Slip‐Dependence of Fault Frictional Stability Under Hydrothermal Conditions.

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. Plain Language Summary: According to the standard model of earthquake nucleation, earthquakes are the result of frictional instabilities along faults. The necessary condition for earthquake nucleation is that the frictional strength of a fault decreases with fault slip velocity ("velocity‐weakening" behavior) or slip distance ("slip‐weakening" behavior). Although extensive laboratory studies have been conducted to investigate the velocity‐dependence of friction in rocks, less attention was paid to the role of slip displacement on fault frictional stability, especially in the presence of hot and pressurized fluids. The latter, difficult to reproduce in the laboratory, is a common condition at seismogenic depths. In this study, we examine how the frictional stability evolves with slip velocity and displacement on simulated faults made of powders of gabbro (a common rock of the oceanic crust) under hydrothermal conditions up to 400°C. We find that the critical velocity for the transition from velocity‐weakening to velocity‐strengthening increases with cumulative slip displacement. In nature, the increase of this critical velocity may result in the uplift, during seismic sequences, of the base of the seismogenic crust (i.e., aftershock hypocenters will be shallower). Our findings suggest that, slip displacement, accompanied by fault mineralogical‐structural evolution, can influence fault frictional stability and earthquake nucleation. Key Points: The role of slip displacement in controlling fault stability under hydrothermal conditions is investigatedThe critical velocity for transition in friction from velocity‐weakening to velocity‐strengthening increases with slip displacementThis frictional transition, attributed to chemical alteration, is accompanied by the transition from localized to delocalized deformation [ABSTRACT FROM AUTHOR]