High-velocity frictional behavior and microstructure evolution of fault gouge obtained from Nojima fault, southwest Japan

High-velocity experiments on fault gouge taken from the Nojima fault that slipped during the 1995 Kobe earthquake were conducted to investigate physical mechanism associated with the slip-weakening behavior. With increasing slip, the friction values of the gouge sheared at 0.62 MPa normal stress and 1.03 m/s slip velocity decrease exponentially from a peak value of more than 0.6 to a steady-state value of 0.2. The textures of the gouge are characterized by grain comminution, oblique and parallel shear planes and localized deformation zone with strongly preferred orientation in the friction weakening stage, and folding and fluttering structures at the steady-state friction stage. Numerical modeling based on the temperature measurements close to the gouge layer shows that the temperature inside the gauge layer did not exceed 400 °C during the experiments. In a slide–hold–slide test, a full strength recovery of the fault gouge was observed only after 12 s slip pause and the slip-weakening curves are the same between the two successive slips. The steady-state coefficient of friction decreased from 0.8 to about 0.2 when the slip velocity increased from 0.006 m/s to 1.03 m/s. This high-velocity weakening feature was observed in a synthetic quartz gouge as well as in the Nojima gouge. Although it is unclear which mechanism causes the weakening among thermal pressurization, silica gel lubrication, flash heating, moisture-draining and so on, the present experimental results suggest that the high-velocity weakening is related to the high heat production rate. Finally, the flow structures observed in the samples deformed up to the final steady-state stages have never been reported in previous slow-rate experiments and could be a key structure characteristic of high-velocity frictional sliding.