Principal Characteristics and Failure Mechanism of the Xiaonanhai Rockfall Triggered by the Qianjiang Ms 6.3 Earthquake in 1856

Based on field geological survey, engineering geological investigation and other related analysis to the Xiaonanhai Rockfall triggered by the Qianjiang Ms 6.3 Earthquake in 1856, principal characteristics and failure mechanism of the rockfall is elaborated. It is held that its forming process consists of four steps, namely weathering and denudation, shocking and projecting, collapse-slide accumulating and dammed lake formation. It shows that the mass is a result of comprehensive action including seismotectonic moving and field stress, rock mass structure, topography and geomorphology, seismic wave. First of all, rock mass near both Dakuayan and Xiaokuayan which developed three groups tectonic joints forming “X” shape collapsed respectively towards their free faces, about 145o~155o direction. Secondly, diameters of rock blocks in accumulation body decrease from northwest to southeast along 150o direction, and it proves indirectly that the direction of rock mass ejecting is along the same direction.