HR: 16:30h
AN: T24B-03    [Abstracts]
TI: Strength of the San Andreas Fault Zone: Insight From SAFOD Cuttings and Core
AU: * Tembe, S
EM: stembe@ic.sunysb.edu
AF: Department of Geosciences, State University of New York at Stony Brook, Stony Brook, NY 11794-2100 United States
AU: Lockner, D A
EM: dlockner@usgs.gov
AF: US Geological Survey, 345 Middlefield Road, MS/977, Menlo Park, CA 94025 United States
AU: Solum, J G
EM: jsolum@usgs.gov
AF: US Geological Survey, 345 Middlefield Road, MS/977, Menlo Park, CA 94025 United States
AU: Morrow, C A
EM: cmorrow@usgs.gov
AF: US Geological Survey, 345 Middlefield Road, MS/977, Menlo Park, CA 94025 United States
AU: Wong, T
EM: teng-fong.wong@stonybrook.edu
AF: Department of Geosciences, State University of New York at Stony Brook, Stony Brook, NY 11794-2100 United States
AU: Moore, D E
EM: dmoore@usgs.gov
AF: US Geological Survey, 345 Middlefield Road, MS/977, Menlo Park, CA 94025 United States
AB: Cuttings acquired during drilling of the SAFOD scientific hole near Parkfield, California offer a continuous physical record of the lithology across the San Andreas fault (SAF) zone and provide the only complete set of samples available for laboratory testing. Guided by XRD clay mineral analysis and velocity and gamma logs, we selected washed cuttings from depths spanning the main hole from 1.85 to 3.0 km true vertical depth. Cuttings were chosen to represent primary lithologic units as well as significant shear zones, including candidates for the currently active SAF. To determine frictional properties triaxial sliding tests were conducted on cylindrical granite blocks containing sawcuts inclined at 30° and filled with 1 mm-thick sample gouge layers. Tests were run at constant effective normal stresses of 10 and 40 MPa and constant pore pressure of 1 MPa. Samples were sheared up to 10.4 mm at room temperature and velocities of 1, 0.1 and 0.01 μm/s. Stable sliding behavior and overall strain hardening were observed in all tests. The coefficient of friction typically showed a modest decrease with increasing effective normal stress and mostly velocity strengthening was observed. Preliminary results yield coefficients of friction, μ, which generally fell into two clusters spanning the range of 0.45 to 0.8. The higher values of friction (~0.7 - 0.8) corresponded to quartzofeldspathic samples derived from granodiorites and arkoses encountered in the drill hole. Lower values of friction (0.45 - 0.55) were observed at depth intervals interpreted as shear zones based on enriched clay content, reduced seismic velocities and increased gamma radiation. Arguments for a weak SAF suggest coseismic frictional strength of μ = 0.1 to 0.2 yet the actual fault zone materials studied here appear consistently stronger. At least two important limitations exist for inferring in-situ fault strength from cuttings. (1) Clays and weak minerals are preferentially lost during drilling and therefore undersampled in the cuttings and (2) cuttings are mixed as they travel up the borehole. To test the validity of this approach sliding tests were conducted on core samples obtained from a prominent fault zone at 2.56 km (10062 ft measured depth). Coefficient of friction was measured to be 0.42-0.5, notably weaker than that for cuttings tested at this depth (~0.6) but similar to values obtained for other shear zones. This difference between core and cuttings from the equivalent depth is likely due to mixing, resulting in the averaging of mechanical properties over a 1 to 10 foot interval. Nevertheless, we find good agreement in the strength of materials obtained from shallow shear zones, an indication that some weak mineral phases are preserved in the cuttings. While our findings indicate that meaningful mechanical data can be derived from the cuttings, it should be noted that these observations do not represent an exhaustive study of SAF frictional strength. We continue to explore the effectiveness of the present technique by a variety of methods. For example, estimates of lost clay fractions determined from XRD analysis of unwashed cuttings can be used in the application of approximate mixing laws to correct friction measurements. In addition, comparisons of strength of cuttings and corresponding sidewall cores will help refine our results. While the analysis of cuttings provides the best fault zone strength data to date, unresolved questions show the importance of collecting continuous core in Phase 3 drilling planned for 2007.
DE: 1859 Rocks: physical properties
DE: 5199 General or miscellaneous
DE: 8010 Fractures and faults
DE: 8034 Rheology and friction of fault zones (8163)
DE: 8163 Rheology and friction of fault zones (8034)
SC: Tectonophysics [T]
MN: Fall Meeting 2005