17:15 〜 18:45
[MGI25-P03] Features of the 2022 Hunga Tonga-Hunga Ha'apai tsunami deposit at Hihifo Peninsula, Tongatapu island
キーワード：tsunami deposit、tephra、volcanic tsunami、Hunga Tonga-Hunga Ha'apai
On 15 January 2022, the Hunga Tonga-Hunga Ha'apai (HT-HH) submarine volcano produced an explosive eruption. The submarine eruption caused a tsunami that affected much of the Pacific region. Tongatapu island located ca. 65 km southeast from the HT-HH volcano was strongly affected by the tsunami and Hihifo Peninsula on the northwestern side of the island was one of the most seriously affected areas. The elevation of the peninsula is mostly less than 12 m with the exception of some hillier sections and the maximum tsunami trace height by the HT-HH tsunami was ca. 20 m on the peninsula (Borrero et al., 2023). The peninsula faces the sea on the western side with a lagoon to the east, and the tsunami overtopped the peninsula from west to east. To investigate features of the tsunami deposits laid down by the overtopping wave, we set a survey line from the beach to the lagoon and conducted an initial field survey on November 2023. The maximum elevation of our survey line was 10 m at 100-200 m from the beach, and from there the elevation decreased towards the beach and the lagoon.
Our results indicate that the thickness of the tsunami deposits is strongly affected by the topography. Tsunami deposits were rarely observed where the elevation decreased as the tsunami flow accelerated. On the other hand, thick tsunami deposits were observed at the point where the topography changed from uphill to flat during the process of wave run-up (i.e., where the accelerated flow turned into a decelerating one). Moreover, one of the most important characteristics of this volcanic tsunami deposit was that it was covered by tephra. Our results show that this tephra layer was often recognized in depressions but in other areas this surface tephra was often reworked by post-depositional processes. These features are important in identifying volcanic tsunami deposits from geological records.
Acknowledgments: This research was supported by SATREPS, JST/JICA and JSPS.
Our results indicate that the thickness of the tsunami deposits is strongly affected by the topography. Tsunami deposits were rarely observed where the elevation decreased as the tsunami flow accelerated. On the other hand, thick tsunami deposits were observed at the point where the topography changed from uphill to flat during the process of wave run-up (i.e., where the accelerated flow turned into a decelerating one). Moreover, one of the most important characteristics of this volcanic tsunami deposit was that it was covered by tephra. Our results show that this tephra layer was often recognized in depressions but in other areas this surface tephra was often reworked by post-depositional processes. These features are important in identifying volcanic tsunami deposits from geological records.
Acknowledgments: This research was supported by SATREPS, JST/JICA and JSPS.