Small-scale lateral shear velocity and anisotropy heterogeneity near the core-mantle boundary beneath the central Pacific imaged using broadband ScS waves

Core-reflected ScS waves from 49 large (M greater than or equal to 5.1) dee p Tonga-Fiji events, recorded in western North America are, used to study a localized region of the core-mantle boundary (CMB) under the central Pacif ic Ocean. A total of 248 observations from the Berkeley Digital Seismic Net work (BDSN), Caltech/United States Geological Survey (USGS) TERRAscope and the Incorporated Research Institutions for Seismology (IRIS) broadband arra ys span epicentral distances of 73 degrees-85 degrees. ScS reflection point s sample a CMB patch southeast of the Hawaiian islands at latitude 4 degree s to 16 degrees N and longitude -156 degrees to -144 degrees W, near the pr oposed source location of the Hawaiian plume. Highly variable ScS travel ti mes, amplitudes, waveforms, and shearwave splitting indicate that the lower most mantle in this region is heterogeneous both laterally and radially. Sc SH-SH differential travel times are on average 4 s larger than predicted by the Preliminary Reference Earth Model (PREM). This is due to delayed ScS a rrivals and is largely accounted for by a model with a strong velocity decr ease in D ". The ScSH-SH residuals also show a spatial trend not accounted for by a radial model, indicating a lateral decrease in lower mantle shear velocity to the northeast. This lateral velocity gradient appears to cause focusing of ScS energy. ScSH/SH amplitude ratios are larger than predicted by PREM or by a model with a strong negative gradient with depth in D ", wh ich enhances ScS. ScS splitting, corrected for lithospheric anisotropy bene ath the receivers, indicates spatial variations in D " anisotropy with a sy stematic change in the orientation of the fast polarization direction from transverse to the ray path (fast ScSH) to parallel to the ray path (fast Sc SV) along a northeast traverse. These spatial trends suggest lateral gradie nts in the boundary layer shear flow on scale lengths of a few hundred kilo meters, which may he related to dynamical flow near the near the root of th e large Hawaiian plume.