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1 Department of Geology and
Geophysics
University of Wisconsin–Madison
1215 W. Dayton
Street
Madison, Wisconsin 53706
(C.T, H.Z.)
2 Lamont Doherty Earth
Observatory
P.O. Box 1000
61 Route 9W
Palisades, New York
10964-1000
(F.W.)
3 U.S. Geological Survey
345
Middlefield Road
Menlo Park, California 94025
(J.H., A.M.)
4 Institute for Geological and Nuclear
Sciences Limited
Dunedin Research Centre
764 Cumberland Street
Private
Bag 1930
Dunedin, New Zealand
(D.E.-P.)
We present a new three-dimensional (3D) compressional wavespeed (Vp) model for the Parkfield region, taking advantage of the recent seismicity associated with the 2003 San Simeon and 2004 Parkfield earthquake sequences to provide increased model resolution compared to the work of Eberhart-Phillips and Michael (1993) (EPM93). Taking the EPM93 3D model as our starting model, we invert the arrival-time data from about 2100 earthquakes and 250 shots recorded on both permanent network and temporary stations in a region 130 km northeast–southwest by 120 km northwest–southeast. We include catalog picks and cross-correlation and catalog differential times in the inversion, using the double-difference tomography method of Zhang and Thurber (2003). The principal Vp features reported by EPM93 and Michelini and McEvilly (1991) are recovered, but with locally improved resolution along the San Andreas Fault (SAF) and near the active-source profiles. We image the previously identified strong wavespeed contrast (faster on the southwest side) across most of the length of the SAF, and we also improve the image of a high Vp body on the northeast side of the fault reported by EPM93. This narrow body is at about 5- to 12-km depth and extends approximately from the locked section of the SAF to the town of Parkfield. The footwall of the thrust fault responsible for the 1983 Coalinga earthquake is imaged as a northeast-dipping high wavespeed body. In between, relatively low wavespeeds (<5 km/sec) extend to as much as 10-km depth. We use this model to derive absolute locations for about 16,000 earthquakes from 1966 to 2005 and high-precision double-difference locations for 9,000 earthquakes from 1984 to 2005, and also to determine focal mechanisms for 446 earthquakes. These earthquake locations and mechanisms show that the seismogenic fault is a simple planar structure. The aftershock sequence of the 2004 mainshock concentrates into the same structures defined by the pre-2004 seismicity, confirming earlier observations (Waldhauser et al., 2004) that the seismicity pattern at Parkfield is long lived and persists through multiple cycles of mainshocks.
Online material: 3D Vp model and earthquake relocations.
This article has been cited by other articles:
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  B. P. Allmann and P. M. Shearer A High-Frequency Secondary Event During the 2004 Parkfield Earthquake Science, November 23, 2007; 318(5854): 1279 - 1283. [Abstract] [Full Text] [PDF]
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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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R. W. Simpson, M. Barall, J. Langbein, J. R. Murray, and M. J. Rymer San Andreas Fault Geometry in the Parkfield, California, Region Bulletin of the Seismological Society of America, September 1, 2006; 96(4B): S28 - S37. [Abstract] [Full Text] [PDF]
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J. B. Fletcher, P. Spudich, and L. M. Baker Rupture Propagation of the 2004 Parkfield, California, Earthquake from Observations at the UPSAR Bulletin of the Seismological Society of America, September 1, 2006; 96(4B): S129 - S142. [Abstract] [Full Text] [PDF]
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P. Liu, S. Custodio, and R. J. Archuleta Kinematic Inversion of the 2004 M 6.0 Parkfield Earthquake Including an Approximation to Site Effects Bulletin of the Seismological Society of America, September 1, 2006; 96(4B): S143 - S158. [Abstract] [Full Text] [PDF]
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B. Mena, E. Durukal, and M. Erdik Effectiveness of Hybrid Green's Function Method in the Simulation of Near-Field Strong Motion: An Application to the 2004 Parkfield Earthquake Bulletin of the Seismological Society of America, September 1, 2006; 96(4B): S183 - S205. [Abstract] [Full Text] [PDF]
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I. A. Johanson, E. J. Fielding, F. Rolandone, and R. Burgmann Coseismic and Postseismic Slip of the 2004 Parkfield Earthquake from Space-Geodetic Data Bulletin of the Seismological Society of America, September 1, 2006; 96(4B): S269 - S282. [Abstract] [Full Text] [PDF]
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J. Murray and J. Langbein Slip on the San Andreas Fault at Parkfield, California, over Two Earthquake Cycles, and the Implications for Seismic Hazard Bulletin of the Seismological Society of America, September 1, 2006; 96(4B): S283 - S303. [Abstract] [Full Text] [PDF]
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E. S. Cochran, Y.-G. Li, and J. E. Vidale Anisotropy in the Shallow Crust Observed around the San Andreas Fault Before and After the 2004 M 6.0 Parkfield Earthquake Bulletin of the Seismological Society of America, September 1, 2006; 96(4B): S364 - S375. [Abstract] [Full Text] [PDF]
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