Internal structure of the San Andreas fault at Parkfield, California

Unsworth, Martyn; Malin, Péter; Egbert, G. D.; Booker, J. R.

doi:10.1130/0091-7613(1997)025<0359:isotsa>2.3.co;2

Cited by 183 publications

(145 citation statements)

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“…A significant number of characterization studies have been conducted along the Parkfield segment of the SAF (e.g., Catchings et al, 2002; Eberhart-Phillips et al, 2003;Thurber et al, 2006;Unsworth et al, 1997). Structurally, both arrays span a section of the SAF that has been described as a flower structure (e.g., Catchings et al, 2002) with several subsidiary branches splaying off of the main fault trace.…”

Section: Discussionmentioning

confidence: 99%

Anisotropy in the Shallow Crust Observed around the San Andreas Fault Before and After the 2004 M 6.0 Parkfield Earthquake

Cochran

Vidale

2006

Bulletin of the Seismological Society of America

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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 (N40ЊW) 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(Li et al., , 2006. Outside of this zone, the fast orientations are scattered with some preference for orientations near N10ЊE, roughly parallel to the regional maximum horizontal compressive stress direction (r 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. IntroductionWe examine crustal shear-wave anisotropy at two locations along the San Andreas fault (SAF) near Parkfield to investigate lateral and possible temporal variation in anisotropy. Local earthquake data were recorded on seismic arrays prior to and following the 28 September 2004 Parkfield M 6.0 earthquake. The data are from one array deployed in October and November 2003, and a second immediately after the M 6.0 mainshock. The 2003 array, referred to hereafter as the SAFOD array, is located adjacent to the SAFOD *Present address: Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California at San Diego, La Jolla, California 92093-0225. drilling site and the 2004 array, referred to as the Parkfield array, is located 12 km to the so...

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Section: Discussionmentioning

confidence: 99%

Anisotropy in the Shallow Crust Observed around the San Andreas Fault Before and After the 2004 M 6.0 Parkfield Earthquake

Cochran

Vidale

2006

Bulletin of the Seismological Society of America

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“…Powered by the California Digital Library University of California Connected RNA fragments the small earthquakes seen in the region may arise at geometric irregularities or at secondary fractures away from the main slip zones, again a result of the complex internal structure of the fault zone 8 .…”

Section: Escholarshiporgmentioning

confidence: 99%

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Robertson¹,

Scott²

2007

Nature

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“…Our preliminary 3-D model (Figure 6) contained the essential features of models from prior MT studies in the Parkfield area [Boudreau, 1989;Park et al, 1991;Unsworth et al, 1997Unsworth et al, , 2000Unsworth and Bedrosian, 2004]. This model also matched constraints from surface geology, bathymetry, and other regional geophysical studies (proprietary well log data [e.g., Prodehl, 1979;Zoback and Wentworth, 1986]) and contains multiple scales extending to 6000 km in all directions from Parkfield.…”

Section: Deriving a 3-d Resistivity Modelmentioning

confidence: 99%

“…We then perturbed this model and used a 3-D MT modeling code [Mackie and Madden, 1993] with Mackie's latest proprietary improvements (R. L. Mackie, personal communication, 2006) to Perturbations of up to 4% or 0.4% at depths up to 11 km were made using the dilatation model in Figure 7 to predict where to make changes. Three lines of squares oriented northeastsouthwest in the northwestern corner of the map show locations of MT soundings from Unsworth et al [1997Unsworth et al [ , 2000. The southernmost MT profile terminates just west of electrode Ff.…”

Section: Linking Telluric Components and Changes Of Resistivitymentioning

confidence: 99%

“…[16] The 3-D model we use here is a synthesis of others' work that is refined with 3-D MT inversion [Mackie and Madden, 1993] of MT data from Unsworth et al [1997]. Our preliminary 3-D model (Figure 6) contained the essential features of models from prior MT studies in the Parkfield area [Boudreau, 1989;Park et al, 1991;Unsworth et al, 1997Unsworth et al, , 2000Unsworth and Bedrosian, 2004].…”

Section: Deriving a 3-d Resistivity Modelmentioning

confidence: 99%

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Electrical resistivity changes not observed with the 28 September 2004 M6.0 Parkfield earthquake on the San Andreas fault, California

Park¹,

Larsen²,

Lee³

2007

J. Geophys. Res.

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[1] An 18 a long experiment to capture changes in electrical resistivity prior to, during, and after a M6 earthquake on the San Andreas fault at Parkfield, California, culminated with an event of that magnitude on 28 September 2004. Resistivity was monitored with a telluric array, giving precisions in measured parameters of 0.2% or less. No coseismic resistivity changes greater than 5% could have occurred or they would have been detected by the array, and any precursory changes would have been much smaller because maximum possible precursory strains were 4000 times smaller than the coseismic strain. Given that the earthquake ruptured directly beneath the telluric array, this experiment places a bound on the amount resistivity might change at the hypocentral location; changes measured with instruments farther from earthquakes can only be smaller. It is still possible that a measurable change with a larger earthquake or other fault geometries are capable of generating larger signals, but our results show clearly that future experiments should be designed with precisions of much better than 0.1% if they are to be successful.

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Internal structure of the San Andreas fault at Parkfield, California

Cited by 183 publications

References 30 publications

Anisotropy in the Shallow Crust Observed around the San Andreas Fault Before and After the 2004 M 6.0 Parkfield Earthquake

Anisotropy in the Shallow Crust Observed around the San Andreas Fault Before and After the 2004 M 6.0 Parkfield Earthquake

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Electrical resistivity changes not observed with the 28 September 2004 M6.0 Parkfield earthquake on the San Andreas fault, California

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