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

Abstract

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...

Passive monitoring of anisotropy change associated with the Parkfield 2004 earthquake

“…For this reason, we interpret the changes to be related to crustal damage occurring at least in the upper 3 km, and potentially deeper down to the middle lower crust. This interpretation also does not contradict results of other recent studies of anisotropy derived from SWS along the SAF [Cochran et al, 2006;Liu et al, 2008], because we believe our observed changes result from propagation through a thin anisotropic layer of limited extent, a case that was not addressed by these previous studies.…”

“…It is completely described by the polarization direction of the leading wave and its delay time with the orthogonally polarized lagging wave [Crampin, 1987]. Liu et al [2008] and Cochran et al [2006] performed temporal analyses of the SWS measurements with a focus on the Parkfield seismic event and were unable to detect any precursory, co-seismic or post-seismic anisotropy change. Zhang et al [2007] also produced a map of SWS anisotropy using a tomography method and found that the distribution of anisotropy in the region was relatively complex.…”

Evidence for ground motion polarization on fault zones of Mount Etna volcano

“…Nevertheless, anisotropy remains an appealing key of interpretation. In recent papers, Boness and Zoback [2006], Cochran et al [2006], and Liu et al [2008] studied the shear wave splitting in the San Andreas fault. They find a fast shear wave The average is made over measures at stations closer than 250 m from the fault trace.…”

Crust and upper-mantle seismic anisotropy variations from the coast to inland in central and Southern Mexico

“…In the case of aligned cracks, the fast direction is associated with the crack orientation whereas the time delay is related to the density and consistency of cracks. Alternately, if anisotropy is the outcome of shear fabric, the anisotropic component depends on the direction of fabric and degree of shearing (Cochran et al 2006).…”