Source characteristics of long runout rock avalanches triggered by the 2008 Wenchuan earthquake, China Shengwen Qi a,b, * , Qiang Xu b , Bing Zhang c , Yuande Zhou d,1 , Hengxing Lan e , Lihui Li a,2 a Key Lab. of Engineering Geomechanics, Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Chaoyang Area, Beijing 100029, China b State Key Lab. of Geo-hard Prevention and Geo-environment Protection, Chengdu University of Technology, Chengdu 610059, China c Centre for Earth Observation and Digital Earth, Chinese Academy of Sciences, Beijing 100190, China d Department of Civil Engineering, The University of Hong Kong, Pokfulam Road, Hongkong, China e State Key Laboratory of Resources and Environmental System, Institute of Geographic Science and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China article info Article history: Available online 1 June 2010 Keywords: 2008.5.12 Wenchuan earthquake Rock avalanche Landslide abstract The May 12, 2008 Wenchuan, China Earthquake which measured M w = 8.3 according to Chinese Earth- quake Administration – CEA (M w = 7.9 according to the USGS) directly triggered many landslides, which caused about 20,000 deaths, a quarter of the total. Rock avalanches were among the most destructive landslides triggered by this seismic event, and have killed more people than any other type of landslide in this earthquake. The Donghekou rock avalanche, one example of a catastrophic avalanche triggered by the Wenchuan earthquake, occurred in Qingchuan and buried one primary school and 184 houses, result- ing in more than 780 deaths, and in addition, caused the formation of two landslide dams, which formed barrier lakes. Combining aerial images (resolution of 0.5 m) with field investigations, this paper lists some parame- ters of 66 cases in one table, and details source characteristics of six typical cases. It has been found that most of the long runout rock avalanches have source areas with high relief and steep inclination, causing the debris in the travel courses to accelerate. There was also a large amount of saturated Holocene-age loose deposits formed by a river or gully that existed in the travel courses. Comparison studies indicate that saturated Holocene loose deposits in the travel courses could be the most important factor for the causes of the long runout characteristic of the rock avalanches especially when they traveled over gentle or even flat ground surfaces. Furthermore, the relationships among the relief slope gradient, runout and covered area are investi- gated, and a threshold line for predicting the maximum horizontal runout distance under certain change in elevation is presented. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction A severe earthquake measuring M w = 8.3 according to Chinese Earthquake Administration – CEA (M w = 7.9 according to the USGS) occurred in Wenchuan County, Sichuan Province, in southwestern China at 2:28 pm on May 12, 2008, and caused more than 69,197 deaths, more than 374,146 injuries with about 18,340 missing, according to official statistics. The epicenter (30.986°N, 103.364°E) was 80 km west-northwest of Chengdu, the capital city of Sichuan, at a hypocenter depth of 19 km (Wang et al., 2009). The Longmenshan fault zone (Fig. 1) is a region with one of the steepest slope-gradient zones in the world. Within a horizontal distance of less than 50 km, the local relief can reach 4000 m (Yin et al., 2009a). Meanwhile, the stream systems are highly developed in this region, and the Minjiang and Fujiang rivers and some other rivers in this region are tributaries of the Yangtze River. The region is characterized by extreme rates of erosion, high mountains and deep valleys (Fu et al., 2009), and the Earth’s endo- genic and exogenic geological processes are both very active. Rock avalanches are landslides that disintegrate into streams of rock fragments that can travel several kilometers on slopes of a few degrees at velocities of hundreds of kilometers per hour, and are the most deadly landslides in terms of loss of life (Keefer, 1984a). The Wenchuan earthquake triggered enormous rock avalanches, and preliminary estimates indicate that there were about 10,000 1367-9120/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2010.05.010 * Corresponding author at: Key Lab. of Engineering Geomechanics, Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Chaoyang Area, Beijing 100029, China. Tel.: +86 10 8299 8055; fax: +86 10 62040574. E-mail addresses: qishengwen@mail.iggcas.ac.cn (S. Qi), xq@cdut.edu.cn (Q. Xu), zb@ceode.ac.cn (B. Zhang), yuande@hku.hk (Y. Zhou), lanhx@lreis.ac.cn (H. Lan), lhli2942@mail.iggcas.ac.cn (L. Li). 1 Tel.: +86 852 2857 8233. 2 Tel.: +86 10 8299 8610; fax: +86 10 62040574. Journal of Asian Earth Sciences 40 (2011) 896–906 Contents lists available at ScienceDirect Journal of Asian Earth Sciences journal homepage: www.elsevier.com/locate/jseaes