Characteristic features of geomagnetic anomaly distribution around Japan.
Hydrographic Department of Japan has been conducting Continental Shelf Surveys Project since 1967 Geomagnetic total intensity measurement, depth sounding, gravity measurement and seismic reflection profiling were carried out along tracks of two nautical miles average spacing・ The survey covered the area within about 100 nautical miles from the coast. Magnetic results of the survey were compiled in a geomagnetic total intensity anomaly map around Japan, and geomagnetic zoning was attempted to investigate local and regional geomagnetic features. Yamato Rise, Yamato Basin, and east of Japan Trench were not covered by the continental Shelf Surveys Project, whereas the airborne magnetic survey of the U.S. Navy (altitude 150ー 180 meters above sea level, average spacing of the tracks is three miles) covered these area, and the results were published in magnetic charts of U S Defense Mapping Agency. These results were incorporated in the anomaly map of this paper Magnetic features of the land area were investigated by the Geological Survey of Japan, and the result of the survey was published in airborne magnetic maps (Okubo et al. 1985). From the magnetic results mentioned above, characteristic features of geomagnetic anomaly distribution in the adjacent seas of Japan are summarized as. (1) Japanese lineations of the Pacific Plate is crossing the axis of the Japan Trench, losing its amplitude toward west, fade away into continental magnetic anomaly at about 100 Km from the trench axis (2) Remarkable positive anomaly zone along Pacific coast of NE Hokkaido and NE Honsyu were inferred as Mesozoic volcanic fronts. The strong westward magnetization vector of the latter was confirmed, which support opening of the Japan Sea correlated with counterclockwise rotation of the NE Japan Arc since the Mesozoic time. (3) Continental slope of the SW Japan is characterized by the smooth geomagnetic anomaly distribution, indicating thick sedimental layer of the area. ( 4) The Japan Sea is dominated by very complicated high amplitude short wave-length geomagnetic anomaly distribution, which is indicating complex geology and complex distribution of the mafic volcanic rocks. Geomagnetic anomaly distribution lack the lineated feature except for the weak EW or NE-SW lineated patterns of the Yamato Rise and Oki Ridge. Geomagnetic anomaly distribution around Japan was compared with the other geophysical data. Among various pair of comparizon, interesting correlation between the seismic activity of the shallow earthquakes and geomagnetic anomaly distribution was found. Positive geomagnetic anomalies of Izu-Ogasawara Arc and that of southern extension of Kamuikotan belt of Hokkaido are characterized by active seismic activity, whereas the positive geomagnetic anomalies along the Pacific coast of NE Hokkaido and NE Honsyu are characterized by low seismic activity.
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| Format: | Journal Contribution biblioteca |
| Language: | Japanese |
| Published: |
1987
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| Online Access: | http://hdl.handle.net/1834/16150 |
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