Earthquake Geology of the Bogd Fault, Gobi Altay, Mongolia

The Gobi-Altay earthquake of December 4, 1957, in southern Mongolia, is one of the largest recorded intracontinental earthquakes and one of four great earthquakes that occurred in this region during the 20th century. The rupture pattern associated with this M8 (published magnitude determinations range from 7.8-8.3) earthquake is complex, involving strike-slip and reverse faulting on several distinct geological structures in a zone about 260 km long and 40 km wide. The principal fault involved in this earthquake is the Bogd Fault, a left-lateral, strike-slip fault that ruptured for a distance of more than 260 km, with reported offsets up to about 6 meters. Earlier workers mapped the surface rupture in detail and conducted preliminary paleoseismic research on several of the faults involved in the rupture. Our new work is focused on acquiring new slip-rate data for the Bogd Fault, determining the ages of prehistoric earthquakes, and analyzing topographic features associated with the rupture to better characterize the slip vectors of the 1957 and earlier earthquakes. We conducted a kinematic GPS survey to measure offset risers on a flight of fluvial terraces in the western part of the 1957 rupture, and collected samples for 10Be analysis to determine terrace ages. At another site farther to the east near Noyan Uul, we collected additional samples for 10Be analysis to constrain the age of an abandoned alluvial fan surface offset from its source by 235 ± 35 m. We also surveyed offset terrace risers near the Bitut River and collected OSL samples to constrain their ages. We conducted paleoseismic studies at four sites and found evidence for the penultimate event at three of these sites, and evidence for three pre-1957 events at two sites. Analysis of OSL and radiocarbon samples from these paleoseismic sites will allow us to constrain the ages of these three prehistoric earthquakes. Analysis of detailed topographic data at several sites suggests the amount of slip on the Bogd Fault has been characteristic on the different fault segments over several earthquake cycles. In addition, these studies will allow us to better constrain the slip rate and the average time interval between earthquakes on the Bogd fault.