Defining physical and mechanical properties of fault gouge and their relationship to fault behavior remains a fundamental step toward understanding earthquakes and faulting. Frictional velocity dependence is considered the most likely mechanism to explain differences between stable sliding (aseismic) and unstable stick-slip (seismogenic) behavior. Materials that exhibit velocity-strengthening behavior produce only inherently stable frictional slip, whereas those that exhibit velocity-weakening are capable of hosting unstable rupture. This work will define the frictional properties of a suite of natural and synthetic mixed-mineralogy gouges, as a function of composition. The experimental plan will focus on testing (1) natural samples of subduction zone materials that have undergone progressive alteration at temperatures up to ~250 degrees C, and (2) a suite of synthetic mixtures of montmorillonite, illite, and quartz, and developing the appropriate constitutive equations to describe their behavior. This work will include: (1) quantitative x-ray diffraction (XRD) to determine the composition of natural gouges, (2) a series of shearing experiments for each material, over a wide range of experimental conditions, and (3) petrographic and SEM characterization of microstructures in experimentally deformed gouges and comparison with natural microstructures observed in the sample localities. The work will provide high-quality measurements of frictional constitutive properties for realistic natural fault gouge materials relevant to seismic faulting, and define variations in these properties with gouge composition. This work will constitute an important step toward a better understanding of fault behavior and its potential variability with depth and along-strike. In addition, detailed characterization of fabric development in experimentally deformed gouges will provide important insights into application of laboratory measurements to natural faults.
