Sedimentary and foraminiferal evidence of the 2011 Tōhoku-oki tsunami on the Sendai coastal plain, Japan

Abstract

The 2011 Tōhoku-oki megathrust earthquake (Mw 9.0) generated a tsunami that reached the Sendai coastal plain with flow heights of ~2 to 11m 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 Tōhoku-oki tsunami deposit and perform a preliminary comparison with sediments deposited by the Jōgan tsunami of A.D. 869.In the coastal forest and rice fields inundated by the Tōhoku-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 1.2km inland, the sand is capped by a thin muddy-sand layer. The tsunami deposit, although highly variable in thickness, is generally thickest (25cm) near the coastal dune and thins to less than 5mm at ~4.5km 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 upward and landward. 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 beach to intertidal origin for the upper unit.