2021 Fiscal Year Final Research Report
Unveiling the mobilization history of tsunami boulders: direct comparison of paleomagnetic and radiometric dating
Project/Area Number |
20K22361
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Research Category |
Grant-in-Aid for Research Activity Start-up
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Allocation Type | Multi-year Fund |
Review Section |
0204:Astronomy, earth and planetary science, and related fields
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Research Institution | The University of Tokyo |
Principal Investigator |
Sato Tetsuro 東京大学, 地震研究所, 特任研究員 (00830085)
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Project Period (FY) |
2020-09-11 – 2022-03-31
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Keywords | 津波石 / 残留磁気 / 放射性炭素年代 |
Outline of Final Research Achievements |
Movement history of tsunami boulders is important for reconstructing repeated tsunami events. Although radiometric dating methods are available for characterizing the movement history, these methods cannot reveal the reworking movements. Paleomagnetic dating methods have been applied in reworking movements. These methods reveal signals associated with remanent magnetization that gradually grew since the reworking event. The methods were verified by comparison to the radiocarbon ages of un-reworked boulders on Ishigaki Island, Japan, while the estimated ages of such boulders based on the classical theory contradicted the radiocarbon ages. Thus, we used a modified dating protocol, and the method addressed the contradiction. The reworking movement was estimated using an additional boulder, whose remanence age linked the boulder to a reworking event of the recent tsunami. Moreover, we could estimated the transported age of boulders on Tongatapu Island, Kingdom of Tonga.
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Free Research Field |
地質学、古地磁気学
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Academic Significance and Societal Importance of the Research Achievements |
巨大地震・津波の再来周期を把握することは、将来起こり得る自然災害の被害を軽減するために重要である。津波で転がった巨礫は景観として印象的なだけでなく、過去の津波災害の危険性を現代に伝承する貴重な試料である。この津波巨礫が何回移動したのかというシンプルでありながら、これまでの技術では解消できなかった課題に、地磁気を用いた年代測定技術を用いて取り組んだ。その結果、石垣島の津波巨礫から少なくとも2回の移動の痕跡を取り出せただけでなく、トンガ王国の津波巨礫の移動してきた年代をも明らかにできた。
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