Effects of normal stress vibrations on frictional healing

We conducted laboratory experiments to study frictional healing and the eff ects of normal stress vibrations on healing. The experiments were carried o ut 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 mu m) quartz powder. We performed slide-hold-slide tests in which s liding surfaces were driven at a constant velocity, halted for a given inte rval, then restarted at the prior driving velocity. Healing varied systemat ically with cumulative displacement, and by conducting several sets of iden tical slide-hold-slides we Calibrated and removed these effects. Forward mo deling of the healing and relaxation curves using the rate-and state-depend ent friction laws shows that a displacement-dependent increase in the param eter. 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 amplitu des ranging from 1 to 13 MPa at a frequency of 1 Hz. Vibrations increased r ates and magnitudes of frictional relaxation and healing, most likely due t o increased gouge compaction. these effects increased with increasing ampli tude of vibration. We performed normal stress step tests and used the resul ts to model the vibrational slide-hold-slide tests. Rate- and state-depende nt constitutive laws cannot adequately describe the behavior we observed ex perimentally because they neglect gouge compaction. Mechanisms such as norm al force oscillations may explain faster fault healing rates than would be predicted by standard laboratory measurements at constant Stress.