Understanding the nature of deformation near plate boundaries is a key element of understanding the nature of the dynamics of plate tectonics. One particularly interesting question is how seismic anisotropy measured in teleseismic body waves can provide constraints on the nature of small-scale (i.e., <10 km) deformation. The San Andreas Fault (SAF) near Parkfield, California provides an ideal zone to study small-scale variations in seismic anisotropy. In this study, we utilized data from the PASO-DOS broadband seismic array. The array has an approximate aperture of 15 km and is diagonally bisected northwest to southeast by the surface trace of the SAF. We performed shear wave splitting analyses to determine fast polarization directions and delay times from 5 high-quality events recorded at the array. Results from the shear wave splitting analysis reveal clear variations in apparent seismic anisotropy across the array. Data for two events from NW backazimuths exhibited first-order variations in seismic anisotropy across the SAF, with a clear, smooth rotation in fast direction from ENE-WSW on the east side of the SAF to WNW-ESE on the west side. Splitting times range from 1.33 to 2.35 sec, average 1.90 sec, but do not show a clear regional trend. Data for three other events with either W or N backazimuths exhibit null measurements. While these variations may be due to strong isotropic lateral heterogeneity, the existence of clear variations in crustal anisotropy as imaged from both receiver function analysis and local S splitting suggests that the variations we have documented are most likely due to effects of seismic anisotropy in the crust. The null splitting results from events with W and N backazimuths are consistent with previous interpretations of E-W fast directions in the mantle across the region. Our results suggest the presence of broad-scale asthenospheric deformation, which generates a pervasive E-W fast direction across the region. The rapid sweep in fast directions near the SAF is consistent with a first-order change in uppermost crustal anisotropy domains across the fault. The implications of our results are threefold: 1) shear wave splitting from teleseismic body waves has the potential to provide unique constraints on deformation at plate boundaries; 2) crustal anisotropy have a much larger influence on shear wave splitting measurements than previously assumed; and 3) backazimuthal variations in shear wave splitting documented at single stations may not necessarily be due solely to variations in anisotropic structure with depth.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- 7230 Seismicity and tectonics (1207;
- 7299 General or miscellaneous;
- 8106 Continental margins: transform;
- 9350 North America