Acoustically-induced slip in sheared granular layers: application to dynamic earthquake triggering

TL;DR

This study uses simulations to explore how transient vibrations can trigger slip in granular layers, revealing a critical strain threshold that causes a sudden transition to slip, with implications for earthquake triggering.

Contribution

It demonstrates a critical vibrational amplitude causing a sharp transition in slip behavior, linking frictional weakening and elastic modulus reduction to earthquake triggering mechanisms.

Findings

Triggered slip occurs at a critical strain of about 10^-6.

A sudden transition from negligible slip to full slip at the critical strain.

Frictional weakening and reduction in elastic modulus are key to the transition.

Abstract

A fundamental mystery in earthquake physics is ``how can an earthquake be triggered by distant seismic sources?'' Here, we use discrete element method simulations of a granular layer, during stick-slip, that is subject to transient vibrational excitation to gain further insight into the physics of dynamic earthquake triggering. Using Coulomb friction law for grains interaction, we observe delayed triggering of slip in the granular gouge. We find that at a critical vibrational amplitude (strain) there is an abrupt transition from negligible time-advanced slip (clock advance) to full clock advance, {\it i.e.}, transient vibration and triggered slip are simultaneous. The critical strain is order of $10^{-6}$, similar to observations in the laboratory and in Earth. The transition is related to frictional weakening of the granular layer due to a dramatic decrease in coordination number and the weakening of the contact force network. Associated with this frictional weakening is a pronounced decrease in the elastic modulus of the layer. The study has important implications for mechanisms of triggered earthquakes and induced seismic events and points out the underlying processes in response of the fault gouge to dynamic transient stresses.