1997 USGS Expedition to Mongolia
Paleoseismic Study of the Gurvan Bulag Thrust Fault: Gobi Altay, Mongolia
- A. Bayasgalan (Informatics Center, Ulaanbaatar, Mongolia and Cambridge University, UK; bayas@esc.cam.ac.uk)
- Kelvin Berryman (IGNS, New Zealand; k.berryman@gns.cri.nz)
- Katherine Kendrick (USGS, Menlo Park, CA; kkendrick@isdmnl.wr.usgs.gov)
- A. Orgil (Technical University, Ulaanbaatar, Mongolia)
- Carol Prentice (USGS, Menlo Park, CA; cprentice@usgs.gov)
- Jeff Ritz (CNRS, Montpellier, France; ritz@sajou.dstu.univ-montp2.fr)
Please send comments or questions to Carol Prentice, US Geological Survey, at cprentice@usgs.gov
Introduction
The Gobi-Altay earthquake of December 4, 1957, in southern Mongolia, is one of the world's largest recorded intracontinental earthquakes, and one of four great earthquakes that have occurred in or near Mongolia this century (Baljinnyam et al., 1993) (Figure 1).
The ruptures associated with this event are well preserved due to the sparse population and cold, arid climate of Mongolia, and remain spectacular (Figure 2A, Figure 2B).
Figure 2A: View of the 1957 surface rupture along the Bogd fault near Ulaan Bulag, Mongolia. Kelvin Berryman for scale. (Photo by C. Prentice, 1995)
Figure 2B: Left-lateral offset along the Bogd fault produced during the 1957 earthquake near Ulaan Bulag, Mongolia. Piercing points along small ridge line offset about 5 m illustrated by Peter Molnar and David Schwartz. (Photo by C. Prentice, 1995)
This rupture is remarkably complicated, involving strike-slip and thrust faulting on several distinct structures within a zone about 250 km long and 40 km wide (Kurushin et al., in press) (Figure 3).
Our work during the 1997 field season focused on one of these structures: the Gurvan Bulag thrust fault (Figure 4A, Figure 4B), a 23-km-long thrust rupture on the southern flank of Ih Bogd, the highest peak of the Gobi Altay.
Figure 4A: Satellite image showing Gurvan Bulag thrust fault disrupting alluvial fans south of Ih Bogd, Mongolia. Blue region at top (north) of image is snow-covered summit plateau of Ih Bogd. Image processed by Informatics Center, Ulaanbaatar, Mongolia.
Figure 4B: View looking north at scarp associated with Gurvan Bulag thrust fault. Bill Lund stands on top of scarp for scale. (Photo by C. Prentice, 1996)
This rupture is subparallel to and about 30 km south of the main, left-lateral strike-slip fault (the Bogd fault). Scarps produced during the 1957 earthquake up to 5.2 m high have been reported along the Gurvan Bulag zone (Kurushin et al., in press).
The complicated rupture of the Gobi-Altay earthquake has been cited as a possible analog for a future earthquake in southern California involving simultaneous rupture of the San Andreas, Sierra Madre - Cucamonga, and other nearby fault zones (Bayarsayhan et al., 1996; Kurushin et al., in press). It also serves as a possible analog for future ruptures in other intracontinental regions such as the New Madrid Seismic Zone of the midwestern U.S. Important questions regarding the Gobi-Altay rupture include the timing of pre-historic earthquakes, whether the rupture complexity seen in 1957 is typical of past earthquakes generated by this segment of the Bogd fault, and whether the temporal cluster of great earthquakes in the larger Mongolia area this century is typical of seismic behavior in this region.
Previous Studies
Immediately after the 1957 earthquake a team of Russian and Mongolian geologists began studies of the rupture zone (Florinsov and Solonenko, 1965). This work was recently revisited and expanded upon by Baljinnyam et al., 1993, who studied a number of the active faults in Mongolia, and by Kurushin et al., in press, who conducted exceptionally detailed field mapping along the Gobi-Altay ruptures. A maximum slip rate along the Bogd fault of 1.2 mm/yr has been estimated based on the offset of alluvial fans dated using 10Be, suggesting long (on the order of 5000 years) recurrence times (Ritz et al., 1995).
Several of us (A.B., K.K., A.O., C.P., and J.R.) participated in last year's paleoseismic research along the Gobi-Altay ruptures (Schwartz et al., 1996; also see EQ-Geonet report sent out by D. Schwartz 9/13/96). Studies conducted during the 1996 expedition will provide constraints on the timing of the penultimate and pre-penultimate earthquakes along the Bogd fault and associated Dalan Türüü thrust. Although the 1996 expedition spent a limited amount of time along the Gurvan Bulag rupture, they were not able to collect data to determine the age of prehistoric earthquakes along this structure.
