Mantle phases recorded at Guam for distances between I" and 52"

Abstract

An analysis of P and S arrivals recorded at Guam from 175 earthquakes which occurred along the seismic belt in the Western Pacific reveals differences between the travel times to that station and those recorded at Marcus, Midway, and Wake islands from earthquakes occurring in the same general area. Three possible explanations are offered to account for the observed discrepancies: (1) eompressional phases observed at Marcus, Midway, and Wake are mantle-guided phases of the Pn type; (2) systematic errors in epicenter determinations of the type reported by Japanese authors for earthquakes in the Kurile, Japan, and Izu trench region are present; and (3) real differences exist in the upper-mantle velocity structure of the two regions in question through which the phases travelled. Either of the last two hypotheses is considered more acceptable than the first. The travel times to Guam are found to be similar for earthquakes north and south of the station, but for earthquakes to the north at distances greater than ~ 26°, energy being transmitted to Guam appears to be absorbed or blocked. This may be explained by a high attenuation zone along paths sub-parallel to the Marianas, Izu, Japan, Kurile trenelis or by a shadow zone at Guam produced by the lateral refraction of energy by the downgoing lithospheric slab.