The Role of Late Quaternary Upper-Crustal Faults in the 12 May 2008 Wenchuan Earthquake

The role of upper-crustal faulting in building and maintaining the extreme relief of the Longmen Shan region at the eastern margin of the Tibetan Plateau has been strongly debated. The M-w 7.9 Wenchuan earthquake of 12 May 2008 ruptured three distinct faults along the plateau margin and thus provides a unique insight into the mechanisms and kinematics of regional crustal deformation. Two of the faults, the northeast-striking Beichuan and Pengguan faults, had been previously recognized as active, with past surface-rupturing earthquakes in the latest Pleistocene to Holocene time. Some of the late Quaternary fault segments were activated in the Wenchuan earthquake, but others, including several with clear evidence of Holocene strike-slip displacement, were not. Instead, in almost all cases, the Wenchuan surface rupture followed geological faults mapped on the basis of bedrock lithology. We infer that active deformation along the plateau margin is accommodated by partial reactivation of a complex network of preexisting faults but that a single through-going structure has not been established-and may never be if the total deformation is strongly rotational and three-dimensional. The earthquake also illustrates that the Beichuan and Pengguan faults are kinematically linked, in part by the northwest-striking Xiaoyudong fault, and that both must be strongly listric in the upper few km of the crust. The complexity of the active (or potentially active) fault network in the Longmen Shan, coupled with rapid postearthquake modification of the surface rupture trace, poses serious difficulties for seismic hazard assessment across the region.