Crustal Seismic Anisotropy in Southern California

Crustal Seismic Anisotropy in Southern California, Figure 1 Figure 1.
Map of station locations and unique anisotropy axis orientations at 36 Ma based on the reconstruction of McQuarrie and Wernicke (2005). Station color-coding corresponds to the crustal blocks. The large arrows show the best fitting block trend-lines rotated back to their orientation at 36 Ma. The rose diagram shows the number of stations with anisotropy trends within each 10° bin when rotated back to their 36 Ma orientations. Vectors show Early Tertiary Farallon-North America relative motion vectors.
Understanding lower crustal deformational processes and the related features that can be imaged by seismic waves is an important goal in active tectonics and seismology. Using data from publicly available stations, we calculated teleseismic receiver functions to measure crustal anisotropy at 38 broadband seismic stations in southern California. Results reveal a signature of pervasive seismic anisotropy located in the lower crust that is consistent with the presence of schists emplaced during Laramide flat slab subduction. Anisotropy is identified in receiver functions by the large amplitudes and small move-out of the diagnostic converted phases. Within southern California, receiver functions from numerous stations reveal patterns indicative of a basal crustal layer of hexagonal anisotropy with a dipping symmetry axis. Neighborhood algorithm searches [Frederiksen et al., 2003] for depth and thickness of the anisotropic layer and the trend and plunge of the anisotropy symmetry (slow) axis have been completed for the stations. The searches produced a wide range of results but a dominant SW-NE trend of the anisotropy symmetry axes emerged among the measurements. When the measurements were assigned to crustal blocks and restored to their pre-36 Ma locations and orientations using the reconstruction of McQuarrie and Wernicke [2005], the regional scale SW-NE trend became even more consistent (Fig. 1). This suggests that anisotropy predates Pacific-North American plate strike slip motion, though a small subset of the results can be attributed to NW-SE shearing that may be related to San Andreas transform motion (Fig 1). We interpret this dominant trend as a fossilized fabric within schists, created from a top-to-the-southwest sense of shear that existed along the length of coastal California during pre-transform, early Tertiary subduction. Comparison of receiver function common conversion point stacks to seismic models from the active LARSE experiment shows a strong correlation between the location of anisotropic layers and “bright” reflectors from Fuis et al. [2007], further affirming these results (Fig. 2).
</p><p>Frederiksen, A. W., H. Folsom, and G. Zandt (2003), Neighbourhood inversion of teleseismic Ps conversions for anisotropy and layer dip, Geophys. J. Int., 155(1), 200-212.
</p><p>Fuis, G. S., M. D. Kohler, M. Scherwath, U. ten Brink, H. J. A. Van Avendonk, and J. M. Murphy (2007), A comparison between the transpressional plate boundaries of the South Island, New Zealand, and southern California, USA: the Alpine and San Andreas Fault systems, in A Continental Plate Boundary: Tectonics at South Island, New Zealand, edited by D. Okaya, T. Stern and F. Davey, pp. 307-327, American Geophysical Union.
</p><p>McQuarrie, N., and B. P. Wernicke (2005), An animated tectonic reconstruction of southwestern North America since 36 Ma Geosphere, 1(3), 147–172.
</p><p>Acknowledgements: This work was funded by a grant from Southern California Earthquake Center and NSF EAR Earthscope Program award #0745588.</p>


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