Paleotsunami History of Hachinohe, Northern Japan: A Multiproxy Analysis and Numerical Modeling Approach

Abstract

\n Paleotsunami studies along the Pacific coast of Tohoku, northern Japan, have been considerably developed recently, particuvclarly after the massive impact of the 2011 Tohoku-oki tsunami. Nevertheless, in the southernmost Shimokita Peninsula, studies pertaining to paleotsunami are underdeveloped, leading to vague understanding of the tsunamigenic sources northward of the Tohoku region, along with incomplete hazard evaluation. Paleotsunami deposits in Shimokita can be related not only to the Japan trench along the Sanriku coast, but also to the Kuril trench along the Pacific coast of Hokkaido. In this study, we unveiled the paleotsunami history of Hachinohe in northern Tohoku. Using a combination of sedimentological, geochemical, paleontological, and mineralogical proxies, we characterized seven sand layers that dated from ca. 2700 to ca. 5500 yr BP based on radiocarbon (14C) ages as event deposits of marine origin. Sedimentological and paleontological evidence coupled with ground penetrating radar imagery revealed a marsh environment comprising successive extinct ponds, controlling the depositional environment. Numerical modeling ruled out the possibility of storms as genetic sources, leading to the conclusion that the presence of event deposits with marine sediments in the study area would be associated with tsunami inundation episodes. Based on 14C dating, the average frequency of recurrence of tsunamis is estimated as 545–736 years with a 2σ error range. This interval is shorter than that estimated in Harashinai, northern Sanriku Coast, near our survey site, which suggests an average of 530 to 956 years (2σ) for the same time frame. The previously recorded limited paleotsunami evidence and absence of an estimated recurrence interval in the Shimokita Peninsula reaffirms the importance of Hachinohe as a tsunami record site for the activity of both trenches.