Seismic reflection (profiler) traverses of the Japan Basin, Yamato Basin, and intervening Yamato Ridge reveal horizontally stratified sediments over weakly stratified sediments. The basement surface is rough in some places and smooth in others and rises with the topography of Yamato Ridge and the lower continental slopes of Siberia and Japan. Compared to the Japan Basin, Yamato Basin has a shallower sea floor and thinner sediments. In each basin, the sediments decrease in thickness outward from a center.

Wide-angle reflection and refraction data from 65 sonobuoy stations made en route give velocities in the sediments that range from 1.6 to 3.2 km/sec. Smooth oceanic basement (or layer 2) has two refracting layers, 3.5 and 5.8 km/sec; rough oceanic basement is typified by the 5.8 km/sec velocity alone. Layer 2 is thicker in the Yamato Basin than in the Japan Basin because of a greater amount of 3.5-km/sec capping material. Layer 3, of velocity about 6.8 km/sec, lies at nearly the same depth beneath the basins and Yamato Ridge.

The results of profiler-sonobuoy measurements combined with the results of earlier two-ship seismic refraction measurements indicate that the Japan Basin and Yamato Basin are underlain by oceanic crust which in turn is covered by sediments (and volcanics?) that have built a shallower sea floor than that in the western North Pacific basin. Yamato Ridge appears to be mainly a pile of volcanics resting on an oceanic layer at normal depth. The crust of Yamato Basin may also have been modified to the extent that it has a thicker than normal layer 3 and a low-velocity mantle.

Other profiler-sonobuoy data, gathered in the strait between Japan and Korea and across the Japan Sea margin of Southwest Japan and Northeast Japan, are presented. A number of profiler crossings of Toyama Channel indicate its formation by turbidity currents. Turbidity flows in this channel and in other channels transport sediment to the abyssal plain.

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