活断層・古地震研究報告，No. 3, p. 285-296, 2003 北海道東部，厚岸町史跡国泰寺跡の泥炭層中において発見された 9 層の津波砂層とその広域イベント対比 Nine tsunami sand deposits in peat layers at the historic site of Kokutaiji Temple in Akkeshi Town, eastern Hokkaido, and their correlation with regional tsunami events 添田雄二 1 ・七山 太 2 ・重野聖之 3 ・古川竜太 4 熊崎農夫博 5 ・堤 康夫 6 ・車塚 洋 7 ・澤井祐紀 8 ・佐竹健治 9 中川 充 10 ・山田悟郎 11 ・桂川 実 12 ・赤松守雄 13 ・石井正之 14 Yuji Soeda 1 , Futoshi Nanayama 2 , Kiyoyuki Shigeno 3 , Ryuta Furukawa 4 , Nobuhiro Kumasaki 5 , Yasuo Tsutsumi 6 , Hiroshi Kurumazuka 7 , Yuki Sawai 8 , Kenji Satake 9 , Mitsuru Nakagawa 10 , Goro Yamada 11 , Minoru Katsuragawa 12 , Morio Akamatsu 13 and Masayuki Ishii 14 1, 11, 13 北海道開拓記念館 (Historical Museum of Hokkaido, soeda@jg8.so-net.ne.jp) 2 海洋資源環境研究部門 (Institute of Marine Resources and Environment, nanayama-f@aist.go.jp) 3, 14 明治コンサルタント株式会社（Meiji Consultant Co., Ltd., shigeno-k@meicon.co.jp） 4 地球科学情報研究部門（Institute of Geoscience, GSJ/AIST, furukawa-r @ aist.go.jp） 5, 6, 7, 12 厚岸町海事記念館（Marine Museum of Akkeshi） 8, 9 活断層研究センター（Active Fault Research Center, GSJ/AIST, yuki.sawai@aist.go.jp, kenji.satake@ aist.go.jp） 10 北海道地質調査連携研究体（Hokkaido Branch, GSJ/AIST, nakagawa.gsj@aist.go.jp） Abstract: We found nine sand layers in trenches excavated at lowland, ca. 2.1 m above sea level, in front of Kokutaiji Temple, Akkeshi town in eastern Hokkaido. Kokutaiji Temple was built in 1804, and their official record “Nikkanki”, as well as timber remnants from this temple, describes the 1843 Tokachi-oki earthquake and tsunami. We correlated volcanic ash layers found in the trenches with Holocene volcanic ashes previously reported from the eastern Hokkaido. Three ash layers just below the ground surface are Ta-a (1739), Ko-c2 (1694), and Ta-b (1667), respectively. An ash layer at 27 cm below Ta-b may be correlated with B-Tm (ca. 1,000 yrs BP), and one 58 cm below B-Tm may be correlated with Ta-c2 (ca. 2,500 yrs BP). Consequently, we estimate that the peat was accumulated during the past 3,500 years. The diatom analysis indicates that (1) fresh water species such as Pinnularia viridis, Navicula elginensis and terrestrial species such as Hantzschia amphioxys, Pinnularia borealis are predominant in almost all the peat samples; (2) marine species such as Paralia sulcata, Cocconeis scutellum, Nitzschia granulata are found in the sand layers; and (3) the individuals from the sand layers are extremely fewer than those from the peat layers. From these facts we conclude that the sand layers are marine event sediments, brought by tsunamis from the ocean. If we compare the stratigraphy of the tsunami events with those reported from Kiritappu Marsh and Lake Harutori-ko, the sand layers in the trenches correspond to regional tsunami events Ts2 to Ts10. Thus, nine giant tsunamis during past 3,500 years brought sediments to Akkeshi area, as well as the other parts of the eastern Hokkaido. キーワード：厚岸町，国泰寺，津波堆積物，古地震，千島海溝，北海道東部，太平洋 Keywords: Akkeshi Town, Kokutaiji, tsunami deposits, paleoseismology, Kuril subduction zone, eastern Hokkaido, Pacific Ocean １．はじめに 北海道東部太平洋沿岸域は千島海溝に面する本邦 屈指の地震津波の多発地帯である（Fig. 1）．同地域に は，縄文海進以降に生じた湿原や海跡湖が多数存在す ることが知られており，近年，過去数千年間に渡って 堆積した泥炭層および湖沼堆積物中の津波堆積物に 関する研究が活発に行われてきている（例えば，七 山・重野，1998 ；1999 ；七山ほか，2000a ,b ；2001a, b, c；2002a. b；西村ほか，2000；平川ほか，2000a, b， 2001；Sawai, 2002; Nanayama et al., 2003）．このうち， 浜中町霧多布湿原地域においては，過去 3,000 年間に 285