Oceanic mantle phases recorded on hydrophones in the Northwestern Pacific at distances between 9° and 40°

Compressional- and shear-wave arrivals recorded on hydrophones at Midway, Wake, and Eniwetok from 85 earthquakes occurring along the circum-Pacific arc extending from the Aleutian Islands through the Solomon Islands indicate significant differences in the velocity structure in the mantle beneath the deep Northwestern Pacific Basin and the shallower East Caroline Basin-Ontong Java Plateau-Nauru Sea area. For paths in the Northwestern Pacific mantle, travel times of apparent first arrivals are less than Jeffreys-Bullen (J-B) P and S times for distances out to 20° and are increasingly greater than J-B times for distances between 20° and 33°. At distances greater than 33°, P arrivals agree well with J-B times and S arrivals are absent. Out to 30°, compressional and shear travel times fit straight lines of 8.28 ± 0.03 and 4.79 ± 0.04 km/sec, respectively. For the shallower area to the south, compressional travel times are close to J-B times for P out to 18° and are greater than J-B times for distances between 18° and 22°. At distances greater than 22°, P arrivals agree well with J-B times. Out to 24°, the compressional travel times fit a straight line of 8.47 ± 0.48 km/sec. For this area S arrivals are unclear or not apparent.