Symmetry and the critical slip distance in rate and state friction laws

We performed laboratory experiments to investigate the processes responsible for rate and state friction (RSF) behavior in fault rocks. We focused on the symmetry of the frictional constitutive response to velocity changes and the mechanics of the critical friction slip distance D_c. Experiments were conducted in double direct shear at 1 and 25 MPa normal stress, at room temperature, and for shearing velocity from 1 to 300 μm/s. We studied three granular materials and bare surfaces of Westerly granite. Ruina's law, which predicts frictional symmetry between velocity increases and decreases, better matches our data than Dieterich's law, which predicts that velocity decreases should evolve to steady state at a smaller displacement. However, for granular shear, in some cases D_c is smaller for velocity increases than for velocity decreases, contrary to expectations from either law. On bare granite surfaces, the frictional response is symmetric for velocity increases/decreases. Two distinct length scales for D_c and two-state variables are required for granular shear in some cases. We hypothesize that asymmetry and two-state behavior are caused by shear localization and changes in shear fabric in fault gouge. Our measurements show that during steady state frictional shear, dilation after a velocity increase is smaller than compaction after a decrease. Normal stress oscillations cause a marked decrease in D _c. Reduction of D_c reduces frictional stability, enhancing the possibility of seismic slip. Our experiments show that shear localization and fabric within the fault gouge can influence the RSF parameters that dictate earthquake nucleation and dynamic rupture. Key Points Ruina law best fits lab data Localization changes the frictional response Localization enhances the possibility of seismic slip