FEATURES OF LANDSLIDES TRIGGERED BY THE WENCHUAN EARTHQUAKE

Over 15,000 landslides were triggered by the Wenchuan Earthquake along the Main seismic fault and back fault of the Longmen Mt.tectonic belt.The area density of landslide coverage,i.e.,the ratio of the landslide distribution and total area,is greater than 50 % in average and 70% in maximum along the Yingxiu-Gaochuan-Chenjiaba-Nanba of the main seismic fault.The enormous loosen rock masses easy for occurring landslides and debris flows will seriously threat the reconstruction and living,especially,during raining seasons.The largest landslide triggered by the Wenchuan Earthquake is located at the Daguangbao,Anxian County,of the main seismic fault.The volume of the Daguangba landslide is 1.1 billion m3,and 4,500 m long from scarp to toe,2,800 m long,1,700 to 2,200 m wide and 200 m in average thickness,and 580 m in max thickness for accumulation.It is also the maximum volume of single landslide in China.Much differences with other conventional landslides,the sliding surfaces are common incomplete and discontinued,and the exits were unclear in the epicenter area of the Wenchuan Earthquake,which illustrates the upper rock mass was strongly vibrated and separated,then thrown and crushed with the bedrock.The five patterns of the landslides are classified as staircase-shaped,convex-shaped,bowel-shaped,slump(vibrated-expansive)-shaped and huge rock-stone in a broad sense.The strong ground motion records and destroyed house cases present that the vertical seismic load is dominated within the epicenter area,i.e.,the vertical is greater than the horizontal of seismic load.It provides an explanation why lots of landslides experienced the processes of broken-thrown-crushed-high speed debris flow.