Co-seismic rupture and displacement in the Xiaoyudong area produced by the 2008 Wenchuan earthquake, China, and its mechanism

The M_W 7.9 Wenchuan, China, earthquake is a large oblique reverse slip shock, whose main fault is dominated by reverse slip with right-lateral strike-slip component. It generated one of the longest and most complicated surface ruptures, and to many of the phenomena, we haven't had an appropriate interpretation or a common understanding, e.g. on the 7 km-long NW-trending Xiaoyudong Fault and the coinstantaneous fracture on the two parallel thrust faults which are 11 km apart on the north of Xiaoyudong area. Field investigation in the Xiaoyudong area shows clear co-seismic rupture and displacement, and on these bases, we analyzed the mechanism of the surface rupture in the Xiaoyudong area. Our study indicates that the change of attitude of Beichuan-Yingxiu Fault (BYF), that is, the ca. 3.5 km step-over in the west of the Xiaoyudong area, is the primary cause of the above complex phenomena. Specific mechanisms are as follows: 1) The dextral strike-slip of the BYF results in compressional uplift in the left-restraining step-over, creating a frontal reverse fault, known as the Xiaoyudong Fault.; 2) The Pengguan Fault, which is parallel to and 11km apart from the BYF, is activated in the north of the step-over by a combination of the increased dip angle in the north of the step-over due to the ca. 3.5 km left step of the BYF and the lateral push of the hanging wall to the footwall of the BYF caused by the dextral strike-slip of the BYF. These results are helpful in deepening our understanding of the dynamic processes that produced surface ruptures during the Wenchuan earthquake. We also suggest that more attention shall be paid to the impact of the dextral strike-slip component, the change of primary fault's attitude and the difference of the rocks of the fault's two walls on the process and distribution of surface rupture.