Voids and Rock Friction at Subseismic Slip Velocity

We found that the amplitudes of transmitted waves across the sliding surfaces are inversely correlated to high slip rate friction, especially when the interfaces slide fast (> 10⁻³ m/s). During the rock–rock friction experiments of metagabbro and diorite at sub-seismic slip rate (~ 10⁻³ m/s), friction does not reach steady state but fluctuates within certain range. The amplitudes of compressional waves transmitted across the slipping interfaces decrease when sliding friction becomes high and it increases when friction is low. Such amplitude variation can be interpreted based on the scattering theory; small amplitudes in transmitted waves correspond to the creation of large-scale (~ 50 μm) voids and large amplitudes correspond to the small-scale (~ 0.5 μm) voids. Thus, large-scale voids could be generated during the high-friction state and low-friction state was achieved by grain size reduction caused by a comminution process. This was partly confirmed by the experiments with a synthetic gouge layer. The result can be interpreted as an extension of force chain theory to high-velocity sliding regime; force chains were built during the high friction and they were destroyed during the low friction. This mechanism could be a microscopic aspect of friction evolution at sub-seismic slip rate.