Crustal structure of the Philippine Sea

The results of twenty-eight seismic refraction profiles recorded in the various physiographic provinces of the Philippine Sea as part of the United States and Japan Science Cooperation Program are presented in four schematic structure sections. The basins of the Philippine Sea have fairly normal oceanic crust that includes, between the sea floor and layer 2, a layer of about 3.5-km/sec velocity controlling the characteristic rough topography. Crustal thickening beneath the Nansei Shoto, Oki-Daito, Kyushu-Palau, and the Honshu-Mariana ridges is associated mainly with an increase in thickness of the 3.5-km/sec layer and a thick underlying section of material with a velocity between 5.5 and 6.0 km/sec. Beneath the Nansei Shoto trench and the Honshu-Mariana trench, there is a tendency for layer 2 to increase and layer 3 to decrease in thickness as the trench is approached from the adjacent oceanic sector.