The starting mechanism and movement process of the coseismic rockslide: A case study of the Laoyingyan rockslide induced by the “5.12” Wenchuan earthquake

Abstract

The “5.12” Wenchuan earthquake in 2008 triggered a large number of co-seismic landslides. The rear boundary or cracks of co-seismic landslide are generally located at the steep free surface of thin or thick mountains. Dynamic process of this kind of landslides could be divided into two parts: the seismic dynamic response of the slope and the movement process of rock mass. Taking the Laoyingyan rockslide as an example, the amplification effect was studied by single-degree-of-freedom system analysis method. Besides, the dynamic process of landslide under seismic loading was simulated by the finite difference method (FDM) and discrete element method (DEM). The amplification coefficient of the rockslide to seismic wave is 1.25. The results show that the critical sliding surface of the Laoyingyan rockslide was formed at the 23th seconds under the action of seismic wave. At the same time, tension failure occurred at the rear edge of the sliding mass and shear failure occurred at the front edge. The maximum displacement was 0.81 m and the initial velocity was 2.78 m/s. During the initiation process of the rockslide, the rock mass firstly broke down along the joints which are along the dip of the rock stratum, and then collapsed bodily along the secondary structural planes. In the process of movement, the maximum velocity of rock mass was 38.24 m/s. After that, the rock mass underwent multiple collisions, including contact, deceleration to 0 and speed recovery after rebound. Finally, due to the constant loss of energy, the rocks stopped and accumulated loosely at the foot of the slope. The longest distance of movement was about 494 m. Besides, the smaller the damping ratio, the farther the rock mass moved. Compared with the results without considering the amplification factor, the movement distance of landslide by considering the amplification factor was more accurate. The study of the Laoyingyan rockslide is helpful to strengthen our field identification of potential co-seismic rockslides. At the same time, understanding its movement and accumulation process can help us better predict the hazard scope of the co-seismic rockslides, and provide a reference for the design of treatment projects.