FAULT WEAR EXPERIMENTS, PART II: TRANSIENT AND STEADY-STATE STAGES UNDER EARTHQUAKE-LIKE SLIP-VELOCITY HISTORY

We investigated rock-wear processes on an experimental fault in a rotary shear apparatus. Sliding occurred between ring samples, about 7 cm in diameter, driven by a 225 kg rotating flywheel, and subjected to normal stresses of up to 7.0 MPa (and one case of 28 MPa). A typical experiment was chronologically conducted as follows: (1) the flywheel speed was raised to a prescribed angular velocity (up to 300 rpm), with the sample disengaged from the flywheel; (2) the motor was disengaged, while simultaneously the flywheel was engaged to the sample; (3) slip initiated if the flywheel exceeded the static friction of the sample; (4) slip seized on the experimental fault, when the kinetic energy from the flywheel was consumed. We continuously monitored the sample slip-velocity, closure across the fault surface, normal stress, shear stress, and temperature.

We present the results of 34 experiments, 19 of them with Sierra White granite samples and 15 with Kasota dolomite samples. In a typical experiment, the velocity rose quickly (<0.1 s) to the maximum slip-velocity, which ranged from 0.07 to ~1.0 m/s, depending on the preselected flywheel speed. Total slip distance in the experiments ranged from 0.016 to 5.55 m. Fault-normal displacements ranged from ~-300 microns (dilation) to ~160 microns (closure). The wear-rate was calculated from the continuously measured closure across the fault blocks, and presented here by the unit W= [(micron of surface wear) / (meter of slip distance)], with maximum value of W being ~20,000 in these experiments. We recognized three distinct modes of wear-rate variations with respect to the measured friction: Mode 1 displays an initial, short, high dilation-rate with corresponding increase of the initial friction (~10%); this transitions to a stage of long, low wear-rate accompanied with a moderate to large friction drop (30-50%). Mode 2 occurs in experiments with relatively higher peak velocities (0.5 to 1.0 m/s) that display an initial high wear-rate, which quickly decays to steady-state stage of low wear-rate. Mode 3, with no discernable wear-rate pattern, was observed in low slip-velocity experiments (<0.1 m/s). Part I of this study includes wear results of experiments with constant slip-velocity (Boneh et al., this meeting).