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

Abstract

Previous research indicates that Yakushima Island, southwestern Japan, may have been struck by a huge tsunami before or soon after the arrival of the Koya pyroclastic flow during the 7.3 ka caldera‐forming Kikai eruption, but this has not yet been confirmed. This paper describes sedimentological and chronostratigraphic evidence showing that Unit TG, one of three gravel beds exposed on the Koseda coast of northeast Yakushima Island and investigated here, is a tsunami deposit. Unit TG is a poorly sorted, 30 cm thick gravel bed overlying a wave‐cut bench and underlying a Koya pyroclastic flow deposit. Sparse wood fragments in Unit TG were dated at 7 416–7 167 cal year BP. The constituent gravel clasts of Unit TG are similar in composition to those of modern beach and river deposits along the Koseda coast. Unit TG also contains pumice clasts whose chemistry is identical to that of pumice derived from the 7.3 ka eruption at Kikai caldera. The long‐axis orientations and composition of gravel clasts in Unit TG suggest that they were transported by a landward‐travelling high‐particle‐concentration flow, which suggests that Unit TG was deposited by a tsunami run‐up flow during the 7.3 ka Kikai caldera eruption, just before the arrival of the major Koya pyroclastic flow at the Koseda coast. Whether the 7.3 ka tsunami was caused by a volcanic eruption or an earthquake remains unclear, but Unit TG demonstrates that a tsunami arrived immediately before emplacement of a Koya pyroclastic flow.

Radiocarbon age offsets of Plants and Shells in Holocene sediments from the Sukumo plain, Southwest Coast of Shikoku, Southwest Japan

“…Angular gravel, shells of rock-reef species, and fragments of coral strongly imply strong currents from outside the inner bay. A similar deposit was also reported in the Oita area (Fujiwara et al 2010;Nakanishi et al 2017c), northwest of the drilling site, and Yakushima Island (Geshi et al 2017;Nanayama and Maeno 2018), southwest of the drilling site ( Figure 1B). They suggest that a reexamination of 7.3 ka tsunami event (Maeno et al 2006;Maeno and Imamura 2007) will be needed, within 300 km from the caldera.…”

Great earthquake at 7.3 ka inferred from tsunami deposits in the Sukumo Bay area, Southwestern Japan

“…BB, beppu bay; BC, bungo channel; OS, osumi strait. The green triangles indicate sites of the older 7.3 ka tsunami deposits on Takeshima Island (Machida & Shirao, 1998) and Yakushima Island (Nanayama & Maeno, 2019). The blue triangles indicate sites of the younger 7.3 ka tsunami deposits named in the text.…”

“…have not yet been reconstructed. Nanayama and Maeno (2019) presented sedimentological and chronostratigraphic evidence showing that Unit TG, one of three gravel beds exposed on the Koseda coast of Northeast Yakushima Island, is a tsunami deposit (Figure 14). Unit TG is a poorly sorted, 30-cm-thick gravel bed overlying a wave-cut bench and overlain by a Koya pyroclastic flow deposit.…”

“…Considering the above findings comprehensively, it appears that at least two tsunamis occurred at the time of the Kikai caldera eruption or immediately afterward. Nanayama and Maeno (2019) proposed that one tsunami (the older 7.3 ka tsunami) occurred during the early climax stage of the eruption, before and after the inflow of the Koya pyroclastic flow, and that another (the younger 7.3 ka tsunami) was larger than the first tsunami and occurred during the late stage of the K-Ah eruption or immediately after the end of the eruption.…”

“…Segment N5 0 was additionally cited by Furumura et al (2011). (b) Topography and bathymetry of southwestern Japan, modified after Nanayama and Maeno (2019) and Nanayama et al (2020). The base map was prepared by Kiyoshi Kishimoto of AIST.…”