Distribution of Active Faults and Recent Earthquake Ruptures along the Gobi-Altai Active Fault Zone, Southern Mongolia

The Gobi-Altai earthquake of 4 December 1957 in southern Mongolia is one of the largest recorded intracontinental earthquakes in the world. The surface ruptures associated with this M8.3 earthquake extend for about 260 km long and 40 km wide, involving strike-slip and reverse faulting (Kurushin et al., 1997). Earthquake ruptures appeared along a limited portion of the entire Gobi-Altai fault zone. West of the 1957 rupture zone, surface ruptures during historical and prehistrical earthquakes have been reported. Khil?fko and others (1985) reported surface deformation along the southern slope of the Bayan Tsagaan Mountains produced during the earthquake of 7 April 1958. They also identified two more Holocene rupture zones: west-northwest of the Bayan Tsagaan rupture near the village of Chandman, and west-northwest of the Chandman rupture near the village of Myangayn. However, previous studies do not discuss relationship between the distributions of active faults and earthquake ruptures along the entire active fault zone, because the exact location of the pre-existing active fault traces has not been mapped on large-scale maps. We have made distribution map of active fault traces based on interpretation of stereo-pair CORONA satellite photographs, which were taken between 1959 and 1972 for military intelligence during Cold War period. We also observed fault scarps in the field and made geomorphic profiles across the fault scarps. We were able to detect a lot of recent surface ruptures including the 1957 earthquake rupture, the Bayan Tsagaan, Chandman and Myangayn ruptures along the active fault traces. Fault scarps of these ruptures cross fluvial terraces of late Pleistocene as well as present riverbeds continuously. This indicates some of the active faults recently ruptured during a relatively short period. On the other hand, there exists an approximately 100-km-long portion between the 1957 and Chandman ruptures that has not displaced recent fluvial terraces, indicating that this portion did not recently rupture, probably during the 19th or 20th century.