Four great and several moderate earthquakes have struck Mongolia and its surrounding area this century, leaving in their wake some of the most spectacular and well-preserved surface faulting in the world [Baljinnyam et al., 1993]. Last August 22 foreign scientists, hosted by a team of Mongolian scientists, explored the extraordinarily well preserved surface ruptures of two major Mongolian earthquakes: the 1967 Mogod earthquake (Mw = 7.0) and the 1957 Gobi-Altay earthquake (Mw = 8.1). The group studied not only extraordinarily well preserved surface ruptures, but also the geomorphology of terrain responding to a cold, dry climate, where erosion is slow and fault ruptures cut pediment surfaces. Mongolia is an excellent laboratory for studying the surface expression of infrequent earthquakes, such as those that occur in the eastern United States.
During the Mogod earthquake of January 5, 1967, strike-slip faulting occurred along a 36-km-long, north trending zone and terminated at its southern end in a 13-km-long southeast-trending thrust rupture. Rupture of apparently frozen ground left huge tension cracks and mole tracks (pressure ridges) that attest to large strike-slip displacement, quantified by one measurement of 3–3.5 m of right-lateral slip. The thrust scarp, which is more than 2 m high along much of its length, corroborated the inference of relatively large slip. Faulting appears to have followed older geologic (Paleozoic?) structures. Throughout much of its length, the rupture seems to have cut pediment surfaces with little sediment on them. Perhaps most remarkable is the nearly complete lack of geomorphic evidence for recent faulting. Participants speculated that the last major event occurred from thousands to tens of thousands of years ago.
The Gobi-Altay earthquake of December 4, 1957, known as the Ih Bogd earthquake in Mongolia [Florensov and Solonenko, 1963, 1965], stands out as one of the great intracontinental earthquakes. Extraordinary preservation of the surface rupture (Figure 1) allows precise measurements of surface displacements, even now, almost 40 years after the event [Baljinnyam et al., 1993; R. A. Kurushin, personal communication, 1995]. Left-lateral strike slip dominates faulting, but components of right-lateral strike-slip, thrust, reverse, and normal faulting occurred on subsidiary faults.
Fig. 1. Photograph looking west-northwest along a segment of the Gobi-Altay rupture, where 6 m of left-lateral and 5 m of vertical slip occurred along an unusually well-preserved fault surface. One workshop participant stands near the top of an offset ridge, another is at its upslope continuation, and another is at its base.
Participants examined strike-slip faulting in three areas along the Bogd fault, and thrust faulting associated with two structures. In its eastern part, strike slip of 3–3.5 m on the Bogd fault left a sharp trace. In the western part, two areas along the Bogd fault display clear strike-slip offsets of 5–6 m (Figure 1), and vertical offsets that vary in amount and sense of displacement along strike. Along the "Toromhon Overthrust" [Florensov and Solonenko, 1963, 1965], scarps from 3 to 6 meters in height attest to large slip, and scalloped surface traces across minor ridges, and valleys indicate a gentle northwestward dip. A low range of hills, the "Dalan Turuu foreberg," apparently formed above a thrust fault that crops out at the northern edge of the hills, several kilometers northeast of the Ih Bogd massif, the highest mountain of the Gobi-Altay. The thrust fault seems to be a splay from the main east-west trending, steeply dipping, oblique thrust/strike-slip fault responsible for the Gobi-Altay rupture. A growing anticline within the foreberg attests to such crustal shortening across the foreberg. Normal faulting across the anticlinal crest that occurred during the earthquake is consistent with active folding from 1957.
As with the Mogod earthquake, scarps from the Gobi-Altay earthquake cross pediment surfaces and even locally outcropping bedrock. Unlike the Mogod earthquake, however, evidence of a previous earthquake could be found in many areas along the fault trace. Some of the workshop participants dug and logged a trench across the rupture where abundant organic material—including a camel bone—may help to date the penultimate earthquake. Many participants speculated that earthquakes in this area may be fairly infrequent, as compared with earthquake activity in the western United States. They also agreed that the slip rates are probably closer to millimeters per year than to centimeters per year. Exposure dating of alluvial fans displaced along the main strike-slip segment of the Bogd fault suggest such low slip rates [Ritz et al., 1995].
The workshop was organized by the Center for Informatics and Remote Sensing of the Mongolian Academy of Sciences, which is based in Ulaanbaatar, Mongolia, under the direction of M. Ganzorig (ganzorig@icm.jinr.dubna.su) and was guided by A. Bayasgalan of that center. Mixing efficient morning and evening logistics with extensive freedom for day-time exploration and punctuated by the occasional, unscheduled encounter with local hospitality, the workshop also provided an unparalleled lesson in how to run a geologic field trip to a remote area.
The 22 foreign participants from 7 countries made several specific recommendations for future study in Mongolia. Because one quarter of Mongolia's ~2 million inhabitants live in its capitol, Ulaanbaatar, and earthquakes, although infrequent on any one fault, appear possible virtually anywhere, a concerted effort should be made to examine geomorphic evidence of past nearby earthquakes near Ulaanbaatar. This information can be used to evaluate seismic risk in the capitol. Second, the excellently preserved surface ruptures should be mapped and regional strains should be estimated. Accordingly, foreign agencies interested in intraplate earthquakes with long recurrence intervals should pursue studies in Mongolia as a natural laboratory for such rare events. The participants also recommended that a program using space geodesy should be launched to provide geodetic estimates for comparison with geologic estimates of strain rates.
Mongolia is an outstanding laboratory for studying the geomorphic response to cold dry climates that characterized much of the world for the last 2–3 m.y. The active faulting provides a record of tectonic processes, and studied together, tectonic and climatic geomorphology may provide constraints on both rates of deformation and climate change. Finally, the hosts in Mongolia were encouraged to lead more such field trips, though it is hard to imagine how this one could have been improved upon.
Florensov, N. A., and V. P. Solonenko (Eds.), The Gobi-Altai Earthquake (in Russian), Akad. Nauk USSR, Moscow, 1963 (English translation, U.S. Dept. Commerce, Washington, D.C., 424 pp., 1965).
Ritz, J. F., et al., Slip rates along active faults estimated with cosmic ray exposure dates: Application to the Bogd fault, Gobi-Altay, Mongolia, Geology, 23, 1019, 1995.
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