Seismic Refraction and Reflection Measurements around Hokkaido.

Seismic refraction and reflection measurements were made across the continental slope off the southeastern coast of Hokkaido and the Kuril-Kamchatka trench. The whole slope is covered by thick low-velocity sediments. Layers with velocities of 5.4 and 5.8 km/sec are found beneath the upper slope. Under the lower deep-sea terrace (1400-2000 m) an unconformity is observed by profiler technique which may show a presence of a large tectonic movement in this area in the past. A structural change is remarkable at water depth between about 4000 and 4500 m, and beneath the land-side slope of the trench a troughlike structure is buried by materials with velocities 4.2 and 2.0 km/sec. Beneath the oceanside slope of the Kuril-Kamchatka trench the crust of the oceanic type is obtained. Many clear faults are observed on this slope by seismic profiler measurements as same as in the Japan and the Izu-Ogasawara trenches.