Paleomagnetic dating for large boulders transported by natural hazards

2023 Fiscal Year Final Research Report

• Project/Area Number: 21K14008
• Research Category: Grant-in-Aid for Early-Career Scientists
• Allocation Type: Multi-year Fund
• Review Section: Basic Section 17040:Solid earth sciences-related
• Research Institution: Waseda University (2022-2023); The University of Tokyo (2021)
• Principal Investigator: Sato Tetsuro 早稲田大学, 教育・総合科学学術院, 助教 (00830085)
• Project Period (FY): 2021-04-01 – 2024-03-31
• Keywords: 年代測定 / 粘性残留磁気 / 巨礫

Outline of Final Research Achievements

The movement history of large boulders is important for reconstructing repeated natural hazards such as landslides and tsunamis. 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. Although some andesitic boulders have shown the multiple magnetic vector components, the unblocking temperature of younger components were higher than the predicted temperature, which indicates the chemical alteration of pre-existing magnetic minerals. To avoid such difficulties, I performed the paleomagnetic analysis of single silicate crystals. Moreover, I estimated the transported age of coral limestone boulders.

Free Research Field

古地磁気学

Academic Significance and Societal Importance of the Research Achievements

巨大地震とそれに伴う地すべりや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．自然災害によって移動した巨礫は景観として印象的なだけでなく，過去の災害の危険性を現代に伝承する貴重な試料だと考えられている．この巨礫がいつ・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．この手法は，これまで石灰岩を用いた研究での成功例が多かったものの，他の岩石種を用いた場合には失敗するケースが報告されていた．そこで，本研究ではこの課題の解決を図り，解決策を提案した．