Seismic anisotropy in the lowermost mantle beneath the Pacific

Onset time differences of up to 3 s are observed between transverse (S^SH) and longitudinal (S^SV) components of broadband S waves at distances of 85° to 120° for paths traversing the lowermost mantle (D") beneath the Pacific. After correction for upper mantle anisotropy, S^SH usually arrives earlier than S^SV with the splitting increasing with distance from 100° to 120°. The data yield two possible models of anisotropy: (1) anisotropy may vary laterally, with transverse isotropy existing in higher-than-average shear velocity regions beneath the northeastern Pacific, or (2) anisotropy may vary with depth, with transverse isotropy concentrated in a thin (100 km) thermal boundary layer at the base of D". A few recordings at distances less than 105° show that S^SV arrives earlier than S^SH, indicating that general anisotropy likely exists in shallower regions of D".