Abstract:

The 2008 Wenchuan Ms 80 earthquake has produced two long NE trending rupture zones as well as a short NWstrike rupture zone on the surface. The latter one is consistent with the Xiaoyudong fault and the distribution of aftershocks, and has been paid less attention. We speculate that this NWdirected fault may play a certain role during the main shock process. To test this idea, we established a seismic source model comprising three faults, the NE trending BeichuanYingxiu fault, the Pengguan fault, and the NW trending Xiaoyudong fault, and calculated Coulomb failure stress change during the source rupture. We considered two cases for the model. One is that the southwestern section of the NE directed BeichuanYingxiu fault served as the initial rupture. And the other is that the NW trending Xiaoyudong fault ruptured first during the main shock process. Under the first condition, most of the Xiaoyudong fault was inhibited by 15 bar Coulomb stress from failure (the stress shadows), though its northwestern section was in the stress trigger region. With the rupture of the BeichuanYingxiu fault spreaded toward NE, the stress shadowed on the Xiaoyudong fault became larger. If such a process was true, then the Xiaoyudong fault should had been stable during the main shock of the Wenchuan Ms 8.0 event and no surface ruptures and small aftershocks would have occurred along this NW trending fault. Evidently this is in contrary to the fact. Under the second condition, the Xiaoyudong fault ruptured first, and then other faults failed successively. In response to the initial rupture on the Xiaoyudong fault, most of the southwestern section of the BeichuanYingxiu fault was in the stress trigger region of 1015 bar, and the parallel Pengguan fault was also brought closer to failure, implying the whole source fault system started to rupture toward northeast, in agreement with the observations. Thus, this study suggests that the NW trending Xiaoyudong fault should have been the initial rupturing member that triggered the NE directed BeichuanYingxiu fault and Pengguan fault to fail leading to the main shock.

Key words: Wenchuan earthquake, NW trending Xiaoyudong fault, Coulomb stress change, initial rupture