The results from seismic refraction profiles made south of Kyushu, Japan, are presented in a structure section crossing lhe outer East. China Sea shelf, the Okinawa trough, the Ryukyu ridge, the Ryukyu trench, the north Amami basin, the Kyushu-Palau ridge, and the western edge of the Shikoku basin. The foundation of the shelf-trough-ridge province is composed of material of velocity near 5.7 kin/sec. Above this material are others of velocity 3.4-4.6 km/sec that represent acoustic basement. The western edge of the Okinawa trough is a buried ridge of subbasement material located near the edge of the East China Sea shelf. Sediments partly fill the trough, the thickest section being beneath the western side slope, which has the appearance of a prograding sedimentary feature. Beneath the continental slope to the Ryukyu trench a thick sequence of sedimenl; in a deep trough with no surface expression lies on a depressed and thinner than normal oceanic crust. It seems likely that the trough of thick sediments, of which the Ryukyu trench is a part, has a structural continuation off the east coast of Kyushu, suggesting that the present day trench evolved from a formerly more extensive trench system. The north Amami basin and the western edge of the Shikoku basin have oceanic crust, with a slightly thicker than normal layer 2, whereas the Kyushu-Palau ridge between them consists of a far thicker section of upper crustal material, which causes a. downward bulge in oceanic layer 3 and prestonably in the mantle below.

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Refraction measurements by the two ship method were conducted in July 1969 in the deep sea terrace of Hidaka , Hokkaido. The general features of the crustal structures derived are similar to those in the adjacent oceanic areas. Thick sediments cover the whole area, especially its northernmost part. The layer with a P-wave velocity of 6km/s is found in all profiles. That is, the crustal structure in this area is continental and similar to that off the Sanriku area.

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. On a revised mapLudwig et al. [1973a] show the general distribution of major structural units off the Philippine Sea coast of southwest Japan by use of seismic reflection measurements combined with other published data. The southwest Japan margin has. a foredeep, the Nankai trough, and a series of intermediatedepth sediment-filled basins that occur off prominent embayments in the coast. This report describes the results of two-ship seismic refraction measurements made across the margin off Tosa embayment, central Shikoku, during a United States-Japan cooperative science program in 1968 (Figure 1). SEISMIC REFRACTION MEASUREMENTS The rather strong drift of the survey ships due to the Kuroshio current made the measurements dincult. The shooting track and drift while each ship was recording on station are shown in Figure 2. Obviously, the refraction profiles are not truly reversed, although we treat them as such 'in the interpretation of the travel time graphs (appendix). The geometry • Lamont-Doherty Geological Observatory contribution 1951.

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