Illuminating Fluid-Induced Fault Reactivation: Laboratory Insights into Injection-Rate Control on Slip Evolution and Seismic Nucleation

Understanding how fluid injection perturbs stressed faults and triggers induced seismicity has become an urgent challenge in geophysics and hazard mitigation. Observations from subsurface fluid injection associated with geoenergy exploitation show that variations in injection pressure and rate and injected volume can strongly modulate seismicity rates and magnitudes. Yet, comparable injection operations may result in stable creep, slow slip, or dynamic rupture, highlighting persistent gaps in our understanding of the physical processes governing fluid-driven fault reactivation.

Here, we investigate fluid-induced fault reactivation through decimetric-scale laboratory experiments on granite samples containing a 45° precut fault. Experiments are conducted in a biaxial apparatus under critically stressed conditions at 3 MPa normal stress, with independent control of normal and shear stresses. Fluids are injected directly into the fault surface while fault slip is measured using fibre-optic sensors (mini-SIMFIP) installed across the fault. Seismic activity is monitored through passive acoustic emission recordings, and repeated active ultrasonic surveys are performed throughout the experiments to track wave velocity changes and map fluid diffusion along the fault.

By systematically varying the injection rate, we observe a clear transition from aseismic creep to slow slip and dynamic rupture. In all cases, fault slip nucleates at the injection point and subsequently propagates within the pressurized region of the fault. High injection rates generate localized overpressure near the injection point, triggering abrupt and seismic fault reactivation. During high-rate injection, we observe a pronounced drop in P-wave velocity, indicating strong mechanical perturbation of the fault zone, followed by a progressive velocity increase as fluids diffuse along the fault. In contrast, low injection rates lead to stable, aseismic slip confined to the pressurized zone, while intermediate rates produce a progressive reactivation sequence in which slip initiates aseismically, evolves into slow slip, and eventually transitions to dynamic rupture as the pressurized region expands.

Our results show that injection rate governs fault slip behavior by controlling where slip nucleates and whether it remains confined to, or propagates beyond, the pressurized zone and accelerates dynamically.