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Title: Anisotropy of anhysteretic remanent magnetization (AARM) reveals cryptic flow fabric of tsunami
Authors: Kon, S
Nakamura, N
Sugawara, D
Goto, K
Chagué-Goff, C
Goff, JR
Keywords: Tsunamis
Magnetic susceptibility
Issue Date: 9-Dec-2013
Publisher: American Geophysical Union
Citation: Kon, S., Nakamura, N., Sugawara, D., Goto, K., Chagué-Goff, C., & Goff, J. (2013). Anisotropy of anhysteretic remanent magnetization (AARM) reveals cryptic flow fabric of tsunami deposits. AGU Fall Meeting, Moscone Centre, San Francisco, California, America, 9–13 December 2013.
Abstract: Sandy tsunami deposits may provide valuable information on tsunami inundation as well as hydrodynamics, such as flow speed. However, if the layer does not have sedimentary structures such as cross laminations, it is difficult to infer the flow direction, which is important to interpret the behavior of the tsunami, such as inflow and outflow as well as repetition of waves. Anisotropy of magnetic susceptibility (AMS), in combination with grain size data, can provide information about the hydrodynamic conditions prevailing during the emplacement of tsunami sequences. It might also allow the reconstruction of transport directions because it provides a cryptic alignment of ferromagnetic and paramagnetic minerals, such as coarse-grained magnetite or platy phyllosilicate minerals (e.g. biotite). These minerals behave differently in different hydrodynamic conditions: for example, platy biotite may deposit in a cryptic micro-ripple. This therefore suggests that the usefulness of bulk AMS together with optical observations is limited in the study of flow fabric in tsunami deposits. The anisotropy of anhysteretic remanent magnetization (AARM) on the other hand isolates the fine-grained magnetite subfabric of needle-shaped inclusions exsolved in silicate minerals. Samples (18) from tsunami deposits, believed to have been laid down by the Jogan event (869 AD), were collected from a section on the Sendai Plain, east Japan. The transport direction in these deposits could not be determined by AMS analysis due to large declination and inclination errors. The AARM technique was thus used to determine the cryptic subfabric of magnetite exsolutions along cleavages in biotite and amphibole. Our scanning electron microscopy (SEM) observations confirmed that the maximum AARM orientation is parallel to the needle-shaped magnetite microexsolutions in biotite and amphibole. We therefore infer that the large error of AMS is caused by the alteration of these paramagnetic minerals, and AARM provides a cryptic alignment of fine-grained magnetite microexsolutions. In order to apply this method to ancient historical Tsunami events, we also collected 40 samples from consecutive sand layers of possible tsunami deposits at 7 sites using 2 m long geoslicers in Rikuzen-Takata, northeast Japan. The AARM and SEM confirmed the tendency of same flow direction of sand layers at each site, suggesting a tsunami origin.
Gov't Doc #: 6291
Appears in Collections:Conference Publications

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