Dynamic fault weakening and strengthening by gouge compaction and dilatancy in a fluid‐saturated fault zone

Fault gouge deformation likely plays a significant role in controlling the strength of mature, large-displacement faults. Experiments show that intact gouge deforms in an overall ductile and stable manner, readily compacting, but dilates and experiences brittle failure under large strain rate. Inelastic gouge compaction and dilatancy are modeled here using a combined Mohr-Coulomb and end-cap yield criterion in a dynamic rupture model of a strike-slip fault with strongly velocity-weakening friction. We show that large shear stress concentration ahead of the rupture associated with the rupture front causes the gouge layer to compact (e.g., by structural collapse and comminution), leading to rapidly elevated pore pressure and significant weakening of the principal fault surface. Shortly after the rupture front passes, strong dilatancy during strength drop and rapid sliding reduces pore pressure and strengthens the fault, promoting slip pulses. Large strain localization in the gouge layer occurs as a result of rapid gouge dilatancy and strain softening. The combination of prerupture weakening from compaction and restrengthening from dilatancy hardening leads to a smaller-strength drop, and limits the stress concentration outside the gouge layer. This leads to a reduction of inelastic shear strain in the damage zone, which is more consistent with geological observations and high-speed frictional experiments. With the presence of well-developed fault gouge, the strength of mature faults may be limited by end-cap rather than Mohr-Coulomb failure; thus, their frictional strengths are significantly smaller than Byerlee friction.