An anisotropic contrast in the lithosphere across the central San Andreas fault

Abstract

Seismic anisotropy of the lithosphere and asthenosphere was investigated with a dense broadband seismic transect nearly orthogonal to the central San Andreas fault (SAF). A contrast in SK(K)S splitting was found across the SAF, with a clockwise rotation of the fast orientation ~26° closer to the strike of the SAF and greater delay times for stations located within 35 km to the east. Dense seismograph spacing requires heterogeneous anisotropy east of the SAF in the uppermost mantle or crust. Based on existing station coverage, such a contrast in splitting orientations across the SAF may be unusual along strike and its location coincides with the high‐velocity Isabella anomaly in the upper mantle. If the Isabella anomaly is a fossil slab fragment translating with the Pacific plate, the anomalous splitting east of the SAF could indicate a zone of margin‐parallel shear beneath the western edge of North America.

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