Mechanical Weakening of Near-Surface Rock Layers Due to the Scattering of Seismic Waves

Date: 4/25/2019

Time: 01:45 PM

Room: Elliott Bay

When an elastic shear wave impinges on a sloped traction-free boundary, mode conversion takes place to satisfy the boundary conditions. For post-critical incidences, scattered energy forms a waveguide parallel to the ground surface. Depending on the angle of incidence and material properties (Poisson’s ratio), these surface shear waves can have amplitude 5⁺ times larger than the incident wave. Furthermore, the converted compressional wave does not carry any energy away from the boundary, and instead creates a zone of concentrated energy that propagates near the surface. Therefore, the resultant wavefield is amplified enough to initiate mechanical weakening (fracturing) of the shallow rock layers. Once weakening is initiated, the phenomenon is self-reinforcing, that is, weakening leads to increasingly larger impedance contrast between the intact and fractured rock; to increasingly larger amplification of the incident waves transmitted and trapped in the weathered rock layer; and hence increasingly larger damage induced by repeated events over large periods of time. In this study, we seek to develop a theoretical model to quantify the thickness and material strength of this weakened zone aka critical zone. We use a hybrid continuum-discrete technique to investigate the response of hillslopes subjected to a seismic motion for a range of material (rock and joint) and excitation (angle of incident, intensity and frequency content) parameters. The results of the systematic study on progressive rock fracturing helps us to characterize the critical zone for various seismic scenarios, to evaluate the earthquake as both a transient preparatory factor and a triggering mechanism of slope failure, and to predict the potential failure mechanisms in hillslopes.

Presenting Author: Kami Mohammadi

Authors