| Abstract: | 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 Group?s 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 ?m/sec (fine
samples) or 0.2 ?m/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 Byerlee?s
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
Hayward Fault. All samples showed velocity strengthening, so that the average a-b values
were all strongly positive. There was no systematic relation between a-b values and
effective pressure. Velocity strengthening behavior is associated with stable sliding
(creep), as observed in the shallow portions of the Hayward Fault. |
| Genre: | USGS Numbered Series |
| ProdID: | 31489 |
| Citation Author: | Morrow, C. A.; Lockner, D. A. |
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| Citation Language: | ENGLISH |
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| Citation Phsyical Description: | 28 p. |
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| Citation Series: | Open-File Report |
| Citation Series Code: | OFR |
| Citation Series Number: | 2001-421 |
| Citation Search Results Text: | Hayward Fault rocks: porosity, density, and strength measurements; 2001; OFR; 2001-421; Morrow, C. A.; Lockner, D. A. |
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| Citation Year: | 2001 |
| Type: | citation/reference |
| Text: | Hayward Fault rocks: porosity, density, and strength measurements; 2001; OFR; 2001-421; Morrow, C. A.; Lockner, D. A. |
| URL (THUMBNAIL): | http://pubs.er.usgs.gov/thumbnails/usgs_thumb.jpg |
| URL (INDEX PAGE): | http://geopubs.wr.usgs.gov/open-file/of01-421/ |
| Date Other: | Fri, 1 Mar 2002 00:00 -0600 |
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