Identifying possible tsunami deposits on the Shizuoka Plain,Japan and their correlation with earthquake activity over the past 4000 years

Abstract

Three washover sand beds, ranging from 15 to 34 cm in thickness, were recorded in a lagoonal mud sequence on the Shizuoka coast of central Japan, which faces the Suruga Trough. The sand beds were composed of well-sorted and well-rounded beach sand derived by a marine inundation. The basal erosion surface, mud clasts, and presence of both inverse and normal grading suggest that the sand beds formed as a result of high-energy deposition. The sand beds are multilayered, with fine alternations of sand sheets and mud drapes, which are consistent with deposition from a long-period wave train. Radiocarbon dates obtained from the three washover deposits are around ad 1000, 3565–3486 cal. yr BP, and 4000 cal. yr BP. Vertical changes in the diatom assemblages suggest a gradually increasing inflow of seawater up to the second sand bed, followed by a rapid change to freshwater conditions just above the bed. The gradual increase of seawater inflow begins again just above the second sand bed. We conclude that this series of sequential environmental changes indicates interseismic subsidence and coseismic uplift. The study area experienced around 1 m of uplift during the ad 1854 Ansei Tokai earthquake (M_w 8.4). In contrast, the area has subsided gradually (c. 6 mm/yr) during the present interseismic period. The youngest sand bed may be correlated with the ad 1096 Eicho earthquake, which caused severe damage along the Shizuoka coast.