Nucleation and Arrest of Fluid‐Induced Aseismic Slip.

Microseismicity associated with fluid pressurization in the subsurface occurs during fluid injection but can also be triggered after injection shut‐in. Understanding the extent and duration of the post‐injection microseismicity is critical to limit the risk of fluid‐induced seismicity and insure the safe utilization of the subsurface. Using theoretical and numerical techniques, we investigated how aseismic slip on a fault plane evolves and stops after a fluid pressurization event. We found that the locking mechanisms controlling the arrest of aseismic slip highly depend on the initial fault stress criticality and the pressurization duration. The absolute arrest time of fault aseismic slip after injection shut‐in is proportional to the pressurization duration and increases significantly with the initial fault stress criticality. Given that microseismicity can be triggered by aseismic slip, these results provide insights into the mechanics controlling the arrest of microseismicity after fluid pressurization as a milestone toward induced seismicity mitigation strategies. Plain Language Summary: Injection of fluid in the subsurface for energy and storage applications can lead to the onset of microseismicity, and possibly to major induced seismic events. Fluid pressurization decreases the shear strength of surrounding faults and slip occurs when the in situ shear stress on a fault reaches its shear strength. The nature of slip (aseismic or seismic) depends on the rate at which it occurs and thus on the stability of the deformation. Understanding the mechanics controlling the onset and arrest of aseismic slip and the transition to seismic slip is therefore key to design mitigation strategies for the safe utilization of the subsurface. In this contribution, we investigate using theoretical and numerical techniques how aseismic slip on a fault plane nucleates and evolves in response to fluid injection and how it stops after injection shut‐in when fluid pressure relaxes. We demonstrate that critically stressed faults prior to injection can slip for a longer time after injection shut‐in than during injection and that the extent of rupture can double in size after the end of injection. These results help to quantify the duration and sphere of influence of fluid injection where microseismicity can occur during and after injection. Key Points: Quantification of fault aseismic slip that continues to propagate away from the former injection point after injection shut‐inArrest time of aseismic slip is proportional to the pressurization durationCritically stressed faults slip significantly longer and at a greater distance than marginally pressurized faults after injection shut‐in [ABSTRACT FROM AUTHOR]