Stratigraphic records of tsunamis along the Japan Sea,southwest Hokkaido,northern Japan |
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| Authors: | Gentaro Kawakami Kenji Nishina Yoshihiro Kase Jun Tajika Keiichi Hayashi Wataru Hirose Tsumoru Sagayama Tatsuya Watanabe Satoshi Ishimaru Ken'ichi Koshimizu Ryo Takahashi Kazuomi Hirakawa |
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| Institution: | 1. Geological Survey of Hokkaido, Hokkaido Research Organization, Sapporo 060‐0819, Japan;2. Docon Co. Ltd, Sapporo 004‐8585, Japan;3. Department of Civil and Environmental Engineering, Kitami Institute of Technology, Kitami 090‐8507, Japan;4. Emeritus Professor of Hokkaido University, Toyohashi 441‐3301, Japan |
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| Abstract: | The stratigraphy of tsunami deposits along the Japan Sea, southwest Hokkaido, northern Japan, reveals tsunami recurrences in this particular area. Sandy tsunami deposits are preserved in small valley plains, whereas gravelly deposits of possible tsunami origin are identified in surficial soils covering a Holocene marine terrace and a slope talus. At least five horizons of tsunami events can be defined in the Okushiri Island, the youngest of which immediately overlies the Ko‐d tephra layer (1640 AD) and was likely formed by the historical Oshima‐Ohshima tsunami in 1741 AD. The four older tsunami deposits, dated using accelerator mass spectrometry 14C, were formed at around the 12th century, 1.5–1.6, 2.4–2.6, and 2.8–3.1 ka, respectively. Tsunami sand beds of the 1741 AD and circa 12th century events are recognized in the Hiyama District of Hokkaido Island, but the older tsunami deposits are missing. The deposits of these two tsunamis are found together at the same sites and distributed in regions where wave heights of the 1993 tsunami (Hokkaido Nansei‐oki earthquake, Mw = 7.7) were less than 3 m. Thus, the 12th century tsunami waves were possibly generated near the south of Okushiri Island, whereas the 1993 tsunami was generated towards the north of the island. The estimated recurrence intervals of paleotsunamis, 200–1100 years with an average of 500 years, likely represents the recurrence interval of large earthquakes which would have occurred along several active faults offshore of southwest Hokkaido. |
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| Keywords: | Hokkaido Japan Sea northern Japan tsunami deposits tsunami recurrences |
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