Effect of confinement on grain fragmentation during frictional instabilities in granular fault gouge

Mature faults contain a granular gouge layer that affects and controls frictional instabilities. The gouge layer is a result of fragmentation and comminution due to friction and wear at the fault damage zone. Here we simulate stick-slip dynamics in a sheared granular fault gouge incorporating particle fragmentation and study how changes in confining stress affect the stick-slip dynamics as well as stress distribution inside the gouge layer and resulting amount of particle fragmentation. Our results show that, with increase of confining stress, the particle fragmentation events become highly correlated with frictional slips, however, the slip friction drops decrease. We show that the porosity and the freedom of particles to rearrange together with particle stress play important roles in changing the dynamics of the system, affecting both the stick-slip dynamics and its relation with fragmentation events. Our results suggest that particle breakage events associated with post-slip particle rearrangements and subsequent stress localization frequently take place at low confinement, but become limited at higher confinements.