Synchronized oscillations and acoustic fluidization in confined granular materials

TL;DR

This study uses numerical simulations to demonstrate how synchronized oscillations at a characteristic frequency in granular fault models can generate acoustic fluidization, weakening the fault and leading to failure.

Contribution

It reveals the mechanism of wave activation and particle oscillations responsible for acoustic fluidization in granular fault systems.

Findings

Particles perform synchronized oscillations at the acoustic fluidization frequency.

Oscillations reorient perpendicular to the fault plane before failure.

External perturbations at the characteristic frequency can induce similar oscillations.

Abstract

According to the acoustic fluidization hypothesis, elastic waves at a characteristic frequency form inside seismic faults even in the absence of an external perturbation. These waves are able to generate a normal stress which contrasts the confining pressure and promotes failure. Here, we study the mechanisms responsible for this wave activation via numerical simulations of a granular fault model. We observe the particles belonging to the percolating backbone, which sustains the stress, to perform synchronized oscillations over ellipticlike trajectories in the fault plane. These oscillations occur at the characteristic frequency of acoustic fluidization. As the applied shear stress increases, these oscillations become perpendicular to the fault plane just before the system fails, opposing the confining pressure, consistently with the acoustic fluidization scenario. The same change of orientation can be induced by external perturbations at the acoustic fluidization frequency.