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Abstract The highly seismic region of Wenchuan earthquake locates in the Longmeng mountainous area，due to the landform condition of deep valleys，the earthquake induced a large numbers of landslide and collapse disasters. The disasters caused serious damage to the roads along the river valley. After the earthquake，site investigation on earthquake-induced landslide and collapse had been conducted along the highway with a total length about 6 056 km. According to geologic tectonics and seismic intensity，the studied area is divided into different sections. In this paper，the seismic landslide and collapse characteristics of each section is summarized and their relationship with the tectonic position，seismic intensity and lithology，the density and average scale of the geological hazards are calculated. According the density and the scale，the whole area is divided into five subareas，i. e. the strongest-developed area，the stronger-developed area，the strong-developed area，the medium-developed area and the less-developed area. The development rules of the earthquake-induced landslide and collapse are shown as follows：(1) Three deep fault zones controlled the distribution of hazards. The Dujiangyan—Zhuyuanba section of the front fault is the clear boundary of less-developed area of the footwall and medium- and strong-developed areas of the hanging wall；but there is not clear boundary at the NE direction of Zhuyuanba. There are significant differences on the two sides of the central fault from Yingxiu to Donghekou. The two strongest-developed areas are all on the hanging wall and strictly limited by the back fault and lithologic boundary；but the boundaries is not clear at the NE direction of Dongkehou. The Maoxian—Wenchuan fault，which is one of the back fault，is the clear boundary of the strongest-developed area and stronger-developed area. The Qingchuan—Pingwu fault，which is one of the back fault，shows a clear effect of hanging wall at the Qingchuan to Shazhou section. (2) There are significant differences among different lithologies. The hazards with the highest density and average largest scale occur in intrusive rocks and hard sedimentary rocks like limestone and dolomite，while those with the lowest density and smallest scale occur in phyllite. Sandy slate and petroclastic rock are in the middle level. (3) There are significant differences among different landforms. The steeper and greater the relative altitude of salley slope is，more developed the disasters are. The positions of steep slope and hard rock are the high incidence area of seismic landslide and collapse；and the failure position of slopes mainly distribute on the upside of slopes，near the point of gradient change. (4) The statistical results of 399 geological sections show the slope gradients of collapse are larger than 40°. (5) Under dynamic conditions，the slope structures have important controlling effect on the development of geological hazards. Generally，soil slopes，strong-weathered rock slopes and slopes with outward-inclined discontinuities are easier to failure.
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Received: 23 March 2011
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2011, Vol. 30 
