On the distribution of deep earthquakes in the Japanese islands and surrounding areas : The minute investigation of the Wadati-Benioff zone

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  • 日本列島とその周辺地域の深発地震の震源分布 : 和達・ベニオフ面の検討
  • ニホン レットウ ト ソノ シュウヘン チイキ ノ シンパツ ジシン ノ シンゲン ブンプ ワダチ ベニオフメン ノ ケントウ

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The spatial distribution of earthquake foci in the Japanese islands and their neighborhoods is described in the paper, based on the data of the Annual Report of Seismicity published by the Meteorological Agency of Japan in 2006. The hypocenters of earthquakes greater than 4.5 in magnitude for those shallower than 100km in depth and those greater than 3 in magnitude deeper than 100km in depth occurred from 1983 to 2005 are shown in the figures, in which summit levels are drawn every 400m on the land and in the sea. The iso-depth contours of deep earthquakes in the Japanese islands are drawn at the bottom of seismicity area. They dip away from the Kuril-Kamchatka trench northwestward , from the Japan trench westward and from the Izu-Ogasawara trench southwestward in general. Similar lines are drawn in Kyushu, Ryukyu islands and surrounding areas. They dip away from the Ryukyu trench northwestward. Being studied closely, the iso-depth contours are not simple as shown by the former authors. They are divided into several segments, in which the contours run linearly or circularly, and displaced several 10km or more near their boundaries. Those displacement lines run in echelon in NW-SE direction in Hokkaido, Kuril islands and their surrounding areas. Each unit is 100 to 400km in width. The major lines running at the east end of Kunashiri island and in the center of Hokkaido are discriminated in the scale. The focal depth lines are curved convexly southeastward under the Kuril basin, suggesting the down-bulge of seismicity area. Those displacement lines run in NW-SE and EW directions in Northeast Honshu and northeast and central parts of the Sea of Japan. Each unit bounded by those displacement lines is 100 to 200km in width. The major displacement line running from the southwest part of Japan Sea to the central part of northeast Honshu is discriminated. The displacement lines run in ENE-WSW direction in Izu-Ogasawara region. Each unit is several tens to 200km in width. Major lines running at the north end and the central part of Izu-Ogasawara islands are discriminated. The displacement lines runs in ENE-WSW direction in Kyushu and Ryukyu region. Each unit cutting by those lines is 80 to 250km in width. Major line running between Osumi islands and Amami-oshima islands is discriminated. Thus the Wadati-Benioff zone is divided into segments cut by nearly vertical lines which suggest block-like structure in the focul zone.

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