Sources and transportation modes of the 2011 Tohoku-Oki tsunami deposits on the central east Japan coast

We evaluated sediment sources of the 2011 Tohoku-Oki tsunami deposits along a 4500 m inland transect from the central east Japan coast, using grain size, mineralogy and foraminifera analyses. The tsunami run-up height in our study area was 13 to 20 m near the coastline, 1.4 to 2.8 m in the middle of the valley and 1.3 m around the inundation limit. The inundation distance was around 4800 m. The deposits are composed of sand and mud and show a fining and thinning trend. Mineral composition of the tsunami deposits was similar to that of coastal dune and beach deposits, indicating that most of the tsunami sediment originated from there. A minor foraminifera population in the deposit (≤ 15 specimens per 5 g of sample compared with ≥ 100 specimens per 1 g for typical marine sediment) indicates that marine sediment source was not dominant. Topography played an important role in sediment transport process and deposit distribution. The run-up wave accelerated faster in concave topography, became more erosive, and resulted in a thin deposit in topographic lows. We infer that the deposits up to 2300 m from the coastline were deposited by bedload and suspension, whereas suspension was responsible for deposits farther inland.