A new application of paleomagnetic viscous dating to tsunami boulders in Ishigaki Island, Japan

2014 Fiscal Year Final Research Report

• Project/Area Number: 25560173
• Research Category: Grant-in-Aid for Challenging Exploratory Research
• Allocation Type: Multi-year Fund
• Research Field: Natural disaster / Disaster prevention science
• Research Institution: Tohoku University
• Principal Investigator: NAKAMURA Norihiro 東北大学, 理学(系)研究科(研究院), 准教授 (80302248)
• Co-Investigator(Kenkyū-buntansha): GOTO Kazuhisa 東北大学, 災害科学国際研究所, 准教授 (10376543) ; SUGAWARA Daisuke 東北大学, 災害科学国際研究所, 助教 (50436078)
• Co-Investigator(Renkei-kenkyūsha): HATAKEYAMA Nobutatsu 岡山理科大学, 情報処理センター, 准教授 (80368612)
• Project Period (FY): 2013-04-01 – 2015-03-31
• Keywords: 津波 / 粘性残留磁気 / 石垣島

Outline of Final Research Achievements

We use temperature-dependent viscous remanent magnetization to estimate the emplacement age of tsunamigenic coral boulders along the shorelines of Ishigaki Island, Japan. The time of their deposition by tsunamis has been estimated using radiocarbon dating. Since the time when the coral skeletons were emplaced on the shorelines as boulders by destructive tsunami waves, a new viscous magnetization was partially overprinted in the boulder parallel to the present geomagnetic field. We analyzed two different boulders (200t dated at 2000 years old and 35t dated at 1771AD: Meiwa tsunami). The result implies that the 200 t coral boulder was immobile when the Meiwa tsunami struck the island and moved the 35 t boulder over the 1.0-km-wide reef. This suggests that the earlier tsunami was larger than the Meiwa tsunami. New application of Neel's theory to tsunamigenic coral boulders gives us an opportunity to ascertain the age and transportation mode of individual boulders in this area.

Free Research Field

地質学・岩石磁気学