HR: 1330h
AN: S42C-0179    [PDF]
TI: Rate dependence of acoustic emissions generated during granular shear
AU: * Mair, K
EM: k.mair@utoronto.ca
AF: Lassonde Institute, University of Toronto, 170 College Street, Toronto, ON M5S 3E3 Canada
AU: Marone, C
EM: cjm@geosc.psu.edu
AF: Dept. of Geosciences, Penn. State Univ., 536 Deike Building, University Park, PA 16802 United States
AU: Young, P
EM: paul.young@utoronto.ca
AF: Lassonde Institute, University of Toronto, 170 College Street, Toronto, ON M5S 3E3 Canada
AB: Earthquake systems are commonly described using rate and state dependent fault models. The parameters in these laws describe the microphysics of slip, however a detailed understanding of the connection between friction parameters and specific micro-processes remains a challenge. We present new laboratory observations using state of the art ultrasonic techniques to gain insights into dynamic processes operating during frictional sliding. Granular layers were sheared under constant normal stress for a range of loading rates in a direct shear apparatus. During experiments, we monitored high frequency acoustic emissions (AE) generated by grain fracture and friction using an array of piezoelectric transducers mounted around the sliding layers. Complete waveforms and event information (AE events/s) were collected for thousands of micro-earthquakes. Perturbations in load point velocity (V) were imposed periodically to determine the friction response and acoustic activity. For a given V, we observed that AE event rate decreased systematically with accumulated slip suggesting sensitivity to gouge layer evolution. Step increases in V induced immediate and sustained increases in acoustic activity; the converse was true for V decreases. Friction response to a velocity perturbation was consistent with previous work. The positive rate dependence of acoustic activity is unsurprising since more slip is covered per unit time at higher V, however our data on the number of events/$\mu$m exhibit a decrease with increasing V indicating a deficit of acoustic activity per unit slip at larger V. Assuming that AE result mainly from grain contact sliding, the 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 frictional contact junction area is reduced at increased sliding velocity. Our data represent unique, direct observations of micro-mechanical contact processes important in friction mechanics and hence are fundamental to earthquake physics.
DE: 5102 Acoustic properties
DE: 5112 Microstructure
DE: 7209 Earthquake dynamics and mechanics
DE: 7215 Earthquake parameters
SC: Seismology [S]
MN: 2003 Fall Meeting