Structural model of 2008 Mw 7.9 Wenchuan earthquake in the rejuvenated Longmen Shan thrust belt, China

Abstract

The 12 May 2008 Wenchuan earthquake ( M w = 7.9) of China occurred in the Longmen Shan thrust belt and is a consequence of the ongoing uplift process of the eastern margin of Tibetan plateau. The coseismic surface ruptures coincide with the pre-existing thrust fault trace. Destruction of buildings and the location of seismically induced landslides are located in the hanging wall of the reactivated thrust faults. In order to detect any indication of coseismic deformation linked to subsurface structures, we interpret the subsurface fault and fold geometries using petroleum seismic reflection profiles, as well as coseismic surface ruptures and seismicity. Our data suggest that the deformation can be divided into two different structural segments related to the regional coseismic deformation during the Wenchuan earthquake. In the southern segment (the Yingxiu–Hongkou–Hanwang segment), two coseismic surface rupture zones coincide with two pre-existing thrust faults (the Yingxiu–Beichuan and Pengguan faults) in the seismic profiles and both coseismic active thrusts become incorporated into the deep main detachment. This through-going thrust fault connecting directly from the hypocenter to the surface break, but cannot be easily extended to the northern segment along the Yingxiu–Beichuan thrust fault zone. In contrast, only one shallow coseismic active thrust fault with oblique-slip occurs as the active passive roof of imbricate thrust sheets in the northern segment (Beichuan–Qingchuan segment). Our results show ramp-flat geometry and the wedge characteristics at the blind front of the rejuvenated thrust belt. We emphasize that there is a potential 15–17 km-deep main detachment associated with this large earthquake in the Longmen Shan belt, and infer that some active displacements along the 6–9 km depth shallow detachment have propagated into the Sichuan basin since late Cenozoic.