An extended theory of paleomagnetic viscous dating and its application to tsunami boulders in Japan and Tonga

2017 Fiscal Year Final Research Report

• Project/Area Number: 15H02986
• Research Category: Grant-in-Aid for Scientific Research (B)
• Allocation Type: Single-year Grants
• Section: 一般
• Research Field: Natural disaster / Disaster prevention science
• Research Institution: Tohoku University
• Principal Investigator: Nakamura Norihiro 東北大学, 高度教養教育・学生支援機構, 教授 (80302248)
• Co-Investigator(Kenkyū-buntansha): 後藤 和久 東北大学, 災害科学国際研究所, 准教授 (10376543) ; 菅原 大助 東北大学, 災害科学国際研究所, 助教 (50436078)
• Project Period (FY): 2015-04-01 – 2018-03-31
• Keywords: viscous remanence / tsunami disaster / fault gouge

Outline of Final Research Achievements

We proposed a new paleomagnetic method to determine the age of emplacement of tsunamigenic boulders, but some boulders showed an anomalously older age than the one of radioisotope method. We have used Neel's theory of magnetic relaxation (i.e., exponential relaxation law), but the theory disagrees with previous laboratory data for magnetic relaxation. Using a stretched exponential relaxation law with variable power exponents, we found that almost all previous data fit this law. This theoretical extension also explained the problem of older ages. However, we still need to answer the further problem that how do we determine the power exponent from individual boulders. In this project, we published two other papers that related to this research. One is that anisotropy of magnetic susceptibility does not always show a paleocurrent flow of sandy tsunami deposits. The other is that slip zones in Nojima fault zone experienced a temperature rise over 400 C using scanning magnetic microscopes.

Free Research Field

Earth and Planetary paleomagnetism