Rediscussion on the geodynamics of the 2008 Wenchuan earthquake: New seismic reflection evidence for crustal structure of the Longmenshan

New evidence concerning the middle and upper crustal structure of the Longmen Shan range where the May 12, 2008 M7.9 Wenchuan earthquake occurred is presented on the basis of seismic reflection surveys. The results of seismic reflection profiles clearly show a relatively complete stratigraphic sequence of the Upper Yangtze Basin since the Cambrian. Folding deformation, along with movements along numerous detachment layers, took place in the strata among the Wenchuan-Maoxian Fault (WMF), the Yingxiu-Beichuan Fault (YBF), and the Guanxian-Anxian Fault (GAF). The WMF is steep in the shallow strata, turning gentler gradually from a depth of 4 km, and more so at depths beyond 7 km. The WMF imbricates with the YBF and GAF, and they do not converge within the top 15 km of the crust. This interpretation disagrees with the viewpoint that the YBF and GAF extend with subvertical trend to a depth of 20 km with a robust relationship between the hypocenters and the subsurface faults and fold geometries. We conclude that the sub-horizontal detachment layers where most aftershocks occurred are the major causative fault, instead of the YBF. The YBF might be an earthquake propagating fault according to our seismic reflection data and the distribution of aftershocks. The overlying rigid and rootless Pengguan complex (similar to 3-4 km) might aggravate the damage to the YBF. Furthermore, there exists a new high-angle, listric structures vertical coseismic fault, which is able to link the seismogenic detachment layer and the shallow YBF together and resulted in huge damage of the YBF.