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

摘要

Core-reflected ScS waves from 49 large (M >= 5.1) deep Tonga-Fiji events, recorded in western North America are used to study a localized region of the core-mantle boundary (CMB) under the central Pacific Ocean. A total of 248 observations from the Berkeley Digital Seismic Network (BDSN), Caltech/United States Geological Survey (USGS) TERRAscope and the Incorporated Research Institutions for Seismology (IRIS) broadband arrays span epicentral distances of 73 deg-85 deg. ScS reflection points sample a CMB patch southeast of the Hawaiian islands at latitude 4 deg to 16 deg N and longitude -156 deg to -144 deg W, near the proposed source location of the Hawaiian plume. Highly variable ScS travel times, amplitudes, waveforms, and shearwave splitting indicate that the lowermost mantle in this region is heterogeneous both laterally and radially. ScSH-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 arrivals and is largely accounted for by a model with a strong velocity decrease 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", which enhances ScS. ScS splitting, corrected for lithospheric anisotropy beneath the receivers, indicates spatial variations in D" anisotropy with a systematic change in the orientation of the fast polarization direction from transverse to the ray path (fast ScSH) to parallel to the ray path (fast ScSV) along a northeast traverse. These spatial trends suggest lateral gradients in the boundary layer shear flow on scale lengths of a few hundred kilometers, which may be related to dynamical flow near the near the root of the large Hawaiian plume.