  Fault-zone fluids control effective normal stress and fault strength. While
most earthquake models assume a fixed pore fluid pressure distribution,
geologists have documented fault valving behavior, that is, cyclic changes in
pressure and unsteady fluid migration along faults. Here we quantify fault
valving through 2-D antiplane shear simulations of earthquake sequences on a
strike-slip fault with rate-and-state friction, upward Darcy flow along a
permeable fault zone, and permeability evolution. Fluid overpressure develops
during the interseismic period, when healing/sealing reduces fault
permeability, and is released after earthquakes enhance permeability. Coupling
between fluid flow, permeability and pressure evolution, and slip produces
fluid-driven aseismic slip near the base of the seismogenic zone and earthquake
swarms within the seismogenic zone, as ascending fluids pressurize and weaken
the fault. This model might help explain observations of late interseismic
fault unlocking, slow slip and creep transients, swarm seismicity, and rapid
pressure/stress transmission in induced seismicity sequences.
