# Induced and endogenous acoustic oscillations in granular faults

## TL;DR

This paper reviews how external perturbations induce acoustic oscillations in granular fault models, affecting their frictional properties and potentially explaining seismic phenomena like fault weakening and earthquake triggering.

## Contribution

It provides a comprehensive review of recent findings on the response of granular systems to perturbations, highlighting non-monotonic weakening effects and re-entrant transitions.

## Key findings

- External perturbations can induce acoustic oscillations in granular faults.
- Frictional weakening may not increase monotonically with perturbation frequency.
- Re-entrant transitions between fluidized and frictional states are observed.

## Abstract

The frictional properties of disordered systems are affected by external perturbations. These perturbations usually weaken the system by reducing the macroscopic friction coefficient. This friction reduction is of particular interest in the case of disordered systems composed of granular particles confined between two plates, as this is a simple model of seismic fault. Indeed, in the geophysical context frictional weakening could explain the unexpected weakness of some faults, as well as earthquake remote triggering. In this manuscript we review recent results concerning the response of confined granular systems to external perturbations, considering the different mechanisms by which the perturbation could weaken a system, the relevance of the frictional reduction to earthquakes, as well as discussing the intriguing scenario whereby the weakening is not monotonic in the perturbation frequency, so that a re-entrant transition is observed, as the system first enters a fluidized state and then returns to a frictional state.