[M-IS23_2AM2] 津波堆積物

[MIS23-12] 岩手県野田村における津波堆積物調査に基づく三陸地方北部の津波履歴の検討

*後藤 和久¹、飯嶋 耕崇¹、西村 裕一²、菅原 大助¹、横山 祐典³、宮入 陽介³、沢田 近子³、中村 有吾² (1.東北大学、2.北海道大学、3.東京大学)

キーワード：津波, 津波堆積物, 野田村, 貞観津波

Along the Sanriku coast, pre-historic tsunami record is still poorly understood in contrast to the well-documented historical tsunamis of past 400 years. AD869 Jogan tsunami is one of these cases. The tsunami affected the Sendai Bay area, as tsunami deposits were reported on Sendai and Ishinomaki Plains, but evidence is unsure if the tsunami was also reached along the Sanriku coast. To explore the paleotsunami histories along the Sanriku coast with emphasis on the possible inundation of AD869 event, we conducted field survey along the coast of Noda Village, Iwate Prefecture. Our survey site is now occupied by paddy and the 2011 Tohoku-oki, 1869 Meiji Sanriku and the 1933 Showa Sanriku tsunamis inundated to this site. We took ~3 m long cores and found several gravel and sand deposits in peat buried by surface paddy soil. Considering the continuous distribution of deposits over 0.7 km from the present shoreline and analytical results of grain size and mineral composition, the deposits are likely formed by the tsunami although further investigation is required. Among these tsunami-like layers, a ~10 cm thick gravel layer is deposited below tephra layers. One of the tephra layers is identified as Baitoushan-Tomakomai tephra (B-Tm) that was deposited in early to middle 10th Century. Volcanic glasses that can be identified as Towada-a tephra (To-a) of AD915 also is observed in patches at the similar horizon as B-Tm tephra. Radiocarbon dating results above the gravel layer is consistent with the tephra chronology. These analytical results as well as tsunami numerical modeling result suggest the inundation of potentially large tsunami before early to middle 10th Century along the northern Sanriku coast.