[S-EM34] 地磁気・古地磁気・岩石磁気

[SEM34-15] 津波性巨礫の粘性残留磁気を用いた津波年代推定

*佐藤 哲郎¹、中村 教博^1,2、後藤 和久²、山田 昌樹³、熊谷 祐穂¹、外崎 貴之¹、箕浦 幸治¹ (1.東北大学大学院理学研究科地学専攻、2.東北大学災害科学国際研究所、3.筑波大学大学院生命環境科学研究科地球進化科学専攻)

キーワード：津波性巨礫、粘性残留磁化、温度時間関係式

Along some coastlines in Japan, there are erratic boulders apparently emplaced by tsunamis. The key to understanding of past tsunami events is the ability to accurately date the emplaced age of boulders. Although radiocarbon is one of powerful tools for dating boulders, subsequent movements and non-organic rocks (sedimentary rocks and volcanic rocks) cannot be dated. To overcome this problem, we use viscous remanent magnetization (VRM). Viscous remanent magnetization (VRM) partially overprints original magnetization in rocks displaced by paleotsunami events. If a magnetic-mineral bearing rock is moved or re-oriented, the magnetism of the smaller magnetic grains re-aligns to the direction of the ambient magnetic field with time. This phenomenon is well known as Neel’s (1949, 1955) single-domain (SD) relaxation theory. Pullaiah et al. (1975) derived a time-temperature (t-T relation) relation by assuming Neel’s theory of magnetite. In principle, an experimental combination of short relaxation time and high temperature for removing VRM can determine the unknown relaxation time (tsunami age) at room temperature. We tested this hypothesis to tsunamigenic boulders consisted of coral limestone in Ishigaki Island, sand stone in Beppu bay and welded tuff in Sanriku coast. The demagnetized paths of all samples have VRM turning points, and their demagnetization temperatures are compatible or higher value of the Neel’s hypothesis.