Mechanism Analysis on Vertical Gradation of Rock Avalanches Triggered by Wenchuan Earthquake, Sichuan, China

Abstract

Study on rock avalanche kinematics is still a hotspot in the world, especially after the Wenchuan earthquake. To examine this mechanism further, some rock avalanches triggered by the Wenchuan earthquake are taken as objects in this paper. After a detailed field investigation of the vertical profiles revealed by natural longitudinal trenches formed in the rock avalanche deposits, an obvious vertical gradation along their sliding paths is observed, and proposed to be divided into five zones from the top to the bottom and named as follows: the air-blast zone, the bouncing and jumping zone, the sliding and deposit zone, the basal travel zone, and the original accumulation zone. Based on the characteristics of these zones, the formation of each zone is discussed in detail with corresponding conclusions proposed: (1) The air-blast zone, as the upper boundary of the area damaged by rock avalanches, is destroyed by wind gusts initiated by a large rapidly moving mass. (2) The bouncing and jumping zone, mainly characterized by the display of accumulated fine grains and isolated blocks, is produced by the co-work of escaping force of entrapped air and debris internal force. (3) The coupling effect of kinematic sieving and momentum transfer is the main reason for the formation of sliding and deposit zone. (4) Debris in basal travel zone goes through a complex dynamical process: collision, dilation, fragmentation, rotation and sliding, and compaction. In this process, both the overburden pressure and the self-excited vibration energized by the undulated slip surface are the main dynamical forces controlling the formation of basal travel zone. And the simple shear and pure shear modes are the main mechanical behaviors, which realize the formation of particle fragmentation, a binary mixture with coarse particles embedded in clay matrix, and alignment distribution of particles in basal travel zone.