Physical and Chemical Properties of Tsunami Deposits in the Northeast Area of Fukushima Prefecture after the Tohoku-Kanto Earthquake

The damage done by deposits left by the tsunami triggered when the 11 March 2011 earthquake struck extended over a wide area in the coastal parts of the Tohoku district of Japan. The objectives of this study are to clarify physical and chemical properties of the tsunami deposits in Minamisoma City in northeast Fukushima Prefecture. From field research and measurements, two kinds of tsunami deposits, a sandy material and a muddy one, were observed. Muddy deposits, composed of small particles, were distributed in areas relatively far from shore. The particles in these deposits were light, and they contained high proportions of organic matter. Sandy deposits, on the other hand, had larger, much denser particles and were observed near the shoreline. The electrical conductivity of the muddy deposits at Kashima was very large. The Cl- and SO42- contents of the deposits were higher than those of paddy soils beneath. From an approximation of the shape of the area inundated by the tsunami and the bulk density and thickness of the deposits, we estimate that a dry mass of 193,000 Mg was deposited.