High-precision estimation of a paleo-tsunami inundation area by identifying tsunami traces beyond sandy tsunami deposits: A case study of the 869 CE Jogan tsunami in Fukushima, northeastern Japan

Previous tsunami deposit studies have considered the distribution of visible sandy tsunami deposits as a tool to establish inundation areas and limits, but use of this area for tsunami magnitude estimation may cause it to be underestimated because the actual inundation limit of a tsunami rarely coincide with the depositional limit of visible sandy tsunami deposits. In this study, we aimed to reconstruct with high accuracy the inundation area of the Jogan tsunami, which occurred off the Pacific coast of the Tohoku region of Japan in 869 CE, by using sedimentological and geochemical methods to identify tsunami traces inland from the depositional limit of the tsunami deposits that are difficult to see by eye. We collected sediment samples at Minamisoma City, Fukushima Prefecture. Based on radiocarbon dating, grain-size analysis of sand, increased amounts of marine-derived elements such as Ca and Sr, and the presence of characteristic biomarkers, visible traces of sandy Jogan tsunami deposits were recognized up to 2280 m from the present coastline. Although no visible sandy tsunami deposits were observed farther inland, at 2790 m from the present coastline, evidence of seawater inundation was inferred from computed tomography image analysis, grain-size distribution of sand, presence of pumice, detection of squalene, and other allochthonous biomarkers. Detection of tsunami traces enables a highly accurate estimation of tsunami inundation areas, leading to a more accurate estimation of the source and scale of earthquakes.