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Poster Abstracts
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MECHANICAL AND MICROSTRUCTURAL OBSERVATIONS OF DYNAMIC WEAKENING IN SMECTITE-
RICH SAFOD GOUGE (A-091)
M.E. French, H. Kitajima, J.S. Chester, and F.M. Chester
We present the results of high-speed rotary shear experiments on gouge from the Central Deforming Zone (CDZ) of the San
Andreas Fault (SAF). Core from the CDZ, which currently creeps aseismically, was recovered during Phase 3 drilling of the
San Andreas Fault Observatory at Depth (SAFOD). The sample tested is a well-foliated smectite – rich gouge that contains
clasts of serpentine, quartz, feldspar, and opaques. The sample was flaked to ~600 µm to preserve the clay fabric; a ~1 mm
thick layer was sheared between gabbro blocks 25 mm in diameter and semi-sealed with a press-fit Teflon ring. Experiments
were conducted at a constant normal stress of 0.3 - 1.5 MPa, velocity of 0.1 - 1.3 m/s, and displacement up to 20 m (we refer to
the local condition at 2/3 the sample radius). We tested room-dry and water-saturated gouge at room temperature and
measured the gouge temperature on a representative suite of samples. Microstructural analyses were conducted on radial cut
thin sections which show slip-perpendicular structures formed under spatial gradients in velocity and displacement.
The CDZ gouge weakens with an increase in velocity. The steady-state coefficient of friction of the wet gouge varies from µ=
0.40 at 0.1 m/s to µ= 0.14 at 1.3 m/s. Temperature within the gouge layer exceeds the water-vapor phase transition in the
water-saturated gouge sheared at 0.35-1.3 m/s. In general, the coefficient of friction and temperature of the dry gouge are
greater than that of the wet gouge. Microstructures show a progression in deformation mechanisms with increasing
displacement and velocity. A clay-foliation develops by ~1 m of displacement. Under the highest velocity and/or
displacement conditions poorly-foliated to nonfoliated regions develop parallel and oblique to the foliation. In some cases the
non-foliated, disaggregated gouge cross-cuts and disrupts adjacent foliated zones and contains fragments of foliated material.
The disrupted zones occur in regions of different displacement and velocity conditions where the temperatures may have
risen high enough to promote pore fluid pressurization and vaporization. Because these experiments are essentially vented,
we expect greater weakening in-situ where fluid escape is limited. Thus, although previous studies have shown this material
to be velocity-strengthening at sub-seismic slip rates, the creeping segment of the SAF and similar clay-rich faults may be
capable of propagating seismic rupture.
THE SALTON SEISMIC IMAGING PROJECT: INVESTIGATING EARTHQUAKE HAZARDS IN THE SALTON
TROUGH, SOUTHERN CALIFORNIA (B-134)
G.S. Fuis, M.R. Goldman, R.R. Sickler, R.D. Catchings, M.J. Rymer, E.J. Rose, J.M. Murphy, L.A. Butcher, J.A. Cotton, C.J.
Criley, D.S. Croker, I. Emmons, A.J. Ferguson, M.A. Gardner, E.G . Jensen, R. McClearn, C.L. Loughran, C.J. Slayday-
Criley, J.R. Svitek, J.A. Hole, J.M. Stock, S. Skinner, N.W. Driscoll, A.J. Harding, J. Babcock, G.M. Kent, A. Kell, and S.H.
Harder
The Salton Seismic Imaging Project (SSIP) is a collaborative effort between academia and the U.S. Geological Survey to
provide detailed, subsurface 3-D images of the Salton Trough of southern California and northern Mexico. From both active-
and passive-source seismic data that were acquired both onshore and offshore (Salton Sea), the resulting images will provide
insights into earthquake hazards, rift processes, and rift-transform interaction at the southern end of the San Andreas Fault
system. The southernmost San Andreas Fault (SAF) is considered to be at high-risk of producing a large damaging earthquake,
yet the structure of this and other regional faults and that of adjacent sedimentary basins is not currently well understood.
164 | Southern California Earthquake Center