Propagation of fluid-driven aseismic slip fronts along crustal faults

Fluid-induced seismicity results from complex interactions between fluid injection, fault friction, and the ambient stress field. While seismic swarms often display spatiotemporal migration1–6, it remains unclear whether this reflects fluid pressure diffusion or stress transfer from aseismic slip7–9. 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 state10–12. However, direct experimental validation of these predictions has been lacking10, 13. Here, we report laboratory experiments on granite samples under upper crustal stress conditions14, 15, where we simultaneously tracked the propagation of fluid pressure and aseismic slip during fluid injection16. 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 slip10–12 and suggest that in critically stressed crustal faults, rupture propagation may commonly outpace fluid diffusion. These findings help reconcile contrasting observations of seismicity migration7–9 and improve our understanding of induced seismic hazards17–20.