HR: 16:00h
AN: T44B-01    [Abstracts]
TI: Structure and Composition of the San Andreas Fault at Seismogenic Depths: Recent Results from the SAFOD Experiment
AU: * Hickman, S
EM: hickman@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025, United States
AU: Zoback, M
EM: zoback@pangea.stanford.edu
AF: Stanford University, Dept. of Geophysics, Stanford, CA 94305, United States
AU: Ellsworth, W
EM: ellsworth@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025, United States
AU: Kirschner, D
EM: dkirschn@eas.slu.edu
AF: Saint Louis University, Dept. of Earth and Atmospheric Sciences, St. Louis, MO 63101, United States
AU: Solum, J
EM: j.solum@shell.com
AF: Shell International Exploration and Production Co., 3737 Bellaire Blvd., Houston, TX 77025,
AB: The San Andreas Fault Observatory at Depth (SAFOD) was drilled into the San Andreas Fault Zone to study the physics of earthquake nucleation and rupture and determine the composition, physical properties, and mechanical behavior of an active, plate-bounding fault at seismogenic depths. SAFOD is located 10 km NW of Parkfield, CA, and penetrates a section of the fault that is moving through a combination of repeating microearthquakes and fault creep. During Phases 1 and 2 in the summers of 2004 and 2005, SAFOD was drilled vertically to a depth of 1.5 km and then deviated to penetrate the active San Andreas Fault Zone at a vertical depth of about 2.7 km. During Phase 3 in the summer of 2007, cores were acquired from holes branching off the main SAFOD borehole to directly sample fault and country rocks at depth. Geophysical logs and cuttings analyses conducted during Phases 1 and 2 define the San Andreas Fault Zone to be relatively broad (~250 m), containing several discrete, highly localized zones only 2-3 m wide that exhibit very low P- and S-wave velocities and low resistivity. Two of these zones have progressively deformed the cemented casing at measured depths of 3194 m and 3301 m, indicating that they are actively creeping shear zones. These active shear zones were targeted for coring during Phase 3. The 3194 m casing deformation zone lies ~100 m above a cluster of repeating M2 earthquakes that form the southwestern boundary of the creeping and microseismically active San Andreas Fault Zone. Casing deformation is most pronounced across the 3301 m zone; hence this zone is believed to accommodate most of the current creep deformation across the San Andreas Fault at this location. During Phase 3 we have obtained core from just outside the geologically defined San Andreas Fault Zone, at the boundary between the Salinian and Great Valley/Franciscan terranes, and from the active deformation zones at 3194 and 3301 m. The cores obtained from these deformation zones exhibit a variety of features indicating pronounced strain localization and probably marked weakening. These include highly sheared and foliated shales and siltstones, cataclasites, veined serpentinite and chert bodies, and compacted, cohesionless serpentine-bearing fault gouge, all juxtaposed in shear zones only a few meters wide. The occurrence of serpentinite is particularly significant, because serpentine and related minerals are widely regarded to be important in controlling frictional strength and the stability of sliding. The Phase 3 core samples will be extensively tested in the laboratory to study the composition, deformation mechanisms, physical properties and rheological behavior of fault rocks from the active traces of the San Andreas Fault at realistic in-situ conditions.
UR: http://www.earthscope.org/
DE: 5104 Fracture and flow
DE: 5112 Microstructure
DE: 7215 Earthquake source observations (1240)
DE: 8159 Rheology: crust and lithosphere (8031)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting