Insights into earthquake nucleation from acoustic emissions and stick-slip instabilities in friction experiments

Developing microphysics-based laws to describe the rheology of dense granular media will be catalytic to a number of problems such as earthquake hazard forecasting. However, current understanding of the link between particle-scale phenomena and macroscopic frictional behavior is limited, as real time imaging of the microscale under geologically relevant conditions is quite demanding. Here, we study the spatial distribution of acoustic emissions (AE) associated with stick slip instabilities in sheared granular layers, in order to better understand the energy budget of deforming fault gouge. Results show the existence of fixed as well as migrating sites that nucleate AE activity, as opposed to randomly placed events. Highly concentrated groups of AEs were observed in experiments with significant particle size reduction. The non-random distribution of AEs confirms that the microstructural state of sheared granular layers during unstable sliding is not homogeneous and evolves with ongoing shear. The sites where failure initiates might be controlled by the stress boundary conditions, but memory effects shared among particles involved in previously failed load-bearing microstructures might play a role as well. These findings may provide a microphysical basis for phenomena such as repeating microearthquakes and migrating seismicity that are observed in natural fault zones.