Hydro-mechanical earthquake cycles in a poro-visco-elasto-plastic fluid-bearing fault structure

Abstract

A major goal in earthquake physics is to derive a constitutive framework for fault slip that captures the dependence of shear strength on fault rheology, sliding velocity, and pore-fluid pressure. In this study, we present H-MEC (Hydro-Mechanical Earthquake Cycles), a newly-developed two-phase flow numerical framework — which couples solid rock deformation and pervasive fluid flow — to simulate how crustal stress and fluid pressure evolve during the earthquake cycle. This unified, continuum-based model, incorporates a staggered finite difference–marker-in-cell method and accounts for inertial wave-mediated dynamics and fluid flow in poro-visco-elasto-plastic compressible medium. Global Picard-iterations and an adaptive time stepping allow the correct resolution of both long- and short-time scales, ranging from years to milliseconds. We present a comprehensive in-plane strike-slip setup in which we test analytical poroelastic benchmarks of pore-fluid pressure diffusion from an injection point along a finite fault width. We then investigate how pore-fluid pressure evolution and solid–fluid compressibility control sequences of seismic and aseismic fault slip. While the weakening phase is controlled by localized compaction of pores and dynamic self-pressurization of fluids inside the undrained fault zone, the subsequent propagation of dynamic ruptures is driven by pore-pressure waves. Furthermore, pore-fluid pressure conditions on the fault and shear strength weakening associated with rapid self-pressurization of fluids control the characteristic slip-weakening distance, the final size of seismic events, and the scaling between slip and fracture energy observed for large earthquakes. Our modeling results demonstrate that fault failure can occur due to poroelastic coupling on a finite-width shear zone, thus highlighting the importance of considering the realistic hydro-mechanical structure of faults to investigate fluid-driven seismic and aseismic slip, either as a natural process or induced by human activities.