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1 Earth and Space Sciences Department
and Institute of Geophysics and Space Physics
University of California
Los
Angeles, California 90095-1567
(E.S.C., J.E.V.)
2 Department of Earth
Sciences
University of Southern California
Los Angeles, California
90089-0740
(Y-G.L.)
* Present address: Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California at San Diego, La Jolla, California 92093-0225.
Local seismic arrays were deployed at two locations along the San Andreas fault (SAF) near Parkfield, California, before and after the 2004 M 6.0 Parkfield earthquake. Using local earthquakes we determine the anisotropic field within 1– 2 km of the main trace of the SAF at the two array locations separated by 12 km. The initial array, near the SAFOD site, was deployed for six weeks in October and November 2003, and the second array, located near the town of Parkfield, was deployed for 3 months following the 28 September 2004 M 6.0 Parkfield earthquake.
We find the fast shear-wave polarization direction nearly fault-parallel
(N40°W) for stations on the main fault trace and within 100 m to the
southwest of the SAF at both array locations. These fault-parallel
measurements span the 100- to 150-m-wide zone of pervasive cracking and damage
interpreted from fault-zone-trapped waves associated with the main fault core
(Li et al., 2004,
2006). Outside of this zone, the
fast orientations are scattered with some preference for orientations near
N10°E, roughly parallel to the regional maximum horizontal compressive
stress direction (
h). In addition, fast directions
are preferentially oriented parallel to a northern branch of the SAF
recorded on stations in the 2004 Parkfield deployment.
The measured anisotropy is likely due to a combination of stress-aligned microcracks away from the fault and shear fabric within the highly evolved fault core. The majority of our measurements are taken outside of the main fault core, and we estimate the density of microcracks from the measured delay times. Apparent crack densities are approximately 3%, with large scatter. The data suggest weak depth dependence to the measured delay times for source depths between 2 and 7 km. Below 7-km source depth, the delay times do not correlate with depth suggesting higher confining pressure is forcing the microcracks to close.
No coseismic variation in the anisotropic parameters is observed, suggesting little to no influence on measured splitting due to the 2004 M 6.0 Parkfield earthquake. However, the premainshock and postmainshock data presented here are from arrays separated by 12 km, limiting our sensitivity to small temporal changes in anisotropy.
This article has been cited by other articles:
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  R. A. Harris and J R. Arrowsmith Introduction to the Special Issue on the 2004 Parkfield Earthquake and the Parkfield Earthquake Prediction Experiment Bulletin of the Seismological Society of America, September 1, 2006; 96(4B): S1 - S10. [Abstract] [Full Text] [PDF]
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Y.-G. Li, P. Chen, E. S. Cochran, J. E. Vidale, and T. Burdette Seismic Evidence for Rock Damage and Healing on the San Andreas Fault Associated with the 2004 M 6.0 Parkfield Earthquake Bulletin of the Seismological Society of America, September 1, 2006; 96(4B): S349 - S363. [Abstract] [Full Text] [PDF]
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