Effect of hydration state on the frictional properties of montmorillonite-based fault gouge

We report on laboratory experiments examining the effect of hydration state on the frictional properties of simulated clay and quartz fault gouge. We tested four mixtures of Ca-montmorillonite and quartz (100, 70, 50, and 30% montmorillonite) at four hydration states: dry (<4.50 wt% water), one water interlayer equivalent (4.5-8.7 wt% water), two layers (8.7-16.0 wt% water), and three layer (>16.0 wt% water). We controlled the hydration state using either oven drying (for <13 wt% H2O) or saline solutions (to achieve >13 wt% H2O under conditions of controlled relative humidity). For each clay/quartz mixture and hydration state, we measured frictional properties over a range of normal stresses (5-100 MPa) and sliding velocities (1-300 mu m/s). We observe a systematic decrease in the coefficient of friction (mu) with increasing water content, normal stress, and clay content. Values of mu for 50/50 mixtures range from 0.57 to 0.64 dry and decrease to 0.21-0.55 for the most hydrated cases (wet). For layers of 100% montmorillonite, mu ranges from 0.41-0.62 dry to 0.03-0.29 wet. As water content is increased from 0 to 20.0 wt%, the friction rate parameter a-b becomes increasingly positive. Variation in a-b values decreases dramatically as normal stress increases. If our experimental results can be applied to natural fault gouge, the combination of stress state, hydration state, and quartz content that facilitates unstable fault behavior implies that the onset of shallow seismicity in subduction zones is more complicated than a simple transition from smectite to illite.