The Effect of Clay Content on the Dilatancy and Frictional Properties of Fault Gouge

Mature fault cores are comprised of extremely fine, low permeability, clay‐bearing gouges. Saturated granular fault materials are known to dilate in response to increases in sliding velocity, resulting in significant pore pressure drops that can suppress instability. Up to now, dilatancy has been measured predominantly in clay‐poor gouges. Clay minerals have low frictional strengths and, in previous experiments, even small proportions of clay minerals were shown to affect the frictional properties of a fault. It is important, therefore, to document in detail the impact of the proportion of clay on the frictional behavior and dilatancy of fault rocks. In this work, a suite of triaxial deformation experiments elucidated the frictional behavior of saturated, synthetic quartz‐clay (kaolinite) fault gouges at effective normal stresses of 60, 25, and 10 MPa. Upon a 10‐fold velocity increase, gouges of all clay‐quartz contents displayed measurable dilatancy with clay‐poor samples yielding comparable changes to previous studies. Peak dilation did not occur in the pure quartz gouges, but rather in gouges containing 10 to 40 wt% clay. The clay content of the simulated gouges was found to control the gouge frictional strength and the stability of slip. A transition occurred at ∼40 wt% clay from strong, unstably sliding quartz‐dominated gouges to weak but stably sliding clay‐dominated gouges. These results indicate that in a low permeability, clay‐rich fault zone, the increases in pore volume could generate pore‐fluid pressure transients, contributing to the arrest of earthquake nucleation or potentially the promotion of sustained slow slip. Plain Language Summary The strength of faults is dependent on the fluid pressure contained within the pore space of fine‐grained fault rocks on which fault slip events such as earthquakes occur. When slip velocity increases at the start of an earthquake, the fault rocks dilate, thereby reducing the fluid pressure and consequently strengthening the fault. If this strengthening is sufficient, an earthquake may arrest. Clay minerals are commonly present within faults, but previous measurements of fault volume changes have focused on clay‐poor materials. In this study, a series of experiments simulated the conditions within a tectonic fault in the Earth's crust. The experiments focused on the effect of changing the proportion of clay on the strength, slip behavior and volume changes of the fault material. All of the experiment materia