FRICTIONAL BEHAVIOR OF LARGE-DISPLACEMENT EXPERIMENTAL FAULTS

The coefficient of friction and velocity dependence of friction of ini tially bare surfaces and 1-mm-thick simulated fault gouges (< 90 mu m) of Westerly granite were determined as a function of displacement to > 400 mm at 25 degrees C and 25 MPa normal stress. Steady state negati ve friction velocity dependence and a steady state fault zone microstr ucture are achieved after similar to 18 mm displacement, and an approx imately constant strength is reached after a few tens of millimeters o f sliding on initially bare surfaces. Simulated fault gouges show a la rge but systematic variation of friction, velocity dependence of frict ion, dilatancy, and degree of localization with displacement. At short displacement (< 10 mm), simulated gouge is strong, velocity strengthe ning and changes in sliding velocity are accompanied by relatively lar ge changes in dilatancy rate. With continued displacement, simulated g ouges become progressively weaker and less velocity strengthening, the velocity dependence of dilatancy rate decreases, and deformation beco mes localized into a narrow basal shear which at its most localized is observed to be velocity weakening. With subsequent displacement, the fault restrengthens, returns to velocity strengthening, or to velocity neutral, the velocity dependence of dilatancy rate becomes larger, an d deformation becomes distributed. Correlation of friction, velocity d ependence of friction and of dilatancy rate, and degree of localizatio n at all displacements in simulated gouge suggest that all quantities are interrelated. The observations do not distinguish the independent variables but suggest that the degree of localization is controlled by the fault strength, not by the friction velocity dependence. The fric tion velocity dependence and velocity dependence of dilatancy rate can be used as qualitative measures of the degree of localization in simu lated gouge, in agreement with previous studies. Theory equating the f riction velocity dependence of simulated gouge to the sum of the frict ion velocity dependence of bare surfaces and the velocity dependence o f dilatancy rate of simulated gouge fails to quantitatively account fo r the experimental observations.