[S-EM15] 地磁気・古地磁気・岩石磁気

[SEM15-P09] トンガ王国本島及びエウア島の津波巨礫の磁気記録とその移動年代

*佐藤 哲郎¹、中村 教博²、長濱 裕幸³、佐藤 雅彦⁴、後藤 和久⁴、渡部 真史⁵、佐竹 健治¹、池田 暁³、Kula Taaniela⁶ (1.東京大学地震研究所、2.東北大学高度教養教育・学生支援機構、3.東北大学大学院理学研究科地学専攻、4.東京大学大学院理学研究科地球惑星科学専攻、5.中央大学理工学部都市環境学科、6.トンガ王国国土天然資源省)

キーワード：津波石、年代測定、残留磁気

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.