Delayed Weakening and Reactivation of Rate-and-State Faults Driven by Pressure Changes Due to Fluid Injection

Fault reactivation induced by pore pressure changes involves complex frictional phenomena because effective normal stresses vary in space and time due to fluid flow and rock deformation. The impact of time-varying normal stresses on fault friction has been characterized in stress-step laboratory experiments and modeled through extended rate-and-state laws that incorporate a stressing-rate dependence of the state variable. Building on these rate-and-state models, we use 2-D poroelastic simulations to understand how the evolution of pore pressures due to fluid injection affects fault strength and reactivation. A sharp increase in pore pressure, associated with fluid injection, leads to an increase in friction coefficient and to a delayed weakening of the fault. Conversely, a sharp pressure decrease during extraction leads to a delayed strengthening. Stressing-rate effects emerge as a purely frictional mechanism that delays or accelerates the onset of fault slip, suggesting that reactivation under fluid injection may occur long after flow rates have decreased and pore pressures have stabilized at the fault. Hence, earthquakes induced by injection may ensue as a deferred process triggered several days later than predicted by simple estimates based on constant friction. The duration of the delay depends on the type of rate-and-state law used in calculations and on a characteristic relaxation time: The memory time over which the state variable evolves under stationary contact. Our results help understand the connection between injection protocols and frictional weakening mechanisms, suggesting that injection processes can be engineered to minimize the risk of induced seismicity for a given injected volume.