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”.