[S-EM15] Geomagnetism, paleomagnetism and rock magnetism

[SEM15-11] Stretched exponential relaxation determines the deposition age of boulders in Lahar deposits of Adatara volcano, Japan

Akira Ikeda¹, *Norihiro Nakamura², Tetsuro Sato³ (1.Dep. Earth Science, Tohoku University, 2.Institute for Excellence in Higher Education, Tohoku University, 3.Earthquake Research Institute, University of Tokyo)

Keywords:Paleomagnetic viscous dating, Lahar, Stretched Exponential relaxation

Neel's relaxation theory of viscous remanence gives the fundamental basis of paleomagnetic viscous dating for determining the emplacement age of tsunami boulders, but there is a problem that the age was estimated old. Introducing a stretched exponential relaxation of viscous remanence might solve this over-age problem. Previous viscous dating of coral boulders needed to determine the stretched exponent (n) using a vibrating sample magnetometer, but corals are difficult to determine the exponent due to their weak magnetization of corals. To overcome this problem, it is necessary to employ volcanic boulders and known ages. In this presentation, we introduce an example of andesitic boulders within Lahar deposits in the Adatara volcano, Japan. The Adatara volcano is active, and Lahar deposits with large andesitic boulders are distributed on the western slope along the Sukawa river. Eighteen lahars were deposited, each at 800-year average intervals during the last 14000 years. Most of the lahars were determined age by using 14C dating for wood fragments within lahars, but the recent lahar deposits were not determined. By calibrating the paleomagnetic viscous dating with the stretched exponent (n) of the viscous remanence of boulders, the stretched exponent-calibrated age was almost the same as the 14C age. The same protocol was applied to the age-unknown boulder in recent lahar, resulting in the age of about 500 years old. The stretched exponent calibration is useful for estimating the deposition age of boulders.