Geographic boundary of the Pacific Anomaly and its geometry and transitional structure in the north

We determine the geographical boundary and average shear velocity structure of the Pacific Anomaly at the base of the mantle based on travel time analysis of ScSH-SH and ScS2 (ScSScS)-SS phases and waveform modeling results. We further constrain the detailed geometry of the northern Anomaly around (20 degrees N, -170 degrees E) and its transition to the surrounding high velocity region along three perpendicular cross sections on the basis of forward waveform modeling of the observed direct S and ScS phases. The observed differential travel-time residuals and waveform modeling results allow the whole geographic boundary of the Anomaly to be delineated and the area of the base of the Anomaly is estimated to be 1.9 x 10(7) km(2). The maximum shear velocity perturbation inside the Anomaly reaches -5% in the lowermost 500 km of the mantle. Waveform analysis suggests that the northern Anomaly reaches 450 km above the CMB with both steeply and shallowly dipping edges and its basal layer extends beneath the surrounding mantle with the degree of extension changing rapidly across a small distance. The inferred characteristics of the Anomaly support the previous suggestion that the Pacific Anomaly represents a chemical anomaly. However, unlike the inferred features of the African Anomaly pointing to an ancient compositionally distinct and geologically stable anomaly, the existence of several separated piles extending into the mid-lower mantle, the complex morphology of the piles with both steeply and shallowly dipping edges and the presence of many ultra-low velocity zones at its base suggest that the Pacific Anomaly likely possesses varying intrinsic compositions and exhibits complex interaction with the surrounding mantle.