Neotectonics of Eastern Tibet and Its Control on the Wenchuan Earthquake

Based on satellite imagery interpretation, land relief amplitude analysis, and field observations, this paper discusses the tectono-morphologic background, neotectonics evolution history and active fault characteristics of eastern Tibet. It is proposed that different blocks in the eastern margin of Tibet-Qinghai Plateau successively extruded eastward during later Cenozoic and the extrusion can be divided into four stages: extrusion of the Chuan-Dian block eastward in early Miocene; intense extrusion of the Chuan-Dian block eastward again in late Miocene; eastward extrusion of the Qing-Chuan block from early Pliocene to early-middle Pleistocene; and tectonic change stage since late Pleistocene to Holocene. Accordingly, morphological boundary of east Tibet underwent change from west to east and from south to north. Latest research results on the active faults and GPS measurements display the kinematic features of East Kunlun-Mingshan-Longmen Mountain strike-slip system. Based on the field survey of earthquake rupture, combined with analysis of ground rupture features in the Wenchuan area, the study proposes a model for seismogenic structure of the Wenchuan earthquake. This study also indicates that almost half speed of movement of an NW-trending wedge-like block toward east was absorbed by the Xianshuihe fault and its westward extension structure. However, the motion of Longmenshan structural belt toward east was likely stopped by the rigid block beneath the Yangtze plate, resulting in that the Longmenshan rapture occurred in a setting of low strain and high stress; thus long-term strain-stress accumulation triggered energy releasing by abrupt thrusting of the steeply west dipping rapture eastward. Therefore, the focal mechanism for the Wenchuan catastrophic earthquake is worth further studying.