PALEOSEISMICITY OF THE GUOQIONG SEGMENT OF THE YUSHU FAULT FOLLOWING THE MW6.9 14 APRIL 2010 YUSHU EARTHQUAKE, QINGHAI PROVINCE, CHINA

The M_w6.9 (M_s7.1) April 14, 2010 Yushu earthquake and aftershocks devastated Yushu Tibetan Autonomous Prefecture (Qinghai Province, China), killed ~3,000 people, injured 12,000 people, and caused extensive damage throughout the region. The earthquake swarm occurred in the central region of the Tibetan Plateau, within the Banyan Har Mountain Range along the Ganzi-Yushu fault system. The focal mechanism for the main shock indicated left-lateral, strike-slip movement with two pulses of rupture along a WNW-ESE-striking, near-vertical fault causing surface ruptures to occur along several segments of the Yushu fault. In October 2011, a trench (CUG2011-1) was excavated (near the village of Guoqiong, 30km-NW of Jiegu town) across the Guoqiong segment of the Yushu fault which had a maximum of ~1.8m left-lateral displacement. The trench was excavated perpendicular to the surface rupture across a Late-Pleistocene alluvial-fan surface. A nearby Holocene stream channel, incised 3m into the fan, was deflected left-laterally ~6m indicating substantial strike-slip displacement since incision. Within the trench, 3 buried A-soil horizons within the downthrown side are regularly-spaced suggesting surface ruptures, associated with similar-sized or larger earthquakes, occurred prior to the 2010 earthquake at fairly regular intervals. Line-length balancing and progressive retro-deformation for the 2010 and 3 previous surface ruptures show ~2m of horizontal shortening perpendicular to the fault and ~1m of vertical displacement. If earlier events were of similar magnitude, then about 1.5m left-lateral displacement also occurred during each previous paleoseismic event. Thus these 4 events could account for the total 6-m of left-lateral displacement of the incised Holocene stream channel.