Abstract
Geophysical and geological studies provide evidence for cyclic changes
in fault-zone pore fluid pressure that synchronize with or at least
modulate seismic cycles. A hypothesized mechanism for this behavior is
fault valving arising from temporal changes in fault zone permeability.
In our study, we investigate the coupled dynamics of rate and state
friction, along-fault fluid flow, and permeability evolution.
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 can be much higher than expected from linear
pressure diffusion, and it scales with permeability enhancement.
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.