Distribution of ultra low velocity zones at the base of the Pacific Anomaly beneath the northeastern Pacific

The velocity structure and geographic distribution of the ultra-low velocity zones (ULVZs) at the base of the Pacific Anomaly are important to the understanding of the origins of the ULVZs and the Pacific Anomaly. Here, we constrain the detailed geometry, velocity structure and geographic distribution of ULVZs at the base of the Pacific Anomaly beneath northeastern Pacific on the basis of travel time analysis and two-dimensional forward waveform modeling. Our seismic observations include direct S, ScS and Sdiff phases recorded in the USArray. Waveform analysis reveals ULVZs are distributed in patches with a thickness of 30~60 km, a shear-velocity reduction of -15% and lateral dimensions of 80 x 120 km located inside the Pacific Anomaly and of 260 x 580 km at the edge of the Anomaly. The geometry, velocity structure and geographic relationship between the ULVZs and the Pacific Anomaly suggest that, although both the Pacific Anomaly and the ULVZs are chemically distinct from the surrounding mantle, they are also chemically distinct from each other.