Investigation of transient friction in rock at low to high slip-rates using a new biaxial machine

Abstract:

Rate-and-state friction (RSF) constitutive relations were developed from experiments on rock that incorporated step-changes in sliding rate at low velocity (< 1 mm/s). High-velocity rotary shear experiments demonstrate pronounced reduction in friction during sustained sliding at high rates (> 0.1 m/s) that appears distinct from RSF. Developing constitutive relations that can describe the transient friction response during a change from sliding at low rates to high rates is necessary to model earthquake nucleation, rupture propagation, and the diversity in modes of fault slip; however, experimental documentation of transient friction between low and high rates is not generally available. We report the results of experiments using a high-speed biaxial apparatus that achieves velocity-steps from low to high rates with high acceleration (up to 100 G) to investigate transient friction behavior and the viability of RSF relations at intermediate and high sliding velocity. Double-direct (unconfined rock-on-rock) shear experiments on granite (Westerly) and quartz sandstone (Tennessee) were conducted at constant normal stresses of 1-20 MPa. Most experiments involve steady-state frictional sliding at 1 mm/s for several mm of slip followed by a velocity step to constant slip rates of 10 to 1000 mm/s for a total displacement of 4 cm. The experiments demonstrate that a high acceleration step to high velocity under moderate normal stress can lead to rapid dynamic weakening (i.e., a weakening distance of ~1 mm). In granite, the magnitude of steady state friction at high rates is proportional to the rate of frictional work done on the surface, consistent with a flash heating mechanism. In contrast, for the sandstone, steps to high sliding rates resulted in strengthening and a change from stick-slip to stable sliding. The observations are qualitatively consistent with RSF type behaviors; i.e., transient friction consists of an instantaneous increase in friction followed by an exponential decay to a new steady-state value and a recovery of the quasi-static frictional strength at the end of slip during deceleration to low sliding rates.