Local variation of inundation, sedimentary characteristics, and mineral assemblages of the 2011 Tohoku-oki tsunami on the Misawa coast, Aomori, Japan

Abstract

The 2011 Tohoku-oki tsunami caused severe damage to the coastal regions of eastern Japan and left a sediment veneer over affected areas. We discuss differences in depositional characteristics of the 2011 Tohoku-oki tsunami from the viewpoint of the sediment source, coastal topography and flow height. The study area on the Misawa coast, northern Tohoku, includes a 20km long coastline with sandy beaches, coastal dunes and a gently sloping lowland. This landscape assemblage provides an opportunity to examine the effects of topography on the characteristics of the tsunami deposit. During field surveys conducted from April 10 to May 2, 2011, we described the thickness, facies, and structure of the tsunami deposit. We also collected sand samples at approximately 20m intervals along 13 shore-perpendicular transects extending up to 550m inland, for grain size and mineral assemblage analysis. The tsunami flow height was estimated by measuring the elevation of debris found in trees, broken tree limbs, or water marks on buildings.The nature of the coastal lowland affected the flow height and inundation distance. In the southern part of the study area, where there is a narrow, 100m wide low-lying coastal strip, the run-up height reached 10m on the landward terrace slopes. To the north, the maximum inundation reached 550m with a run-up height of 3.2m on the wider, low-lying coastal topography. The average flow height was 4–5m.The tsunami eroded coastal dunes and formed small scarps along the coast. Immediately landward of the coastal dunes the tsunami deposit was more than 20cm thick, but thinned markedly inland from this point. Close to the dunes the deposit was composed largely of medium sand (1–2 Φ) with planar and parallel bedding, but with no apparent upward fining or coarsening. The grain size was similar to that of the coastal dune and we infer that the dunes were the local source material for the tsunami deposit at this point. The mineral assemblage of the tsunami deposit was dominated by orthopyroxene and clinopyroxene and was also similar to the dune and beach sand. At sites more than half the inundation distance inland, the thinner tsunami deposit consisted mainly of fine sand (2.375 Φ) with some upward fining. The difference in grain size and sedimentary characteristics was probably caused by differences in sediment transportation and depositional processes. We infer that the well-sorted, finer sediments were deposited out of suspension, whereas the relatively coarse sands were laid down from traction flows. The depositional characteristics of the 2011 Tohoku-oki tsunami deposit appeared to have been affected mainly by the coastal topography and the extent of erosion at any one point, as opposed to flow height.