Modified Torsional Kolsky Bar Experiments Elucidate the Relationship Between Work and Velocity Weakening Behavior of Westerly Granite and SAFOD Gouges

Abstract:

High velocity friction experiments on rock gouge and bare rock have demonstrated that, for velocity weakening materials, the value of the coefficient of friction (µ) decreases rapidly as sliding velocities reach seismic slip rates. Several mechanisms have been proposed as sources of dynamic weakening, including flash heating or melting of asperities, caused by insufficient heat diffusion due to large amounts of work generated over a short period of time. Furthermore, during individual rupture events, the bulk shear stress and strain are commonly localized along a principle slip zone (PSZ), located within the fault core zone. To investigate the frictional and microstructural behavior or rock gouge under seismically relevant normal stresses, slip velocities and accelerations, experiments using a modified torsional Kolsky bar apparatus were conducted on SAFOD Interlaboratory Comparison gouge samples at normal stresses from 46 MPa to 121 MPa, slip velocities up to 5.3 m/s, and slip distances up to 4.5 mm. The dynamic friction coefficient (µ_d) peaked within the first 100 µm of slip followed by a rapid decrease after a critical slip distance for all experiments with slip velocities above 1.2 m/s, on trend with expectations of dynamic weakening. µ_d decreased with increasing slip velocity for both SAFOD and Westerly granite and µ_d decreased with increasing normal stress for Westerly granite while SAFOD µ_d exhibited no pressure dependence. SEM analysis of sheared gouge revealed fully developed principle slip zones exhibiting significant grain size reduction to sizes <<1 µm in all Westerly samples with average slip velocities ≥ 2 m/s irrespective of normal stress, with no evidence of melting. Computed temperatures during the experiments are consistent with flash heating being the dominant weakening mechanism, although grain size reduction associated with initial acceleration likely also plays a role. Initial results also suggest that width of the PSZ decreased with increasing slip velocity, slip distance, total power, and work and µ_d decreases with increasing work for SAFOD and Westerly gouge.