Influence of roughness and slip velocity on the evolution of frictional strength

Surface roughness and slip velocity play a critical role in determining the strength of crustal faults and their potential seismic response. We examine these controls through slide-hold-slide (SHS) experiments on bare sandstone fractures of variable roughnesses and slip velocities. These experiments explore the effects of frictional healing and frictional relaxation quantified through rate-and state-dependent friction law (RSF). Frictional healing rates (β) range between 0.0020 and 0.0074 and frictional relaxation rates (β_c) between 0.0058 and 0.0097. Increases in surface roughness and shear velocity each accelerate healing and relaxation, whereas elevated normal stresses promote accelerated healing but suppress relaxation. Fracture contact area is closely correlated with changes in frictional healing rate with the evolution of protrusion playing a key role in this frictional response. The number of time-binned AE ring-down counts increase with increasing strength as observed during reactivation – and therefore serve as a reliable indicator of increased strength gain. The logarithmic relationship between hold-time and evolution in the contact area is confirmed by correlations with seismic moment independently measured from the absolutely calibrated AE data. This correlates with an observed increased RSF-b evocative of elevated frictional recovery during hold that translates to a more rapid and intense energy release.