[S-EM13] Geomagnetism, paleomagnetism and rock magnetism

[SEM13-P05] Paleomagnetic Dating on Tsunami Boulders from the Kingdom of Tonga

*Tetsuro Sato¹, Norihiro Nakamura², Hiroyuki Nagahama³, Masahiko Sato⁴, Kazuhisa Goto⁴, Masashi Watanabe⁵, Akira Ikeda³, Kenji Satake¹, Taaniela Kula⁶ (1.Earthquake Research Institute, The University of Tokyo, 2.Institute for Excellence in Higher Education, Tohoku University, 3.Department of Earth Science, Tohoku University, 4.Department of Earth and Planetary Science, The University of Tokyo, 5.Faculty of Science and Engineering, Chuo University, 6.Ministry of Lands and Natural Resources)

Keywords:Tsunami boulder, Age estimation, Remanent magnetization

Viscous remanent magnetization partially overprints the natural remanent magnetization in reworked boulders. The magnitude of such magnetization depends on the time passed since reworking, as well as ambient temperature, for which there is a well-known theoretical time-temperature relationship of remanent magnetization. Most references used the paleomagnetic dating protocol, which is applicable to single-domain particle assemblages, while experimental data often disagree with other dating applications (e.g., radiocarbon and U-series dating). Here, we use modified time-temperature relationship based on a stretched exponential relaxation function that accounts for the complex magnetic structures. Samples of tsunami boulders from the Kingdom of Tonga have chemical-viscous remanent magnetization, and their non-exponential relaxation is well fitted by the stretched exponential relaxation function. Combining the modified time-temperature relationship of remanent magnetization, which considers the observed stretching exponent value, and sea level change data, we estimate the reworked age of the tsunami boulders.