Frictional restrengthening in simulated fault gouge: Effect of shear load perturbations

Laboratory friction experiments are important for understanding fault restr engthening (healing) between failure events. To date, studies have focused mainly on time and velocity dependence of friction for small perturbations about conditions for steady state sliding. To investigate healing under a w ider range of conditions, as appropriate for the interseismic period and dy namic rupture on seismogenic faults, we vary shear load for holds tau (hold ), hold time t(h), load point velocity V, and initial gouge layer thickness T-0. We shear layers of granular quartz in a biaxial testing apparatus at room temperature and humidity. In addition to conventional slide-hold-slide (CSHS) healing tests, we perform tests in which shear stress is rapidly re duced prior to each hold. Identical slip histories are used in all experime nts. Our CSHS tests show time-dependent healing, where Delta mu is the diff erence between peak static friction and prehold sliding friction, consisten t with previous work. For a given t(h) we find a systematic increase in pea k static strength and Delta mu with decreasing tau (hold) (for t(h) = 100 s , Delta mu = 0.007 for CSHS tests and 0.05 for tau (hold) = 0 tests). Signi ficantly, healing tests at zero shear stress show decreasing static frictio nal yield strength with increasing t(h); thus we observe time-dependent wea kening in this case. We vary initial layer thickness (0.5-3 mm) and find gr eater healing for thicker layers. Numerical simulations using rate and stat e friction laws show that neither the Dieterich nor Ruina evolution laws pr edict our experimentally observed healing rates for the full range of condi tions studied. Our results have significant implications for the mechanics of deformation within granular media. We present a micromechanical model ba sed on stress chains, jamming, and time-dependent unjamming of sheared gran ular layers. As applied to earthquakes, our data indicate that coseismic st ress drop is expected to have an important effect on fault healing rates an d static yield strength.