Stress Perturbation From Aseismic Slip Drives the Seismic Front During Fluid Injection in a Permeable Fault.

Fluid pressure changes affect fault stability and can promote the initiation of earthquakes and aseismic slip. However, the relationship between seismic and aseismic fault slip during fluid injection remains poorly understood. Here, we investigate, through 3‐D hydromechanical modeling, the spatiotemporal evolution of seismicity and aseismic slip on a permeable, slip‐weakening fault subjected to a local injection of fluid, under different prestress conditions. The model results in an expanding aseismic slip region, which concentrates shear stress at its edge and triggers seismicity. The aseismic slip dominates the slip budget, whatever the initial fault stress. We find that the seismicity is collocated with the aseismic rupture front rather than with the fluid pressure diffusion front. On faults initially far from failure, the aseismic rupture front is located behind or at the pressure front. On faults initially closer to failure, the model predicts that both the rupture front and the seismicity outpace the pressurized zone, resulting in a sharp increase of the migration velocity and released moment of the seismicity. Insights gained from this modeling study exhibit various features that are observed in sequences of induced earthquakes in both field experiments and natural reservoir systems and can help guide the interpretation of past and future observations of induced seismicity. Plain Language Summary: The injection of fluids deep below ground can induce earthquakes, but it can also trigger slow deformations. Studying the relationship between fluid perturbation and seismic and aseismic deformations is fundamental to understand the mechanisms of injection‐induced seismicity in order to mitigate seismic risk. Here, we present results of computer models of the response of a fault to fluid injection. We show that, in the presence of induced aseismic slip, the seismicity is not directly induced by the elevated fluid pressure but by the stresses generated by the expansion of the aseismic slip region. The seismicity initiates and diffuses along the edge of the stress‐increase zone, rather than along the edge of the zone of elevated fluid pressure. We find two different behaviors depending on how stressed the fault is before injection. For a fault initially far from failure, the aseismic slip and the seismicity are confined in the pressurized zone. On the contrary, if a fault is initially close to failure, both the aseismic slip front and the seismicity front accelerate and outpace the fluid pressure front. Thus, the transient stress‐increase associated with the aseismic slip and the initial stress conditions are key factors that control the triggering of seismicity. Key Points: 3‐D hydromechanical models show that injection‐induced seismicity along a permeable fault is not limited to the pressurized volumeSeismicity follows the migration of the shear stress concentration near the aseismic slip front rather than the fluid pressure frontOnce the shear stress front outpaces the pressure front, the seismicity migration accelerates and cannot be explained by fluid diffusion [ABSTRACT FROM AUTHOR]