Geographic boundary and shear wave velocity structure of the "Pacific anomaly" near the core - Mantle boundary beneath western Pacific

We determine the geographical boundary and shear-velocity structure of a very-low velocity province at the base of the Earth's mantle beneath western Pacific (we term it the "Pacific anomaly") based on the waveform modeling and travel time analysis of ScSH-SH phases. Our seismic data are from the China National Digital Seismographic Network, the F-net in Japan, the Global Seismographic Network and several PASSCAL arrays. The observed ScS-SH differential travel-time residuals allow the northwestern geographic boundary of the anomaly to be clearly defined. The seismic data also suggest that the average shear-velocity reduction inside the anomaly reaches - 5% in the lowermost 300 km of the mantle. Waveform modeling of the seismic data sampling the edge of the anomaly suggests that the northwestern boundary is best characterized by a shear-velocity model with a velocity jump of about 2% at about 100-145 km above the core-mantle boundary and a thin (30-km thick) basal layer with a shear wave velocity reduction of - 13%. Stacked seismic data sampling the middle of the anomaly, however, show no evidence for any internal discontinuity with a velocity decrease greater than - 2% in the middle of the anomaly. Overall, the seismic data sampling the base of the "Pacific anomaly" can be explained by a negative shear-velocity gradient from 0% to - 1% (top) to - 13% (bottom) in the lowermost 220 km of the mantle, similar to those of a very-low velocity province beneath the South Atlantic Ocean and the Indian Ocean. Such a strong negative shear-velocity gradient can be explained by partial melting of a compositional anomaly produced early in the Earth's history located within a bottom thermal boundary layer. Our travel time data also exhibit small-scale variations inside the anomaly, indicating existence of internal small-scale seismic heterogeneities inside the "Pacific anomaly".