IMPLICATIONS OF COULOMB PLASTICITY FOR THE VELOCITY DEPENDENCE OF EXPERIMENTAL FAULTS

Simulated fault gouges often deform more stably than initially bare su rfaces of the same composition. It is important to understand why the sliding stability is enhanced because the presence of gouge on natural faults may have the same effect as seen in experiments, and thus expl ain the absence of earthquakes at shallow depths. Gouge stabilization in experiments has been attributed to positive contributions to veloci ty dependence within gouge layers from either dilation (MARONE et al., 1990) or grain fracture (BIEGEL et al., 1989). In this study we test the hypothesis that some aspects of gouge and initially bare surface v elocity dependence are identical by measuring the time-dependent const itutive parameter b. An important result follows however from stress a nalysis: if both sample configurations are frictional in the Mohr-Coul omb sense, each configuration is required to deform on planes of disti nctly different orientation. The measured strength and velocity depend ence will reflect this geometric difference. Our observed values of b for simulated granite and quartz gouge are two to two and a half times smaller than b for initially bare surfaces. This difference is comple tely accounted for if gouge is represented as a cohesionless-Coulomb p lastic material. The analysis demonstrates the following points: 1) go uge deformation is fully consistent with Coulomb plasticity, 2) observ ed gouge velocity dependence is a function of observed strength and 3) the constitutive parameter b is the same for both bare surfaces and g ouge. Furthermore, the results suggest that there is no time-dependent strengthening associated with stabilizing effects in gouge. These obs ervations provide a framework for understanding how slip on initially bare surfaces and gouge deformation are related.