Rock avalanches triggered by oblique-thrusting during the 12 May 2008 Ms 8.0 Wenchuan earthquake, China F.C. Dai a, ⁎, X.B. Tu a , C. Xu a , Q.M. Gong b , X. Yao c a Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029, PR China b College of Architecture and Civil Engineering, Beijing University of Technology, Beijing 100022, PR China c Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing 100081, PR China abstract article info Article history: Received 20 November 2010 Received in revised form 20 May 2011 Accepted 23 May 2011 Available online 12 June 2011 Keywords: Wenchuan earthquake Rock avalanches Surface rupture Oblique-thrust faulting 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. © 2011 Elsevier B.V. All rights reserved. 1. Introduction At 14:28 (Beijing time) on 12 May 2008, the surface wave magnitude (Ms) 8.0 Wenchuan earthquake, of which the epicenter is located at 31.0° N. and 103.4° E. with a focal depth of 14 km (CENC, 2008), occurred in the Longmenshan mountain region of China, the transition zone between the Tibetan Plateau and the Sichuan basin, resulting in extensive damage (Fig. 1). It was estimated that the earthquake claimed 69,227 lives, injured 374,643 people, and 17,923 listed as missing; ~ 20,000 deaths were as a direct result of landslides (Yin et al., 2009). The total number of landslides mapped from aerial photographs and satellite images for the earthquake is over 56,000 (Dai et al., 2011) and will underrepresent those landslides with size below the resolution of the images used for identification. After the Wenchuan earthquake, a number of researchers have published their work associated with some large rock avalanches, containing estimates of volumes, distances, and possible mechanisms for long runout (e.g., Huang, 2009; Huang and Li, 2009; Wang et al., 2009; Yin et al., 2009; Chigira et al., 2010; Lourenco et al., 2010). Strong ground motion generated by the earthquake and topographic amplification of strong motion were emphasized as the trigger of the rock avalanches (e.g., Huang, 2009; Yin et al., 2009). Buildup of excessive pore water pressure generated by undrained loading in loose deposits filled valleys (Wang et al., 2009; Chigira et al., 2010) were stressed to be responsible for the high mobility of the rock avalanches. Based on the field observations, we found that oblique- thrust faulting should be a controlling factor in the initiation of some rock avalanches, the source areas of which were traversed by the surface ruptures. The spatial relationship between the source areas of the rock avalanches and the surface rupture and topographic characteristics of landslide deposits near the oblique-thrust fault are thus important aspects in the analysis of the triggering mechanism of rock avalanches. This paper attempts to summarize the main characteristics of some rock avalanches possibly triggered by oblique-thrust faulting during the 2008 Wenchuan earthquake and to unravel the triggering mechanism of these rock avalanches. We briefly describe the tectonic and geologic settings, surface ruptures associated with the Wenchuan Geomorphology 132 (2011) 300–318 ⁎ Corresponding author. Tel.: + 86 10 82998319; fax: + 86 10 62010846. E-mail address: daifc@mail.iggcas.ac.cn (F.C. Dai). 0169-555X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2011.05.016 Contents lists available at ScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph