Structural Feature and Shear-Velocity Structure of the"Pacific Anomaly"

Seismic tomography has revealed two broad, seismically slow anomalies in the lower mantle, with one beneath the Pacific Ocean and the other beneath Africa. Here we term them the "Pacific Anomaly" and the "African Anomaly". Detailed mapping of the geometric and velocity features of the anomalies is crucial to further understanding of the origin, formation and dynamical evolution of these anomalies. In this study, we constrain structural feature and shear velocity structure of the Pacific Anomaly on the basis of forward travel time and waveform modeling of seismic data sampling a great arc across the anomaly from eastern Asia to southern South America. Our collected dataset consists of direct S, Sdiff, ScS, SKS and SKKS phases recorded in the Global Seismographic Network, the China National Digital Seismographic Network, the F-net in Japan and several temporary seismic arrays, from earthquakes occurring in the Solomon Islands, the Tonga-Fiji Islands, and the southern East Pacific Rise. After corrected for the effects of earthquake mislocation and the seismic heterogeneities outside the Pacific Anomaly, seismic observations suggest that the Pacific Anomaly along the great arc consists of at least two separated portions with a 740-km wide gap between them. The western portion of the anomaly is about 1050 km wide, extends at least 730 km above the core-mantle boundary (CMB) and exhibits a trapezoidal shape with lateral dimension increasing slightly with depth. The eastern portion of the anomaly has an 1800-km wide base and reaches at least 340 km above the CMB beneath the mid-Pacific. The waveforms and travel times for the seismic data sampling the western portion of the anomaly can be best explained by a negative shear velocity gradient from -3.0% at the top (730 km above the CMB) to -3.5% at 100 km above the CMB and an average shear velocity reduction of - 5% in the bottom 100 km of the mantle. The seismic data sampling the eastern portion of the anomaly can be explained by a uniform velocity reduction of -3%. Waveform modeling further suggests a 100-km thick low-velocity layer with a shear velocity reduction of -10% located at the edge of the western portion of the anomaly. Combining the latest results from others, we present a general picture of structural and velocity structures of the Pacific Anomaly. The structural and velocity features suggest that the Pacific Anomaly represents a cluster of metastable thermo-chemical piles.