Paleoseismic Analysis of the Egiin Davaa Fault, Hangay Mountains,
Whitman College, Walla Walla WA 99362
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.