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

Abstract

We evaluated sediment sources of the 2011 Tohoku-Oki tsunami deposits along a 4500m 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 20m near the coastline, 1.4 to 2.8m in the middle of the valley and 1.3m around the inundation limit. The inundation distance was around 4800m. 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 5g of sample compared with ≥100 specimens per 1g 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 2300m from the coastline were deposited by bedload and suspension, whereas suspension was responsible for deposits farther inland.