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The Relationship Between Upper Mantle Anisotropic Structures Beneath California, Transpression, and Absolute Plate Motions

Minoo Kosarian, Paul M. Davis, Toshiro Tanimoto, & Robert W. Clayton

Published 2011, SCEC Contribution #1482

We calculated SKS splitting parameters for the California Integrated Seismic Network. In southern California, we also estimated splitting in the upper 100 km using azimuthal anisotropy determined from surface waves. The inferred splitting from surface waves in the mantle lithosphere is small (on average <0.2 sec) compared with SKS splitting (1.5 sec) and obtains a maximum value (0.5 sec) in the transpressive region of the Big Bend, south of, and aligned with, the San Andreas Fault (SAF). In contrast, the SKS splitting is approximately E-W and is relatively uniform spatially either side of the Big Bend of the SAF. These differences suggest that most of the SKS splitting is generated much deeper (down to 300-400 km) than previously thought, probably in the asthenosphere. Fast directions align with absolute plate motions (APM) in northern and southeastern California but not in southwestern California. We interpret the parallelism with APM as indicating the SKS anisotropy is caused by cumulative drag of the asthenosphere by the over-lying plates. The discrepancy in southwestern California arises from the diffuse boundary there compared to the north, where relative plate motion has concentrated near the SAF system. In southern California the relative motion originated offshore in the Borderlands and gradually transitioned onshore to the SAF system. This has given rise to smaller displacement across the SAF (160-180 km) compared with central and northern California (400-500 km). Thus, in southwestern California, the inherited anisotropy, from prior North American APM, has not yet been overprinted by Pacific APM.

Kosarian, M., Davis, P. M., Tanimoto, T., & Clayton, R. W. (2011). The Relationship Between Upper Mantle Anisotropic Structures Beneath California, Transpression, and Absolute Plate Motions. Journal of Geophysical Research: Solid Earth, 116(B08307). doi: 10.1029/2010JB007742.