2020 Fiscal Year Final Research Report
Paleomagnetic geochronology and its applications to tsunami boulders and fault gouges
| Project/Area Number |
18K03755
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| Research Category |
Grant-in-Aid for Scientific Research (C)
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| Allocation Type | Multi-year Fund |
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| Section | 一般 |
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| Review Section |
Basic Section 17030:Human geosciences-related
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| Research Institution | Tohoku University |
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Principal Investigator |
Nakamura Norihiro 東北大学, 高度教養教育・学生支援機構, 教授 (80302248)
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| Co-Investigator(Kenkyū-buntansha) |
後藤 和久 東京大学, 大学院理学系研究科(理学部), 教授 (10376543)
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| Project Period (FY) |
2018-04-01 – 2021-03-31
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| Keywords | 古地磁気学 / 津波性巨礫 / ナノバブル |
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| Outline of Final Research Achievements |
We have developed a paleomagnetic viscous dating with a stretched exponential relaxation for various grain-size distributions of magnetite. Also we have developed the effective method of reductive chemical demagnetization with a nano-bubble generator. These two methods allowed us to determine the date of tsunamigenic boulder. Examination of the tsunamigenic coral boulders, whose emplacement ages are determined by radiocarbon, has revealed the age estimated from paleomagnetic viscous dating indicates an older age. The stretched exponential relaxation method we have developed succeeded to explain the older ages. Our newly-developed nano-bubble method for reductive chemical demagnetization of volcanic rocks revealed two components, the characteristic remanence during the eruption and the viscous remanence acquired after the emplacement. We found that many boulders are distributed along the coral reefs of the western coast of Eua island where is located western side of the Tonga trench.
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| Free Research Field |
地球電磁気学
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| Academic Significance and Societal Importance of the Research Achievements |
三陸沿岸には多くの巨礫があり，その巨礫を祀る神社が多い。これは巨大津波を経験した先人が，未来の私たちに宗教儀式として，津波の危険性を知らせるためのタイムマシーンなのだろう。しかし，津波の危険性は理解できても，「いつ・どれぐらいの規模」の津波が襲ったのかの定量的なデータはわからない。さらに，巨礫が移動したことから規模は推定できても，「いつ」については推定する術がこれまでなかった。本研究にいたるこれまでの地磁気による年代推定研究により，巨礫ひとつから「いつ・どれぐらいの規模」の津波が発生したことを推定できる手法を開発した。これにより，宗教的なお祀りだけでなく，津波に関する定量的な知識が得られる。
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