Evidence on the Koseda coast of Yakushima Island of a tsunami during the 7.3 ka Kikai caldera eruption

A significant discovery on Yakushima Island, Japan, reveals evidence of a massive tsunami occurring around 7.3 thousand years ago during the Kikai caldera eruption. The study identifies Unit TG, a gravel bed on the Koseda coast, as a tsunami deposit, characterized by its poor sorting and thickness, overlying a wave-cut bench and underlying a Koya pyroclastic flow deposit. Radiocarbon dating of wood fragments within Unit TG suggests deposition between 7416–7167 cal year BP, aligning with the timing of the Kikai eruption. The gravel clasts in Unit TG match modern beach and river deposits along the coast, while pumice clasts indicate a link to the 7.3 ka eruption. The study proposes that Unit TG was deposited by a landward-travelling high-particle-concentration flow, likely from a tsunami run-up during the eruption, just before the arrival of the Koya pyroclastic flow. While the exact cause of the tsunami—whether volcanic or seismic—remains uncertain, this finding underscores the immediate impact of the eruption on the island's geology.

Nanayama, F., & Maeno, F. (2018). Evidence on the Koseda coast of Yakushima Island of a tsunami during the 7.3 ka Kikai caldera eruption. Island Arc, 28(2). Portico. https://doi.org/10.1111/iar.12291