17:15 〜 18:45
[MGI25-P04] Initial results of a two-thousand years paleotsunami history of Tongatapu Island, Kingdom of Tonga
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 volcanoes. Indeed, the 2022 Hunga Tonga–Hunga Haʻapai (HTHH) eruption generated a large tsunami with a maximum runup height of 20 m in Tonga (Borrero et al., 2023). However, only limited paleotsunami studies have been carried out on Tongatapu Island (e.g. Frohlich et al., 2009; Lavigne et al., 2021). Lavigne et al. (2021) reported possible evidence of a large tsunami in the 15th century, but the paleotsunami history on the island is not well known.
We carried out field surveys mainly in the Kanokupolu area, where a large tsunami boulder was reported by Borrero et al. (2023). This site is located 200 m inland from the shoreline and 10 m above sea level. Based on a trench survey beneath the boulder, we identified at least 6 event layers (Event layers 1 to 6 from the top). These event layers included abundant shell and coral fragments and some or all of them are potentially transported by tsunamis. Event layer 1 is on the ground surface and is interpreted as the 2022 HTHH tsunami. The tsunami boulder appears to be associated with Event layer 2, and so it seems reasonable to propose that it was transported by this event. In addition, Event layer 6 is directly covered by a tephra layer and as such may well have been associated with a submarine volcanic tsunami similar to HTHH.
Preliminary radiocarbon measurements on molluscs and coral fragments were conducted to identify ages of Event layers 2 and 6. Radiocarbon datings on those sediments suggested that Event layers 2 and 6 were deposited about 200 and 1800 years ago, respectively. Although the local reservoir effect (Delta R) in Tonga has been reported previously (e.g., Petchey et al., 2023; Fukuyo et al., 2020;2023), further investigations are underway to refine the ages of tsunami deposits. In any case, this is the first report of potential tsunami deposits older than the 15th century in Tonga.
Acknowledgments: This research was supported by SATREPS,JST/JICA and JSPS.
We carried out field surveys mainly in the Kanokupolu area, where a large tsunami boulder was reported by Borrero et al. (2023). This site is located 200 m inland from the shoreline and 10 m above sea level. Based on a trench survey beneath the boulder, we identified at least 6 event layers (Event layers 1 to 6 from the top). These event layers included abundant shell and coral fragments and some or all of them are potentially transported by tsunamis. Event layer 1 is on the ground surface and is interpreted as the 2022 HTHH tsunami. The tsunami boulder appears to be associated with Event layer 2, and so it seems reasonable to propose that it was transported by this event. In addition, Event layer 6 is directly covered by a tephra layer and as such may well have been associated with a submarine volcanic tsunami similar to HTHH.
Preliminary radiocarbon measurements on molluscs and coral fragments were conducted to identify ages of Event layers 2 and 6. Radiocarbon datings on those sediments suggested that Event layers 2 and 6 were deposited about 200 and 1800 years ago, respectively. Although the local reservoir effect (Delta R) in Tonga has been reported previously (e.g., Petchey et al., 2023; Fukuyo et al., 2020;2023), further investigations are underway to refine the ages of tsunami deposits. In any case, this is the first report of potential tsunami deposits older than the 15th century in Tonga.
Acknowledgments: This research was supported by SATREPS,JST/JICA and JSPS.