Effects of normal stress vibrations on frictional healing

We conducted laboratory experiments to study frictional healing and the effects of normal stress vibrations on healing. The experiments were carried out using a servo-controlled double-direct shear apparatus on 10 cm x 10 cm blocks separated by a 3 mm-thick gouge layer of fine-grained (grain size of 75-212 /zm) quartz powder. We performed slide-hold-slide tests in which sliding surfaces were driven at a constant velocity, halted for a given interval, then restarted at the prior driving velocity. Healing varied systematically with cumulative displacement, and by conducting several sets of identical slide-hold-slides we calibrated and removed these effects. Forward modeling of the healing and relaxation curves using the rateand state-dependent friction laws shows that a displacement-dependent increase in the parameter b can account for our observations. To study the effects of vibration, we varied the mean normal stress of 25 MPa during holds by double amplitudes ranging from 1 to 13 MPa at a frequency of 1 Hz. Vibrations increased rates and magnitudes of frictional relaxation and healing, most likely due to increased gouge compaction. These effects increased with increasing amplitude of vibration. We performed normal stress step tests and used the results to model the vibrational slide-hold-slide tests. Rate- and state-dependent constitutive laws cannot adequately describe the behavior we observed experimentally because they neglect gouge compaction. Mechanisms such as normal force oscillations may explain faster fault healing rates than would be predicted by standard laboratory measurements at constant stress.