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P-wave velocity of the uppermost mantle and crustal thickness in the central Vanuatu Islands (New Hebrides Island arc)

DEPARTMENT OF GEOLOGICAL SCIENCES CORNELL UNIVERSITY, ITHACA, NEW YORK 14853
OFFICE DE LA RECHERCHE SCIENTIFIQUE ET TECHNIQUE OUTRE-MER (ORSTOM), NOUMEA CENTER, B.P. A5, NOUMEA, NEW CALEDONIA

Abstract

The P-wave velocity of the uppermost mantle and crustal thickness were determined for the central Vanuatu (formerly New Hebrides) Islands using data from a local seismograph network. Two different methods and data sets were used in this investigation. The first method used travel-time plots for earthquakes located well outside of the seismograph network. The second method used time-term analysis on sets of earthquakes. Both methods yielded a mantle velocity of 7.6 (±0.1) km/sec and a crustal thickness of 27 (±2) km for the region located just trenchward of the volcanic arc. Published refraction studies have demonstrated that the mantle velocity near the trench has a more typical value of 8.0 km/sec. This contrast in mantle velocities between the trench and the volcanic arc has also been reported for the Japanese and Kurile-Kamchatka Island arcs and, therefore, may be a general feature of subduction zones.