The poroelastic stress and pore pressure effects on delayed seismicity based on fully coupled fluid-solid simulations and rate-and-state friction laws

SUMMARY Fluid injection into the subsurface can trigger moderate-magnitude earthquakes days to months after shut-in, complicating hazard assessment. To investigate the governing mechanics, we implemented a fully coupled hydromechanical model that couples Darcy flow, poro-viscoelastic deformation and rate-and-state fault friction on a planar fault, allowing two-way feedbacks between pore pressure, volumetric strain and fault slip and the simulation of both aseismic and seismic transients. Compared with decoupled or one-way approaches, the fully coupled formulation generally yields longer post-injection delays, owing to poroelastic stress contributions and a more realistic evolution of volumetric strain. After shut-in, a slow poroelastic redistribution of volumetric compression broadens and migrates along the fault, constructively overlapping regions of elevated shear stress and reduced effective normal stress. This causes the nucleation of a delayed rupture away from the well, indicating that the point of peak instantaneous pressure does not necessarily coincide with the location of maximum coseismic slip. By scanning permeability and injection rate we construct an empirical injection-rate (IR)–permeability (k) phase diagram that delineates regimes of immediate, delayed and no induced seismicity; this diagram is offered as a conceptual, physics-informed screening tool that requires site-specific calibration. Our results indicate that two-way hydromechanical coupling and fault slip evolution should be considered when assessing post-injection seismic hazard and in the design of spatially distributed monitoring.