Preliminary study of coseismic fault gouge occurred in the slip zone of the Wenchuan Ms 8.0 earthquake and its tectonic implications

Intensive coseismic surface deformation was produced by the 12 May 2008 Wenchuan Ms 8.0 earthquake along the pre- existing NE-trending Yingxiu-Beichuan and the Guanxian-Anxian faults,in the Longmen Shan fault zone.The post-earthquake geological investigation shows that the most of surface ruptures occurred along the carbonaceous argillite,shale and coal-bearing layers, which are low-frictional layers to form slip interface within the fault zone.We found that fresh fault gouges with a thickness of 1～2 cm occurred along the co-seismic surface rupture zone,which suggests that the coseismic slip concentrated in a narrow slip zone of thickness ranging from 0.5～2 cm although the coseismic vertical and horizontal dislocations are larger than 6 meters.These fresh fault gouges within co-seismic slip zone are resulted from both thermal pressurization and frictional melting during rapid faulting movement at the shallow crust level.