汶川地震极重灾区地质背景及次生斜坡灾害空间发育规律

5·12汶川大地震造成大量的次生斜坡灾害，本次研究区域为汶川大地震的11个重灾区，包括汶川、北川、青川、安县、平武、茂县、江油、彭州、什邡、绵竹、理县等市县。通过对重灾区航片、卫片、雷达图像的解译研究发现，重灾区次生斜坡灾害的主要灾种表现为崩塌、滑坡以及崩塌、滑坡高速运动解体形成的碎屑流（个别地方由于水的参与表现为泥石流）以及它们堵江形成的堰塞湖。研究发现地震次生斜坡灾害的发育具有明显的丛集性规律。从区域上看，次生斜坡灾害明显呈带状，沿龙门山断裂带展布，并主要受北川—映秀断裂控制。各灾种的发育在不同地段发育的规模、频率差别较大。以灾害分布面积来排序，汶川县灾害面积最大，为131.55km2，其次为北川县，为45.57km2，其余9个县（市）灾害面积相差不大，均介于6~17km2，其中理县灾害面积最小，为6.25km2。各灾种的发育在不同地段发育的规模、频率差别较大。青川县、平武县灾种主要为滑坡，汶川县、茂县、安县、理县灾种主要表现崩塌转化的碎屑流，北川的主要灾种则为碎屑流，其次为滑坡，什邡、彭州、绵竹、江油等地主要灾种为崩塌。
灾种发育的这种地域性差别主要受控于地层岩性，除此而外，还与构造特征、地形地貌等因素紧密相关。研究表明：岩性对灾害种类的展布有决定性控制作用。统计发现，岩性越坚硬，崩塌、碎屑流发育率越高，滑坡则在软岩地区、较软岩地区和较坚硬区发育率最高，泥石流则在软岩地区最为发育。地形地貌对次生斜坡灾害的发育有重要影响，统计表明，崩塌、碎屑流以及泥石流在1200～2000m坡段范围内发育率最高，其次为800～1200m坡段；而滑坡则在800～1200m坡段范围发育率最高。对坡度而言，除11°~20°坡度范围外，崩塌和碎屑流的发育率总体具有随坡度增高而增大的特点；而滑坡和泥石流的发育率呈现典型的单峰特征，在1°~20°范围内发育率最大。坡向对地震次生斜坡灾害的发育影响不明显。
地震次生斜坡灾害的发育规律表明，地震斜坡灾害的发生主要受控于活动构造本身，并沿活动构造呈带状展布，同时受场地条件如岩性、地形地貌等因素的强烈控制。

SLOPE INSTABILITIES IN THE SEVEREST |DISASTER AREAS OF 5·12 WENCHUAN EARTHQUAKE

The 5·12 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.