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Research Papers

Laboratory Observations of Fault Strength in Response to Changes in Normal Stress

[+] Author and Article Information
Brian Kilgore

U.S. Geological Survey,  Earthquake Science Center, Menlo Park, CA 94025 e-mail: bkilgore@usgs.gov

Julian Lozos

Department of Earth Sciences,  University of California, Riverside, CA 92521 e-mail: jlozo001@ucr.edu

Nick Beeler

U.S. Geological Survey,  Earthquake Science Center, Menlo Park, CA 94025; U.S. Geological Survey,  Cascades Observatory, Vancouver, WA 98683 e-mail: nbeeler@usgs.gov

David Oglesby

Department of Earth Sciences,  University of California, Riverside, CA 92521 e-mail: david.oglesby@ucr.edu

J. Appl. Mech. 79(3), 031007 (Apr 05, 2012) (10 pages) doi:10.1115/1.4005883 History: Received July 01, 2011; Revised November 07, 2011; Posted February 06, 2012; Published April 04, 2012; Online April 05, 2012

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 new 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: a set of double direct shear experiments indicates that the shear strength of a fault responds immediately, and then is followed by a prolonged slip-dependent response, while a set of shock loading experiments indicates that there is no immediate component, and the response is purely gradual and slip-dependent. In our new, high-resolution experiments, 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, and verify an immediate increase in the area of contact between the roughened sliding surfaces as normal stress increases. However, the shear strength of the fault does not immediately increase, indicating that the new area of contact between the rough fault surfaces does not appear preloaded 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.

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Copyright © 2012 by American Society of Mechanical Engineers
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Figures

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Figure 1

Effect of changes in normal stress on shear stress for bare granite at room temperature and 5 MPa normal stress from the prior study of Linker and Dieterich [17]. Shear stress versus load point displacement for changes in the normal stresses of 1, 2, 5, 10, 20, and 40% of the ambient value. Note that the 40% change is shown at a compressed vertical scale compared to the other tests. Linker and Dieterich [17] interpret the response to have 3 components. First, there is an instantaneous change marked by the open circle. This is a machine effect due to misalignment of the loading frame or Poisson expansion of the fault surface as the fault normal stress is changed. Next, there is an immediate change from the open circle to the solid circle; this is interpreted as an immediate increase in contact area due to the increased normal stress. Subsequently, there is a further prolonged increase in shear resistance with displacement.

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Figure 2

Response of shear resistance of a bimaterial interface of WC and 4340 structural steel to changes in normal stress at high stress from the prior study of Prakash [19]. Shear (solid line) and normal (dotted-dashed line) stress with time during imposition of a large reduction in normal stress. The normal stress change does not induce a significant immediate change in shear resistance, rather, virtually the entire response is slow.

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Figure 3

The biaxial experimental geometry used, consisting of applied shear and normal forces. Instrumentation includes a fault slip sensor, a fault normal displacement sensor, and a pair of acoustic transmitters/receivers.

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Figure 4

Representative data showing (a) step changes in normal stress, and the subsequent response of (b) shear stress, (c) acoustic transmissivity, and (d) fault closure, responding to the change in fault normal stress, plotted versus load point displacement × load point velocity (1 μm/s). (b) A line with slope equal to the estimated stiffness of the loading system (∼0.75 MPa/μm) is shown for reference. (e) Observed fault slip during a step up in normal stress (time = 0) from 5 MPa to 7 MPa.

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Figure 5

Forward model of the relations, Eqs. 2a and 2b developed by Linker and Dieterich [19], to simulate their experimental observations. Parameters used in these simulations of our experiments are f0 =0.7, a = 0.008, b = 0.01, dc = 0.5 μm, α = 0.2 and k = 0.75 MPa/μm. Note the linear rise in shear stress in response to the step change in the applied normal stress. In the shear stress plot is line with slope equal to the estimated stiffness of the loading system (∼0.75 MPa/μm) is shown for reference.

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Figure 6

Forward models of the relations 2a, 7, and 8 developed by Prakash [19] to simulate his experimental observations. Parameters used in the simulations of our observations and testing procedure are the same as in Fig. 5: f0  = 0.7, a = 0.008, b = 0.01, dc = 0.5 μm, and k = 0.75 MPa/μm. Note the gradual nonlinear rise in shear stress in response to the step change in the applied normal stress. In the shear stress plot a line with slope equal to the estimated stiffness of the loading system (∼0.75 MPa/μm) is shown for reference.

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