[MIS12-P06] Paleotsunami History in Hachinohe, Aomori Prefecture, northern Japan
Keywords:Paleotsunami research, Tsunami sedimentology, Numerical simulation, Geochemistry
The study site is located on a marine terrace surface elevated between 5-10 MAMSL, on the Kamikita coastal plain (Miyauchi, 1985) and 1.2 km inland from the present shoreline; at the same time, this surface is positioned between the Oirase and Gonohe rivers. We have identified eight sand layers, interbedded with mud and occasional tephra layers. Based on tephra analysis, some of them were possible to confirm as the To-a and To-cu tephra at the top and bottom of the sequence, respectively, giving a range of 6 thousand years of geological history, while the last 1000 years has been apparently eroded, removed or disturbed by human activity. To-a tephra layer contained many diatoms, which implies a water flow or shallow water body as deposition environment. Soils from the top of the section.
The stratigraphic correlation shows that four of the eight sand layers can be well traced landward, and grain analysis shows upward and landward fining. Diatom analysis reveals the presence of marine spices in some sand layers, on the other hand, freshwater and reworked freshwater extinct species were found in the uppermost sand layer (under To-a), suggesting a freshwater inundation event as a genetical source for that. CT number profiles (Hounsfield Units) evidence density changes related to lithological variations between the sand and mud layers. Moreover, XRF core scanner (ITRAX) shows that most of the sand layers are strongly related to a marine origin and exposes concentration of Ti that could be related to heavy minerals concentration and, in consequence, suggesting a sedimentary environment of high energy. Therefore, described evidence suggest a marine origin to the seven sand sheets.
In order to clarify the origin of the sand layers, we performed the numerical simulation to differentiate tsunami sands from storm sands proposed by Watanabe et al. (2018), by comparing wave penetration capacity for each phenomenon; the calculation showed that the only way to carry marine sediments on such further inland is with a tsunami event, if we assume present topography.