SKS Splitting from Ocean Bottom Seismometer Data Offshore Southern California

Abstract:

SKS arrivals from ocean bottom seismometer (OBS) data of the ALBACORE
experiment 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.