Reduction of friction on geological faults by weak-phase smearing

• Experimental investigation of effect of weak phase additive on fault gouge friction.
• Small amounts of added graphite cause large decrease in friction coefficient.
• Microstructural study shows graphite becomes smeared over slip plane.
• Accounting for smear effect causes weak-phase addition to follow a linear mixing law.
• Smear effect may contribute significantly to low friction in natural faults.

Most common crustal rock types display friction coefficients of 0.6 or higher, but some faults must be frictionally weak as they slip when the stress state is unfavourably-oriented (i.e. the resolved shear stress is low for a given normal stress across the fault surface). A role for low-friction minerals and high pore fluid pressures, either separately or in combination, is frequently invoked to explain such slip, but volume fractions of dispersed weak phases often seem to be present in fault gouge in amounts too small to produce significant mechanical weakening. By means of mechanical tests on synthetic fault gouge and microstructural study of run products, we show that the effective area of an embedded weak phase (graphite) on a slip plane can be substantially increased by mechanical smearing, and that the enlarged area of the weak phase on the slip plane follows a linear mixing law. This allows a relatively small volume fraction of the initially dispersed weak phase to have a disproportionately large effect, provided the smearing is concentrated into a narrow planar slip zone or into an interconnected network of them.