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Laboratory observations of fault strength in response to changes in normal stress

Brian D. Kilgore, Julian C. Lozos, Nicholas M. Beeler, & David D. Oglesby

Published May 2012, SCEC Contribution #1516

Changes in fault normal stress can either inhibit or promote rupture propagation, depending on the fault geometry and on how fault shear strength varies in response to the normal stress change. A better understanding of this dependence will lead to improved earthquake simulation techniques, and ultimately, improved earthquake hazard mitigation efforts. We present the results of high resolution laboratory experiments investigating the effects of step changes in fault normal stress on the fault shear strength during sliding, using bare Westerly granite samples, with roughened sliding surfaces, in a double direct shear apparatus. Previous experimental studies examining the shear strength following a step change in the normal stress produce contradictory results: suggesting either that the shear strength of a fault responds immediately and then is followed by a prolonged slip-dependent response, or that there is no immediate component and the response is purely slip-dependent. We observe that the acoustic transmissivity and dilatancy of simulated faults in our tests respond immediately to changes in the normal stress, consistent with the interpretations of previous investigations, verifying an immediate increase in the area of contact between the roughened sliding surfaces as normal stress increased. However, the shear strength of the fault does not increase immediately indicating that the new area of contact between the rough fault surfaces does not appear pre-loaded with any shear resistance or strength. Additional slip is required for the fault to achieve a new shear strength appropriate for its new loading conditions, consistent with previous observations made during shock loading.

Kilgore, B. D., Lozos, J. C., Beeler, N. M., & Oglesby, D. D. (2012). Laboratory observations of fault strength in response to changes in normal stress. Journal of Applied Mechanics, 79(3), 031007. doi: 10.1115/1.4005883.