A Microstructural Study of Simulated and Natural Fault Gouge Using Digital Image Techniques

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Recent large-displacement experiments with simulated gouge indicate that velocity-dependence of gouge friction evolves to, and out of velocity weakening behavior, dependent upon cumulative slip history. Such rate-dependence transitions appear to coincide with certain shear localization and delocalization microstructures. Do the microstructures appear and disappear in a systematic way with cumulative displacement? The PI's propose to investigate the question through specific models with built-in tendency for shear localization and recurrent delocalization. In one model the state of gouge is determined by interaction of shear bands and surfaces at the scale of gouge thickness, in another the observed rate-dependence transitions result directly from grain-scale processes. They will investigate the validity of the models in both simulated and natural fault gouge. Simulated gouge samples will be produced by a series of rotary shear experiments; natural gouges will be collected from previously studied San Gabriel andPunchbowl fault outcrops in southern California. The rate-and state-dependent friction laws are to a large extent empirical expressions of the processes of cataclasis, healing, and development of internal structures in gouge. Microstructural parameters known to control these processes include particle size, particle size distribution, gouge layer thickness, porosity, and gouge fabrics. The coupling between microstructural and mechanical analysis is weak due to difficulties in working with randomness, variability, and discontinuity that is inherent in brittle deformation microstructures. Do cataclasticmicrostructures offer significantly more information than have been acquired so far by traditional methods? The PI's propose to use digital image techniques, which allow matrix-type manipulations of image data and application of statistical, massive and sequential data analyses. They believe that the project has clear implications for the physical definition of the evolution laws in friction constitutive equations, laboratory-to-field scaling relations, and computer simulation of gouge evolution. Furthermore, large-scale fault mechanics investigations (e.g. SAFOD) are expected to benefit from the portability and efficiency of digital image techniques that will be developed by this project.