Data from seven closely spaced seismic refraction profiles made off the Philippine Sea coast of central Shikoku, Japan, reveal a large thickness of sediment beneath the continental slope that does not slope simply and constantly toward the ocean basin. There is a major ridge in the sediment approximately halfway between the continental shelf and the Nankai trough. The Tosa basin thus developed shoreward of this ridge has been filled by sediment whose surface is a terrace at a depth of 1000 meters. The Nankai trough is a depressed section of oceanic crust within the Shikoku basin.
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Yoshii et al. (1973) studied this question.