Paleoseismic Analysis of the Egiin Davaa Fault, Hangay Mountains,

Mongolia

Juliana Williams

Whitman College, Walla Walla WA 99362

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The Hangay Dome, a zone of upwarping and moderate extension in central Mongolia, is bordered by the Baikal Rift zone to the north, a system of left-lateral strike-slip faults to the south, and a system of right-lateral strike-slip faults to the west. The Egiin Davaa fault in the Hangay Mountains is a northeast-striking normal fault, with an upthrown southeastern block, typical of faults in the southern part of the Hangay Dome. The fault, at least 80 km long, consists of three strands which are progressively younger to the southwest. Only the southwesternmost strand ruptured in the last earthquake, producing a scarp with five segments. Of these segments, the dominant three strike at N61°E, and the two minor segments strike between N15°E and N20°E. The N61°E segments experienced almost entirely normal motion, whereas the two minor segments experienced oblique motion. The fault is associated with Quaternary basalts, including a complex of four small cinder cones located at one of the bends in the fault. The recent scarp crosses alluvial fans along most of its length, but where it cuts bedrock the estimated dip is 43° NW. The average displacement along the 42-km-long recent scarp is approximately 3 meters, with a range of 1 to 5 meters. Although previous researchers proposed that this scarp was produced by an earthquake 300 to 500 years ago, radiocarbon samples collected from two trenches across the scarp suggest that the last earthquake occurred between 4355 and 6840 radiocarbon years ago. Based on comparison with the older expression of the fault to the northeast, where glacial drift is offset by approximately 8 to 10 meters, the recurrence interval is estimated to be approximately 2 to 3 meters of displacement about every 5000 years.