Seismic refraction experiment at the amami plateau

Seismic refraction profiles were shot at the Amami Plateau, an anomalous bathymetric high among many in the West Philippine Basin. A model of the crustal structure beneath the Amami Plateau consisting of materials with compressional wavespeeds 2.0, 2.8-3.8, 5.4-5.6, and 6.7 km/sec, with thicknesses 0.3-0.7, 1.0-2.0, and 3.0-4.0 km, layered from the top, was obtained. This indicates that the 5.4-5.6 km/sec layer characterizes the structure of the plateau and implies that a wavespeed of less than 5.6 km/sec occupies more than half the crust. It is suggested, therefore, that the crustal structure is different from that of normal oceanic. The sedimentary shear wavespeed was found to be 0.39 km/sec and Poisson's ratio 0.48 from P-PS delay time produced by phase conversion at the base of the sediment.