Indexed on: 14 Jan '20Published on: 12 Jan '20Published in: arXiv - Physics - Geophysics
Fluids play an essential role in controlling effective normal stress and hence fault strength. While most earthquake models assume a fixed pore fluid pressure distribution, geologists have documented fault valving behavior, that is, cyclic changes in fluid pressure and unsteady migration of fluids along fault zones. Here we quantify fault valving through 2-D antiplane shear simulations of earthquake sequences on a vertical 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 fault zone permeability is reduced by healing and sealing processes, and is released immediately after earthquakes when permeability has been enhanced by shearing of the fault zone. In our simulations, large earthquakes are often triggered by arrival of a fluid-driven aseismic slip front that infiltrates the base of the previously locked seismogenic zone. This occurs as ascending fluids from depth pressurize and weaken the fault, initiating aseismic slip and enhancing permeability, thereby facilitating further advance of pressurized fluids and aseismic slip (and possibly foreshocks/microseismicity if heterogeneity were accounted for). This phenomenon might help explain observations of fault unlocking in the late interseismic period, slow slip events and creep transients, and rapid pore pressure and stress transmission in induced seismicity sequences.