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.

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