HR: 0800h
AN: S21A-0242    [Abstracts]
TI: Frictional Strength of Hayward Fault Gouge
AU: * Morrow, C
EM: cmorrow@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025, United States
AU: Moore, D
EM: dmoore@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025, United States
AU: Lockner, D
EM: dlockner@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025, United States
AB: A recent 3-D geologic model of the Hayward fault in the San Francisco Bay Region shows that a number of different rock units are juxtaposed across the fault surface as a result of lateral displacement. The fault gouge formed therein is likely a mixture of these various rock types. To better model the mechanical behavior of the Hayward fault, which is known to both creep and have large earthquakes, frictional properties of mixtures of the principal rock types were determined in the laboratory. Room temperature triaxial shearing tests were conducted on binary and ternary mixtures of Great Valley Sequence graywacke, Franciscan jadeite-bearing metagraywacke, Franciscan pumpellyite-bearing metasandstone, Franciscan melange matrix, serpentinite and two-pyroxene gabbro. The gouge samples were crushed and sieved (<150 μm grains), then applied in a 1-mm layer between saw-cut sliding blocks. Each sample assemblage was saturated and sheared at constant pore water pressure of 1 MPa and normal stress of 51 MPa. Coefficients of friction, μ, ranged from a low of 0.38 for the serpentinite to a maximum of 0.85 for the gabbro. While the serpentinite and the Franciscan melange matrix were relatively weak, all other rock types obeyed Byerlee's Law. The friction coefficient of mixtures could be reliably predicted by a simple average based on dry weight percent of the end member strengths. This behavior is in contrast to some mixtures of common gouge materials such as montmorillonite+quartz, which exhibit non- linear frictional strength trends with varying weight percent of constituents. All materials tested except serpentinite were velocity strengthening, therefore promoting creeping behavior. The addition of serpentinite decreased a-b values of the gouge and increased the characteristic displacement, d_c, of strength evolution. Because temperature strongly influences the mechanical properties of fault gouge as well as speeding chemical reactions between the constituents, elevated temperature experiments simulating deeper seismogenic regions of the Hayward fault are planned for the near future.
DE: 5104 Fracture and flow
DE: 8118 Dynamics and mechanics of faulting (8004)
SC: Seismology [S]
MN: 2007 Fall Meeting