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Oceanic mantle phases recorded on seismographs in the Northwestern Pacific at distances between 7° and 40°

HAWAII INSTITUTE OF GEOPHYSICS UNIVERSITY OF HAWAII, HONOLULU, HAWAII 96822

Abstract

Body-wave data recorded on hydrophones at Midway, Wake, and Eniwetok and on seismographs at Midway, Wake, and Marcus indicate the following for the mantle under the Northwestern Pacific Basin: a zone of low P-wave velocity with a thickness of about 75 km beginning in the uppermost 100 km of the Earth, perhaps very close to the M discontinuity; a wave guide immediately below the M discontinuity which propagates 3 to 8 Hz energies of compressional and shear phases with velocities of 8.28 ± 0.03 and 4.79 ± 0.04 km/sec, respectively, to great distances (observed to 33°, with the wave guide for compressional phases being considerably more efficient than the wave guide for shear phases); and, an asthenosphere of low Q for frequencies of 3 to 8 Hz as compared to that for material above and below.

Similar data for the East Caroline Basin-Ontong Java Plateau-Nauru Sea area indicate the following: a wave guide immediately below the M discontinuity which propagates 3 to 8 Hz energies of compressional phases with a velocity of 8.47 ± 0.48 km/sec to at least 22°; a weaker wave guide for shear phases than observed for the area to the north; an asthenosphere of low Q for frequencies of 3 to 8 Hz as compared to that for material above and below; and a velocity distribution somewhat different from that for the area to the north since, for normal refracted-mantle P phases recorded in the distance range of 22° to 40°, travel-time residuals are generally slightly negative for the area to the north and slightly positive for the area to the south.

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