The effect of shear load on frictional healing in simulated fault gouge

We report on frictional strengthening (healing) in granular quartz gouge as a function of time of true stationary contact. To distinguish between the slip-dependent [Ruina, 1983] and time-dependent [Dieterich, 1979] friction constitutive laws, we designed tests similar to conventional slide-hold-slide (SHS) tests except that shear load was completely removed prior to holds. We find large healing values (0.033-0.054 for holds of 10(1)-10(2) s) compared with quasi-static SHS experiments, and our data indicate time-dependent weakening in contrast with strengthening observed from SHS tests. Gouge layer compaction increases with increasing hold time, comparable to observations from conventional SHS tests. Our data indicate that purely time dependent processes have a minor influence on healing under the conditions studied and/or that such effects are efficiently erased by particle rearrangement during removal/reapplication of shear load. The data are not adequately described by either the time (Dieterich) or slip (Ruina) dependent state evolution laws.