SLOPE INSTABILITIES IN THE SEVEREST |DISASTER AREAS OF 512 WENCHUAN EARTHQUAKE

The 512 M8.0 Wenchuan Earthquake induced a lot of slope instabilities (including landslide reactions). Study areas of this paper are 11 severest disaster counties including Wenchuan, Beichuan, Qingchuan, Anxian, Pingwu, Maoxian, Jiangyou, Pengzhou, Shifang, Mianzhu, Lixian. The geological settings of the areas including it's the topography and geomorphology, geologic structure, and lithology are detailed in this paper. The interpretation and study on aerial photographs, satellite images and radar images of the areas show that slope instabilities occurred in the forms of rock fall, landslide and debris flow. The main form of slopes instabilities was debris flow, which was resulted from high-speed movement of rock fall mass or sliding mass. Sometimes, debris flows turned into mud rock flows if there were water participation. Barrier lakes formed when slope instability plugged up rivers. It is found that the distribution of slope instability has the obvious feature of cluster grouping. Slopes instabilities were all located along the Longmen Shan fault zone, and especially controlled by the Beichuan-Yingxiu Fault which is one of the three sub-faults of Longmen Shan fault. According to the distributed area of slopes instabilities, Wenchuan ranks the first with area of 131.55 km2. Beichuan ranks the second with area of 45.57 km2. The other nine counties (or cities) with similar areas varied from 6 km2 to 17 km2, where Lixian had the least area of 6.25 km2. Each type of slope instability has different scales and frequencies in different counties. Landslide is the main form of slope instability in Qingchuan and Pingwu. Debris flow is the predominant form in Wenchuan , Maoxian, Anxian and Beichuan. Rock fall is the predominant form in Shifang, Pengzhou, Mianzhu and Jiangyou. The differences mentioned above are principally controlled by stratigraphy and lithology, and also have close relationship with geological structure, topography, and geomorphology. It has been shown that the lithology has crucial influence on the distribution of different form of slope instability. After GIS based statistical analysis, it is concluded that: (1) The harder the lithology is, the higher the incidence ratio of rock falls and debris flows is; (2) The incidence ratio of landslides is higher in the areas with soft rock, secondary soft rock and secondary hard rock; (3) The incidence ratio of mud rock flows is highest in the areas with soft rock. Topography and geomorphology have an important influence on the distribution of different form of slope instability. It is indicated that the rock fall, debris flow and mudflow have the highest incidence ratio in slopes with an elevation from 1200 m to 2000 m. Landslides have the highest incidence ratio in slopes with an elevation from 800 m to 1200 m. Incidence ratio of rock fall and debris flow increases with slope gradient increasing except 11 to 20. However, incidence ratio of landslides and mudflow presents typical single peak characteristic, becomes highest in the areas with slope gradient from 11 to 20 . Statistics indicated that the influence of slope aspect is not obvious on the slope instability induced by the earthquake.