HR: 0830h
AN: GP41C-0053    [PDF]
TI: Stratigraphic Correlation of Core Samples From the Osaka Bay off Kobe Based on Magnetic Properties and its Implication for Tectonic Activity of the Osaka-wan Fault for the last 6300 years
AU: * Iwaki, H
EM: iwaki@geor.or.jp
AF: Geo-Research Institute, 4-3-2 Itachibori, Nishi-ku, Osaka, 550-0012 Japan
AU: Hayashida, A
EM: ahay@doshisha.ac.jp
AF: Science and Engineering Research Institute, Doshisha University, Kyo-Tanabe_@, Kyoto, 610-0321 Japan
AU: Kitada, N
EM: kitada@geor.or.jp
AF: Geo-Research Institute, 4-3-2 Itachibori, Nishi-ku, Osaka, 550-0012 Japan
AU: Ito, H
EM: ito@geor.or.jp
AF: Geo-Research Institute, 4-3-2 Itachibori, Nishi-ku, Osaka, 550-0012 Japan
AU: Suwa, S
EM: suwa@geor.or.jp
AF: Geo-Research Institute, 4-3-2 Itachibori, Nishi-ku, Osaka, 550-0012 Japan
AU: Takemura, K
EM: takemura@bep.vgs.kyoto-u.ac.jp
AF: Institute for Geothermal Sciences, Graduate School of Science, Kyoto University, Noguchibaru, Beppu_@, Oita, 874-0903 Japan
AB: Detailed correlation of bore-hole cores that collected around an active fault provides fundamental data for estimating vertical displacement of the strata generated by the fault movement. In order to recognize characteristic horizons for correlations, measurements of magnetic properties can be applied to core sediments, even if they seem homogeneous by visual inspections. For example, variation of initial susceptibility is often used for correlation of core samples, for its measurement can be made readily and rapidly. In this study, we have made detailed analysis of lithology and magnetic properties of bore-hole cores from sub-bottom sediments of Osaka Bay, central Japan, which were recovered from two sites at the both sides of the Osaka-wan fault off Kobe. These cores are composed of massive marine clay of the Holocene, intercalating a widespread tephra layer, K-Ah tephra, dated at 6300 14C years BP. We obtained u-channel samples from these cores and measured low-field magnetic susceptibility and anhysteretic remanent magnetization (ARM) by pass-through method. While we also measured natural remanent magnetization (NRM), reliable records of paleosecular variation were not obtained due to physical distortion of the core samples. The magnetic concentration parameters, however, show characteristic variations which are useful for stratigraphic correlations. The curve of ARM intensity corresponded to that of initial susceptibility. It is suggested that relative variation of ARM intensity supports the correlation obtained from the initial susceptibility measurement. The horizon of the K-Ah ash indicates a peak of initial susceptibility and ARM intensity, providing a key for correlation between the two sequences. The two parameters show increased value at the interval below the K-Ah ash at both sites, suggesting enhanced input of magnetic minerals into the Osaka Bay before 6300 yrs BP. Minor variations of the magnetic parameters were observed at several horizons, showing similar features at both sites. Based on these features and several radiocarbon dates, we established high-resolution correlation of the two sequences and compared accumulation history between both sides of the Osaka-wan fault. Our result delineates that the Osaka-wan fault has been active twice for the last 6300 years. It is also concluded that ARM intensity variation can improve the accuracy in correlating bore-hole cores based on initial susceptibility measurement.
UR: http://www.geor.or.jp/
DE: 1525 Paleomagnetism applied to tectonics (regional, global)
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2003 Fall Meeting