Abstract: Xiaonanhai landslide in Chongqing city is a high landslide occurred in relatively low seismic intensity regions, its causes have been puzzled. This paper is based on the related research materials of Qianjiang Xiaonanhai, through the recovery of the Xiaonanhai slope instability analysis, to calculate the seismic force’s critical conditions that make rock mass collapse sliding. Only after the topographic ampliflcation effects of seismic waves can landslide start. In order to further validate the calculated conclusions, by using UDEC software, a typical two-dimensional space model of the Xiaonanhai is established. Applying the corresponding seismic force on the slope to simulate the whole process of landslide, as to research in the mechanism that a steep slope parallel to the surface of the structure controlling the high rocky slopes triggered by topographic amplification effect under earthquake. The results show that, in the process of seismic wave propagation, the high and steep terrain with a parallel slope controlled by steep structure plane, have a significant amplified effect on seismic wave. Amplify peak acceleration-the increasing amplitude forces rock mass along the controlling structure planes which parallel with slope to crack quickly-slip along the gently dipping levels-high-speed detach from the slipping source region-huge potential and kinetic energy drive rock mass to do long-distance movement; meanwhile, accompanied by disintegration, collisions between particles and scraping effect, even with two-phase or three-phase fluid properties, which is the movement mode of this slope. Analysis reveals that the particle acceleration in the slope body has terrain amplification effects under the action of seismic force. Comparing the speed amplification factor of monitoring points between structural surface and bedrock, it is proved that the slide starts with a greater acceleration, when faces with discontinuities structural surface which parallels, to the slope, the dynamic response of slope becomes strong and eventually lead to slope’s instability.

Key words: Xiaonanhai seismic landslide, gently inclining slope outside rock landslide, formation mechanism, discrete element simulation