“5·12” Wenchuan large earthquake has the characteristics of high seismic magnitude,strong destructive, and serious secondary geological hazard. Through the field geological investigations of geohazards in Tang Jiashan dammed lake(Beichuan to Yuli) before and after the earthquake, it was found that the amount of rockfall,landslide,debris flow changed into 7,51,10 from 5,3,0 separately after earthquake;geohazards development density and modulus changed into 1.35 and 104 from 0.26 per kilometer and 460 thousand m3/km.The effecting of slope failure induced by earthquake was quite significant. Based on the field geological investigations and subsection analysis, the following recognitions were obtained which induced:1) The distance to triggering seismic fault is shorter and geohazards devel opment density is higher, as far as Tang Jiashan dammed lake(Beichuan to Yuli) is concerned, the earthquake geohazards development strongly within limits of 3 km distance to the hanging wall of triggering seismic fault, middle development within limits from 3 km to 6 km, weakly development within limits from 6 km to 20 km, it that the distance beyond 20 km has not influence. 2) The earthquake geohazards mainly development in oblique~bedding bank slope and the geohazards mainly are large and huge landslide, and river blocking up happens frequently; the earthquake geohazards are no development in the whole in oblique~conversing bank slope, and the geohazards mainly are rockfall which mostly occur at thin mountain ridge and steep terrain. 3) The whole reactivation of large accumulation body induced by earthquake is mainly controlled by terrain gradient and micro morphologic unit.Terrain gradien beyond 40° and turning position from gentle slope to steep slope are sufficient conditions for that old landslide or accumulation body wholely or locally losing stability induced by earthquake.
