VELOCITY ANISOTROPY IN THE SEA OF JAPAN AS REVEALED BY BIG EXPLOSIONS

Seismic waves generated by two explosions of dynamite, 5 tons each, in the Sea of Japan off the coast of northern Honshu were observed at more than 100 temporary and permanent seismological stations in the Hokkaido, Honshu, Sado, and Oki islands. A purpose of these measurements was to extend our investigation of lateral variation in Pn velocity which has been found around northeastern Japan in the previous explosion experiments. In fact, a lateral variation in Pn velocity by about 5% was confirmed in regions of the uppermost mantle below the Sea of Japan and the Honshu island, although the boundary where the velocity change takes place was not determined.The measurements have also revealed an indication that the upper mantle just beneath the Moho interface under the area in the southeastern half of the Sea of Japan is anisotropic with respect to P-wave velocity. The velocity variation in the anisotropy is approximately 0.4km/sec (i.e., 5%) about a mean velocity of 7.94km/sec. The direction of the maximum velocity is 141°E of north which corresponds roughly to a direction perpendicular to the general trend of northern Honshu as well as to magnetic lineations.

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