Fluid-driven earthquake sequences and aseismic slip in a poro-visco-elasto-plastic fluid-bearing fault structure

There is a growing interest in understanding how geologic faults respond to transient sources of fluid. However, the spatio-temporal evolution of sequences of seismic and aseismic slip in response to pore-fluid evolution is still poorly constrained. In this study, we present H-MEC (Hydro-Mechanical Earthquake Cycles), a newly-developed two-phase flow numerical code — which couples solid rock deformation and pervasive fluid flow — to simulate how crustal stress and fluid pressure evolve during the earthquake cycle on a fluid-bearing fault structure. This unified 2D numerical framework accounts for full inertial (wave) effects and fluid flow in a finite difference method and poro-visco-elasto-plastic compressible medium with rate-dependent strength. An adaptive time stepping allows the correct resolution of both long- and short-time scales, ranging from years to milliseconds during the dynamic propagation of dynamic rupture. We present a comprehensive plane strain strike-slip setup in which we test analytical benchmarks of pore-fluid pressure diffusion from an injection point. We then investigate how pore-fluid pressure evolution and solid–fluid compressibility control sequences of seismic and aseismic slip on a finite fault width. While the onset of fluid-driven shear cracks is controlled by localized collapse of pores and dynamic self-pressurization of fluids inside the undrained fault zone, subsequent dynamic ruptures are driven by solitary pulse-like fluid pressure wave propagating at seismic speed. Furthermore, shear strength weakening associated with rapid self-pressurization of pore-fluid can account for the slip–fracture energy scaling observed in large earthquakes. This numerical framework provides a viable tool to better understand fluid-driven dynamic ruptures — either as a natural process or induced by human activities — and highlight the importance of considering the realistic hydro-mechanical structure of faults to investigate sequences of seismic and aseismic slip.