Using magnetic fabric to reconstruct the dynamics of tsunami deposition on the Sendai Plain, Japan - The 2011 Tohoku-oki tsunami

The magnetic fabric and grain size of sand deposits emplaced during the 2011 Tohoku-oki tsunami were studied in five trenches along a 1800m long shore normal transect on the Sendai plain as well as in a near shore sedimentary infill of a scour depression. The magnetic susceptibility in all deposits is due to ferromagnetic minerals (mainly magnetite) making the anisotropy of magnetic susceptibility (magnetic fabric) suitable for fabric analyses. The dominant magnetic fabric is planar in all trenches and stronger in finer-grained inland deposits than in the coarser sediments. This planar fabric is related to tractive shearing of the bedload basal portion of the tsunami flow that led to the deposition of traction carpet layers. Among the various fabric parameters used for this study, the vertical evolution of the shape factor (q) of the magnetic ellipsoid in each trench follows the evolution of the magnetic lineation (L) and foliation (F). These parameters provide information on the hydrodynamic energy (flow speed) fluctuations during the emplacement of the tsunami deposit. For the most proximal deposits, characterized by well-sorted reworked beach sand with minor fluctuations in grain-size distribution, the magnetic fabric is sensitive to hydrodynamic energy variations during sedimentation. Reconstruction of tsunami flow orientation in the sediments, based on the orientation of the mean K_max calculated for each trench, appears to be unambiguous only for the sandy infills of small topographic depressions. The variations in flow direction indicators elsewhere could be related to local variation of the flow and to micro-topographic effects. These findings are encouraging for the use of the magnetic fabric proxy in the study of paleotsunami deposits.