LAUSR

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. Plain Language Summary Directional dependence of seismic wave speeds, referred to as anisotropy, can illuminate preferred orientations or fabrics in the Earth organized by deformation. Seismic anisotropy near the sharply defined central segment of the San Andreas fault was investigated with a new dense temporary seismic transect. A contrast in uppermost mantle anisotropy across the fault was identified, with nearly fault parallel orientations only on the east side of the fault. We suggest that development of asymmetric anisotropy about the central San Andreas may arise due to fault-parallel movement of a fossil slab beneath the western edge of North America.