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The Scale-Dependence of Fault Roughness and Asperity Strength

Christopher A. Thom, Emily E. Brodsky, & David L. Goldsby

Published August 14, 2016, SCEC Contribution #6805, 2016 SCEC Annual Meeting Poster #034

The frictional properties of fault surfaces are controlled by the collective behavior of asperity contacts. For a single asperity, the force of friction is given by Ff = τa ∙ A, where τa is the shear strength of an asperity and A is its contact area [Bowden & Tabor, 1950]. While a distribution of contact sizes is expected on a frictional interface, the asperity shear strength is commonly assumed to be constant. Recent work on the scale-dependent roughness of natural faults, however, suggests that asperity strength may also be scale-dependent [Brodsky et al., 2016]. In order to directly test the link between scale-dependent fault roughness and strength, we are conducting nanoindentation tests over a wide range of indentation depths on a sample of the Corona Heights Fault surface, whose roughness is well-characterized down to nanometer length scales [Thom et al., 2015]. For the Corona Heights Fault, roughness data collected via atomic force microscopy suggest that indentation hardness, which is proportional to yield stress, should decrease as a power-law function of length scale as L-0.24. Nanoindentation tests performed on non-geologic materials often show an ‘indentation size effect’ (ISE), whereby hardness increases with decreasing indentation depth [Pharr et al., 2010] in a manner consistent with this prediction. To more generally investigate scale-dependent strengths of geologic materials, we have performed nanoindentation tests on single crystals of synthetic quartz and San Carlos olivine to depths ranging from 30 to several hundred nanometers. To date, these tests reveal a constant hardness with no ISE for quartz. In contrast, the measured hardness of olivine decreases with indentation size as a power law, with an exponent in agreement with the value predicted above for a natural crustal fault. If a similar scaling of material strength exists for faults like Corona Heights, as revealed by nanoindentation of natural fault surfaces, then our results will suggest that using a single value of τa may not be appropriate when considering multi-asperity friction on faults.

Key Words
roughness, asperity, strength, nanoindentation

Thom, C. A., Brodsky, E. E., & Goldsby, D. L. (2016, 08). The Scale-Dependence of Fault Roughness and Asperity Strength. Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)