抄録
During the high-resolution side-scan sonar survey off Kozu Sima, Nii-jima and Miyake Sima Islands, a northern part of the Izu-Mariana Ridge, various kinds of bedforms such as 2D dunes, 3D dunes and sand ribbons, were observed on the sea floor at a depth of 100-400 m. These bedforms display sharp profiles, which are interpreted as fresh bedforms produced and maintained by the Kuroshio Current. The distribution of these bedforms indicates that the Kuroshio Current passes a narrow channel between southern coast of Kozu Sima and northern end of Kozu-nakase to enter the study area, and that the flows over Kozu-nakase is too weak to produce large bedforms on the sea floor. The flow passing the narrow channel extends to develop very large dunes on the sea floor at a depth of 350 m, which have the maximum wavelength of c. 500 m and height of c. 15 m.The flow velocities estimated from the necessary conditions for dune formation and the observed dune morphology show that i) the surface flow velocity required to develop dunes on a initially flat bed is 2-3 m/s, and ii) that required to maintain the observed very large dunes is 1 m/s. These values are fairly consistent with the observed flow velocity in this area.
