalan Türüü foreberg lies very near the largest compressional jog in the Bogd rupture, and the strike-slip component west and east of it must be transformed into east-west shortening in this area. The southeast end of the thrust rupture marking the edge of thrust deformation in the region of the foreberg intercepts the Bogd strike-slip rupture at a steep angle (Fig. 44). The absence of a clear strike-slip offset on the westward continuation of the strike-slip rupture west of this junction and the presence of decreasing vertical components toward the west concur with the inference that this part of the foreberg formed by a slice of crust advancing onto the area to the east as if pushed from behind by the Ih Bogd massif. The north-northeast shortening along much of the foreberg, however, does imply some counterclockwise rotation of the upper crustal slice. Thus, superficial similarities of the forebergs may obscure somewhat different relationships to their underlying faulting. The Dalan Türüü results, at least partly, from a short zone of convergence, and the Hetsüü is, loosely speaking, more of a “spin-off” from the main rupture. Much of the topography north of the Bogd rupture, particularly between Ih Bogd and Baga Bogd might also reflect the thrusting of detached upper crustal slices, and hence other forebergs (Figs. 51 and 62). As discussed further in the section on “Differences between the 1957 and preceding earthquakes,” scarps that apparently did not rupture in 1957 can be recognized at the bases of such hills on the Landsat imagery and aerial photographs. Toromhon Overthrust The Toromhon Overthrust is a puzzle. For a distance of only 13 km, faulting is as spectacular as along nearly every other portion of the 1957 rupture that we visited. The form of the east-facing trace over the topography (Figs. 86–89) attests to a westward dip, and the tension cracking of the hanging wall at the scarp (Figs. 91 and 92) concurs with thrust faulting (Fig. 3). Scarps higher than 3 m typify much of the rupture (Figs. 86–88 and 92); in one place the height reaches 6 m (Fig. 87). Strike-slip components of 1–2 m (Figs. 84 and 85), and rarely even 3 m (Figs. 86–88 and 92), vary according to the local strike: right-lateral slip on north-northeasterly–striking traces and left-lateral on east-southeasterly ones. Thus, by any standard, faulting was major. Yet, the trace dies out before intercepting the Bogd rupture in the north, and it dwindles to a minor scarp, tens of centimeters high only a few kilometers southeast of Site 39 (Plate 1) where deformation is especially impressive (Fig. 92).

Surface rupture of the 1957 Gobi-Altay earthquake As discussed by Baljinnyam et al. (1993), the sense of slip along most of the overthrust is opposite to the present topography, so that the flanks of ridges elevated in 1957 commonly lie below the opposite flanks, except at the scarp (Figs. 84–89). Thus, slip on this fault has not played a major role in shaping the present topography. Although evidence of an earlier Quaternary rupture is sparse, this fault did not form in 1957. Baljinnyam et al. (1993) described one locality Paleozoic and Mesozoic sedimentary rock appeared to be offset right laterally by ~200 m. Vertical components of slip comparable to that required to create the summits of Ih Bogd and Baga Bogd, 2,000–3,000 m, however, almost surely have not occurred. Thrust faulting along the southern margin of Ih Bogd Discontinuous thrust ruptures follow the southern margin of the Ih Bogd–Dulaan Bogd massif. A minor arcuate zone, the Tsagaan Ovoo–Tevsh uul rupture (Fig. 2, Plate 1), extends west from the Toromhon Overthrust. Displacements are small compared with elsewhere; only in one short section did the vertical component reach 2 m. Perhaps most interesting about this rupture is the possibility that part of it represents the growth and propagation of a fold, without as yet complete detachment of the two flanks by a throughgoing fault (Figs. 93 and 94). There appears to be a region with no clear rupture, ~50 km long, between the Tsagaan Ovoo–Tevsh uul and Gurvan bulag ruptures and south of the Ih Bogd summit plateau, but we cannot be certain that faulting did not occur between them. N. A. Florensov and V. P. Solonenko’s team did not map a trace, and we did not examine the terrain in this area. The most impressive thrust or reverse slip occurred along the Gurvan bulag rupture (Figs. 95–97). Both V. P. Solonenko and we saw scarps with vertical components of 4–5 m (Figs. 98 and 99), with lower scarps characterizing the eastern and western ends of this rupture. In addition, M. A. and V. P. Solonenko discovered a gully, excavated by fresh stream incision, across the scarp in the summer of 1958 that revealed a cross section into the fault. Their measured dip of the fault of 40° in the wall of the gully, decreasing to 30° at its base (Florensov and Solonenko, 1963, 1965, Fig. 130), removes any doubt of thrust faulting. Moreover, as Florensov and Solonenko (1963, 1965) noted, the Gurvan bulag rupture lies along the base of low hills that mark another foreberg (Figs. 95–97) similar in many respects to those north of Ih Bogd and Baga Bogd. West of the Gurvan bulag rupture, another zone of thrust faulting follows the southern edge of the Ölziyt uul, where nearly 2 m of vertical slip occurred (Figs. 2 and 100, Plate 1). The gap between the Ölziyt uul and Gurvan bulag ruptures may be due less to an absence of faulting and more to thick young sediment and weakening by fluids, suggested by numerous springs in this area. (Bulag means spring in Mongolian.) We suspect that the rupture at depth is continuous beneath this area where the fan is relatively steep, but surface ruptures have not been recognized. Vertical components, which reach their maxima along the

65

Bogd rupture north of Ih Bogd, are also greatest along the Gurvan bulag rupture, directly beneath the Ih Bogd summit plateau, and they are smaller along the Tsagaan Ovoo–Tevsh uul rupture, where altitudes to the north are relatively low. This observation obviously suggests that the high mountain has been built by repeated slip with a spatial distribution similar to that in 1957. We infer similar thrust faults south of Baga Bogd, from prominent scarps visible on the Landsat imagery (Fig. 64). In their flight over the surface faulting in January 1958, Solonenko et al. (1960, Fig. 9) saw only a short rupture (~10 km) with apparently fresh deformation southeast of Baga Bogd, compared with their recognition of clear evidence of slip along both the Gurvan bulag and Tsagaan Ovoo–Tevsh uul ruptures. Moreover, a careful search south of Baga Bogd in 1958 by one of Florensov and Solonenko’s teams, including R. A. Kurushin, revealed no evidence of fresh surface faulting. This observation is a reminder that surface faulting in 1957 did not mimic exactly all of deformation associated with building of the Ih Bogd and Baga Bogd massifs. Summit plateau of Ih Bogd Surface deformation was widespread on the Ih Bogd summit plateau, but a search in 1958 by one of Florensov and Solonenko’s teams, including Kurushin, revealed no evidence for comparable deformation on Baga Bogd. The deformation on Ih Bogd includes both deep seated faulting and widespread superficial deformation reflected by landslides and cracking of the surface, particularly near the edges of steep slopes. The clearest surface faulting trends northwest across the summit plateau in parallel zones. Measuring strike-slip offsets was difficult in 1958 and has become almost impossible because the ruptures are wide and the surface of the plateau lacks distinctive features offset by it. We suspect that the relatively wide surface traces (Figs. 102–109) and the apparent disruption of blocks of sod (Fig. 107) owe their complexity to the fracturing of a layer of permafrost, a process that complicates surface faulting elsewhere in Mongolia (Baljinnyam et al., 1993). The large, sharply preserved vertical components, consistent offsets, and linear traces, however, (Figs. 103–106) attest to significant slip, and the senses and orientations of tension gashes and mole tracks lend credibility to Luk’yanov’s (1965) inference of left-lateral strike slip of ~1.5 m. Elsewhere surface cracking is evident both on aerial photographs (Figs. 108 and 109) and on the ground. There is no obvious consistent orientation of this cracking, and we suspect that much of it reflects superficial deformation in response to perturbations in either static or dynamic stresses caused by the proximity to steep topographic slopes. The most impressive of this superficial deformation is the Bitüüt landslide, described in some detail by Florensov and Solonenko (1963, p. 310–318; 1965, p. 328–337). This huge landslide, which transferred ~140 × 106 m3 of material, formed where the main strike-slip rupture that crosses the Ih Bogd summit plateau follows a steep slope above the Bitüüt valley. Most of the other superficial deformation shows no simple relationship to known faults.

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R. A. Kurushin and Others

IMPLICATIONS FOR CHARACTERISTIC EARTHQUAKES, TRIGGERING OF RUPTURES, AND EARTHQUAKES ELSEWHERE Evidence and characteristics of the previous earthquake Many sections of the surface rupture associated with the 1957 earthquake seemed so sharp and fresh that we often wondered if we would have recognized an active fault trace before that earthquake occurred. At the same time, one need not walk far along the Bogd rupture from the Toromhon Overthrust without finding evidence of scarps that predate the 1957 earthquake. Such scarps are most easily recognized where there was a vertical component of slip, but in a few places large strike-slip offsets also imply the occurrence of a previous earthquake. In two areas along the Bogd rupture we surveyed offsets that must represent more slip than occurred in 1957. At Site 6, slip of 10.4 ± 1.5 m is approximately twice that of the areas farther east (5.5 ± 0.5 m) (Figs. 26–30) or west (5.8 ± 1.5 m) (Site 5). At Site 27 (Fig. 59), the amount of offset is less accurately measured, but also exceeds that in 1957 by about two times; Florensov and Solonenko (1963, 1965) reported an offset of 8.85 m of one gully, but adjacent gullies were offset much less (3–4 m) (e.g., Baljinnyam et al., 1993, p. 31–32). The 8.85 m offset, in fact, need not accurately represent the total offset of the gully, because subsequent flow has modified the drainage in this area. In any case, our measurements of 1957 offset of 3.5 ± 1.5 m (Site 27) and 4.1 ± 1.0 m (Site 28) are close to half of what Florensov and Solonenko reported. What is perhaps most noteworthy is that offsets in 1957 from the two areas are resolvably different (5–7 m at Site 6 versus 3–4 m at Site 27), but both seem to require only one earlier event. Evidence of slip prior to 1957 is particularly clear at two sites where faulting has exposed bedrock. At each, mildly weathered bedrock underlies deeply pitted rock, darkened by desert varnish (Figs. 17 and 24). Again apparent offsets both in 1957 and before seem to be of comparable magnitude. Finally, although a large number of gullies reveal offsets of 5–6 m near Site 5, an aerial photograph over this area (Fig. 20) shows that some of the large gullies have been offset approximately twice that amount. Evidence of previous earthquakes is particularly clear where slip in 1957 included a vertical component. In some areas, scarps several meters in height appear to record several events (e.g., Sites 13 and 19; Figs. 39 and 44). In others (Fig. 37, Site 17 on the Dalan Türüü foreberg, and Fig. 49), however, offsets in 1957 appear to account for only half of the heights of the scarps. These repetitions of different amounts and types of faulting on different portions suggest that the earthquake that immediately preceded the 1957 earthquake was associated with variability in style and amounts of slip similar to that in 1957. Thus, insofar as the 1957 earthquake was a “characteristic earthquake,” one whose rupture repeats in successive earthquakes, an important characteristic is the variation in slip along the rupture.

Differences between the 1957 and preceding earthquakes Perhaps as important as the evidence of a previous earthquake are the areas where such evidence is absent. We saw no clear evidence of a previous Quaternary rupture in the westernmost 25 km of the Bogd rupture, where the offset was typically 3–3.5 m. Similarly, convincing evidence for a previous rupture near Baga Bogd was sparse, and perhaps absent. There is no doubt that a scarp existed before 1957 in many areas, such as along the Hetsüü foreberg (Figs. 64–66), and nowhere did we see a suggestion that a new fault formed in 1957. Yet, throughout most of its length, the scarp marking the Bogd rupture north and east of Baga Bogd seems to have formed entirely in 1957. Only in the area of Site 34 (Fig. 74), the most spectacular scarp in this area, was there a suggestion that perhaps part of it formed before 1957; we disagree among ourselves about this. Similarly, although geologic offsets attest to pre-1957 slip on the Toromhon Overthrust, the only locality where we saw evidence of a scarp possibly associated with a previous Quaternary earthquake was in the knot where two strands intersect (Fig. 90), the most complicated faulting that we observed along the Toromhon Overthrust. Finally, the relief along the Gurvan bulag zone requires thrust or reverse slip of tens of meters, if not much more, but we saw no evidence of a scarp before 1957 as fresh as those along the Bogd rupture. Only in one locality along the Tevsh Ovoo–Tsagaan uul rupture, at Site 40, did we see a suggestion of an eroded scarp on the hanging wall of the 1957 rupture. The amplitude of the eroded scarp is significantly smaller than the throw in 1957, suggesting that slip in the preceding earthquake might have been less than that in 1957. Yet, because the footwall had been eroded, and because the apparently eroded scarp might reflect warping of the hanging wall in 1957, we cannot assign much significance to this suggestion of prior slip. Apparently only the Bogd rupture west of the Toromhon Overthrust ruptured in the most recent important event prior to 1957. In addition to faults that ruptured in 1957 but apparently not in an immediately preceding earthquake, there are several apparently active faults in the Ih Bogd region that did not rupture in 1957. The clearest of these follows the southern margin of Baga Bogd (Fig. 64). A sharp break in slope between the rugged terrain of the Baga Bogd massif and the flatter terrain south and southwest implies that an active fault bounds the massif. In addition, along much of the area north of the Bogd rupture, particularly in the area between Ih Bogd and Baga Bogd, the topography as seen on the Landsat imagery suggests that small “forebergs” have developed (Figs. 51 and 62). These features stand out as breaks in slope, but we saw no evidence that they had ruptured in 1957. A particularly good example of such a scarp can be seen just east of the Bitüütiyn am and west of the Dalan Türüü foreberg; reverse faulting has created a clear scarp bounding the eastern side of an inactive alluvial fan (Fig. 42). This scarp is clear in the field, but we saw at most only a hint of a very minor rupture that could have occurred in 1957.

Surface rupture of the 1957 Gobi-Altay earthquake “Characteristic” earthquakes The apparent repetitions of earthquakes in some areas, the absence of such evidence along some portions of rupture, and the absence of rupturing of active faults among others that did rupture call attention to peculiarities associated with other major earthquakes in continental regions, but sometimes overlooked in efforts to seek oversimplified patterns. The common occurrence of fresh scarps superimposed on older scarps with similar offsets has for many years underlain the most common method of estimating recurrence intervals of large earthquakes (e.g., Wallace, 1970), which eventually became formulated into the idea that some faults rupture with “characteristic earthquakes” (e.g., Schwartz and Coppersmith, 1984). The similarity of recent and older offsets along much of the Bogd rupture suggests that in 1957 that fault ruptured with an earthquake similar to the immediately preceding earthquake. Hence, not only did different magnitudes of slip occur along different portions in 1957, but the variation in magnitudes along the fault seem to characterize slip within these portions. Yet, it seems obvious that previous major earthquakes responsible for the pre-existing scarps along the Bogd rupture were not identical to that in 1957. This is not unusual; previous events along the San Andreas fault in California, for instance, suggest that slip along various portions repeats in recurring earthquakes, but not all portions rupture in each earthquake (e.g., Fumal et al., 1993; Sieh and Jahns, 1984). Moreover, the clearly active fault traces within or adjacent to the rupture zone of the 1957 earthquake, but apparently without slip in 1957, also remind us that even the same portion of a fault that ruptures with a characteristic displacement may involve different patterns of local strain release with different earthquakes. Coulomb stress changes and triggering of one fault by slip on another The concurrent rupturing of faults with different orientations and senses of slip makes the 1957 Gobi-Altay earthquake a laboratory for understanding interactions among neighboring active faults elsewhere where a major strike-slip fault system is paralleled by an abutting thrust fault system. Thus, the observed surface faulting carries implications that go beyond understanding the 1957 earthquake alone. As discussed in a following section, one similar setting is in southern California. To examine fault interactions, we calculated changes in elastic stresses throughout the region caused by slip on one of the ruptures, in order to examine its potential effects on other faults in the region. The tendency of rocks to fail under brittle conditions is thought to be a function of both shear and confining stresses, commonly formulated as a Coulomb criterion (e.g., Harris and Simpson, 1992; Jaumé and Sykes, 1992; King et al., 1994; Stein et al., 1992, 1994). To quantify the role played by slip on one plane triggering slip on another fault, we calculated the change in Coulomb failure stress ∆σf, acting on suitably oriented faults in the crust:

67

∆σf = ∆τs + µ · ∆σn, where ∆τs is the change in static shear stress, ∆σn is the change in confining normal stress, and µ is the coefficient of static friction. As discussed more fully elsewhere (e.g., King et al., 1994), the Coulomb stress change depends on the relative orientations of the fault plane that slips first and the plane of the secondary fault, on the senses of slip on the primary and secondary faults, on the amount of slip on the main rupture, and on the effective coefficient of friction on the secondary fault plane. We used an elastic half-space with Poisson’s ratio v = 0.25 and Young’s modulus E = 7 × 1010 Pa. We examined results for an effective coefficient of friction µ = 0.0, 0.4, and 0.75 but found the results changed only in detail. Hence, we illustrate results for µ = 0.75. Because secondary shocks can occur on small isolated faults, which may exist with a wide variety of orientations throughout the crust, the secondary faults most likely to slip are those optimally oriented for failure by the regional stress field perturbed by stress changes caused by the preceding earthquakes (e.g., Stein et al., 1992). Thus, the optimal orientation for secondary fault planes depends on the orientation and magnitude of the regional deviatoric compressive stress σo, the stress change caused by the main rupture, and the coefficient of friction µ. Lacking a useful bound on regional stresses in the Gobi area at the time of 1957 earthquake, we examined two end-member cases: without a regional stress field, and with regional compression of magnitude σo = 10 MPa oriented N53°E, approximately parallel to the average compressive strain associated with the earthquake (discussed below). In practice, results are insensitive to the magnitude of the regional stress as long as it is larger than the earthquake stress drop ∆τ. (See King et al., 1994, for a detailed sensitivity analysis.) We used surface slip associated with the Gobi-Altay earthquake (Table 5) to assess possible fault interaction and triggering in this rupture sequence. Two obvious possibilities are that strikeslip along the Bogd rupture triggered thrust slip along the Gurvan bulag and surrounding ruptures, or thrust slip on the Gurvan bulag zone triggered strike-slip along the Bogd rupture. As discussed below, there are reasons for discounting the latter of these cases and retaining only the former. Nevertheless, we address both because, even if inapplicable in detail, the second case might be appropriate for other similar events, or to other parts of the Bogd rupture. Strike slip on the Bogd rupture triggering Gurvan bulag– Ölziyt Uul thrust slip. The relocated epicenter of the earthquake, uncertain by tens of kilometers, but < 50 km, lies near the west end of the Bogd rupture (Chen and Molnar, 1977), suggesting that strike slip began there and propagated eastward. We considered a case in which this strike slip, as it propagated eastward, perturbed the static stress on the Gurvan bulag–Ölziyt uul thrust fault system and triggered slip on it (Figs. 113 and 114). Although field observations revealed large thrust or reverse, but negligible strike-slip, components on the Gurvan bulag and Ölziyt uul ruptures, we considered the possibility of both types of slip. In the absence of regional compression, strike slip on the

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Figure 113. Map views of calculated Coulomb stress changes (in MPa) at a depth of 12.5 km for optimally oriented faults in an elastic half space caused by the slip on the Bogd rupture (Table 5). We assume slip on the Bogd rupture from the surface to a depth of 15 km. Contour interval is 2 MPa. (a) Calculated Coulomb stress changes on optimally oriented secondary strike-slip faults, for which we assume no regional stress field. (b) Calculated Coulomb stress changes on optimally oriented secondary strike-slip faults for which a maximum regional compressional stress is oriented N53°E. (c) Calculated stress changes on optimally oriented secondary dip-slip faults for which a maximum regional compressional stress is oriented N53°E.

Surface rupture of the 1957 Gobi-Altay earthquake

69

Figure 114. Cross section of Coulomb stress changes (in MPa) on optimally oriented secondary faults caused by the Bogd strike-slip fault without regional stresses. A hypothetical cross-sectional plane of the Gurvan-bulag rupture is also shown (dashed). The concentration at the base of the strike-slip rupture is due to the abrupt termination of 3.5 m of slip. Tapering it to vanish gradually over a few kilometers will reduce the concentration but not the contours a few kilometers from the base of the fault. The contoured values are in MPa units.

Bogd rupture enhanced the potential for strike slip on the Gurvan bulag system (Fig. 113a). In the presence of a regional stress field oriented northeast-southwest, however, that potential is reduced, and for the values used here, it is negligible (Fig. 113b). Thus, the absence of a measurable strike-slip component on the Gurvan bulag system is not surprising. North of Ih Bogd where the Bogd fault dips south, a component of dip slip enhances the Coulomb stress for thrust slip on the Gurvan bulag zone (Figs. 113c and 114). This increased stress results from a combination of strike slip on a plane dipping south at the bend in the fault northeast of Ih Bogd and the component of reverse slip (south side up) on the Bogd rupture. The reverse component decreases the normal stress near where the Bogd and Gurvan bulag faults approach one another at depth. Simultaneously, the passage of the strike-slip component of slip through a compressional bend and on a nonvertically dipping fault also produces a proclivity for the thrust fault system to rupture, largely by increasing the shear stress on the fault. These two effects add, and in this case each contributes, about 5 MPa Coulomb stress change on the thrust fault system. In our calculations, a stress concentration develops at the base of the Bogd fault (Fig. 114), but a smoothly decreasing amount of slip near the base of the rupture would not affect the contours of stress change farther from the base. An increase in the Coulomb stress change of 5 MPa or more on the thrust system would have brought it closer to rupture. Uncertainties in the dip of the Bogd rupture in this area contribute uncertainties in Coulomb stress changes of tens

of percent. Because of the fault geometry, rupture of the thrust fault system would presumably have started at the bottom and propagated upward toward the south (Fig. 111). The calculations shown in Figure 113 also predict Coulomb stress to increase in the thrust system south of the Baga Bogd region, but this thrust system apparently did not rupture during the 1957 event. We presume that this is an example of how earthquakes do not repeat exactly from one to the next. Thrust slip on the Gurvan bulag zone triggering strike slip on the Bogd rupture. Thrust slip on the Gurvan bulag thrust fault should alter the Coulomb stress on the Bogd fault by as much as 5 MPa, perhaps enough to trigger strike slip on the Bogd fault system (Figs. 115 and 116). As for the first case discussed previously, uncertainties in dips make this estimate uncertain by tens of percent. Also, again a more smoothly varying slip with dip will remove the stress concentration at the base of the fault in Figure 116, but the magnitude of stress change several kilometers from the fault should be the same for the average dip slip used here. Thus, a precursory thrust slip could trigger strike slip like that along the Bogd rupture. This case is consistent with the epicenter located north of Ih Bogd reported by Richter (1958) for the main shock, although the more precise relocation of Chen and Molnar (1977) seems to rule out such a sequence of events and cause-and-effect relationship between them. Moreover, fault plane solutions based on initial motions of P waves, and therefore relevant to the initial rupture but not necessarily the entire rupture, reveal two nodal planes, neither

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Figure 115. Map view of calculated Coulomb stress changes sampled at a depth of 12.5 km for optimally oriented secondary strike-slip faults in an elastic half-space caused by thrust slip on the Gurvan bulag–Ölziyt uul faults. The thrust faults are assumed to rupture from the surface to a depth of 15 km. No regional stress was included. Contour interval is 2 MPa.

Figure 116. Cross section of Coulomb stress changes on optimally oriented secondary faults caused by the Gurvan bulag and Ölziyt uul thrust faults. No regional stress is included. A hypothetical cross-sectional plane of the Bogd rupture is also shown (dashed). The contoured values are in MPa units.

of which is parallel to the Gurvan bulag thrust fault (Okal, 1976; work of L. M. Balakina in Florensov and Solonenko, 1963, 1965). One nodal plane strikes east-southeast and dips south at about 55°, and the other strikes northeast and dips southeast. We include discussion of this case here because it is easy to imagine that future earthquakes in similar settings with thrust and reverse faulting could undergo such a sequence. Moreover, we cannot be sure that triggering of this type did not occur at the west end of the rupture. A scarp on the south side of the Bayan Tsagaan nuruu (Fig. 4) attests to reverse slip in that area (Baljinnyam et al., 1993; Khil’ko et al., 1985). Although there is no suggestion that this scarp ruptured in 1957, the possibility of blind thrusting there should not be overlooked, and it is possible that

thrust slip beneath the Bayan Tsagaan nuruu triggered strike slip on the Bogd rupture. The generalities implied by these calculations also apply, in principal, to similar fault systems in which the thrust fault system may involve gently dipping faults at depth, although a study using actual fault geometry in such a case would allow more direct insights. In short, when a thrust fault system lies directly adjacent and approximately parallel to a major strike-slip fault system, some interaction and triggering by mechanisms described here is likely over many earthquake cycles. In some cases, the triggering effects may involve aseismic slip on one or the other fault system, and the timing of triggering may not always be nearly instantaneous (as it appears to have been in the case of the 1957 Gobi-

Surface rupture of the 1957 Gobi-Altay earthquake Altay event). Aperiodicity of rupture in such fault systems almost surely is not fully explained by the simple fault interactions that we have posed here. A shortcoming of our calculations made here is that they include neither dynamic stress changes associated with waves propagating in the near field of the ruptures nor lateral variations in stress. The number of possible changes or variations make it unlikely that one could choose the correct one for the GobiAltay earthquake without additional information on the dynamics of slip during the earthquake. Relevance to earthquakes elsewhere The pattern of faulting associated with the 1957 Gobi-Altay earthquake resembles a combined rupture of the San Andreas fault in central California simultaneously with thrust ruptures at the edge of the Los Angeles Basin (Fig. 117) (Bayarsayhan et al., 1996). As in the Ih Bogd region, a major strike-slip fault, the San Andreas fault, bounds a high terrain on one side, the San Gabriel Mountains. A series of thrust faults, collectively called the Sierra Madre–Cucamonga fault, bound the high area on the other side. Clearly, there is a qualitative similarity between the San Andreas and Bogd faults, and between the Sierra Madre–Cucamonga fault and the Gurvan bulag and Tsagaan Ovoo–Tevsh uul zones. The similarities do not stop with the relationships of faults to topography and tectonic style. The variations of slip along the San Andreas fault in the 1857 Fort Tejon earthquake (Sieh, 1978; Sieh and Jahns, 1984), the only major earthquake in historic time to rupture this part of the San Andreas fault, resemble those along the Bogd rupture. In the northwestern part of the 1857 rupture, right-lateral slip increased from 3–4 m to 8–10 m of slip, which characterized the 110-km-long central part of the rupture, and then decreased to 3–4 m along the 90-km-long part

71

nearest Los Angeles, before decreasing to 1 m at the southeast end (Fig. 117). Moreover, different amounts of slip seem to characterize previous earthquake ruptures along these portions (Sieh and Jahns, 1984), as we inferred also for the Bogd rupture west of the Toromhon Overthrust. The significance of repeated but different amounts of slip along the faults requires some discussion. Two simple explanations can be offered. (1) Different portions of the faults rupture with different recurrence intervals, so that adjacent portions do not always rupture together. This is the common explanation given to the ruptures along the San Andreas fault. Recurrence intervals at Pallett Creek, north of the San Gabriel Mountains, are so short that the portion with 8–10 m in 1857 could not rupture every time the section including Pallett Creek ruptured (Sieh et al., 1989). Because slip has been relatively small along sections where earthquakes recur more frequently, and because the San Andreas fault often is treated literally as a plate boundary, a common, implicit assumption is that the long-term slip rate does not vary along the fault. (2) The other, if extreme, interpretation of repeated variations in slip along ruptures treats such variations as indicative of long-term variations in rates, and in amounts of cumulative slip, along the fault. In a following section, we argue that the slip rate along the Bogd fault may indeed vary spatially. Although we do not doubt that earthquakes along different portions of the San Andreas fault recur with different frequencies, its interception with other faults, both strike-slip and thrust, and its variation in orientation require that the slip rate vary along the series of traces assigned the common name of “San Andreas.” Although the rate of about 34 mm/yr seems well determined for Central California (Sieh and Jahns, 1984), a lower rate closer to 20–25 mm/yr may apply to the southern section where slip of only 3 m occurred in 1857 (e.g., Brown, 1990; Molnar and Gipson, 1994). Because the largest offsets in 1857 formed where the long-

Figure 117. Map of active faulting and surface ruptures of major earthquakes in southern California. Thrust systems lie south of the San Gabriel Mountains, and the San Andreas fault passes north of this range. Measured offsets in meters, associated with the 1857 earthquake (Sieh, 1978), are shown along the rupture, and dots denote the epicenters of other major earthquakes showing thrust faulting in and near the Los Angeles Basin (e.g., Dolan et al., 1995; map from Bayarsayhan et al., 1996).

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term slip rate is highest, it may be premature to assume that the smaller offsets north of the San Gabriel Mountains are simply a result of local variations in recurrence intervals. Regardless of whether or not the long-term slip rate varies along the San Andreas fault by as much as 30–50% in this area, however, the lower amount of slip adjacent to the thrust systems for both the southern San Andreas fault and the Bogd fault constitutes a similarity that seems noteworthy to us. The thrust systems also share similar dimensions and amounts of rupture. Among the earthquakes within the Los Angeles Basin during the last 30 years (e. g., Dolan et al., 1995), the 1971 San Fernando earthquake, which ruptured the main thrust fault dipping beneath the San Gabriel Mountains (Fig. 117), stands out as the largest. From an analysis of acceloragrams, Heaton (1982) estimated a maximum slip of about 3 m of oblique thrust slip at depth, although surface faulting indicated less slip. The seismic moment for the 1994 Northridge earthquake is consistent with a similar amount of slip, if its fault plane dips south. Although smaller earthquakes have ruptured other portions of Sierra Madre–Cucamonga fault (Dolan et al., 1995), Wesson et al. (1974) pointed out that both previous and future earthquakes could rupture the entire Sierra Madre–Cucamonga fault, ~90 km long. This length is comparable with that from the west end of the Gurvan bulag rupture to the east end of the Tsagaan Ovoo zone. When we have presented this similarity between the GobiAltay rupture and what might be a more disastrous earthquake for southern California than is commonly considered (Bayarsayhan et al., 1996), others have quickly pointed out differences between them. For instance, the San Andreas fault is cited as a plate boundary, not an intracontinental fault in a complex network of faults, as the Bogd fault might be. Moreover, slip rates along the faults in California are much faster than along those in the Gobi-Altay. To address the first, we emphasize that the San Andreas fault does not define the Pacific–North America plate boundary. Deformation is diffuse in the western United States, and even where slip on the San Andreas fault is fastest, it accounts for only ~70% of the relative plate rate. Perhaps more important, however, is the irrelevance of plate boundaries in such a discussion. Indeed, earthquakes involving multiple ruptures on faults with very different orientations seem to be more common in intracontinental settings than at plate boundaries. The most impressive and best documented multiple ruptures include the July 23, 1905, Bulnay earthquake in northern Mongolia, which seems to have ruptured three distinct faults 370, 80, and 20 km in length, plus many other more minor faults (Baljinnyam et al., 1993), the 1927 Tango earthquake in Japan, which ruptured two nearly orthogonal faults (Richter, 1958, p. 573–578), and the 1932 Changma, China, earthquake, which ruptured four roughly parallel, en echelon reverse faults and an oblique strike-slip fault (Meyer, 1991; Peltzer et al., 1988). Such conjugate faulting is virtually impossible at a plate boundary, where horizontal components of slip vectors for separate segments must be parallel. Yet, multiple ruptures clearly have occurred where plate boundaries with different orientations meet. For instance, the aftershocks of the 1964 Alaskan earthquake and their

fault plane solutions (Stauder and Bollinger, 1966) virtually require rupture of two nearly orthogonal faults: a gently dipping thrust fault, and a nearly vertical fault dominated by a vertical component of slip. The relevance of slip rate to sizes and complexities of earthquakes seems likewise irrelevant to the similarity of the 1957 earthquake to potential earthquakes in southern California. Although the three intracontinental earthquakes cited above as rupturing separate faults ruptured faults with relatively low slip rates, other multiple ruptures have been associated with faults slipping at centimeters per year. The 1987 Superstition Hills earthquakes in southern California involved slip on both the San Jacinto fault, whose slip rate is ~10 mm/yr (Sharp, 1981), and an orthogonal strike-slip fault (Hudnut et al., 1989; Sharp et al., 1989). The 1950 Assam earthquake, which occurred at the eastern end of the Himalaya where the slip rate is at least 10–25 mm/yr (Lyon-Caen and Molnar, 1985), seems to have ruptured both the main thrust fault and a strike-slip fault at the east end of the range (Chen and Molnar, 1977). Moreover, there is no known correlation of maximum magnitudes of earthquakes with the long-term slip rates on which they occur. The discontinuous nature of the Bogd rupture, with many splays and subparallel ruptures, contrasts with the San Andreas fault, whose trace seems smooth with only rare step-overs, and might constitute an excuse for ignoring the relevance of the Gobi-Altay earthquake to hazards in southern California. First, as argued above, most of the jogs, splays, small basins, and hills bounded by subparallel ruptures almost surely reflect only superficial deformation (depths, <1–3 km) and not deep-seated variations in orientation and dip of the Bogd fault. Large earthquakes nucleate at depths below a few kilometers. The absence of long, straight ruptures along the Bogd fault is almost surely due to the small amount of cumulative slip, estimated below to be <10 km, and short duration of slip. In the course of some 300 km of slip on the San Andreas fault, such complexities in the rupture probably have been smoothed out, both by repeated earthquakes with slightly different surface ruptures and by more rapid erosion than in Mongolia’s cold, dry climate. Although we are aware of no evidence that demonstrates simultaneous ruptures of the San Andreas and nearby thrust faults in southern California, we consider the evidence for such a heterogeneous Bogd rupture to be irrelevant to the question of a similarity with possible future earthquakes in southern California. IMPLICATIONS FOR RECURRENCE INTERVALS OF MAJOR EARTHQUAKES AND RATES OF DEFORMATION IN THE GOBI-ALTAY Seismic moment tensor and regional deformation The offsets measured along the different sections can be combined to estimate the seismic moment tensor of the earthquake, from which we may estimate the contribution of the earthquake to the regional strain of the Gobi-Altay and its partitioning

Surface rupture of the 1957 Gobi-Altay earthquake into various elements. The seismic moment tensor (Gilbert, 1970) is given by

(

Moij = Mo ni b j + n j bi

)

and therefore cancel. Using the summary of amounts of slip on different ruptures in Table 5,

(1)

where n and b are unit vectors normal to the fault plane and parallel to the slip vector, respectively, with the 1-, 2-, and 3-directions east, north, and up, and Mo is the scalar seismic moment (Aki, 1966): Mo = µ Α ∆u, where µ is the shear modulus (= 3.3 × 1010 N m–2), A is the rupture area, and ∆u is the average slip on the fault (or portion of the fault). The moment tensor measures the contribution of the earthquake to the regional strain. Kostrov (1974) showed that N separate ruptures, with different orientations, fault areas, and amounts of slip in a region, account for regional strain of

Moij

 −0.70 −1.20 5 1 = . ×  −1.20 −0.07   0.09 −0.21

ε ij =

m =1

2 µV

,

(2)

where V is the volume of the region over which the contribution to the strain is estimated and Moij, m is the seismic moment tensor for the m-th rupture. We may also consider an asymmetric moment tensor: * = M nb Moij o i j

(3)

Assuming that the faults that ruptured do not rotate with respect to one another or to the surrounding regions (Jackson and McKenzie, 1988), Molnar (1983) showed that the deformation gradient can be calculated from the asymmetric moment tensor, in like manner to Kostrov’s (1974) method for estimating strain: N

∂ui = ∂x j

∑ M*

m =1

oij ,m

µV

(4)

The displacement of one side with respect to another may then be estimated from the dot product of the velocity gradient with a vector, Wj, oriented perpendicular to the region, and whose length equals the width of the region: ∆ui =

∂ui Wj ∂x j

(5)

We have measured offsets, orientations, and lengths of numerous portions of the 1957 rupture, summarized in Table 5. To estimate scalar moments, we must also assume widths for each rupture. Let us, for the moment, simply assume that all ruptures extended to a depth of 20 km. For the calculations using equations (1) to (5), the values assumed for the width and for µ appear in both the numerators and denominators of (2) and (4)

0.09  −0.21 × 10 20 Nm  0.77

(6)

Uncertainties for each element are approximately 15–20% of values given in (6). The maximum eigenvalue of the moment tensor, 8.3 × 1020 Nm, reflects nearly horizontal contraction oriented N52°E, which is partitioned approximately equally into perpendicular extension and crustal thickening. We may also use M*oij to estimate the average displacement across the region that ruptured in 1957. Again using the summary in Table 5,

N

∑ Moij ,m

73

Moij

 −1.8 −6.5 =  0.3 −0.2   −0.5 −0.1

1.0  −0.9  × 10 20 Nm  2.0

(7)

Let us consider a region 260 km long, 30 km wide, and 20 km thick, so that V = 1.6 × 1014 m3, and a vector 30 km in length and oriented N15°E, perpendicular to regional trend of the GobiAltay, Wj = (7.8, 29.0, 0). From (5), the region to the south-southwest of the deformed region moved 3.8 (± 0.7) m eastward. The orientation of the slip vector (N90°E) differs from that of maximum strain (N52°E), because the rate of left-lateral slip on approximately east-west planes exceeds the conjugate rightlateral component. Although irrelevant to all calculations made previously, the values of the scalar moment and maximum eigenvalue open a question. First, we argue that the rupture does not reach a depth of 20 km, for if it did, the strike-slip and thrust planes would intersect one another. Thus, the widths of ruptures that we have used here and the resulting scalar moment (5.1 × 1020 Nm) and maximum eigenvalue of the moment tensor (8.3 × 1020 Nm) are all upper bounds. Similarly, the scalar moment and maximum eigenvalue imply upper bounds for the corresponding moment magnitudes (Hanks and Kanamori, 1979) of Mw = 7.8 and 7.9. Yet, both the scalar moment and the maximum eigenvalue are smaller than the scalar seismic moments determined from seismic waves: 13 ± 5 × 1020 Nm (Chen and Molnar, 1977) and 18 ± 4 × 1020 Nm (Okal, 1976). These estimates of seismic moments are based on different seismograms and different approaches to their analysis. Thus, it is not clear why both Okal (1976) and Chen and Molnar (1977) obtained estimates that are larger than the surface faulting suggests. Perhaps, they both used values of Q, the measure of attenuation of seismic waves, that are too small and overcorrected for attenuation. Yet, Chen and Molnar’s estimates of moments based on longer periods, which require smaller corrections for attenuation, are larger than those based on shorter periods. Perhaps slip at depth was greater than that at the surface, or the rupture was longer than that revealed by the surface trace. In any case, finding suffi-

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cient deformation in the epicentral region of the earthquake to account for the measured scalar moments seems impossible to us. Rates and amounts of slip and recurrence intervals between major earthquakes The high flat summit plateaus of Ih Bogd and, to a lesser extent, of Baga Bogd suggest that they once lay at lower elevations and have been elevated to their present heights fast enough that erosion has not destroyed their gentle regional surfaces (Baljinnyam et al., 1993; Florensov and Solonenko, 1963, 1965). The especially flat surface of Ih Bogd and its eastward continuation toward Dulaan Bogd virtually eliminates the possibility of erosion and uplift of the underlying rock being in steady state. This flat surface (Figs. 118–120) contrasts with the more dissected, but nevertheless regionally uniform elevation of Baga Bogd (Fig. 64). The basement below the basins to the north and south does not appear to be buried by thick sediment. Just north of Ih Bogd’s summit plateau, the Bitüütiyn am debouches onto an alluvial fan at an elevation of 2,000 m, more than 500 m above the Nuuryn Höndiy. Yet, where the river debouches and crosses the fault, metamorphic basement crops

out in walls of the valley beneath the fan deposits north of the fault. Although late Cenozoic deformation surely elevated the basement of the Bitüütiyn am’s alluvial fan, the Ih Bogd massif does not appear to have been thrust onto the Nuuryn Höndiy enough to flex it down. Thus, we assume that the difference in altitudes between the summit plateaus and the basins to the north and south (1,500–2,500 m) approximates the total vertical component of slip on the faults bounding the Ih Bogd and Baga Bogd massifs. The vertical components of slip in 1957 qualitatively mimic the heights of adjacent summit plateau and high ridges, suggesting that repetitions of similar earthquakes have built the topography. The largest vertical components in 1957, averaging 2–3 m and reaching 5 m along the Gurvan bulag rupture, lie due south of Ih Bogd’s summit plateau. Correspondingly, along the Bogd rupture, consistently large vertical components, of 1–3 m, were observed only in the areas just north of Ih Bogd and northeast of Baga Bogd, where altitudes are greatest. The smaller vertical components along the Bogd rupture both east and west of Ih Bogd, north of Dulaan Bogd uul and of Noyon uul, respectively, and the relatively small vertical component along the Tsagaan Ovoo-Tevsh uul rupture south of Dulaan Bogd correlate with

Figure 118. Landsat Thematic Mapper image of the Ih Bogd summit plateau (in the left center) and surroundings (Orog Nuur, the Dalan Türüü foreberg, and the Gurvan bulag rupture zone). Note the flatness of the summit plateau, covered by snow. The cross near the center marks 45°N, 100.5°N. North is toward the top.

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Figure 119. Photograph looking south across the Nuuryn Höndiy at Ih Bogd and its flat summit plateau. Photograph by P. Molnar, August 21, 1993.

these ridges being significantly lower than Ih Bogd. Although in detail the vertical components in 1957 are not at the same scale with adjacent regional variations in altitude, the larger and smaller vertical components qualitatively correlate with adjacent higher and lower mountains. Moreover, the approximately similar magnitudes of slip on the Bogd rupture associated with the 1957 and the previous earthquake suggest that at least some aspects of the 1957 rupture do repeat. We may use this similarity of slip in 1957 with total vertical components of slip and the rate of slip determined by Ritz et al. (1995) for one area to estimate the total slip on the various faults of the region, other rates of slip, and recurrence intervals of major earthquakes. The ratio of vertical components along the Bogd rupture north of Ih Bogd (1.5–2 m) to the difference in elevation of 2,000–2,500 m, between Ih Bogd’s summit plateau and the basins to the north and south, suggest that approximately 1,000–1,500 repetitions of earthquakes like that in 1957 would create the present height of the Ih Bogd summit plateau. Baljinnyam et al. (1993) speculated that earthquakes like that in 1957 might recur approximately every 1,000 yr and that the average slip rate along the Bogd fault might be several millimeters per year. Ritz et al. (1995), however, showed that alluvial fans seemed to be offset only about 100 m since 80 ka north of Noyon uul, near Site 5 (Fig. 20), where the fault trace is especially clear and where slip in 1957 was its largest, 5–7 m. They used cosmogenically produced 10Be to date cobbles on the abandoned fans and matched the displaced fans to the current valley head to determine the amount of offset. Their upper bound on the slip rate of 1.2 mm/yr and slip of 6 m in 1957 yield recurrence intervals for this area closer to 5,000 yr than to 1,000 yr. Using the inference that the immediately preceding earthquake to rupture the Bogd fault was similar to that in 1957, let us suppose that Cenozoic slip on the Bogd fault occurred in comparable earthquakes: 5–7 m of strike slip north of Noyon uul and 3–3.5 m north and east of Ih Bogd, and a vertical component of

~2 m on both the Bogd fault north of Ih Bogd and on the Gurvan bulag zone to its south. With 1,000–1,500 predecessors of the 1957 earthquake needed to create the present relief, the total Cenozoic slip on the Bogd fault north of Noyon uul would be only ~5–9 km, and that north of Ih Bogd would be only 3–5 km. These total offsets would have accumulated in 5–10 m.y., a period much shorter than the duration of India’s penetration into the rest of Asia. The various assumptions made here obviously contribute additional uncertainties to these estimates, but it seems unlikely that the total Cenozoic displacement along the Bogd fault reaches 20 km, or is as little as 2 km, or that deformation began as long ago as at 20 Ma. (It has not escaped our notice that the Tibetan Plateau may have risen abruptly 1–2.5 km between 5 and 10 Ma [Harrison et al., 1992; Molnar et al., 1993], and that most of the high mean elevation of the Tien Shan may have formed since 10 Ma [Abdrakhmatov et al., 1996.]) IMPLICATIONS FOR INTRACONTINENTAL MOUNTAIN BUILDING Regional tectonics of the Ih Bogd region The similarity of the slip pattern in 1957 to the heights of mountains extends to differences in style of deformation. The steep slopes on the north and south margins of Ih Bogd’s summit plateau (Figs. 118 and 119) apparently have formed by rapid slip on faults with reverse, or perhaps thrust, components dipping beneath the massif. Farther east, where the mean heights decrease toward Dulaan Bogd, the regional topography suggests a broad fold or warp, which is seen most clearly from the air (Fig. 120) (Florensov and Solonenko, 1963, 1965). The northern margin of this gentle fold is steep, presumably because a vertical component of slip has occurred on the Bogd fault, although the vertical component in 1957 was small and in places even opposite in sense to the relief. Correspondingly, the topography south of Dulaan Bogd

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Figure 120. Aerial photograph looking east-southeast along the eastern part of the Ih Bogd summit plateau. Baga Bogd forms the skyline in the upper left distance. Note the smooth surface that forms a gentle fold on the south (right) side and a steeper edge on the north side. Photograph by N. A. Florensov in January 1958 (Fig. 60 of Florensov and Solonenko, 1963, 1965).

is less dramatic than that south of Ih Bogd, and the 1957 Tsagaan Ovoo-Tevsh uul rupture is not very impressive. More important, the most impressive deformation along the Tsagaan Ovoo rupture seems to result from localized folding rather than faulting (Figs. 93 and 94). Thus, the more gradual emergence of the Dulaan Bogd than that of the Ih Bogd massif from their surroundings to the north and south appears to reflect different amounts of slip on the causative reverse, or thrust, faults at depth. The fault dipping north beneath Ih Bogd has broken the surface, and slip is relatively large. The fault beneath Dulaan Bogd apparently has not yet reached the surface, and a fault-propagation fold (Suppe, 1985, p. 348–352) seems to follow the edge of the ridge. The distance between separate surface ruptures north and south of Ih Bogd, 20–25 km, requires that the Ih Bogd massif be part of a small crustal block. Above, we estimated the dip of the Bogd fault north of Ih Bogd to be 75–80°. Although at one locality along the Gurvan bulag rupture (west of Site 44) Florensov and Solonenko (1963, 1965) measured a dip of only 40° at the surface decreasing to 30° near the base of a recently incised gully, this estimated dip may be too gentle. Gently dipping faults commonly make very arcuate scarps across irregular landscapes, but the Gurvan bulag zone, if scalloped in places, is relatively straight (Figs. 95 and 96). We suspect that the dip at depth is closer to 45–55°. With dips of 75–80° and 45–50°, these bounding faults, if planar, would intersect at a depth of ~20 km, within the crust. Although we cannot rule out through-going faults to such depths, a more plausible state is that the faults merge into distributed deformation in the lower crust, at a depth shallower

than 30 km. Accordingly, crustal blocks like the Ih Bogd massif would be separated from the mantle lithosphere by a ductile lower crust, as is commonly assumed for wide zones of deformation. Assuming that the strength of the mantle lithosphere exceeds that of the crust (e.g., Molnar, 1992), such blocks should not be amenable to treatment as rigid blocks, an inference corroborated by the internal deformation within the Ih Bogd massif. At the eastern end of the Ih Bogd massif (Fig. 120), the Earth’s surface rises smoothly northward approximately 800 m over a distance of 10 km to form a gentle warp. Such warping requires strain that is small compared with that associated with most folds near the Earth’s surface. The implied strain of ~0.01, nevertheless, is much too large to be sustained by elastic strength. Moreover, if this warping represents concentric folding of the upper crust, then either the material near the core of the fold, at the base of the upper crust, must be compressed by ~0.01, or the layers that are folded must slip with respect to one another. For such crustal-scale folds, slip of the folded upper crust with respect to the lower crust and upper mantle would occur by shear within the lower crust, presumably by ductile deformation. Thus, the growing crustal-scale fold at the eastern end of the Ih Bogd massif provides a prototype for Argand’s (1924; Argand and Carozzi, 1977) “basement folding,” which in turn is surely a reflection of flow in the lower crust. The variations in vertical, and therefore presumably horizontal, components along the thrust or reverse faults south of Ih Bogd and Dulaan Bogd require a counterclockwise rotation of the mas-

Surface rupture of the 1957 Gobi-Altay earthquake sif relative to the area to the south, about a vertical axis east of Dulaan Bogd. We treat this rotation as if one block, the Ih Bogd massif, rotated; the secondary ruptures across the top of the Ih Bogd massif, however, are aligned with regional topography and with variations in the character of slip along both the Bogd fault system and the Gurvan bulag–Ölziyt uul thrust system, suggesting that the massif is cut into long narrow blocks, each of which rotates with respect to the area to the north. Consistent with the regional variations in elevation and presumably horizontal shortening, the difference between about 2.5 (±1.0) m of convergence across the Gurvan bulag rupture and only 1 (±0.5) m across the Tsagaan Ovoo–Tevsh uul rupture implies an axis of rotation between Ih Bogd and Baga Bogd, ~75 (±30) km east of Ih Bogd. The magnitude of rotation in 1957 would be 3 (±2) × 10–5 [= 2.5 m ÷ 75 km]. If such increments occur at a rate corresponding to once every 5,000 yr, the estimated recurrence interval for slip on the Bogd fault, the rotation rate would be 6 (±4) × 10-9 yr–1 [= 0.3° (±0.2°) m.y.–1], a rotation rate one or two orders of magnitude smaller than those in rapidly deforming continental regions (e.g., Kissel and Laj, 1988; Luyendyk, 1991). Correspondingly, assuming 1,000–1,500 repetitions of that in 1957, the total rotation of Ih Bogd would be 0.03–0.05, or 2–3°, too little to be measurable with paleomagnetism. Although the amount and rate of rotation are small, not only is this process common in intracontinental deformation, but it may also contribute to the variation in slip along the Bogd rupture. The difference in strike slip along the Bogd rupture between 3–3.5 m north and east of Ih Bogd and 5–7 m west of Ih Bogd and north of Noyon uul appears to be due, at least in part, to deformation within the Ih Bogd massif. Left-lateral slip across the massif, of ~1.5–2 m, clearly contributes to the difference in slip (5–7 m versus 3–3.5 m) east and west of the Hüühniy höndiy (Fig. 2, Plate 1). Yet, the approximate longitudinal correspondence between the locus of the step from 3–3.5 m to 5–7 m along the Bogd fault and the western end of the Gurvan bulag rupture suggests that a fraction of the difference in strike slip might be absorbed by the counterclockwise rotation of the Ih Bogd massif. This inference requires the axis of rotation to lie south of the Bogd rupture, for instance at the latitude of the Gurvan bulag rupture. If it lay 75 km east of the Gurvan bulag rupture and 25 km south of the Bogd rupture, 2.5 m of north-south convergence at the Gurvan bulag rupture would, by rotation of the Ih Bogd massif, account for a difference in left-lateral slip of ~1 m along the Bogd rupture. The combination of left-lateral strike slip along the eastsoutheast Bogd rupture and thrust or reverse slip on the Gurvan bulag and Tsagaan Ovoo–Tevsh uul ruptures requires that the overall slip vector for the material south of Ih Bogd, relative to the area north of it, be approximately east. In the parlance now commonly applied to regions of oblique convergence, the slip is partitioned into a nearly pure strike-slip component along the east-southeast–striking Bogd rupture and a nearly pure convergent component at the thrust system to its south.

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Regional tectonics of the Baga Bogd region The similar west-northwest orientations of the Ih Bogd and Baga Bogd massifs, comparable heights, and geometry of faulting with large components of strike slip to the north and apparently reverse faulting to the south make the Ih Bogd and Baga Bogd massifs appear to be nearly identical manifestations of the overall oblique left-lateral convergence across the Gobi-Altay (Bayasgalan, 1995). The large thrust slip on the Toromhon Overthrust, however, demonstrates that the Ih Bogd and Baga Bogd massifs move as separate entities, with respect to the areas north and south of them. Moreover, in detail, their independent movements are implied also by the different styles and orientations of structures surrounding the massifs. Despite the apparent continuity of the Bogd rupture north of Ih Bogd and Baga Bogd, the orientation of this rupture and the sense of slip near Baga Bogd differ significantly from those near Ih Bogd. North of Ih Bogd, it trends approximately N95–100°E along most of its length, and strike slip dominates the sense of slip. North of Baga Bogd, however, the Bogd rupture seems to consist of two very different parts. Northwest of Baga Bogd, the vertical component is small, but its orientation of N75°E is distinctly different from that north of the Ih Bogd massif. Northeast of Baga Bogd, where the orientation of N110°E is closer to that of the Bogd rupture, slip includes a large reverse or thrust component. Thus, it appears that whereas the Ih Bogd massif moved east-southeast with respect to the area to its north in the 1957 earthquake, the Baga Bogd massif seems to have moved east-northeast with respect to that area. The orientations of active faults south of Ih Bogd and Baga Bogd also differ. Unlike the approximately east-west–trending thrust zones south of Ih Bogd (Gurvan bulag and Tsagaan Ovoo–Tevsh uul), the escarpment south of Baga Bogd that seems to mark an active fault trends west-northwest (Fig. 64). Although this fault did not break in 1957, its sharp delineation of the topography makes it appear active. If slip on it has been nearly purely reverse, as seems to be the case for the Gurvan bulag rupture, the slip vector for the Baga Bogd massif relative to the area south of it should be roughly southwest, not south as for Ih Bogd. These different orientations suggest that counterclockwise rotation, of the type inferred for the Ih Bogd massif, may not play a role as important in the deformation near Baga Bogd. Moreover, the large components of both strike slip and reverse slip northeast of Baga Bogd rule out a partitioning into strike slip north of the Baga Bogd massif and thrust or reverse slip south of it. In part, because slip in 1957 did not occur both north and south of Baga Bogd, we cannot know well the long-term slip vector between material north and south of Baga Bogd. Nevertheless, the orientations of oblique left-lateral/reverse slip north of Baga Bogd and apparently largely reverse slip south of the massif associated with previous earthquakes suggest that this slip vector, like that for the Ih Bogd region, is east-northeast.

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Relationship to the rest of the Gobi-Altay and to intracontinental deformation in general The Ih Bogd and Baga Bogd massifs behave as small crustal blocks caught in a regional shear zone where both strike slip and convergence occur (Bayasgalan, 1995). They move separately from one another and in detail they respond differently to the forces applied to them. Yet, the overall east-northeast orientation of oblique convergence of the area to the south with respect to that to the north appears to be the same for both. The Ih Bogd massif does not behave as a rigid block, but rather as a deforming block of upper crust. Near Ih Bogd, slip is partitioned into nearly pure strike slip along the Bogd fault and nearly pure reverse, or thrust, faulting along the Gurvan bulag and Tsagaan Ovoo–Tevsh uul ruptures. Variations in rates and amounts of slip characterize both ruptures, as must occur where blocks rotate with respect to neighboring regions. The decrease in altitude from the maximum on the summit of Ih Bogd to gradually lower values to the east seems to constitute a long-term manifestation of varying amounts of slip, which, in turn, results more from different slip rates, than from different dates of initiation of slip at constant rates. The varying magnitude of slip along the reverse fault system south of Ih Bogd seems to show different stages of development, with slip beneath the lower eastern part of the area confined to the axis of a growing fault-propagation fold that has not yet broken the Earth’s surface. Thus, the Ih Bogd region illustrates manifestations of continuous, if inhomogeneous, deformation at a spectrum of scales. The Gobi-Altay consists of anastomosing ranges approximately parallel to those of Ih Bogd and Baga Bogd and built by slip on ~3–4 fault systems similar to those that ruptured in 1957. If the slip rate inferred by Ritz et al. (1995) applied to each, the overall strike-slip rate would be 3–4 mm/yr, and the total leftlateral slip would be 20–40 km. The variations along strike, illustrated well by the differences between Ih Bogd and Baga Bogd, imply that the other ranges of the Gobi-Altay would similarly consist of separate crustal blocks, most of which would undergo internal deformation. Many would rotate with respect to the areas adjacent to them. We presume that each of the three to four subparallel ranges would consist of blocks, like the Ih Bogd massif, that move with respect to the surroundings and with respect to each other. These blocks, in turn, would be separated from each other by basins that would also behave as crustal blocks. Thus, the belt roughly 250 km wide and 700 km long would consist of 15–20 such uplifted blocks with dimensions of 20–50 km by

100–150 km separated by a comparable number of blocks that currently form intermontane basins. Clearly, a treatment of the region in terms of rigid blocks, even if we ignore the internal deformation that obviously occurs within the Ih Bogd massif, would require many more parameters than could be usefully determined. With internal deformation of blocks, continuous deformation provides a more appropriate description of the kinematics of relative movements across the region than the many tens of vectors needed to describe translations and rotations of blocks. ACKNOWLEDGMENTS

We are grateful to M. Ganzorig, V. M. Kochetkov, and U. Sukhbaatar for their help in organizing field work; V. S. Baskakov, Enhtsetseg, M. S. Kustov, V. N. Nazarov, A. F. Osipov, I. S. Sukhanov, and V. V. Sukhoparov for logistical assistance; J. A. Jackson, C. S. Prentice, and T. Rockwell for exceptionally thorough reviews, and the many local people who always took it upon themselves to look after us when we strayed close to their yurts. Molnar also thanks the Woods Hole Oceanographic Institution, where he was a guest investigator, for help of many kinds in assembling this manuscript. Both field work and analysis were supported largely by the Russian-Mongol Geophysical Expedition and by the National Science Foundation under grant EAR-9206063. Contribution 9279 of the Woods Hole Oceanographic Institution. APPENDIX A. SITES WHERE DETAILED TOPOGRAPHIC MAPS AND PROFILES WERE MADE TO QUANTIFY OFFSETS ALONG THE SURFACE RUPTURE OF THE 1957 GOBI-ALTAY EARTHQUAKE IN MONGOLIA For most sites, two, and for some three or more, figures are shown. A map, usually labeled (a), shows the data used. On the map, dark lines show profiles drawn to illustrate the topography and to measure offsets. These profiles, usually labeled (b), demonstrate offsets. Where a vertical offset is being illustrated, commonly only one profile across the scarp is shown, but for strike slip, two parallel profiles are shown, with one of them translated with respect to the other. For some areas, more than one set of profiles are shown. For many areas, a block diagram, usually labeled (c) presents an oblique view of the mapped topography. We show such diagrams to provide objective views of the landscapes that we have mapped.

Site 1 (on facing page). 45°10.18′N, 99°16.08′E, Z (altitude) = 2,170. Bogd Rupture along the southern foot of the Bahar uul, where the rupture consists of only one strand (Figs. 8–10). A relatively wide gully with a deep young channel crosses the scarp between adjacent ridges (Fig. A1a). Deflections of contours along y ≈ 11 m, particularly those for 2.5 m < z < 4.5 m in the western part of the area, can be explained either by left-lateral slip or by a vertical component with the south side up (Fig. 9). The closer spacing of contours parallel to the fault trace on the left side of the map rather than above or below the trace, however, implies that any vertical component must have involved uplift of the north, not south, side. Thus, these deflections imply left-lateral slip of 3–5 m. The relatively deeply and freshly incised main channel of the gully shows, at most, only a slight vertical offset. The deflections of con-

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tours south of the trace near x = 45 m show that the channel once lay east of its present course on the south side of the fault. Farther east, the trace follows a steep southwest slope, and slumping of material has modified the topography there. By matching the western slopes of the gully defined by profiles along y = 10 m, south of the fault trace, and along y = 13 m, north of it (Fig. A1b), and the present channel on the north side to the abandoned channel on the south side, we estimate a horizontal separation of the profiles of 4.0 ± 0.8 m. The relatively large uncertainty includes the difficulty of matching the profiles on the east side of the gully and uncertainties in the position of the channel before the earthquake. The trend of the gully is oblique to the fault trace. Correcting for an obliquity of 75° between structures and the fault trace and a distance of 3 m between profiles reduces the horizontal separation to 3.2 m. The profiles are separated vertically by 0.9 m, but the southward slope of the regional topography accounts for ~0.5 m of the measured vertical separation of 0.9 m, leaving a vertical component of slip of only 0.4 ± 0.2 m. The block diagram (Fig. A1c), viewed from the northwest toward N147°E, at an angle of 20° from the horizontal shows a north facing scarp on the west (right) side of the gully, which is not due to a vertical component, but rather to the left-lateral slip of 3.2 ± 0.8 m.

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Site 2 (on this and facing page). 45°09.00′N, 99°31.75′E, Z = 1,710 m. Bogd rupture, 4 km east of Ulaan bulag höndiy (Figs. 12 and 13). V. P. Solonenko took the well-known photograph (Fig. 13) of a remarkably sharp, clean scarp at this site, for which the left-lateral offset was particularly clear in 1958 and had remained so in 1993. Trifonov (1985) and Trifonov and Makarov (1988) reported horizontal offsets of only 2.7–3.3 m for this area and argued that larger offsets implied multiple earthquakes, an inference with which we do not agree. The comparable amounts of horizontal slip and spacing between gullies has ponded the drainage so that on the upstream, south side, deposits filling the gullies have left flat floors (Fig. A2a). Nevertheless, except for the bottoms of gullies, profiles just north (y = 0.5 m) and south (y = –1 m) of the fault trace at y = 0 m can be matched well with 5.1 ± 0.5 m of strike-slip restoration (Fig. A2b). We consider the sharpness of the scarp and the comparable 5 m separations of features with different dimensions at this site to require at least 4.5 m of offset in 1957. Three hills of different widths are aligned in such a restoration (Fig. A2b). Because both the shapes of the hills and the slopes across the fault differ from one another, correcting for the 1.5 m distance between the profiles is difficult. Nevertheless, the correction appears to be small, and the vertical separation of only 0.4 m provides a sensible estimate of the vertical component of slip. The block diagram (Fig. A2c) is viewed toward N32°W, and from 35° from the horizontal.

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Site 3. 45°09.00′N, 99°31.83′E, Z = 1,705 m. Bogd rupture, 4.2 km east of Ulaan bulag höndiy and ~200 m east of Site 2 (Figs. 12 and 14). A photograph in Baljinnyam et al. (1993, Fig. 32a) shows this area viewed from the west-southwest. The scarp between Site 2 and Site 3 is continuous and very clear (Fig. 12), but it curves somewhat so that the local strikes differ by 13°. The map (Fig. A3a) shows left-lateral separation of a small ridge, at x = 9 m on the south side, and of a much larger ridge on the eastern edge of the map. The combination of the strike-slip and vertical components, however, have dammed the northerly flowing drainage and ponded sediment on the south side. A restoration of profiles along y = 6 m and y = 8 m shows 5.0 ± 0.5 m of left-lateral slip (Fig. A3b). The restored profiles require only a small vertical separation of 0.3 m, but the northward plunge of ~0.2 of the ridge on the east side suggests that the northern profile at y = 8 m should have lain 0.4 m below the southern profile at y = 6 m before 1957. Thus, there appears to have been a vertical component of ~0.7 (± 0.3) m with the northern side up. The nearly identical strike-slip components for Sites 2 and 3 but somewhat different vertical components are consistent with the slightly different local strikes of the rupture and the resulting greater extensional component at Site 3.

Site 4. 45°08.50′N, 99°39.00′E, Z = 1,690 m. Bogd rupture, north of Taryatiyn uul, ~1 km east of Sevsüüliyn bulag (Fig. 15). The fault trace crosses a gentle north-south ridge that defines northerly trending drainage. Whereas the regional topography slopes northward, the ridge shows no such northerly plunge, perhaps because of a vertical component of slip (Fig. A4a). Approximately 300 m to the west, a south-facing scarp attests to a vertical component of about 1 m. In addition, a trough parallel to the fault trace seems to mark a zone of extension, or small graben, particularly in the western part of Fig. A4a where 24 m < y < 32 m. Thus, both the horizontal and the vertical components of slip are less clearly defined than in many of the other areas studied. Nevertheless, the left-lateral separation of the segments of ridge north and south of the scarp is clear (Fig. A4a). Profiles along y = 19 m, south of the scarp, and along y = 31 m, north of it, suggest a strike-slip offset of 6.4 m and a vertical separation of only 0.3 m (Fig. A4b). The match of the profiles, however, is poor in part because the ridge north of the trace is narrower than south of it and in part because the ridge does not plunge northward as the regional topography does. The 6.4 m separation was obtained simply by matching the crests of the ridge, but the obliquity of the ridge north of the scarp, of 79°, makes this estimate too large. Assuming the ridge to have projected at this angle across the region between the profiles, 12 m apart, both reduces the offset by 2.4 m, and adds uncertainty to the estimate: 4.0 ± 1.5 m. Projecting the contours of either the area on the west side of the map or of the ridge near the center (Fig. A4a) suggests a vertical offset of closer to 0.5 m at this locality, rather than either 1 m or 0.3 m. We assume a value of 0.5 ± 0.3 m.

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Site 5 (on this and facing page). 45°07.67′N, 99°51.58′E, Z = 1,790 m. Bogd rupture, on the north slope of Noyon uul, 1 km west of the Nurgiyn am in an area of relatively large offsets (Fig. 20). Northerly sloping gullies on a steep slope were displaced left-laterally in 1957 (Fig. A5a), and the northern side of the trace moved up with respect to the southern side. Gullies and the intervening ridge south of the trace are aligned perpendicular to the trace, but some gullies converge north of the trace. Thus, estimating the offset is complicated by different orientations of features north and south of the fault trace (along y ≈ 24 m). The ridge south of the scarp and approximately parallel to x = 24 m lies ~6 m to the east of its northward continuation, at x = 18 m north of the trace. The relatively shallow gully at x = 18.5 m on the south side lies only 4.5 m east of its northward continuation, at x = 14 m. The deeper gully emanating from the southern edge of the map at x = 35 m, however, is separated by 11 m from its current northward continuation at x = 24 m just north of the trace. This larger offset suggests that two events may be responsible for it. Profiles along y = 29 m, just north of the trace, and south of it along y = 15 m (Fig. A5b) show the difficulty in determining the offset. Aligning the segments of the ridge with a 5.8 m shift, mismatches the relatively small western gully by nearly 2 m and the deeper, eastern gully by ~5 m. We conclude that the offset in 1957 was 5.8 ± 1.5 m, but quantifying the offset at this locality is perhaps not as convincing as elsewhere. Because of the steep northward slope, the northern profile must lie below the southern profile. For an average slope of 0.5 (Fig. A5a) and a distance of 14 m between the profiles, the northern profile should have lain 7 m lower than the southern profile before slip occurred. The measured vertical separation of the profiles of 5.2 m (Fig. A5b), therefore, implies a vertical component of 1.8 m. Given the suggestion that the eastern gully has been offset approximately twice as much as the ridge adjacent to it, this vertical component may be the result of two events, with that associated with the 1957 earthquake only ~1 ± 0.5 m. The block diagram (Fig. A5c) is viewed toward N148°E and 10° from the horizontal.

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Site 6. 45°06.3′N, 99°56.6′E, Z = 1,760. Bogd Rupture ~3 km west of Site 7 and Öndgön Hayrhan, and just west of Uhaagiyn am (Gate Valley) (Fig. 22). The fault has displaced a small stream valley, an adjacent terrace, and a higher ridge farther east (Fig. A6a). A stream enters on the south of the mapped area near x = 35 m, and where the stream follows and crosses the fault near y = 14 m, it has incised a narrow gully, which then flows north along x ≈ 17 m. This stream has cut a channel into a terrace on the east side marked by the wide space between the contours for z = 4.0 m and z = 4.5 m where 35 m < x < 50 m. The east edge of the terrace slopes up onto a ridge on the east edge of the map. The offsets of these features are clearly larger than those that we had mapped elsewhere. To quantify that offset we constructed profiles along x = 10 m and x = 17 m (Fig. A6b). The match of the eastern slopes of both the young gully and the higher bank east of the terrace yield a horizontal separation of 10.4 (±1.5) m. We suspect that this large offset represents slip in two earthquakes. No vertical component is required, but note that the width of the disrupted zone is several meters, with troughs (e.g., near x = 8 m, y = 14 m) and perhaps a graben on the crest of the ridge on the right edge of the map. Thus the lack of a vertical component is not well constrained (0 ± 2 m).

Surface rupture of the 1957 Gobi-Altay earthquake

Site 7 (on this and next page). 45°06.13′N, 99°59.50′E, Z = 1,680. Bogd Rupture on south slope of Öndgön Hayrhan, 5.5 km west of the Hüühniy höndiy, and 200 m west of Site 8 (Figs. 25–27). A spectacular offset shows both a vertical component with the south side uplifted and a large left-lateral component. Because the combination of large components on a nearly vertical plane has dammed drainage from the steep slope to the north, matching profiles can be difficult. The left-lateral offset enhances the apparent vertical component on features that slope east-southeast, such as at 9 m < x < 16 m on the north side and at 15 m < x < 33 m on the south side. Nevertheless, the vertical component is clear even where the topography slopes west-northwest, such that 35 m < x < 40 m. Note also that a narrow ridge approaches the scarp on the north side near x = 9–10 m, on the left side of the mapped area (Fig. A7a), and continues near x = 14–15 m on the south side. We suspect that there has been erosion of the saddle near x = 10 m, y = 5–6 m, because the slope is steeper near the scarp than farther north. Allowing for such steepening, the projection of the gentler slope between the z = 6.5 and z = 7.5 contours suggests that before the earthquake, the downslope continuation of the ridge at the fault trace near y = 3.5 m should have been at a height of approximately z = 5 m, not the observed height of z = 10 m. To quantify the displacement accurately, we constructed profiles along y = 6 m, at the foot just north of the steep scarp, and along y = 3 m across the top of the uplifted block just south of the scarp (Fig. A7b). Matching the profiles clearly requires both strike-slip and vertical components. To match them we assumed that the slopes on the two sides of the ridge on the west side of the mapped area and the westward slope on the east side match. The restored positions yield a left-lateral offset of 5.5 (±0.5) m, and a vertical separation of 4.1 (±1.0) m. To estimate the vertical component of slip, we should correct the vertical separation for the southward slope of the ridge on the west side, which suggests that the profile along y = 6 m should have lain 1 (±0.5) m higher than that along y = 3 m. Thus, we estimate a vertical component of 5.1 ± 1.1 m. The block diagram (Fig. A7c) is viewed toward N114°W and 30° from the horizontal.

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Site 8. 45°06.17′N, 9959.58’E, Z = 1,740 m. Bogd rupture approximately 200 m east of Site 7, on the south side of the Öndgön Hayrhan (Fig. 25). Determining the strike-slip offset along much of this scarp is difficult, because a vertical component has caused the uplifted southern side to dam the south-flowing drainage. Sediment has ponded behind these gullies (as within the 3-m contour in Fig. A8a), and determining thalwegs of the gullies on the north side near the scarp is difficult. Nevertheless, the combination of vertical and left-lateral offsets have isolated a low, elongated hill on the west side of the area mapped (Fig. A8a). The top of the hill, at x = 9 (±2) m, apparently was the southern continuation of the ridge on the north side, whose crest plunges south at x = 3 m, suggesting left-lateral offset of 6 (±2) m. The approximate left-lateral separation of profiles at y = 9 m and y = 15 m of 5.4 (±1) m yields a lower bound for the offset in 1957 (Fig. A8b), because the trend of the gully to the north is not perpendicular to the scarp. With an angle of ~75° between them, the extrapolation of this trend across the 6-m north-south distance separating the profiles implies an additional 1.6 (±1.0) m of left-lateral slip, and hence an offset of 7.0 (±1.4) m. The prominent north-facing scarp in an area where the regional topography slopes steeply southward implies that a large vertical component accompanied the strike-slip component. Extrapolating the southward plunging ridge along x = 3 m across the fault trace (Fig. A8a) suggests that at y = 9 m the height should have been only 1 m prior to 1957, and not that of 6 m for the elongated hill on the south side of the fault trace. Thus these observations suggest a large vertical component of slip of 5 m, with the north side down. Similarly, extrapolating the contours for 0 m < z < 16 m along the right edge of the map (Fig. A8a) across the fault trace to y = 9 m yields an expected pre-1957 height of z = 5 m at y = 9 m, not the measured height of 10 m. Thus, it appears that not only a large strike-slip component of 7.0 (±1.4) m developed in 1957, but also that a comparable vertical component formed, 5.0 (±1.0) m.

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Site 9. 45°06.33′N, 100°00.33′E, Z = 1,700 m. Bogd rupture, 1 km west of the Hüühniy höndiy (Woman’s Valley) and just west of Öndgön Hayrhan, where the offset seems to have reached its maximum (Figs. 25, 31, and 32). East of this locality, offsets of 3–4 m characterize the displacement, but west of it, offsets of 5–6 m appear to be common. This site lies near where the scarp bifurcates (Figs. 25 and 31). The northern strand continues westward with a strike of N75°W but becomes difficult to trace a few hundred meters to the west. The area that we mapped lies on the southern strand, which continues around the south side of Öndgön Hayrhan. A gully crosses the trace nearly perpendicular to it in the center of the map (Fig. A9a). The horizontal separation of the thalweg of 3.9 (±0.5) m appears to define the strike-slip component (Fig. A9b). A clear north-facing scarp formed (Fig. 32), suggesting that slip included a substantial vertical component with the north side down. The scarp on the west side of the area mapped (4 m < x < 12 m) is nearly 2 m high (Fig. 32), and approximately 2.4 m separate the profiles at y = 11 m and y = 17 m vertically (Fig. A9b). When displaced both vertically and horizontally by these amounts, the profiles match, defining the strike-slip offset of 3.9 (±0.5) m (Fig. A9b). In the eastern part, the scarp is much less clear, in part because two splays (along y = 14.5 m and y = 10.5 m, Fig. A9a) make a compound scarp that is not easily discerned from the contours. The northerly component of the topographic slope, of about 0.4 (Fig. A9a), however, calls for a difference of about 2.4 m in heights of profiles 6 m apart, equal to that measured (Fig. A9b). Thus, the height of the scarp appears to result from the strike-slip offset of topography that includes a relatively steep northwestward regional slope. The vertical component of slip must be small: 0.0 ± 0.5 m.

Surface rupture of the 1957 Gobi-Altay earthquake

Site 10. 45°06.08′N, 100°04.75′E, Z = 1,680 m. Bogd rupture between the Hüühniy höndiy and Zaraagiyn höndiy, and only about 300 m west of Site 11 (Fig. 33). We mapped left-lateral offsets of two gullies and the ridge between them (Fig. A10a). All are oriented obliquely to the fault trace, making the estimation of offsets more difficult than if they crossed the trace perpendicular to it. Profiles parallel to the trace at y = 20.5 m and y = 24 m overlap when the southern profile is displaced to the west 4.2 m and down 0.7 m (Fig. A10b). With orientations of ~60° to the fault trace and a separation of the profiles of 2.5 m, the measured horizontal separation overestimates the offset by 1.25 m, suggesting left-lateral slip of 3.0 ± 0.8 m. With a regional slope of ~0.25, the separation of 2.5 m suggests that the northern profile lay 0.6 below the southern profile before the earthquake. Hence, if there was a vertical component, it was small, 0.1 ± 0.3 m.

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Site 11 (on this and facing page). 45°05.83′N, 100°05.00′E, Z = 1,690 m. Bogd rupture, only about 300 m east of Site 10, and also between the Hüühniy höndiy and Zaraagiyn höndiy (Figs. 33 and 34, Plate 1). A northerly plunging gully in the center of the mapped area (Fig. A11a) and the ridge just east of it have been offset left laterally 3 to 4 m. Profiles along y = 17 m and y = 13 m show strike-slip and vertical separations of 3.5 m and 2.0, respectively (Fig. A11b). Neither the ridge nor the gully is straight, however, and overall, neither is oriented perpendicular to the fault. Yet, in the areas near where they approach the two profiles they are essentially perpendicular to the fault. Hence, correcting for a possibly oblique intersection of the fault and offset features is difficult, and rather than try to do so, we merely assign a relatively large uncertainty to the offset, 1 m. Matching the two profiles (Fig. A11b) requires that the southern profile be lowered 2.0 m to overlap the northern profile. Because of a northward regional slope of ~0.4, however, the southern profile should have stood ~1.6 m above the northern profile before the rupture formed. Thus, slip in 1957 included only a small vertical component, 0.4 (±0.2) m, with the northern the upthrown block. The block diagram (Fig. A11c) is viewed toward N33.5°W and 25° from the horizontal.

Surface rupture of the 1957 Gobi-Altay earthquake

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Site 12. 45°05.42′N, 100°08.58′E, Z = 1,910 m. Bogd rupture ~2 km east of a large landslide that predates the 1957 earthquake and crosses the fault, and ~200 m east of the Zaraagiyn höndiy (Hedgehog Valley; Figs. 35 and 36). The rupture cuts a gently northward sloping surface (Fig. A12a), and a vertical component of slip of 1.3 (±0.4) m with the south side up can be inferred from a north-south profile across the scarp (Fig. A12b). Two northward flowing gullies have been offset left laterally 3.5 (±0.5) m (Fig. A12c). The western gully, the clearer of the two, is incised into the upthrown southern block at the scarp (x = 13–14 m, y = 16–18 m), and just north of it. Its former continuation on the north side, however, emanates from the scarp at x = 9–10 m and continues north and then north-northeast. The different orientations of the eastern gully, north-northwest on the northern side and north-northeast on the southern side of the scarp, prohibit the separation of the thalwegs from constraining the offset as well as that of the western gully does. Nevertheless, an offset of 3.5 (±0.5) m matches the topography (Fig. A12b).

Surface rupture of the 1957 Gobi-Altay earthquake

Site 13. 45°03.83′N, 100°16.17′E, Z = 1,870 m. Bogd rupture zone, approximately 5.5 km west of Urd Burgasny am (South Bush Gorge) and 2.5 km east-southeast from Ulyastay am (Poplar Gorge; Fig. 39). The topographic map at the base of a high scarp reveals only a suggestion of strike-slip displacement (Fig. A13a). The map shows a small gully flowing north-northeast down the scarp along x = 10–12 m (Fig. A13a). Its continuation seems offset approximately 3 (±1) m, for it appears to emanate from the foot of the scarp at x = 7 m and continue north-northeast before curving to the north (Fig. A13a). Flow from the gully on the south side now continues north-northeast, perpendicular to the scarp, and seems to have cut a smaller gully a few meters north of the scarp as shown by the deflection of the z = 3.75 m contour near x = 11 m, y = 18 m. We also constructed a long, north-south profile across the entire scarp to show the cumulative vertical component of slip (Fig. A13b), which shows a total vertical component of ~6 m. Slip in 1957 was less, 2 ± 1 m, but defining that offset more precisely is difficult.

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Site 14. 45°02.42′N, 100°20.83′E, Z = 1,965 m. Bogd rupture, a 2.5 km west of where the Bitüütiyn am debouches from Ih Bogd into the Nuuryn Höndiy (Valley of Lakes; Fig. 40). Although left-lateral strike slip is obvious in the field, recognizing it with only topographic contours requires guiding explanation. First, a vertical component of slip (~1.5 m) with the north side up helped dam the gully on the west side of the mapped area (8 m < x < 14 m, y < 16 m, Fig. A14a); ponding of sediment on the south side of the fault trace obscures the thalweg. The vertical component enhances apparent left-lateral components of west-facing slopes, but obscures those of east-facing slopes. Second, the difference in the northerly to north-northeasterly orientations of gullies and ridges south of the scarp (along y = 17 m) from northeasterly orientations north of it introduces an uncertainty in projected positions of features from one side of the rupture to the other. Third, a young channel has developed along the rupture on the east edge of the mapped area (y = 16 m) and migrated westward by headward erosion to capture a more gently sloping gully on the northern side that flows north-northeast. The horizontal distance between the thalweg on the northern side at x = 10 m and its deepest part on the south side at x = 13 m (Fig. A14a) suggests left-lateral slip of 3–4 m. Matching profiles north and south of the fault in the western edge of the region mapped corroborates left-lateral slip of 3.9 ± 1 m (Fig. A14b). In the eastern half, the gully, which enters in the south at x = 27 m and exits in the north at x = 37 m, trends northeast near the fault trace (Fig. A14a). The northeast-trending thalwegs also can be aligned by ~3 m of left-lateral restoration, but this match is not very convincing. Despite the relatively large uncertainty in the offset, slip as large as 5 m or as small as 2 m is not permitted. The two profiles show a vertical separation of 1.1 m (Fig. A14b). With 5 m between them and the northward regional slope, the northern profile should have lain ~0.5 m below the southern profile before 1957, implying 1.6 ± 0.5 m of vertical slip with the north side up.

Surface rupture of the 1957 Gobi-Altay earthquake

Site 15. 45°03.75′N, 100°27.92′E, Z = 1,380 m. Near the northwest end of the Dalan Türüü foreberg (Fig. 42). At the western end of the foreberg, the scarp crosses an alluvial fan that has been dissected by recent erosion (Fig. A15a). A profile oriented perpendicular to the scarp (Fig. A15b) shows a vertical component of only 1.0 ± 0.2 m, notably less than that farther east and south.

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Site 16. 45°03.17′N, 100°28.83′E, Z = 1,400 m. Dalan Türüü foreberg, where the scarp defines the base of the hills and ~2 km southeast of Site 15 (Fig. 42). The recent offset is clearly defined where slip on the fault has brought part of the relatively flat area at the foot of the hills (13 m < x < 16 m) up with the upthrown block (Figs. A16a and A16b). Assuming slip on a planar fault, extrapolating the gentle slope of the downthrown block beneath its apparent continuation on the upthrown block yields a vertical component of 2.2 ± 0.5 m (Fig. A16b). The greater horizontal separation between the 4.25 m and 4.5 m contours than between 1.0 m and 1.25 m contours and lower pairs suggests that there has been some deformation of the upthrown block. This warping of the frontal surface of the upthrown block would then lead to a small overestimate of the vertical component of slip at this locality, but there seems no escaping a larger offset here than at Site 15. The block diagram (Fig. A16c) is viewed toward N97°W and 5° from the horizontal.

Surface rupture of the 1957 Gobi-Altay earthquake

Site 17 (on this and next page). 45°02.50′N, 100°31.17′E, Z = 1,365 m. Northeast edge of the Dalan Türüü foreberg (Figs. 42 and 43). The scarp with a local north-northwest strike, crosses low hills and alluvial fans, but is not straight. Instead, it seems to be discontinuous with steps of tens of meters from one part to another (Fig. A17a). Three profiles across the scarp drawn perpendicular to the local strikes show uplift of the west relative to the east side (Figs. A17b, A17c, and A17d). The vertical components of 1.8 m for the southern profile (Fig. A17d) and 1.9 m for the central profile (Fig. A17c) appear to reflect slip during the earthquake in 1957. The low area on the middle profile at 19 m < x < 28 m (Fig. A17c) marks a gully crossing the profile obliquely. The larger vertical component of 3.2 to 3.7 m for the northern profile (Fig. A17b), drawn down the nose of a ridge that plunges northeast, may reflect either slip in two earthquakes or erosion of the base of the scarp from the gully southeast of the ridge. We suspect that the vertical component in 1957 was closer to 2 ± 0.5 m than to 3.5 m. The block diagram (Fig. A17e) is viewed toward N135°W and 5° from the horizontal.

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Surface rupture of the 1957 Gobi-Altay earthquake

Site 18 (on this and next page). 45°01.83′N, 100°33.00′E, Z = 1,345 m. Near southeast end of the Dalan Türüü foreberg (Fig. 42). The local strike of the scarp of N10°W is unequivocally different from that of the Bogd rupture. The surface rupture lies at the base of the hills and is not easily discerned from the topographic contours (Fig. A18a) without an image of the free face on the scarp (Fig. A18c). Yet, where 25 m < x < 30 m, the spacing between contours on the upthrown block near the scarp is somewhat wider than that higher on the block (x < 25 m), implying that before the earthquake the low, gently sloping part merged with the flatter surface on the downthrown block (x > 33 m). A profile perpendicular to the scarp at y = 30 m shows this decreasing slope on the upthrown block (Fig. A18b) and indicates a vertical component of ~2 ± 0.5 m. The block diagram (Fig. A18c) is viewed toward N55°W and 10° from the horizontal.

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Site 19. 45°00.00′N, 100°35.50′E, Z = 1,480 m. Near the central part of the rupture that defines the northeastern edge of a faulted and offset fan, formed by deposits of the Huustiyn am (Fig. 44). A prominent scarp striking N45°E crosses an alluvial fan southeast of the Dalan Türüü foreberg, northwest of Site 20 (Fig. 44). A profile of elevations across this scarp yields a vertical separation of 10.8 m (Fig. A19), which is much higher than the offset that seems to have formed in 1957. (We made no map here.) Estimating the vertical component in 1957 is difficult. There is a clear change in the slope of the scarp near x = 45 m (Fig. A19), which defines part of the rupture of 1957. Above and southwest of the scarp, the slope is relatively gentle. The height of the scarp projected above the regional slope of the downthrown block is 3.2 m. If, however, we extrapolate the steeper slope of the surface just above the scarp and use it to define the local slope before the earthquake in 1957, the estimated difference in height is only 1.1 m (Fig. A19). Given the difficulty in determining the shape of the profile before 1957, we deduce only that a vertical component of slip of 1 to 3 m, or 2 ± 1 m, occurred in 1957 at this locality.

Surface rupture of the 1957 Gobi-Altay earthquake

Site 20. 44°59.50′N, 100°36.00′E, Z = 1,500 m. Southeast end of the rupture that defines the northeastern edge of an old fan formed by deposits of the Huustiyn am, and only about 100 m north of the Bogd rupture (Fig. 44). We saw no evidence of a previous scarp or of a foreberg at all in this area (Fig. A20a). The orientation of the scarp of ~N25°W differs by more than 50° from both the local and the regional trends of the Bogd rupture. We mapped only a short portion of the scarp where recent erosion has not destroyed it (Fig. A20a). The deformation includes a substantial vertical component at the scarp, of ~1.8 m (Fig. A20b), but the surface of the upthrown block also shows mild deformation. Because of this, we extended the mapped area 30 m southwest of this deformation. The northeastward projection of the slope of the undisturbed surface southwest of the scarp overlies the approximately parallel surface of the downthrown block by 2.7 ± 0.5 m, which is the component of vertical displacement that we attribute to the 1957 earthquake (Fig. A20b). The block diagram (Fig. A20c) is viewed toward N160°E and 10° from the horizontal.

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Site 21. 44°59.33′N, 100°38.33′E, Z = 1,555 m. Bogd rupture, 0.5 km west of the Övtiyn am (Inheritance Gorge; Figs. 47 and 48). A strike-slip component is clear (Fig. 48). In the center of the mapped area, two gullies converge just south of the scarp (x = 23 m, y = –1 m; Fig. A21a). Their continuation to the north has been displaced ~3 m to the west. Profiles parallel to the scarp and 2 m apart show a left-lateral separation of 2.2 m between the segments of the gully (Fig. A21b). Because both the gully to the north and the main tributary south of the rupture are oriented obliquely to the scarp, this 2.2 m separation underestimates the offset. A difference of ~60° in the orientations of the gullies and the fault trace and a separation of profiles from one another by 2 m imply that the total left-lateral offset should be ~1 m greater than the 2.2 m separation, or 3.2 ± 0.5 m. The vertical component of slip in this area is small. The southern and northern profiles can be matched to one another by displacing one 0.8 m with respect to the other. In the eastern part of the region, where the regional slope is 0.5, the southern profile should have lain 1 m higher than the northern one prior to 1957. The measured difference of 0.8 m suggests that the northern side of the fault may have moved up a small amount with respect to the southern side, but the resulting estimated vertical component of 0.2 ± 0.2 m is too small to be resolved by these data.

Surface rupture of the 1957 Gobi-Altay earthquake

Site 22. 44°58.50′N, 100°43.83′E, Z = 1,565 m. Southern branch of the Bogd rupture zone south of the hill Ulaan Huts, and just east of the Baruun huuray sayr (Fig. 50). As can be seen in the photograph (Fig. 50), the Bogd rupture zone consists of two (or more) splays that bound another small basin. The strand along the southern edge of the basin trends ~N115°E with a small strike-slip component. A detailed topographic map (Fig. A22) of one strand shows a small gully offset left laterally 1.3 ± 0.3 m. Fig. A22 shows parallel lines drawn along the thalwegs. With a tape measure, we measured similar offsets along this scarp farther northwest. The spacing of contours suggests a vertical component of 0.3 ± 0.2 m with the north-northeast side down.

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Site 23. 44°57.17′N, 100°53.00′E, Z = 1,770 m. Bogd rupture, near a saddle between Dulaan Bogd and an isolated high terrain in which Paleozoic rock crops out, and between the Züün huuray sayr and Shavinahinyn sayr (Figs. 51 and 52). The strike-slip component on the Bogd rupture associated with slip during the 1957 earthquake is especially clear on aerial photographs, and this site was chosen along a section where the scarp is particularly sharply defined. In the center of the mapped rupture (Fig. A23a), a small gully has been displaced left laterally 2.9 ± 0.5 m (Fig. A23b). Deeper gullies east and west of this small gully show a comparable offset, but variations in orientations near the fault make inferring the offset more precisely difficult. The vertical separation of 2.1 m between profiles 4 m apart is partly the result of the regional northward slope. Correction for a slope of ~0.25 near x = 50 m (Fig. A23a) makes the vertical component of slip (south side up) only 1 ± 0.5 m. The block diagram (Fig. A23c) is viewed toward N152.5°W and 10° from the horizontal.

Site 24. 44°56.50′N, 100°56.50′E, Z = 1,650 m. Bogd rupture, northeast of Dulaan Bogd, and 3 km west of the Shavinahinyn sayr (Fig. 54). Four northerly trending gullies have been offset by the 1957 surface rupture, where it trends N103°E (Fig. A24a). The match of profiles measured 2 m north and south of the scarp indicates 3.6 ± 1 m of left-lateral offset (Fig. A24b). The three eastern gullies (at x = 17 m, 43 m, and 54 m on the northern profile, along y = 2 m) are separated by this amount from their continuations south of the fault. The westernmost gully (at x = 10 m on this profile) trends northeasterly near the scarp, and estimating the offset of it is less straight forward than for the others. The relative heights of the profiles suggest a small vertical component with the north side uplifted <1 m with respect to the south side. Elevations along the southern profile (y = –1 m) stand about 0.2 m higher than those along the northern profile (y = 3 m). As the profiles are separated by 4 m, however, an extrapolation of the northward regional slope of ~0.15 (Fig. A24a) implies that the southern profile lay 0.6 m above the northern profile, 4 m from it. Thus, the northern flank seems to have moved up with respect to the southern ~0.4 ± 0.2 m, even though the higher, mountainous terrain lies to the south and the basin to the north. The sense of the vertical component of displacement is opposite to that of the regional topography. The block diagram (Fig. A24c) is viewed toward N32°W and 20° from the horizontal.

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Site 25. 44°56.50′N, 101°01.33′E, Z = 1,600 m. Bogd rupture, 1.5 km west of where the Toromhon sayr crosses the scarp (Fig. 55). A small north-northeasterly trending gully enters the map on the south side at x = 8 m and seems to have been offset 5 to 6 m (Fig. A25a). Lines along the thalwegs north and south of the fault scarp, at ~y = 17.5 m, drawn on the map (Fig. A25a) show ~5.5 m of left-lateral separation. Yet, a low ridge in the center of the map, indicated by parallel lines along its crest, shows only ~3 m of separation. Whereas we had believed the offset here to be to be 5–6 m (Baljinnyam et al., 1993), a more objective assessment is between 3 and 6 m, which we simply call 4.5 ± 1.5 m. A vertical separation of ~0.3 (±0.1) m can be inferred from the contours. The block diagram, viewed toward N46°W and 15° from the horizontal (Fig. A25b), also shows these different apparent offsets and the small vertical component of 0.3 m, near y = 17.5 m.

Surface rupture of the 1957 Gobi-Altay earthquake

Site 26. 44°56.33′N, 101°02.45′E, Z = 1,607 m. Bogd rupture, 1 km west of Toromhon sayr and 0.5 km east of Site 25 (Figs. 55 and 56). A deep gully sloping west approximately parallel to the rupture meanders near x = 30 m (Figs. A26a and 56), and left-lateral slip, along y ≈ 15 m, displaced the southern edge of the meander as well as the topography adjacent to it. Profiles parallel to the scarp on the two sides indicate 4.3 ± 0.5 m of left-lateral slip (Fig. A26b). The vertical separation of ~0.3 m agrees with the expected difference in elevations for a regional slope of 0.2, as measured from the hill on the right, and a distance between profiles of 1.5 m. Thus, there was no measurable vertical component, 0.0 ± 0.2 m. The block diagram (Fig. A26c) is viewed toward N148°E and 20° from the horizontal.

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Site 27. 44°56.17′N, 101°03.58′E, Z = 1,560 m. Bogd rupture just east of the Toromhon sayr and just west of where the Toromhon Overthrust approaches it (Figs. 58 and 59). We mapped two gullies offset by slip on the fault in 1957. Our purpose here is to show that the offset in 1957 was only ~3 to 4 m, and not the much larger value of 8.85 m suggested by V. P. Solonenko, in Florensov and Solonenko (1963, 1965) for the eastern gully mapped here (See Figs. 40 and 41 of Baljinnyam et al., (1993). Both a vertical component with the south side up and a left-lateral component have constructed north-facing scarps (Fig. A27a), which have blocked south-flowing drainage. Ponding on the north side of the fault makes defining thalwegs difficult. The western gully, entering the northern edge of the mapped area near x = 17 m is the smaller of the two gullies. An extrapolation of the gully to the scarp implies a left-lateral offset of only ~3 m. The larger gully enters the northern edge near x = 44 m, and extrapolating it to the scarp suggests a larger offset of 7 m or 8 m (Fig. A27b), as Solonenko inferred. To present this in another way, we constructed profiles along y = 11 m, just south of the scarp, along y = 14 m, just north of it, and along y = 20 m, north of where ponding has obscured the original topography (Fig. A27b). Profiles along y = 11 m and 14 m are separated ~2.9 ± 1 m. The thalweg of the western gully, along y = 20 m, is nearly aligned with that along y = 14 m (Fig. A27b). The trend of the gully differs from that of the fault trace by 75° to 80° (Fig. A27a), implying that 0.6 m should be added to the separation of the profiles 3 m apart. Thus, the offset in 1957 was 3.5 ± 1.5 m. The thalweg of the larger, eastern gully, however, lies ~7 m west of its continuation south of the fault. The greater offset of the larger gully implies that it has been displaced twice, in 1957 and in a previous event. The vertical separation profiles along y = 11 m and y = 14 m of 0.5 m shows that the south side moved up relative to the north side. The contours on the south-plunging ridge in the center (x = 30 m, Fig. A27a) and north of the fault trace define a southward slope of 0.5. Thus, the profile along y = 14 m should have been 1.5 m higher, not 0.5 m lower, than that along y = 11 m before 1957. Accordingly, there seems to have been a vertical component of 2 ± 1 m with the south side up.

Surface rupture of the 1957 Gobi-Altay earthquake

Site 28. 44° 5.08′N, 101°04.75′E, Z = 1,580 m. Bogd rupture, approximately 1km east of where the Toromhon Overthrust approaches it (Figs. 58 and 60). The north-facing scarp striking N99°E reveals both vertical and left-lateral separations (Figs. 60 and A28a). Near the eastern end of the area surveyed, two gullies converge just south of the scarp. The ~3–4 m left-lateral offset during the earthquake caused the continuation of the western gully north of the fault to be abandoned and placed its southern part against the northern continuation of the eastern gully (at x = 42 m, y = 11 m). Erosion during the 36 years following the earthquake deepened this gully, but the correlation of the western channels is unambiguous. The axis of a larger gully, which lies west of the main hill in the center of the contour map (Fig. A28a), seems to be displaced more than that of the smaller channel in the eastern part and may reflect the occurrence of two events since its entrenchment. Profiles along y = 9 m and y = 13 m (Fig. A28b) show a left-lateral separation of 2.1 m, which underestimates the offset. Both the topographic contours on the west side of the map and the offset gully trend northeast, oblique to the strike of the scarp. With a difference in trend of 60° ± 15°, the distance of 4 m between the profiles implies that 2 ± 1 m should be added to yield a displacement of 4.1 ± 1 m. In the field in this area, we estimated typical offsets of 3.5 to 4 m. The match of the thalwegs of the gully also yields a fit of the eastward sloping topography on the west side of the map, limiting the vertical separation of the profiles to 1 m (south side up). As is clear from the contour map (Fig. A28a), however, restoration of 4.1 m of a left-lateral offset does not align contours on the left side of the map. With an eastward slope of ~0.15, the additional 2 m shift of the profiles (Fig. A28b) reduces the vertical separation by 0.3 m, implying a vertical component of 0.7 ± 0.3 m. This component is inadequate to restore the crest of the ridge in the center of the mapped area (Fig. A28a), suggesting that the relief there results from two events.

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Site 29 (on this and facing page). 45°00.89′N, 101°29.65′E, Z = 1,591 m. Hetsüü foreberg, in its eastern part (Figs. 64–66). This profile crosses the scarp on the northern margin of the Hetsüü foreberg. We found the scarp neither continuous nor simple, and it was a difficult task to find a place where slip was localized on one plane, instead of two or more parallel ruptures. Like profiles across the Dalan Türüü foreberg, strike-slip components were not obvious, and vertical components were defined by narrow surfaces that had merged with gently sloping fans before the earthquake. Such a surface can be seen on the mapped area (Fig. A29a) approximately at 22 m < y < 26 m. A profile, which passes up a gully on the south side, shows the vertical component to be 1.8 ± 0.3 m (Fig. A29b). The block diagram (Fig. A29c) is viewed toward N137°W and 10° from the horizontal.

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Site 30 (on this and facing page). 44°59.61′N, 101°32.23′E, Z = 1,725 m. Hetsüü foreberg, northern part of the right-lateral rupture (Figs. 64 and 65). Slip includes a right-lateral component and a smaller vertical component with the west side up. Profiles taken along x = 12 m and x = 16 m show an approximate horizontal separation of 3 to 4 m (Fig. A30b). Ridges and valleys mapped in this area cross the fault at different angles (Fig. A30a), particularly for the ridge on the south side of the map, near y < 15 m, but also for the valley near the north edge. Moreover, the rupture is not straight, making the construction of profiles parallel to it difficult. The most clearly offset features are the ridges near 35 m < y < 40 m and 50 m < y < 55 m. Matching these ridges on profiles along x = 12 m and x = 16 m yields a right-lateral separation of 3.3 ± 0.6 m (Fig. A30b). Because of the relief, determining a vertical component accurately is difficult. Nevertheless, in the region 35 m < y < 40 m, contours are nearly uniformly spaced with a distance of 4 m between contours differing by 2 m. At the fault scarp, however, ~5–6 m of horizontal distance separate contours differing by 2 m, implying 0.5–1 m of vertical offset and suggesting a vertical offset of 0.7 ± 0.3 m. The block diagram (Fig. A30c) is viewed toward N135°E and 10° from the horizontal.

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Site 31 (on this and facing page). 44°59.40′N, 101°32.20′E, Z = 1,749 m. Hetsüü foreberg, northern part of the right-lateral rupture (Figs. 64 and 65), and 300 m south of Site 30. Slip includes a right-lateral component and a smaller vertical component with the west side up (Fig. A31a). A profile made across an area where the scarp traverses a smooth westerly dipping surface (at y = 2 m) constrains the vertical component to be 0.8 ± 0.3 m (Fig. A31b). Profiles along x = 15 m and x = 18 m indicate right-lateral slip, but the amount is difficult to estimate because the topography varies so much perpendicular to the profiles. The profile along y = 2 m shows that in that area the vertical separation of the profiles results from the regional slope, but in the central part of the profiles along x = 15 m and x = 18 m, the deep gully requires lower elevations along the western profile. To estimate the strike-slip offset, we matched only the southern parts of the profile (Fig. A31c). The poorer match farther north may suggest that the estimated horizontal slip of 3.0 m is too small, but the width of the deformed zone in that area is greater than the 3 m distance between the profiles. Nevertheless, the misfit is a reminder that the uncertainty is not small, ~1 m. The block diagram (Fig. A31d) is viewed toward N55°W and 20° from the horizontal.

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Site 32. 44°57.97′N, 101°31.39′E, Z = 1,874 m. Bogd rupture along the northern front of Baga Bogd, south of the Hetsüü foreberg. Along much of the north side of Baga Bogd, the rupture is neither straight nor localized. Site 32 was chosen because the scarp is sharper than in most of this region (Fig. A32a). A profile across the scarp shows a vertical component of slip of 1.9 ± 0.3 m (Fig. A32b). The relatively gentle topography on both sides, however, does not permit a reliable estimate of the horizontal component of slip.

Surface rupture of the 1957 Gobi-Altay earthquake

Site 33 (on this and next page). 44°54.37′N, 101°41.89′E, Z = 2,124 m. Bogd rupture, on the east side of Züün hanh sayr (Fig. 69). A clear vertical scarp faces north-northeast (Fig. A33a). Farther west, erosion and deposition has destroyed the scarp, and this area lies at the edge of the active stream bed. A profile constructed along x = 6 m shows a vertical component of 1.2 ± 0.4 m (Fig. A33b). The contours trending parallel to the y-axis, N22°E, north and south of the scarp define the eastern edge of an abandoned channel of the Züün hanh sayr that appears to be offset left laterally. A profile constructed parallel to the scarp along y = 23 m lies ~1.2 m below and 1.8 (±0.4) m east of another along y = 29 m (Fig. A33c). Although the thalweg appears to trend obliquely to the fault trace, the contours perpendicular to the scarp (Fig. A33a) suggest that the separation of them defines the offset.

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Surface rupture of the 1957 Gobi-Altay earthquake

Site 34 (on this and next page). 44°52.74′N, 101°48.65′E, Z = 1,716 m. Bogd rupture, on the northeast slope of Baga Bogd, along the west side of Burgastayn am (Fig. 74). A vertical component of 2.6 ± 0.4 m has created a huge scarp (Figs. A34a and 75). To quantify that component, we constructed a profile (Fig. A34b) obliquely across the scarp and parallel to the west edge of a deep channel, so that it does not cross the topography that developed before the scarp formed. A comparison of profiles along y = 23 m and y = 33 m, north and south of the scarp, shows a left-lateral separation not only of the channel, but of other adjacent topography of 2.1 ± 0.5 m (Fig. A34c). Because all of these features trend obliquely across the fault, the north-south separation of the profiles underestimates the offset. With an obliquity of 15° and a separation of 10 m between profiles, an additional 2.7 ± 1 m should be added, implying a strike-slip offset of 4.8 ± 1.1 m. The block diagram (Fig. A34d) is viewed toward N119°W and 10° from the horizontal.

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Surface rupture of the 1957 Gobi-Altay earthquake

Site 35 (on this and next page). 44°49.43′N, 101° 58.02′E, Z = 1,725 m. Bogd rupture, on the northeast slope of Bulgan uul, southsouthwest of the spring called Ulaan Shand (Fig. 76). The scarp is not straight, and farther east it splays into more than two widely separated strands. We chose this locality because slip is localized in this area, but in fact, on the eastern side of the area mapped, two separate strands make for a wider zone of deformation than to the west, where only one clear scarp could be seen (Fig. A35a). Two profiles at x = 6 m and x = 32 m, drawn approximately perpendicular to the scarp, show that a vertical offset of 2.6 ± 0.4 m occurred (Fig. A35b). The broader zone of deformation for the eastern profile (x = 32 m) is a manifestation of the two strands converging in this area. No strike-slip component could be identified with confidence, but it could be as large as 1–2 m. The block diagram (Fig. A35c) is viewed N170°W and 2° from the horizontal.

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Site 36. 44°55.67′N, 101°03.33′E, Z = 1,595 m. Toromhon Overthrust, near its northern end, where the fault scarp descends toward the Builsny Zadgay sayr and Bogd rupture (Figs. 80 and 81). A. Cisternas and H. Philip, in Baljinnyam et al., (1993), inferred a large right-lateral component of slip (~4 m) at this site, where the local strike of N45°E is more northeasterly than farther south. Our purpose here was to evaluate that inference with objective data. The scarp is clear in photographs (Fig. 81 and Fig. 43 of Baljinnyam et al., 1993) and can be seen on the topographic map (Fig. A36a) as a sharp break in contours along the band 15 m < x < 18 m, especially in the section 20 m < y < 40 m where the contours southeast and northwest of the scarp are oriented approximately perpendicular to the scarp. The deflections of the contours indicate a vertical separation resulting from both right-lateral slip and a vertical component with the northwest side up. The fault trace cuts the nose of a ridge plunging southeastward near y = 63 m. Elevations along the nose of the ridge decrease toward the southeast, but the nose has been offset right laterally only a small amount. To quantify the amount of slip, we constructed profiles parallel to the y-axis where x = 13 m, 15 m, and 25 m (Fig. A36b). The profiles along x = 13 m and x = 15 m overlap where 20 m < y < 45 m, but because the height of the ridge at y = 63 m decreases southeastward the profile along y = 15 m is lower in that section. The southwestern portion (20 m < y < 45 m) of the profile along x = 25 m is parallel to the others and nearly 2 m below it. Restoring it vertically by ~2 m or horizontally by about 6 m matches it with the profile along x = 15 m. The crest of the ridge near y = 63 m, however, is displaced only 0.6 ± 1 m right laterally. The corresponding vertical component necessary to match the profiles is 1.6 ± 0.5 m. Thus, we conclude that the 4 m estimate of right-lateral strike-slip displacement suggested in Baljinnyam et al. (1993) is incorrect.

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Site 37. 44°55.17′N, 101°02.83′E, Z = 1,620 m. Toromhon Overthrust, ~1 km south-southeast of Site 36, and where the strike of the overthrust is nearly north-south (Fig. 82). The fault scarp, striking N5°E, crosses a gently southwestward sloping surface (Fig. A37a). Although the width of disrupted topography is more than 5 m, the scarp remains steep in places, with free faces (see Baljinnyam et al., 1993, Fig. 42). The close spacing of contours on the western, upthrown block near the scarp suggests some minor warping of its top surface. A profile across the region perpendicular to the scarp and along y = 15 m shows the vertical component to be 3.0 ± 0.5 m (Fig. A37b), if this suggested minor warping is ignored. Thus, if the fault dipped west at 45°, the displacement at the surface in 1957 would be 4.2 m. No strike-slip component can be identified, but were there a right-lateral component, as can be seen to the northeast (Site 36), it would require a yet greater vertical, and hence thrust, component. The block diagram (Fig. A37c) is viewed toward N40°W and 10° from the horizontal.

Surface rupture of the 1957 Gobi-Altay earthquake

Site 38 (on this and next two pages). 44°52.71′N, 101°00.75′E, Z = 1,666 m. Toromhon Overthrust, northeast branch, along right (eastern) side of the Toromhon sayr (Figs. 86–88). Thrust slip on the Toromhon Overthrust produced a very large scarp in this area. Photographs taken by N. A. Florensov in January 1958 (Figs. 86 and 87) and in October 1958 in Florensov and Solonenko (1963, 1965, Fig. 69) show the vertical component to be as large as 6 m. We mapped a large area (Fig. A38a) that includes two relatively large valleys dammed by the scarp (along y ≈ 120 m and y ≈ 30 m), where both vertical and strike-slip components could be measured accurately. In the northern part of the mapped area, the local strike of the rupture is N38°E, but in the southern part it is N10°E. In both areas, the scarp is defined clearly by a ridge of uplifted material on the west slope of a northerly trending ridge. Thus, the fault, which dips west, has created local topography of a sense opposite to that forming the larger-scale landscape. Unfortunately, in this area the trace of the scarp is not straight, and in some areas deformation was spread over a zone tens of meters in width. In particular, several traces cross the ridge in the middle of the mapped area, near y = 90 m, and between x = 10 m and x = 35 m (Fig. 88). Thus, in that area, little quantitative analysis can be made. To quantify the vertical component, we constructed two profiles approximately perpendicular to the local strikes of the rupture and along the floors of the dammed valleys (Figs. A38b and 38c). The northern profile (Fig. A38b) near y = 120 m and west of the rupture crosses steep topography, and only a short continuation west of the fault could be mapped. Nevertheless, a component of 4.0 (±1.0) m with the west side up, similar to that inferred from the photograph of the area (Fig. 86), is clear. The southern profile, near y = 30 m (Fig. A38c), is also short, and ponding of sediment behind the scarp makes the projection of the profile to the scarp uncertain. We show a vertical component of 5 m (Fig. A38c), but the slopes on the west and east are different. Perhaps a better estimate of the vertical component is 6 m made from the photograph (Fig. 87). Defining the horizontal component is yet more difficult. Consider first the southern part of the mapped region (Fig. A38a). Deflections of contours are significantly greater on the southern slope of the valley, than on the northern slope, implying a right-lateral component of slip. A pair of profiles parallel to the trace are separated both vertically and horizontally by 2.8 m (Fig. A38d), which is less than the vertical offset estimated for the profile in Figure A38c. Correcting for the distance of

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12.1 m between the profiles and a slope of ~0.1 (Fig. A38a) yields a vertical component of 4.0 m, consistent with that of ~5 m inferred from the perpendicular profile (Fig. A38c). Thus, the relatively well-defined horizontal separation of 2.8 m seems to provide an accurate estimate of the right-lateral offset along this part. The strike of the rupture in the northern part of the mapped region is distinctly different from that in the southern part (Fig. A38a). Moreover, defining the strike-slip component is yet more difficult, in part because the shape of the offset ridge varies along the strike of the fault. Near the southern end of the profiles, the contours for z = 5 m to z = 9 m are nearly perpendicular to the fault trace, but near the northern end, they are oblique to the trace. Thus, at the southern end, we may determine the strike-slip offset simply by imposing a vertical separation of the profiles of 3.6 m and then matching these parts of the profiles (Fig. A38e). The match along those parts of the profiles yields a horizontal separation of 4.7 m, with an uncertainty of at least 1.2 m given simply by the ratio of the uncertainty in the vertical component (0.4 m) to the slope of the topography parallel to the trace (0.33). In the northern part of the profiles, however, the slope across the fault is steeper. As a result, matching the profiles to one another should be possible with less than 3.6 m of slip. A match is obtained with 1.8 m of vertical separation and 2.1 m of horizontal separation (Fig. A38f). The difference between 4.7 m and 2.1 m for the same profile is a statement of how uncertain this right-lateral offset must be. Accordingly, we report a range of values that includes both: 4.0 ± 2.0.

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Site 39 (on this and facing page). 44°50.01′N, 101°00.10′E, Z = 1,905 m. Toromhon Overthrust, northwest-southeast branch (Fig. 89). This portion of the Toromhon Overthrust trends southeast, unlike the main zone farther north. A major scarp with considerable deformation over a broad zone characterizes this portion. The scarp that we mapped (Fig. A39a) was chosen because of its relative simplicity (Fig. 89), but its crest is laced with tension cracks and disruption that makes the width of deformation greater than is common. Moreover, with the substantial uplift of the southwestern block, a gully has cut a deep channel into it in the center of the mapped area. A profile along the western margin of the mapped area (Fig. A39b) shows a large vertical component: 5.0 ± 0.4 m. Profiles constructed along y = 15 m and y = 32 m indicate a left-lateral separation of 3.0 ± 0.8 m (Fig. A39c). This separation is not well defined because the topography northeast of the fault differs from that southwest of it, but the channel on the southwest side is clearly offset left laterally from the deepest part on the northeast side. The block diagram (Fig. A39d) is viewed toward N90°W and 10° from the horizontal.

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Site 40. 44°49.09′N, 100°48.04′E, Z = 1,914 m. Middle part of the rupture that extends from Tsagaan Ovoo–Tevsh uul (Figs. 93 and 94). In 1993, we quickly measured a profile (but no map) across a scarp found on the aerial photographs. This scarp strikes approximately east-west for a short distance but seems to be curved, convex to the south. Where we measured the profile, the local strike is N115°E. The scarp seems to be cut into bedrock, and at this particular locality the vertical component of displacement in 1957 seems to have been ~2.5 ± 0.5 m (Fig. A40). The total vertical separation of 4.2 m may be the result of warping of the surface, or might be due to erosion of an older scarp. As discussed in the text, however, the height of the scarp decreases rapidly westward. Thus, assigning significance to any of the scarp heights is risky.

Surface rupture of the 1957 Gobi-Altay earthquake

Site 41. 44°49.9′N, 100°26.9′E, Z = 2,100. East end of the Gurvan bulag rupture (Fig. 95). We measured this site near the eastern end of the Gurvan bulag scarp where the vertical component is smallest. We saw no evidence to constrain the strike-slip component, which therefore seems to be <1 m. In the map of the region, note the closer contours near y = 15 m in contrast to those north or south of this band (Fig. A41a). A profile down the nose of a ridge in the middle of the area reveals a small vertical component of only 0.7 ± 0.2 m (Fig. A41b).

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Site 42. 44°49.8′N, 100°22.9′E, Z = 2,060. Near east end of the Gurvan bulag scarp and 5.3 km west of Site 41 (Fig. 95). We mapped an area between two gullies where the scarp is straight and relatively simple (Fig. A42a). No obvious strike-slip component could be recognized. Warping of the upper block just north of the scarp is suggested by a profile perpendicular to the scarp along x = 8 m (Fig. A42b). An extrapolation of the slopes above and below the scarp, however, demonstrates a vertical component of 1.5 ± 0.2 m. The block diagram (Fig. A42c) is viewed toward N20°W and 5° from the horizontal.

Surface rupture of the 1957 Gobi-Altay earthquake

Site 43. 44°50.1′N, 100°19.9′E, Z = 2,060. Near the center of the Gurvan bulag rupture (Figs. 95, 96, and 98). The scarp crosses a large, mildly dissected alluvial fan (Fig. A43a). We made no attempt to map details in the relief on the fan. Instead we measured only high points that seem to define the surface before dissection began in order to smooth minor relief over a large area. The profile along x = 32 m shows a vertical component of 4.0 ± 0.3 m (Fig. A43b). We saw no evidence of a strike-slip component, 0.0 ± 2.0 m.

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Site 44 (on this and facing page). 44°50.3′N, 100°18.8′E, Z = 1,940. Near the center of the Gurvan bulag rupture, 1.4 km west of Site 43 (Figs. 95, 96, and 99). A very spectacular scarp faces south where a small gully traverses the south front of a low east-west trending hill south of Ih Bogd (Fig. 99). Relief is steep, particularly where the gully crosses the scarp, and we made no attempt to map the details of relief there (Fig. A44a). Moreover, steep slopes east and west of the area mapped on the upthrown block and both pre- and post-1957 erosion of the downthrown block limited the area that we could map meaningfully. Our purpose was to constrain the vertical offset here, where it exceeds that elsewhere along the Gurvan bulag rupture. A profile along x = 11 m shows a vertical component of 5.2 ± 0.5 m (Fig. A44b), significantly higher than most portions of the fault, but not obviously different from other localities near this area. Again, no strikeslip component could be recognized (0.0 ± 2.0 m). The block diagram (Fig. A44c) is viewed toward N15°W and 5° from the horizontal.

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Site 45 (on this and facing page). 44°54.9′N, 100°06.1′E, Z = 1,880. In the eastern part of the Ölziyt Rupture (Fig. 100). We mapped a part of a clear scarp that crosses an alluvial fan (Fig. A45a). The height seemed typical of scarps seen east and west of this area for a few kilometers. A profile constructed to avoid a small gully in the middle of the fan shows a vertical offset of 1.9 ± 0.3 m (Fig. A45b). No evidence of a strike-slip component could be seen, either here or in other areas along this rupture: 0.0 ± 2.0 m. The block diagram (Fig. A45c) is viewed toward N35°W and 2° from the horizontal.

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Site 46 (on this and facing page). 44°57.4′N, 100°07.2′E, Z = 2,040. Northeast of the Ölziyt uul and southeast of the Bulagtay Valley (Fig. 101). We found a short rupture trending west-northwest in the valley between the Ih Bogd massif and the Ölziyt uul, where in the field it seemed that a right-lateral strike-slip offset had formed. Finding definitive evidence for a strike-slip offset proved difficult, in part because of a small vertical component. We mapped a section where a gully crosses the trace nearly perpendicular to the scarp (Fig. A46a). A profile perpendicular to the scarp defines a clear vertical component of 0.7 ± 0.2 m, with the southwest side up (Fig. A46b). Profiles constructed parallel to the scarp southwest and northeast of it (x = 20 m and x = 23 m) can be matched well without more than a negligible rightlateral strike-slip component: 0.1 ± 0.4 m (Fig. 46c). The block diagram (Fig. A46d) is viewed toward N152°W and 15° from the horizontal.

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