SKS Splitting from Ocean Bottom Seismometer Data Offshore Southern California

SKS arrivals from ocean bottom seismometer (OBS) data of the ALBACOREexperiment offshore Southern California are analyzed for shear wave splitting. The ALBACORE (Asthenospheric and Lithospheric Broadband Architecture from the California Offshore Region Experiment) project involved deployment of 34 OBSs for 12 months in a region extending up to 500 kilometers west onto the oceanic Pacific plate. Splitting fast directions are similar to on-land directions, WSW-ENE, and exhibit similar delays, 1.1-1.4 seconds. A numerical method to remove S-wave interference with SKKS arrivals by f-k velocity filtering is tested with synthetic and the observed OBS data. The fast directions are at 45 degrees to the direction of absolute plate motion (APM) of the Pacific plate suggesting that either frozen-in anisotropy from paleo-spreading dominates over APM effects in the asthenosphere or that deeper mantle shearing has occurred unrelated to APM. A toroidal flow around slab rollback would be consistent with the splitting results if the slab rollback resulted in large-scale secondary flow well into Pacific plate, or if the source of the splitting extended much deeper than 200 km.