Fault-Valve Instability: A Mechanism for Slow Slip Events

Geophysical and geological studies provide evidence for cyclic changes in fault-zone pore fluid pressure that synchronize with or at least modulate slip events. A hypothesized explanation is fault valving arising from temporal changes in fault zone permeability. In our study, we investigate how the coupled dynamics of rate and state friction, along-fault fluid flow, and permeability evolution can produce slow slip events. Permeability decreases with time, and increases with slip. Linear stability analysis shows that steady slip with constant fluid flow along the fault zone is unstable to perturbations, even for velocity-strengthening friction with no state evolution, if the background flow is sufficiently high. We refer to this instability as the "fault valve instability." The propagation speed of the fluid pressure and slip pulse, which scales with permeability enhancement, can be much higher than expected from linear pressure diffusion. Two-dimensional simulations with spatially uniform properties show that the fault valve instability develops into slow slip events, in the form of aseismic slip pulses that propagate in the direction of fluid flow. We also perform earthquake sequence simulations on a megathrust fault, taking into account depth-dependent frictional and hydrological properties. The simulations produce quasi-periodic slow slip events from the fault valve instability below the seismogenic zone, in both velocity-weakening and velocity-strengthening regions, for a wide range of effective normal stresses. A separation of slow slip events from the seismogenic zone, which is observed in some subduction zones, is reproduced when assuming a fluid sink around the mantle wedge corner. Slow slip events are observed in subduction zones worldwide. Their mechanism is not well understood, but geophysical and geological research suggests a relation with recurring changes in fluid pressure within the fault zone. Here we explore the fault valve mechanism for slow slip events using mathematical and computational models that couple fluid flow through fault zones with frictional slip on faults. The fault valve mechanism (arising from cyclic changes in the permeability or resistance to fluid flow) produces pulses of high fluid pressure, accompanied by slow slip, that advance along the fault in the direction of fluid flow. We quantify the conditions under which this occurs as well as observable properties like the propagation speed and rate of occurrence of slow slip events. We also perform simulations of subduction zone slow slip events using fault zone and frictional properties that vary with depth in a realistic manner. The simulations show that the fault valve mechanism can produce slow slip events with approximately the observed rate of occurrence, while also highlighting some discrepancies with observations that must be addressed in future work. We analyze the dynamics of fault slip with fault-zone fluid flow and fault-parallel permeability enhancement with slip and sealing with time Fault-valve instability produces unidirectional aseismic slip and pore pressure pulses even with velocity-strengthening friction Subduction zone earthquake cycle simulations show that the fault-valve instability can produce slow slip events below the seismogenic zone