SURFACE RUPTURES ASSOCIATED WITH THE 14 APRIL, 2010 YUSHU EARTHQUAKES, EASTERN TIBETAN PLATEAU, QINHAI PROVINCE, CHINA

Surface ruptures were associated with temporally and spatially separate earthquakes of the swarm of earthquakes near Yushu, China. The main earthquake (M_S7.1; M_W 6.9;17.0-km fixed depth), which occurred at 07:49 am local time on 14 April, 2010 (23:49:39 UTC on 2010-04-13) and/or a large aftershock (M_W5.2; ~10.0-km depth) which occurred 23 minutes later (00:12:25 UTC on 2010-04-14) resulted in surface rupture for ~30-km along a N52W-trend. Field examination of offset fences, roads, streams, alluvial fans, and terraces near the SE-terminus of the rupture near Guoqiong documented progressive SE-decrease of left-lateral, strike-slip displacement from ~1.75-m to <0.1-m over a 1-km distance. Orientations of en echelon ground cracks and soil-horizon folds are, respectively, consistent with their formation normal to the extensional (S_hmin) and contractional (S_Hmax) axes related to these earthquakes and the observed left-lateral displacement along the surface rupture.

En echelon ground cracks and soil-horizon folds also occur along an ~N20W-trending zone in the foothills south of Yushu extending SE ~10-km from a knob (located ~2-km due south of and overlooking the city center) across the river to near Shangushi where ~0.5-m reverse displacement on a steeply SW-dipping fault rapidly dies out. This surface rupture is near an earthquake (M_W5.3; ~10.0-km depth) which occurred 12 minutes after the main event and ~20-km to the SE. An ~N20W-trending slight bulge with en echelon ground cracks along most of it length is consistent with displacement along a reverse fault with a left-lateral component along most of the length. However, at the knob, overlooking Yushu, where surface effects end, orientations of en echelon ground cracks and soil-horizon folds are, respectively, consistent with their formation normal to the extensional (S_hmin) and contractional (S_Hmax) axes related to a short right-lateral strike-slip fault. Thus surface ruptures, related to the Yushu swarm, appear to have developed during earthquakes along 3 different types of faults: a near-vertical, left-lateral strike-slip fault related to the main event and/or Guoqiong aftershock; a SW-dipping reverse fault with a left-lateral component related to the Machang earthquake; and a short, right-lateral fault on the knob.