The effect of clay content on the dilatancy and frictional properties of fault gouge.

Abstract

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 only 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 behaviour and dilatancy of fault rocks. In this work, a suite of triaxial deformation experiments elucidated the frictional behaviour of saturated, synthetic quartz-clay (kaolinite) fault gouges at effective normal stresses of 60 MPa, 25 MPa 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 20 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.

Rupture and Afterslip Controlled by Spontaneous Local Fluid Flow in Crustal Rock

“…Our observations conducted in initially intact rock are directly relevant to rupture and slip in “immature” fault systems, or faults that have been substantially sealed or healed during the interseismic period. In more “mature” fault rocks containing clay‐rich gouges, dilatancy is also significant (e.g., Ashman & Faulkner, 2023), and we expect a similar qualitative behavior to that observed in intact rocks, albeit with different magnitudes for fluid pressure drops.…”

Fracture Permeability Enhancement During Fluid Injection Modulated by Pressurization Rate and Surface Asperities

“…Fluid pressurization of the fracture requires injection of fluid volume from the pump, according to the compression‐induced pressurization principle of fluids (Ji, Kluge, et al., 2022; Kestin, 1979). Particularly, the elevation of fluid pressure is linearly correlated with the increase of injected fluid volume when the sample volume is constant, and thus any changes in sample volume result in variations of the injected fluid volume (cf., Ashman & Faulkner, 2023; Samuelson et al., 2009). The slip displacement starts to increase when the fluid pressure reaches the critical activation fluid pressure.…”

Pore Pressure Drop During Dynamic Rupture and Conditions for Dilatancy Hardening

“…Grain comminution is associated with widening of the particle‐size distribution, which in turn can inhibit dilatation (Bedford & Faulkner, 2021; Ferdowsi & Rubin, 2020; Guo & Morgan, 2006; Mair et al., 2002). Indeed, mature faults, for which variations in particle size may exceed four orders of magnitude (Chester et al., 2005), and clayey ultracataclastic gouges exhibit reduced dilatation (Ashman & Faulkner, 2023; Delle Piane et al., 2017; Mair & Marone, 1999). Present results neglecting grain comminution and using a narrow grain size distribution are therefore relevant rather for relatively young faults with small accumulated displacement or for a rupture of fresh rock.…”