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.

The presence of clay minerals has the potential to influence the seismic cycle, and earthquake nucleation in particular, through the frictional strength and stability of the gouge, and pore fluid pressure changes resulting from dilation or compaction during slip velocity transients (

<p>Many natural fault cores comprise volumes of extremely fine, low permeability, clay-bearing fault rocks. Should these fault rocks undergo transient volume changes in response to changes in fault slip velocity, the subsequent pore pressure transients would produce significant fault weakening or strengthening, strongly affecting earthquake nucleation and possibly leading to episodic slow slip events. Dilatancy at slow slip velocity has previously been measured in quartz-rich gouges but little is known about gouge containing clay. In this work, the mechanical behaviour of synthetic quartz-kaolinite fault gouges and their volume response to velocity step changes were investigated in a suite of triaxial deformation experiments at effective normal stresses of 60MPa, 25MPa and 10MPa. Kaolinite content was varied from 0 to 100wt% and slip velocity was varied between 0.3 and 3 microns/s.</p><p>Upon a 10-fold velocity increase or decrease, gouges of all kaolinite-quartz contents displayed measurable volume change transients. The results show the volume change transients are independent of effective normal stress but are sensitive to gouge kaolinite content. Peak dilation values did not occur in the pure quartz gouges, but rather in gouges containing 10wt% to 20wt% kaolinite. Above a kaolinite content of 10wt% to 20wt%, both dilation and compaction decreased with increasing gouge kaolinite content. At 25MPa effective normal stress, the normalised volume changes decreased from 0.1% to 0.06% at 10wt% to 100wt% kaolinite.  The gouge mechanical behaviour shows that increasing the gouge kaolinite content decreases the gouge frictional strength and promotes more stable sliding, rather than earthquake slip. Increasing the effective normal stress slightly decreases the frictional strength, enhances the chance of earthquake nucleation, and has no discernible effect on the magnitude of the pore volume changes during slip velocity changes.</p><p>Low permeabilities of clay-rich fault gouges, coupled with the observed volume change transients, could produce pore pressure fluctuations up to 10MPa in response to fault slip. This assumes no fluid escape from an isolated fault core. Where the permeability is finite, any pore pressure changes will be mediated by fluid influx into the gouge. Volume change transients could therefore be a significant factor in determining whether fault slip leads to earthquake nucleation or a dampened response, possibly resulting in episodic slow slip in low permeability fault rock volumes.</p>

<p>Field observations have shown that mature fault zones are rich in clay minerals (e.g. MTL in Japan, Punchbowl Fault in USA, and Alpine Fault Zone in New Zealand). Most mature fault zones are also seismogenic, which is at odds with the velocity strengthening behaviour observed for clay-bearing material in rock deformation experiments. The measurements of rate and state friction in clay-bearing material show that most clay-bearing material would favour aseismic creep when the experiments are conducted at room temperature. To address this disparity between experimental and field observations, a set of controlled friction experiments were devised to investigate the effect of varying temperature conditions on the frictional properties of clay-bearing fault gouges.<br>The velocity-step friction experiments were conducted in a triaxial deformation apparatus at an effective normal stress of 90MPa and ambient temperatures that increased from room temperature (23°C) to 180°C in increments of 40°C. In order to measure the rate and state frictional properties of the fault gouges, the imposed slip velocity was stepped between 0.3-3 μm/s. The simulated quartz-clay fault gouges had controlled clay (kaolinite) contents in increments of 25wt% from 0-100wt%. Preliminary results show that by increasing the ambient temperature during fault slip, the rate and state friction parameter [a–b] consistently decreases significantly in clay-bearing fault gouges, often from a velocity strengthening [a–b] value to a weakening [a–b] value. This is consistent with the previous, limited studies of clay-bearing material at elevated temperatures. In the clay-poor gouges, the velocity weakening [a–b] parameter is accompanied by dynamic stick-slip behaviour, whereas in clay-rich gouges the velocity weakening [a–b] parameter shows initially unstable slip that is dampened and arrests to aseismic slip. The elevated temperatures in fault zones at depths up to ~6km, as investigated in this study, can therefore lead to unstable fault slip in clay-rich material that is velocity strengthening at room temperature. It is proposed that elevated temperatures are an important component of seismogenic slip occurring in clay-rich material, as is observed in natural faults.</p>