Seismic and aseismic slip driven by ascending fluids and overpressure pulses on faults

Abstract

It is well known that fault zones are conduits for fluids. In particular, elevated pore-fluid pressure can neutralize the strengthening effect of normal stress and bring the fault system closer to failure. Despite this, most earthquake cycle models prescribe a constant effective normal stress and neglect the evolution of fluid pressure and transport properties. In this study, we present a hydro-mechanical earthquake cycles (H-MECs) model on a 2-D antiplane strike-slip fault with rate-and-state friction, full inertia effects, and poro-visco-elasto-plastic rheology. As a proxy for metamorphic reactions, a source of fluids is imposed at bottom of the fault causing fluids to ascend along the seismogenic zone. We further adopt a permeability evolution law in which permeability increases with fault slip and decreases due to healing and sealing processes. Our results show that fluid overpressure at the base of the seismogenic builds during the late interseismic period, when the fault has low permeability, weakening the fault and triggering slow-slip transients and earthquakes. When the healing time is shorter than the average recurrence interval of earthquakes, overpressure pulses facilitate the propagation of fluid-driven aseismic slip and their ascent through the seismogenic zone, thus causing swarm seismicity. For healing times of the order of a few years, overpressure pulses trigger long-term slow slip events, whereas for even longer healing times, fluid-driven aseismic slip causes a transient unlocking of the fault, without causing any seismic event. As a result, our models show that charge and discharge processes of fluid pressure and relative changes in fault strength influence the timing, slip behavior, stress transfers, stress drop, and other rupture properties. Accounting for viscoelastic deformation and poroelasticity effects incorporating the two-way coupling of solid and fluid phases brings earthquake cycle simulations much closer to reality, allowing greater consistency with experimental and geologic constraints on fault zone structure and dynamics.