Propagation ofPwaves and Rayleigh waves in Melanesia: Structural implications

Abstract

The records of the Orstom seismological stations in New Caledonia and New Hebrides were used for a study of the propagation of P waves and Rayleigh waves in the South Pacific seismic belts, particularly in the New Hebrides area. After a brief study of the seismicity of the New Hebrides arc, it is shown that P arrivals from deep-focus earthquakes in Fiji and from the Longshot explosion are late in Melanesia, and that local differences exist between stations. A study of P-wave propagation from normal earthquakes of the New Hebrides implies an important difference between the stable areas of New Caledonia in the Loyalty Islands and the seismic area of New Hebrides. It is shown that the propagation has an azimuthal influence, and this influence is interpreted as a lithospheric plate effect and as a root of mountain effect. In the second part of the paper, Rayleigh-wave dispersion is studied by a systematic investigation of influence coefficients of layer parameters. The structural framework of the area between Australia and the andesite line is presented. In this area the crust is thicker than in oceanic regions, being about 20 km on the average, about 15 km in basins, and 25–30 km under the Coral Sea platform and ridges. Theoretical models to explain body-wave and Rayleigh-wave propagation give results in fair agreement with observations in the New Hebrides anomalous area. It is found that there is a crust where P-wave velocity is high on a low-velocity upper mantle. Speed increases with depth from 7.4 km/sec at a depth of 20 km to 8.1 km/sec at a depth of 120 km. In the third part of this paper the results and geological implications are discussed in the sea-floor spreading and continental-drift concepts. There is fair agreement with the hypothesis that spreading occurs from the Indian Antarctic and Pacific Antarctic ridges. The area to the east of New Hebrides (North Fiji basin, Fiji Islands, and Lau ridge) would be a large stable plate between two mobile plates of lithosphere converging and thrusting under the Tonga-Kermadec and New Hebrides arcs. According to the propagation of seismic waves under New Caledonia and the Loyalty Islands and to geological observations, it is suggested that New Caledonia and Norfolk ridge would be an island arc whose activity ceased in the Oligocene.