[S-EM15] Geomagnetism, paleomagnetism and rock magnetism

[SEM15-P09] Paleomagnetic Dating on Tsunami Boulders from Tongatapu Island and Eua Island, the Kingdom of Tonga

*Tetsuro Sato¹, Norihiro Nakamura², Hiroyuki Nagahama³, Masahiko Sato⁴, Kazuhisa Goto⁴, Masashi Watanabe⁵, Kenji Satake¹, Akira Ikeda³, Taaniela Kula⁶ (1.Earthquake Research Institute, The University of Tokyo, 2.Institute for Excellence in Higher Education, Tohoku University, 3.Department of Earth Science, Tohoku University, 4.Department of Earth and Planetary Science, The University of Tokyo, 5.Faculty of Science and Engineering, Chuo University, 6.Ministry of Lands and Natural Resources)

Keywords:Tsunami boulder, Age estimation, Remanent magnetization

On January 15th, 2022, the eruption of the Hunga-Tonga volcano generated a violent underwater explosion, and the tsunami produced by the eruption affected the Pacific coasts. For informing the prevention of future tsunami hazards, the age estimation of geological records of tsunamis is important. Large tsunami boulders exist at the coastline of Tongatapu Island and Eua Island, the Kingdom of Tonga. Using the boulders on Tongatapu Island, the numerical model reported by previous research finds that a slump in an eastward direction away from the volcano produces a tsunami wave with peak-to-trough amplitudes of 14 m at 100 m water depths off western Tongatapu Island. The reworking ages of seven tsunami boulders on Tongatapu Island obtained from the comparison of uranium/thorium dating of attached marine organisms, numerous modeling, and sea level change are either within the past 7 k.y. or ca. 122 ka, rather than at intermediate times when sea level was 15–120 m lower than present level. However, the detailed transportation timing of boulders on Tongatapu Island is still unclear. Moreover, we found that the eight huge boulders on the northern coastline of Eua Island, and the biggest boulder size is 4x3x3 m. Although the source of tsunami is unknown (i.e., earthquake at the subduction zone or volcanic eruption), we use paleomagnetic dating protocol to reveal the reworked age of tsunami boulders. Most rocks contain magnetic particles that record the geomagnetic field, and a subset of constituent magnetic particles in a reworked boulder is expected to acquire a secondly magnetic component called viscous remanent magnetization. The magnitude of such magnetization depends on the time passed since reworking. Traditional paleomagnetic dating based on the theory of viscous remanent magnetization for fine magnetic particles has shown anomalously old ages, so we applied a different protocol which is applicable to admixture of magnetic particle sizes. Based on this method, we found that the reworked age of boulders on Tnagatapu Island is within the past 7 k.y. In this presentation, we will also show the result of boulders from Eua Island.