GEOMETRY AND KINEMATICS OF POTENTIALLY ACTIVE FAULTS IN CENTRAL MONGOLIA: IMPLICATIONS FOR THE SEISMIC SAFETY OF ULAANBAATAR

Four large earthquakes (M>7) have occurred in Mongolia since 1900. Fault-plane solutions and field observations indicate the earthquakes were either along NW-SE striking, right-lateral strike-slip faults or E-W striking, left-lateral strike-slip faults. These earthquakes fortunately occurred in sparsely populated regions and caused little damage to Ulaanbaatar, the capitol of Mongolia. The closest of these large earthquakes was located 300 km to the west of Ulaanbaatar (i.e., the 1967, Mogod earthquake, M = 7.1). Three larger earthquakes were located 600 to 800 km to the west and southwest of Ulaanbaatar (i.e., the 1905 Bulnay, 1905 Tsetserleg, 1957 Gobi-Altai, M>8 earthquakes).

An area of 100,000 km² in size, located between Ulaanbaatar and the four large earthquakes (M>7) was studied using state-of-the-art remote sensing techniques and geological field mapping to characterize the recent seismic activity and kinematics of previously misinterpreted and unidentified faults. Two sets of potentially active right-lateral and left-lateral faults, with similar orientations to the faults associated with the Mogod, Bulnay, Tsetserleg and Gobi-Altai earthquakes, have been identified within 200 km of Ulaanbaatar. Earthquakes generated by these nearby faults could adversely impact the many multi-story, soviet-era apartment buildings that serve as housing for the citizens of Ulaanbaatar. This investigation revealed clues of how the tectonic stress is accommodated between the historically large earthquake faults, located further to the west and southwest, and the potentially active faults located nearer to Ulaanbaatar. Besides a similar spatial orientation and kinematics of the two sets of faults, it was discovered that these faults terminate with similar fault geometries. The NW-SE striking, right-lateral strike-slip faults and E-W striking, left-lateral strike-slip faults terminate into extensional and compressional faults due to a change in strike of the faults. This observation indicates that there is a spatial and temporal relation between many of the strike-slip, extensional and compressional faults of central Mongolia.