[M-IS20] Tsunami deposit

[MIS20-04] Unraveling the Spatial Complexity of Tsunami Deposits: Insights from Trench Investigations

*Yuichi Nishimura¹, Yasuhiro Takashimizu², Takashi Ishizawa³, Daisuke Sugawara³ (1.Graduate School of Science, Hokkaido University, 2.Faculty of Education, Niigata University, 3.International Research Institute of Disaster Science, Tohoku University)

Keywords:tsunami deposit, trench, pre-historical earthquake

Trench excavation was conducted with the aim of observing tsunami deposits in Atsuma Town, Hokkaido. In the eastern part of the Iburi region, including Atsuma Town, widespread distribution of tsunami deposits from the Keicho Sanriku Earthquake in 1611 has been revealed (Nishimura et al., JpGU 2023). The trench was excavated at locations with tsunami deposit layer thicknesses ranging from 10-20 cm, based on information from a broad survey. The trench dimensions were set at 9m along the tsunami inundation direction, 6m perpendicular to it, and with a depth of approximately 2m. The deepest layers date back to around 3500 years ago (Ishizawa et al., JpGU 2024), containing five layers of volcanic ash and pumice from the 17th century to 2700 years ago within peat. Tsunami deposits primarily consist of well-sorted fine sand. On the 9m-long trench wall, variations in layer thickness were observed ranging from 1cm to 22cm. One contributing factor to the thickness variability is the deposition of eroded peat blocks along with sand. Some blocks, measuring about 50cm in width and 20cm in height, are present without disrupting multiple layers of volcanic ash. Within the trench, the continuous deposition of B-Tm volcanic tephra beneath the tsunami deposits suggests the tsunami was powerful enough to erode peat blocks at a seaward position from the trench and transport them inland. On a portion of the trench wall, bedding and grading structures were observed in the tsunami deposit. Additionally, the grain size of tsunami deposits tends to be coarser in locations with thicker layers. This can be explained by assuming that after the tsunami deposits filled the surface depressions, the entire area was covered with blown sand, containing finer particles. Notably, at this site, there is minimal soil layer between the tsunami deposits and the overlying the 1663 Us-b pumice layer, implying that the tsunami deposits were exposed on the surface for several decades. The opportunity to extensively observe tsunami deposits in trenches is limited. Therefore, the trench excavation has been utilized to confirm and develop investigation techniques for tsunami deposit research. This includes validating dating methods for tsunami deposits (Ishizawa et al., 2024 JpGU) and explorimg the practical application of ground-penetrating radar. Additionally, with the aim of contributing to regional disaster prevention, explanatory meetings for residents were