SEDIMENTARY AND FORAMINIFERAL EVIDENCE OF THE 2011 TOHOKU-OKI TSUNAMI ON THE SENDAI COASTAL PLAIN, JAPAN

The 2011 Tohoku-oki megathrust earthquake (Mw 9.0) generated a tsunami that reached the Sendai coastal plain with flow heights of ~2 to 11 m above TP (Tokyo Peil). We examined the tsunami deposit exposed in 14 shallow trenches along a ~4.5 km transect perpendicular to the coast. We primarily document the stratigraphical, sedimentological, foraminiferal and geochemical characteristics of the Tohoku-oki tsunami deposit and perform a preliminary comparison with sediments deposited by the Jogan tsunami of A.D. 869. In the coastal forest and rice fields inundated by the Tohoku-oki tsunami, a poorly-sorted, dark brown soil is buried by a poorly-sorted, brown, medium-grained sand deposit. In some trenches located more than 1.2 km inland, the sand is capped by a thin muddy-sand layer. The tsunami deposit, although highly variable in thickness, is generally thickest (25 cm) near the coastal dune and thins to less than 5 mm at ~4.5 km inland. The tsunami deposit was discriminated from the underlying soil by the appearance of recent and fossil foraminifera and a pronounced increase in grain size that fined upwards and landwards. The recent foraminifera preserved in the sandy facies of the deposit are rare and showed evidence of prolonged subaerial exposure (e.g. pitting, corrosion, fragmentation). Recent foraminifera likely originated from coastal dune and beach sediments that were breached by the tsunami. Calcified and sediment in-filled, fossil foraminifera are abundant and were eroded from sedimentary units and transported by fluvial or wave activity to Sendai Bay. Trends associated with test size (e.g. decreasing concentration of large test sizes with distance inland) are in agreement with grain size data. At two locations a decrease in total organic carbon and an increase in δ¹³C were found in the tsunami sand compared with the underlying soil, supporting a marine or beach origin for the upper unit.