Tsunami evidence in Arahama - A.D. 869 and 2011

Core collected by the survey team 1.6 km (1 mi) inland at Arahama on the Sendai coastal plain contains sandy deposits from the A.D. 869 Jōgan tsunami about 40 cm beneath the March 11, 2011, tsunami deposits. We chose the site on the basis of a report of the Jōgan tsunami deposits by Yuki Sawai (Active Fault Research Center) and colleagues in the Annual Report on Active Fault and Paleoearthquake Researches and numerical modeling of Jōgan tsunami inundation by Kenji Satake (Active Fault Research Center) and colleagues. The relative magnitudes of the Jōgan and 2011 tsunamis cannot be inferred from the thickness of each deposit, which is affected by local topography as well as sediment grain size and tsunami flow speed.