| Summary: | Detailed magnetic surveys were carried out at sea around the northern part of Japan as part of “The Basic Map of the Sea Project” by the Hydrographic Department of Japan from 1968 to 1972. Tracklines are spaced every two nautical miles in most part of the surveyed areas, where geomagnetic total intensity was measured by means of proton magneto-meters. Magnetic anomalies of the surveyed areas show four significant features as follows. i ) Lineated patterns of magnetic anomalies found at the north-west Pacific basin have proved extending over the continental slope off the Tohoku District across Japan Trench. ii) Remarkable positive anomaly zones exist along the Pacific coast of Japan. They are parallel to the southern part of the Kuril-Kamchatka Trench and to the northern part of Japan Trench. iii) In the Japan Sea, anomaly patterns are extremely complicated, positive and negative anomalies being distributed like a patch works. iv) Continental shelf of the Okhotsk Sea is characterized by wide and smooth negative anomalies with locally very strong negative values found at limited areas. The positive linear anomalies along the Pacific coast of Japan were examined by calculating numerically the magnetizations which are thought to cause the anomalies. From the results it is found that the one on the coast of the southeast Hokkaido can be explained by a normally magnetized horizontal layer, but that the other along the east coast of Tohoku District can be explained by a magnetized body having a westward component of magnetization. This facts have led us to a conclusion that the Tohoku District must have turned counter-clockwise after the layer causing positive anomalies had been formed. This conclusion coincides well with the palaeomagnetic results from rock samples on land obtained by Kawai, et. al. (1962). Published
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