U.S. Geological
Survey
Open-File Report 01-421
Hayward Fault Rocks: Porosity, Density, and Strength Measurements
By
C.A. Morrow and D.A. Lockner
Porosity,
density and strength measurements were conducted on rock samples collected
from the Hayward Fault region in Northern California as part of the Hayward
Fault Working Groups efforts to create a working model of the Hayward
Fault. The rocks included in this study were both fine and coarse grained
gabbros, altered keratophyre, basalt, sandstone, and serpentinite from
various rock formations adjacent to the Hayward Fault. Densities
ranged from a low of 2.25 gm/cc (altered keratophyre) to 3.05 gm/cc (fine
gabbro), with an average of 2.6 gm/cc, typical of many other rocks. Porosities
were generally around 1% or less, with the exception of the sandstone
(7.6%) and altered keratophyre (13.5%). Failure and
frictional sliding tests were conducted on intact rock cylinders at room
temperature under effective pressure conditions of up to 192 MPa, simulating
depths of burial to 12 km. Axial shortening of the samples progressed
at a rate of 0.1 µm/sec (fine samples) or 0.2 µm/sec (porous
samples) for 6 mm of displacement. Velocity stepping tests were then conducted
for an additional 2 mm of displacement, for a total of 8 mm. Both peak
strength (usually failure strength) and frictional strength, determined
at 8 mm of displacement, increased systematically with effective pressure.
Coefficients of friction, based on the observed fracture angles, ranged
from 0.6 to 0.85, consistent with Byerlees Law. Possible secondary
influences on the strength of the Hayward rock samples may be surface
weathering, or a larger number of pre-existing fractures due to the proximity
to the |
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Maintained by: Carolyn Donlin
Created: 12-11-01
Last modified: 12-18-01 (cad)