While the notion that injecting fluids into the subsurface can
reactivate faults by reducing frictional resistance is well established,
the ensuing evolution of 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 upon slip acceleration could help prevent
large-scale earthquakes.