Influence of dilatancy on the frictional constitutive behavior of a saturated fault zone under a variety of drainage conditions

We use numerical simulations to investigate how fault zone dilatancy and pore fluid decompression influence fault strength and friction constitutive behavior. Dilatant hardening can change the frictional response and the effective critical stiffness, K-cr, which determines the transition from stable to unstable sliding in velocity weakening fault zones. We study the frictional shear strength response to numerical velocity stepping experiments and show that when the duration of pore fluid decompression is long compared to the time necessary for frictional evolution (as dictated by rate and state friction) both the effective critical slip distance (D-C') and the effective shear strength direct effect (A') are increased. We investigate the role of fault zone permeability (k), dilatancy coefficient (epsilon), and the magnitude of shearing velocity of the fault zone (v(lp)) and compare results using the Dieterich and Ruina state evolution laws. Over the range from k = 10(-15) to 10(-21) m(2), D-C' increases from 25 mu m to similar to 2 mm and A' increases from 0.15 to similar to 5 MPa. We vary epsilon from 10(-5) to 10(-3) and the size of the velocity perturbation from 3X to 1000X and find large increases in the values of D-C' and A', which may lead to inhibition of unstable sliding. Our results indicate that spatial variations, with either depth or lateral extent, in epsilon and k could result in significant changes in the drainage state in fault zones. Such variation may lead to spatial variation of the nucleation and propagation of earthquakes based upon the drainage capabilities of the fault zone.