[M-IS15] Tsunami deposit: research progress after the 2011 Tohoku-oki earthquake and prospects

[MIS15-07] Features of 17th-century tsunami deposits formed by tsunami crossing river channel and detection using X-ray computed tomography

*Kohei Iwaki¹, Yasuhiro Takashimizu², Yuichi Nishimura³, Takashi Chiba⁴ (1.Graduate School of Science and Technology, Niigata University, 2.Mathematical and Natural Sciences, Institute of Humanities, Social Sciences and Education, Niigata University, 3.Graduate School of Science, Hokkaido University, 4.Faculty of Bioresource Sciences, Akita Prefectural University)

Keywords:Tsunami deposit, X-ray CT, Grain size distribution

Tsunami deposits distributed during run-up show inland fining and thinning trends. However, the effect of abandoned river channels within the run-up area on deposit features is unknown and was investigated in this study. Tsunami deposits are an effective index by which to estimate the inundation area of historical and paleo tsunamis. However, the actual inundation area and distribution limit of tsunami deposits are distinct; the former is generally wider than the latter. Therefore, this study also verified the effectiveness of X-ray computed tomography (CT) analysis for reconstructing the actual distribution of the deposits. The study was conducted in a coastal lowland of the Yufutsu Plain in Hokkaido, Japan, interspersed with back marshes, ponds, and depressions (interpreted as traces of abandoned river channels) behind coastal sand dunes. Samples extracted using a geoslicer and peat sampler were used for core, X-ray CT, and grain size analyses, and the weight of clastic grain contents was measured using organic matter treatments. The lithofacies of the tsunami deposits changed from sand to sandy peat with gradually decreasing sand content. However, X-ray CT analyses identified the presence of clastic materials beyond the area in which sand was visually identifiable, and a positive correlation existed between the mean CT value and the clastic material content of the tsunami deposits, suggesting that the mean CT value could be used as a proxy for the tsunami deposit distribution. The thickness of the tsunami deposits along shore-normal transects generally presented inland thinning part with local coarsening trends in abandoned channels (topographical depressions), which was indicative of the sudden deposition of transported grains under tsunami deceleration during run-up into these channels. The mode grain size and mud content of the tsunami deposits along the shore-normal transect generally indicated inland fining and increasing trends, respectively, which also suggested deceleration of the tsunami run-up flow. However, the mode grain size became coarser after the tsunami flowed across abandoned channels. Thus, these coarse materials may have originated from the channel deposits, a possibility which should be explored in further research.