[M-GI25] Holocene paleoenvironment, paleoclimate, and paleohazards in the Pacific Islands

[MGI25-02] Preliminary Results of the Distribution of Tsunami Deposits on Tongatapu Island, Kingdom of Tonga

*中田 光紀¹、石澤 尭史²、Goff James³、Faoliu Savelinga⁴、Vailea Penikolo⁴、笠井 克己¹、池永 有弥¹、Penisoni Maofa⁴、横山 祐典¹、宮入 陽介¹、後藤 和久¹ (1.東京大学、2.東北大学、3.University of New South Wales、4.Tonga Geological Services)

キーワード：津波堆積物、南太平洋、トンガ、放射性炭素年代測定

Tongatapu Island is at high risk from large tsunamis because of its closeness to the Tonga Trench and numerous submarine active volcanoes. Indeed, the 2022 Hunga Tonga–Hunga Haʻapai (HTHH) eruption triggered a massive tsunami with a maximum runup height of 20 m in Tonga (Borrero et al., 2023). Despite this risk, paleotsunami research on Tongatapu Island remains limited (Frohlich et al., 2009; Lavigne et al., 2021). Although Lavigne et al. (2021) identified potential evidence of a large 15th century tsunami, the paleotsunami history on the island is still not well understood. Recently, we conducted field surveys in Kanokupolu, located in the northwest of Tongatapu Island, and discovered six possible tsunami deposits (Event layers 1 to 6 from the top). Preliminary radiocarbon dating was conducted on Event layers 2 and 6, and those were deposited about 250 and 2000 years ago, respectively (Nakata et al., 2024 JpGU). However, since the survey was limited to Kanokupolu, the distribution of the tsunami deposits over the Tongatapu Island remained unclear. Therefore, we conducted additional field surveys and radiocarbon dating at Kala’au, located in the southwest of Tongatapu Island, and Haveluliku, located on the east of the island. At Kala’au, we excavated a trench directly beneath a tsunami boulder (470 tons) that we had discovered. The boulder is located 250 m inland from the coast, at an elevation of about 10 m. In this trench, we identified five possible tsunami deposits containing coral and shell fragments. At Haveluliku, we excavated a trench directly beneath a tsunami boulder (40 tons) that was reported by Lavigne et al. (2021), which is located 400 m inland from the coast at an elevation of about 30 m. In our survey, we also identified an event layer containing coral and shell fragments directly beneath the tsunami boulder, similar to the findings of Lavigne et al. (2021). Radiocarbon dating will be performed for the event layers that we discovered at Kala’au and Haveluliku for detailed correlation of event layers among sites.

Acknowledgments: This research was supported by SATREPS,JST/JICA and JSPS.