2021 Volume 79 Issue 2 Pages 47-69
In this study, the distribution and characteristics of the tsunami deposits by the 2011 off the Pacific coast of Tohoku and A.D. 869 Jogan earthquakes were identified in Hirota Bay, Sanriku Rias through high-resolution seismic profiles and core samples. The seabed sediments of Hirota Bay are composed of sand layer and silt layer, and separated by a well-defined boundary. The tsunami deposit by the 2011 Tohoku-oki earthquake on the Hirota Bay seafloor exhibit the characteristics of: (1) the bed separated from lower layer by a clear distinct boundary, (2) coarser grain size than ordinary bay-floor sediment, (3) composed by more than two sand layers, (4) parallel lamination, and (5) broad distribution within Hirota Bay. The volume of the 2011 tsunami deposit is 2.6×106 m3, about four times larger than the coeval onshore tsunami deposit in the Rikuzentakata Plain. The total volume of these onshore and offshore tsunami deposits is greater than the total volume of coastal erosion ranging from the beach ridge to shallow-marine. Hence, the sediment sources of the 2011 Tohoku-oki tsunami deposit may also include the materials from the coasts (east and west sides of bay) and deeper seafloor of Hirota Bay, which are not included in the calculation for the volume of erosion. Another event layer occurs beneath the 2011 Tohoku-oki tsunami deposit in Hirota Bay. This layer is interpreted as A.D. 869 Jogan tsunami deposit based on their distribution, sediment facies and radiocarbon-ages.