Rock avalanches triggered by oblique-thrusting during the 12 May 2008 Ms 8.0 Wenchuan earthquake, China

The 12 May 2008 Wenchuan earthquake (Ms 8.0) triggered over 56,000 landslides over a broad area, including shallow, disrupted landslides, rock falls, deep-seated landslides, and rock avalanches. Among these landslides, rock avalanches are of great concern because of the large volume of displaced material, the long runout distance and the resulting fatalities. In this paper, some rock avalanches (including the Woqian, the Tangjiawan, the Hongyancun, the Xiejiadianzi, and the Shuimogou rock avalanches) were selected for detailed study. Mapping of coseismic surface rupture has been carried out at regional and site scales. The minor scarps or stepped topographic features observed at the source areas of the rock avalanches correspond spatially to the surface rupture, indicating that they should be created by thrusting of a fault that ruptured during the 2008 Wenchuan earthquake. Avalanche deposits of some rock avalanches are shifted northeastward, the same direction with the strike–slip of the fault rupture, compared with the sources of the rock avalanches, indicating that the component of strike–slip of the fault rupture exerted a horizontal initial velocity on the failed mass. For the Woqian rock avalanche, the source rock on the hanging wall was pushed northeastward, creating a hummocky topographic feature because of the right-lateral slip of the fault rupture. These rock avalanches are, therefore, triggered by oblique-thrust faulting. In addition, the source rock mass was of poor quality resulting from the shearing and brecciation associated with fault movement in history, rendering the rock slopes prone to failure and contributing to the high degree of disintegration of the failed rock mass.