Fault hydromechanics control the emergence of seismic and aseismic slip patterns

Understanding why some fault systems produce seismic swarms while others evolve into foreshock–mainshock– aftershock sequences remains a central challenge in seismology. Fluids are known to play a key role, but numerical models are still far from reproducing the full complexity of solid–fluid interactions and the entire range of fault slip behavior. Here, we use a fully coupled 2D poro-visco-elasto-plastic fault model with rate-and-state friction to explore how pore-fluid pressure and permeability structure jointly shape fault slip behavior. The model resolves off-fault poroelastic deformation and fluid flow, and incorporates a slip- and time-dependent permeability evolution law. By systematically varying the along-strike widths of low-permeability barriers and over-pressurized patches relative to the nucleation size, as well as the healing timescale of permeability, we map out a continuum of dynamic regimes — from distributed, similar-sized clustering to system-size ruptures. Low-permeability barriers act as seismic asperities that promote rupture, whereas high pore-fluid pressure patches favor aseismic slip that redistributes stress. Following seismic ruptures that damage and unseal the fault zone, long permeability-healing times allow the resulting high-permeability pathways to remain open over multiple events, enhancing fluid redistribution and favoring swarm-like activity. In contrast, short healing times rapidly reseal these damaged pathways after each event, trapping fluids between barriers, amplifying stress concentrations, and promoting large, system-size dynamic ruptures. These results show how evolving fault-zone hydromechanics can generate diverse seismic sequences from common underlying physics, providing a framework for interpreting the natural variability in fault slip modes.