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Science 16 November 2001:
Vol. 294. no. 5546, pp. 1513 - 1515
DOI: 10.1126/science.1065100
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Reports

Steep-Dip Seismic Imaging of the Shallow San Andreas Fault Near Parkfield

J. A. Hole,1* R. D. Catchings,2 K. C. St. Clair,1 M. J. Rymer,2 D. A. Okaya,3 B. J. Carney1dagger

Seismic reflection and refraction images illuminate the San Andreas Fault to a depth of 1 kilometer. The prestack depth-migrated reflection image contains near-vertical reflections aligned with the active fault trace. The fault is vertical in the upper 0.5 kilometer, then dips about 70° to the southwest to at least 1 kilometer subsurface. This dip reconciles the difference between the computed locations of earthquakes and the surface fault trace. The seismic velocity cross section shows strong lateral variations. Relatively low velocity (10 to 30%), high electrical conductivity, and low density indicate a 1-kilometer-wide vertical wedge of porous sediment or fractured rock immediately southwest of the active fault trace.

1 Department of Geological Sciences, Virginia Tech, Blacksburg, VA 24061-0420, USA.
2 U.S. Geological Survey, MS 977, 345 Middlefield Road, Menlo Park, CA 94025, USA.
3 Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089, USA.
*   To whom correspondence should be addressed. E-mail: hole{at}vt.edu

dagger    Present address: Columbia Natural Resources, 900 Pennsylvania Avenue, Charleston, WV.

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THIS ARTICLE HAS BEEN CITED BY OTHER ARTICLES:
Arkosic rocks from the San Andreas Fault Observatory at Depth (SAFOD) borehole, central California: Implications for the structure and tectonics of the San Andreas fault zone.
S. D. Springer, J. P. Evans, J. I. Garver, D. Kirschner, and S. U. Janecke (2009)
Lithosphere 1, 206-226
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Distributed deformation in the lower crust and upper mantle beneath a continental strike-slip fault zone: Marlborough fault system, South Island, New Zealand.
C. K. Wilson, C. H. Jones, P. Molnar, A. F. Sheehan, and O. S. Boyd (2004)
Geology 32, 837-840
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A Look Inside the San Andreas fault at Parkfield Through Vertical Seismic Profiling.
J. A. Chavarria, P. Malin, R. D. Catchings, and E. Shalev (2003)
Science 302, 1746-1748
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High-Resolution Seismic Velocities and Shallow Structure of the San Andreas Fault Zone at Middle Mountain, Parkfield, California.
(2002)
Bulletin of the Seismological Society of America 92, 2493-2503
Reply to "Comment on 'Faulting Apparently Related to the 1994 Northridge, California, Earthquake and Possible Co-Seismic Origin of Surface Cracks in Potrero Canyon, Los Angeles County, California,' by R. D. Catchings, M. R. Goldman, W.H.K. Lee, M. J. Rymer, and D. J. Ponti," by Brian J. Swanson, Allan E. Seward, Perry L. Ehlig, and James E. Slosson.
R. D. Catchings, M. J. Rymer, and M. R. Goldman (2002)
Bulletin of the Seismological Society of America 92, 2539-2550
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