Pacific large-low-shear-velocity province bounded by an extensive high-velocity anomaly observed with NECESSArray

Lateral variation in shear-wave velocity across the western edge of the Pacific Large-Low-Shear-Velocity Province (LLSVP) is investigated by analyzing differential travel-times of ScS--S. By using data from the events occurred in the South Pacific recorded at the broadband seismic stations of NECESSArray and F-net in Japan Islands, we explore a broad region of the lowermost mantle beneath central to western Pacific (latitude -10^o to 30^o, longitude 130^o to 170^o). So far, we collected over 500 seismograms for NECESSArray and over 300 for F-net, from 23 events. The differential travel time residuals were calculated with respect to PREM. The residuals were corrected for the heterogeneity above the lowermost 250 km (D'' layer) of the mantle using various global tomography models. A large contrast in the differential time residuals is clearly observed. Very large positive residuals of up to 6.2 seconds are observed for the rays reflected at the LLSVP, while the rays sampled the region adjacent to the LLSVP show significant negative residuals of down to -4.2 seconds, indicating the very high shear-velocity anomaly of up to 3 percent along the ray paths in the bottom 250 km of the mantle. The inferred fast shear-velocity anomaly is much stronger than that evaluated from the tomography models.