Geological record of 18th-century tsunami in Tsugaru Peninsula facing to Japan Sea

Tsunamis, storms, and flooding cause transport of allochthonous materials, these materials become incorporated into the sedimentary record as event deposits, resulting in anomalous layers of coarse grained sand in low-energy coastal environments such as coastal marshes, ponds and lakes. The Sotomeyachi site is a narrow lowland behind beach ridges along a small creek connecting to Lake Jusan on west coast of Japan Sea, Northeast Japan. Stratigraphic and paleontological investigations using core samples ranging in depth from 0 to 200 cm revealed two events deposits (E1 and E2) within peaty clay. Sedimentary structures in the event deposits are characterized by mud (rip-up) clasts, mud drape, and erosional lower stratigraphic contacts. In addition, we measured the diatom assemblage in the E1 and E2 deposits because it may indicate a critical local environmental change. Although fresh-brackish species were predominant in the E1 deposit, the proportion of brackish water taxa (Mastagloia smithii) increased below the sand-peaty clay boundary. Brackish and brackish-marine species were generally dominant in the E2 deposit, indicating rapid deposition of allochthonous detritus during inundation by seawater. Sediments below the E2 sand layer, composed of sandy silt with freshwater-brackish and marine species, included many skins of brackish bivalve (Corbicula japonica), which are regarded to have been formed under similar environmental conditions of the present-day Lake Jusan. Radiocarbon ages indicates that constrained ages show the E1 and 2 deposits were formed between 1842-1962 CE and 1709-1787 CE, respectively.