SEISMOGENIC STRUCTURE AND RUPTURE MECHANISM OF THE M_S 8.0 WENCHUAN EARTHQUAKE

A composite multiple seismogenic source model is presented to interpret the preparation and generation of the Wenchuan earthquake,based on the results of investigations on the coseismic surface rupture,aftershock distribution and crustal deformation of the Wenchuan earthquake and the studies on the tectonic deformation conducted before the earthquake. It is assumed that the differences in the structure and property of the lithosphere beneath the west Sichuan plateau,Longmen Shan and Sichuan Basin led to the differences in deformation behavior and stress accumulation between them,and their joint actions resulted in the high accumulation and abrupt release of stress in the Longmen Shan Fault zone,witch eventually generated the Wenchuan mega-earthquake. As a high-friction high-angle listric thrust,the Longmen Shan Fault zone is not liable to deformation(but high stress accumulation)to form slip zones before earthquake,or to micro-rupturing to form seismic gaps before earthquake. When stress accumulated exceeded the strength of the Longmen Shan Fault zone,burst occurred,creating the huge earthquake. Coseismic deformation and energy release were concentrated mainly in the Longmen Shan Fault zone to offset the pre-earthquake slip deficit and seismic gap.The high-angle listric thrust structure of the seismogenic fault of the Wenchuan earthquake played an important role in controlling the preparation and generation of the Wenchuan earthquake. The decrease of normal stress resulting from increased displacement rate on the fault plane was the major contributor to the rupture of the high-angle listric thrust fault.