Slip-Dependence of Fault Frictional Stability Under Hydrothermal Conditions

Abstract

In the rate-state friction law framework, the transition from velocity weakening (V-W) to velocity strengthening (V-S) behavior marks the base of the seismogenic crust. Here we investigate the role of fault slip displacement under hydrothermal conditions in controlling the V-W to V-S transition. We shear simulated gabbro gouges at slip velocities ranging from 16 nm/s (∼50 cm/year) to 10 μm/s (∼8 cm/day) under hydrothermal conditions (300–400°C temperature; 30 MPa pore fluid pressure). We observe that cumulative fault slip increases the critical velocity for the V-W to V-S transition. The transition is accompanied by localized to distributed deformation mode, the formation of smectite-type clays and occurrence of intergranular mass transfer. Our results provide insights into understanding the deepening and shallowing of V-W/V-S boundary (lower limit of the seismogenic zone) following a mainshock. Besides strain rate effects, slip-enhanced chemical alteration and grain size-sensitive deformation may temporarily contribute to the shallowing process.