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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
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