High-Resolution Seismic Velocities and Shallow Structure of the San Andreas Fault Zone at Middle Mountain, Parkfield, California

Catchings, R. D.

doi:10.1785/0120010263

Cited by 64 publications

(60 citation statements)

References 27 publications

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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. At the Parkfield array, instruments were deployed along the main fault strand and a secondary branch to the north of the main 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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“…Numerous empirical and laboratory studies elsewhere have shown that Vp and Vs within fault zones can result in seismic velocity reductions of 20%-50% [for example, see Aki and Lee, 1976;Wang et al, 1978;Mooney and Luetgert, 1982;Thurber, 1983;Mooney and Ginzburg, 1986;Jarchow et al, 1994;Catchings et al, 2002;Mooney et al, 2007]. Such large reductions in velocity result from reductions in the shear modulus of the faulted rocks [Nur and Simmons, 1969].…”

Section: Velocity Structure and Reservoir-induced Seismicitymentioning

confidence: 99%

Seismicity, faulting, and structure of the Koyna‐Warna seismic region, Western India from local earthquake tomography and hypocenter locations

et al. 2014

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Although seismicity near Koyna Reservoir (India) has persisted for~50 years and includes the largest induced earthquake (M 6.3) reported worldwide, the seismotectonic framework of the area is not well understood. We recorded~1800 earthquakes from 6 January 2010 to 28 May 2010 and located a subset of 343 of the highest-quality earthquakes using the tomoDD code of Zhang and Thurber (2003) to better understand the framework. We also inverted first arrivals for 3-D Vp, Vs, and Vp/Vs and Poisson's ratio tomography models of the upper 12 km of the crust. Epicenters for the recorded earthquakes are located south of the Koyna River, including a high-density cluster that coincides with a shallow depth (<1.5 km) zone of relatively high Vp and low Vs (also high Vp/Vs and Poisson's ratios) near Warna Reservoir. This anomalous zone, which extends near vertically to at least 8 km depth and laterally northward at least 15 km, is likely a water-saturated zone of faults under high pore pressures. Because many of the earthquakes occur on the periphery of the fault zone, rather than near its center, the observed seismicity-velocity correlations are consistent with the concept that many of the earthquakes nucleate in fractures adjacent to the main fault zone due to high pore pressure. We interpret our velocity images as showing a series of northwest trending faults locally near the central part of Warna Reservoir and a major northward trending fault zone north of Warna Reservoir.

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“…This network was extensively used to image the San Andreas Fault (SAF) using man-made explosions and earthquakes (Ben-Zion & Malin 1991;Catchings et al 2002;Thurber et al 2004).…”

Section: E T H O D O L O G Ymentioning

confidence: 99%

Passive seismic imaging with directive ambient noise: application to surface waves and the San Andreas Fault in Parkfield, CA

Roux

2009

Geophysical Journal International

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International audienceThis study deals with surface waves extracted from microseismic noise in the (0.1–0.2 Hz) frequency band with passive seismic-correlation techniques. For directive noise, we explore the concept of passive seismic-noise tomography performed on three-component sensors from a dense seismic network. From the nine-component correlation tensor, a rotation algorithm is introduced that forces each station pair to re-align in the noise direction, a necessary condition to extract unbiased traveltime from passive seismic processing. After rotation is performed, the new correlation tensor exhibits a surface wave tensor from which Rayleigh and Love waves can be separately extracted for tomography inversion. Methodological aspects are presented and illustrated with group-speed maps for Rayleigh and Love waves and ellipticity measurements made on the San Andreas Fault in the Parkfield area, California, USA

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High-Resolution Seismic Velocities and Shallow Structure of the San Andreas Fault Zone at Middle Mountain, Parkfield, California

Cited by 64 publications

References 27 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

Seismicity, faulting, and structure of the Koyna‐Warna seismic region, Western India from local earthquake tomography and hypocenter locations

Passive seismic imaging with directive ambient noise: application to surface waves and the San Andreas Fault in Parkfield, CA

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