Propagation of fluid-driven aseismic slip fronts under upper crustal conditions

Fracture mechanics models suggest that aseismic slip may either lag behind or outpace the fluid pressure front, depending on injection conditions and the fault's initial stress state. However, direct experimental validation of these predictions has been lacking. Here, we report laboratory experiments on granite samples under upper crustal stress conditions, where we simultaneously tracked the propagation of fluid pressure and aseismic slip during fluid injection. We show that aseismic slip lags the pressure front at low injection rates and low initial stress but rapidly outpaces it under high-rate injection or near-failure initial stress conditions. The transition is governed by a dimensionless loading parameter that integrates injection rate, fault strength, and initial shear stress. Our results provide direct support for fracture mechanics models of fluid-induced aseismic slip and suggest that in critically stressed crustal faults, rupture propagation may commonly outpace fluid diffusion.