Earthquake systems are commonly described using rate and state dependent fault models; however, the connection between rate and state friction parameters and specific microprocesses remains a challenge. We present new laboratory observations using modern ultrasonic techniques to reveal dynamic processes operating during frictional sliding. Granular layers were sheared under constant normal stress for a range of loading rates. During experiments, we monitored high-frequency acoustic emissions (AE) generated by grain fracture and friction using an array of piezoelectric transducers surrounding the layers. Complete waveforms and event information were collected for thousands of microearthquakes. Perturbations in imposed load point velocity (V) produced a friction response consistent with previous work. For a given V, AE per sec decreased with accumulated slip, suggesting sensitivity to gouge layer evolution. Step increases in V led to immediate and sustained increases in AE per sec; the converse was true for V decreases. The positive rate dependence of AE per second is unsurprising because more slip is covered per unit time at higher V; however, AE per unit slip decreases with increasing V, indicating a deficit of acoustic activity at a faster slip rate. Assuming that AE result mainly from grain contact sliding, acoustic activity is proportional to the real area of contact between sliding particles. Our results qualitatively agree with previous experiments carried out on bare rock surfaces and support ideas that the frictional contact junction area is reduced at increased sliding velocity. We highlight a new way to visualize micromechanical contact processes important in frictional mechanics and highly relevant to earthquake physics.

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