Tsunami deposits recognized in Okushiri Island, southwestern Hokkaido, Japan

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  • Kase Yoshihiro
    Geological Survey of Hokkaido, Hokkaido Research Organization
  • Nishina Kenji
    Geological Survey of Hokkaido, Hokkaido Research Organization
  • Kawakami Gentaro
    Geological Survey of Hokkaido, Hokkaido Research Organization
  • Hayashi Keiichi
    Geological Survey of Hokkaido, Hokkaido Research Organization
  • Takashimizu Yasuhiro
    Mathematical and Natural Sciences Course, Faculty of Education, Institute of Humanities, Social Sciences and Education, Niigata University
  • Hirose Wataru
    Geological Survey of Hokkaido, Hokkaido Research Organization
  • Sagayama Tsumoru
    Geological Survey of Hokkaido, Hokkaido Research Organization
  • Takahashi Ryo
    Geological Survey of Hokkaido, Hokkaido Research Organization
  • Watanabe Tatsuya
    Geological Survey of Hokkaido, Hokkaido Research Organization Present address: Kitami Institute of Technology
  • Koshimizu Ken'ichi
    Geological Survey of Hokkaido, Hokkaido Research Organization
  • Tajika Jun
    Geological Survey of Hokkaido, Hokkaido Research Organization Present address: Docon Co. Ltd.
  • Ohtsu Sunao
    Geological Survey of Hokkaido, Hokkaido Research Organization
  • Urabe Atsushi
    Research Institute for Natural Hazards & Disaster Recovery, Niigata University
  • Okazaki Noritoshi
    Geological Survey of Hokkaido, Hokkaido Research Organization
  • Fukami Hiroshi
    Geological Survey of Hokkaido, Hokkaido Research Organization
  • Ishimaru Satoshi
    Geological Survey of Hokkaido, Hokkaido Research Organization

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Other Title
  • 北海道南西部奥尻島で発見された津波堆積物
  • ホッカイドウ ナンセイブ オクシリトウ デ ハッケン サレタ ツナミ タイセキブツ

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Abstract

<p>Okushiri Island, located in southwest Hokkaido, Japan, was hit by an earthquake-generated tsunami in July 1993. An excavation survey revealed several event sand beds, interbedded with peaty sediments, in the lowland at the southernmost part of the island. These beds consist of relatively well-sorted fine- to medium-grained sand similar to sandy shoreline deposits; they are normally graded, fining and thinning landwards. Some beds show partial erosional bases and ripple cross-laminations, implying landward-directed paleocurrents, and grain fabrics also indicate the landward dispersal of clastics. The sand beds yield marine organic-walled microfossils such as dinoflagellate cysts and foraminiferal linings. These findings indicate that the sand beds are of tsunami origin. The results of carbon-14 dating and tephrochronology indicate that at least six tsunami events, including the AD 1741 Oshima-Ohshima and the AD 1993 Hokkaido Nansei-Oki tsunamis, have hit the island over the past 3000-4000 years.</p>

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