Browsing by Author "Iijima, Y"

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    Magnetic anisotropies for tsunami deposits: application to the 3.11
    (Japan Geoscience Union, 2013-05-24) Kon, S; Nakamura, N; Goto, K; Sugawara, D; Iijima, Y; Chagué-Goff, C; Goff, JR
    Tsunami deposits consist of well-sorted fine sand intercalating with non-marine black organic mud. It is difficult to reveal a transport direction of the deposit if the deposit showed no sedimentary fabrics,such as ripples. The proxy of anisotropy of magnetic susceptibility (AMS) appears to be a promising tool for the study of flow fabrics in recent-tsunami deposits such as Sumatra tsunami (Wassmer et al. 2010). The AMS fabric might allow us to reconstruct transport directions of unconsolidated tsunami sediments during emplacement because AMS provides a cryptic alignment of ferromagnetic and paramagnetic minerals. Such cryptic minerals, such as magnetite or phyllosilicate minerals, would behave as a different emplacement mode in a different hydrodynamic condition. In the AMS fabrics of volcanic rocks, there are large discrepancies between the magnetic lineation and the framework-forming silicate linear fabric. This suggests that the uncorroborated use of bulk AMS to detect flow fabric in tsunami deposits has risks. In this article, we show that the anisotropy of anhysteretic remanent magnetization (AARM) may resolve the difficulties. The combination of inundation eye-witness, SEM, AMS, and AARM confirms the flow pattern of recentand paleo-tsunami deposits from the geoslicer sampleing at Rikuzen-Takata city, Japan during 2011, 11th March Tohoku tsunami. We determined if the sandy deposits are of tsunami from these magnetic anisotropies. © 2013, Japan Geoscience Union.
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    Possible paleo-tsunami deposits at Rikuzentakata City, Japan
    (National Committee of Japan for IGU, 2013-08-05) Iijima, Y; Sugawara, D; Goto, K; Chagué-Goff, C; Hayase, R; Hashimoto, K; Kon, S; Nakamura, N; Goff, JR
    Rikuzentakata City, NE Japan, has been repeatedly suffered by tsunami inundations including 1896 Meiji-Sanriku, 1933 Showa-Sanriku, 1960 Chilean Tsunami, and 2011 Tohoku-Oki Tsunami. Up to 30 cm thick sand layer was deposited by the 2011 tsunami in this city (Naruse et al., 2012). Our study indicates that historical and prehistoric tsunamis also left deposits in this area. Nevertheless, previous studies of paleo-tsunami deposits in this area are limited (Haraguchi et al., 2006, Imaizumi et al., 2007), because of the difficulty of finding paleo-tsunami deposits along this ""ria"" coast. We conducted a field survey using a geoslicer to acquire sediment cores in order to explore the magnitude and history of tsunamis in this area. Overall 10 cores, each 2 m long and 12 cm wide were acquired during the survey. The sedimentary sequences were mostly composed of peaty soil, which was thought to have been deposited in a marsh environment, however these soils units were inter-fingered by numerous 1-15 cm thick sand layers. According to initial work including grain size analysis, some of the sand layers deposited 1.4 km from the present shoreline are identified as having a possible tsunami origin because they tend to show upward fining characteristics, indicating rapid sedimentation from suspended load. We will also present the preliminary results of tephra chronology, radiocarbon and 210Pb dating, and diatom analysis.