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 ...

Author: R. A. Kurushin

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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.


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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.


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.

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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.)


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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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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.


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-

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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.)


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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 (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.


47

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.


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 (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.

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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

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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.

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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.)


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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).

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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


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.

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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


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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-

62


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.

64


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.

66


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:

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∆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.


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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

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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,

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