HR: 0800h
AN: T41B-0580    [Abstracts]
TI: Frictional Behavior of Materials in the 3D SAFOD Volume
AU: * Carpenter, B M
EM: bcarpent@geosc.psu.edu
AF: Department of Geosciences Pennsylvania State University, 541 Deike Building, University Park, PA 16802,
AU: Marone, C J
EM: cjm@geosc.psu.edu
AF: Department of Geosciences Pennsylvania State University, 541 Deike Building, University Park, PA 16802,
AU: Saffer, D M
EM: dsaffer@geosc.psu.edu
AF: Department of Geosciences Pennsylvania State University, 541 Deike Building, University Park, PA 16802,
AB: Laboratory data for fault zone materials acquired at hypocentral depths are necessary for improving our understanding of the relationship between fault zone properties and earthquake physics. These data will illuminate the processes that determine fault strength and stability, and will allow for better assessment of seismic hazard. Here, we report on experiments designed to investigate the frictional behavior of materials in the 3D SAFOD volume including wall and fault rock. We conducted experiments on fault zone materials recovered from SAFOD drilling, and surface samples of rock formations in the 3D crustal volume surrounding the SAFOD drill hole, selected using recent detailed cross sections. Samples from San Andreas drilling range in depth (MD) from 1.4 km to 4.1 km. Surface samples include country rock, wall rock, and fault rock. We also conducted a set of experiments on synthetic mixtures of clay, quartz, and talc. We conducted shearing experiments in double direct shear, under both room conditions and in a true-triaxial pressure vessel. Layers of pulverized fault and surface rock were sheared at constant effective normal stress between rough rigid forcing blocks. Once steady state friction was obtained, shear velocity was stepped between 1 and 300 microns/s. At room temperature and humidity, normal stress ranged from 5 to 100 MPa. Under saturated conditions in a pressure vessel with pore and confining pressures applied, normal stress ranged between 5 and 20 MPa. Results for SAFOD borehole samples of granodiorite, arkosic metasandstone and siltstone indicate coefficients of sliding friction (we assume zero cohesion for the granulated layers) ranging from 0.52 to 0.66. Surface samples show consistent coefficients of friction ranging between 0.55 and 0.68. A serpentinite melange has a coefficient of friction of 0.25 to 0.30 under dry conditions, and 0.20 to 0.25 under saturated conditions. Velocity stepping tests indicate that steady-state friction values were reached and that the fault rocks exhibit slip- rate and history-dependent friction behavior similar to that documented for simulated fault gouge. A sudden increase in load point velocity results in an immediate increase in friction followed by a displacement-dependent decay to a new steady-state level. Measurements of steady-state friction as a function of slip velocity show velocity-weakening frictional behavior for some SAFOD materials at low normal stress. The serpentinite melange shows velocity strengthening behavior under dry conditions and becomes increasingly velocity strengthening under saturated conditions. Our ongoing work includes a comparison of SAF related materials under dry and saturated conditions to synthetic mixtures in an effort to gain insight into fault behavior. Future work will attempt to determine the effects of temperature and pore/confining fluids on the strength and second-order frictional characteristics of fault zone materials.
DE: 3902 Creep and deformation
DE: 5199 General or miscellaneous
DE: 8163 Rheology and friction of fault zones (8034)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting