Constraining Fault Friction and Stability With Fluid‐Injection Field Experiments.

While the notion that injecting fluids into the subsurface can reactivate faults by reducing frictional resistance is well established, the ensuing evolution of the slip is still poorly understood. What controls whether the induced slip remains stable and confined to the fluid‐affected zone or accelerates into a runaway earthquake? Are there observable indicators of the propensity to earthquakes before they happen? Here, we investigate these questions by modeling a unique fluid‐injection experiment on a natural fault with laboratory‐derived friction laws. We show that a range of fault models with diverging stability with sustained injection reproduce the slip measured during pressurization. Upon depressurization, however, the most unstable scenario departs from the observations, suggesting that the fault is relatively stable. The models could be further distinguished with optimized depressurization tests or spatially distributed monitoring. Our findings indicate that avoiding injection near low‐residual‐friction faults and depressurizing during slip acceleration could help prevent large‐scale earthquakes. Plain Language Summary: Fluid injections into the Earth's crust are common practice in the exploitation of subsurface energy resources such as geothermal energy, shale gas, and conventional hydrocarbons. These injections can perturb nearby fault structures and hence induce earthquakes and transient slow slip. Understanding what controls the stability (i.e., the propensity to generate earthquakes) and spatial extent of the fault response as well as identifying precarious faults is crucial to minimize the seismic hazard associated with these industrial practices. Here, we take a step toward this goal by modeling a unique experiment, in which water was injected into a natural fault and the resulting slip measured directly at depth. We first show that multiple models can explain the observations equally well, while pressure is increased in the experiment. In these models, how stable the fault response is with further injection and how large of a zone is reactivated compared to the fluid‐affected region depends on frictional properties. We then demonstrate that the slow slip response to a decrease in injection pressure further constrains the range of admissible models. Our work suggests that it may be possible to identify potentially hazardous faults with optimally designed injection tests without inducing damaging earthquakes. Key Points: Multiple frictional models with different stability reproduce the slip observed during the pressurization stage of a field experimentThe depressurization phase provides additional constraints on hydromechanical parameters and hence fault stabilityFault stability and the spatial extent of slip relative to the pressurized region depend on residual friction versus initial stress levels [ABSTRACT FROM AUTHOR]