SEISMIC WAVE ATTENUATION IN THE UPPER MANTLE BENEATH

Barazangi, M., Isacks, B., Dubois, J. and Pascal, G., 1974. Seismic wave attenuation in the upper mantle beneath the southwest Pacific. Tectonophysics, 24 : 1-1 2. A detailed study of P and S seismic waves produced by hundreds of earthquakes located along the New Hebrides, Fiji plateau, Solomon, New Britain and New Guinea seismic zones and recorded at stations in Fiji, New Hebrides and New Caledonia shows signific,ant variations in the amplitudes and frequencies of the waves. The data define zones of anomalou$ly high seismic wave attenuation in the uppermost mantle beneath the Fiji plateau and the Woodlark basin. The active volcanoes of the New Hebrides arc seem to mark the western boundary of the high-attenuation zone that exists beneath the Fiji plateau. Our observations are in agreement with the hypothesis that the Fiji plateau and the Woodlark basin are young oceanic features foiined by crustal extension. In addition, a zone of high attenuation exists in the uppermost mantle somewhere between the northeastern coast of Australia and the New Hebrides arc; the data further require that this zone be located not closer than about 400 km to the New Hebrides trench. Observations at New Caledonia stations of very low frequency shear waves from intermediate-depth earthquakes in the westward dipping Tonga-Kermadec seismic zone yield an estimate of Q, of about 10 in the zone of extremely high attenuation located beneath the Lau basin, the actively spreading inter-arc basin behind the Tonga arc.