Two slip regimes in sheared granular fault

Abstract

In the stick-slip cycles of faults, the evolving contact state inside the gouge is a major contribution to the evolution of fault friction. However, the microscopic contact changes within stick-slip cycles associated with the possible existence of multiple fault slip regimes are still obscure. By simulating a sheared fault containing a granular gouge, we examine the particle-level contact evolution inside the gouge and the corresponding gouge stress field heterogeneity at fault slip. We find two regimes of fault slip driven by the heterogeneity of the gouge stress field: in the case of weak gouge stress field heterogeneity, local contact rearrangements give rise to macroscopic friction coefficient drop and accumulated gouge stress field heterogeneity (dispersive regime); in the instance of strong stress field heterogeneity, catastrophic failure occurs via contact rearrangement sweeping the whole fault and homogenizes the stress field distribution in the gouge (pervasive regime). In the language of self-organized criticality, the accumulation of gouge stress field heterogeneity is an inevitable consequence of increased entropy in the preparation stage of a large earthquake, which eventually destroys the long-range stress field correlations in the gouge and resets at the beginning of a new cycle of a large earthquake.