A new mechanism of slip weakening and strength recovery of friction associated with the mechanical consolidation of gouge

I present results of a series of slide-hold-slide tests on granite surfaces separated by a layer of simulated granite gouge. The shear stress during the hold period (tau(hold)) was controlled and used as a major control parameter of the experiments in addition to the time of hold. Time-dependent fault healing, which has been observed in many previous similar experiments, was observed only at very high tau(hold), above 90% of the sliding friction. A time-independent strengthening termed "consolidation strengthening" occurred at all tau(hold). With a magnitude proportional to the reduction of tau(hold). The magnitude of this strengthening was as large as 25% of the residual friction. The consolidation strengthening was found to be erased by subsequent slip in quite a different manner than is time-dependent healing. The friction decreased linearly with slip displacement at a rate independent of the magnitude of the strengthening and hence larger slip displacement is required to erase larger consolidation strengthening. On the basis of these results together with earlier authors' results of the measurements of porosity change under similar conditions, I conclude that the time-independent strengthening is due to the occurrence of gouge particle rearrangement to tighter packing enabled by decreased shear stress (under a constant normal stress) and that the subsequent slip weakening is due to unconsolidation of gouge enabled by particle rearrangement (dilatancy) during shearing. The rate- and state-dependent friction (RSF) law was extended to incorporate the present findings.