FAULT ZONE STRENGTH AND FAILURE CRITERIA

This paper discusses Coulomb failure criteria for brittle deformation of intact rock and fault gouge. Data are presented from laboratory exp eriments designed to identify the critical gouge layer thickness requi red to effect a transition from the standard Coulomb criterion to a mo dified failure law (referred to as Coulomb plasticity) appropriate for simple shear of a gouge layer. Experiments were carried out using ten sion fractures and quartz powder to simulate granular fault gouge. Fra ctures sheared without gouge obey the standard Coulomb law. A 0.6mm-th ick gouge layer was required to effect the transition to Coulomb plast icity. I test and reject the hypothesis that fault zone strength and a pparent coefficient of internal friction can be predicted from fractur e of intact rock simply by accounting for differences in the failure l aws and without considering variations in the Coulomb parameters. The data presented indicate that the stress state required for Coulomb pla sticity is not developed within very thin gouge layers. This work impl ies that brittle fault zones have lower friction than predictions base d on the strength of intact rock. However, the magnitude of this weake ning effect is small (for example, a coefficient of sliding friction o f 0.75 would be reduced to 0.6) and thus it is not an independent expl anation of the apparent weakness of mature faults.