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Abstract

A method to construct the history of past crustal deformation based on diatom analysis was developed, and the amount of coseismic and postseismic deformation associated with the 17th Century megathrust earthquake in Toberi marsh, Tokachi, Hokkaido, was estimated. The cores including the tsunami deposits at an altitude of 2.5-3.0 m (TP) inferred that subside trend continued before the earthquake and it turned to uplift trend after the event. Since both of the trends are gradual, the coseismic deformation does not seem to be significant. On the other hand, Pseudopodosira kosugii, which distributes the intertidal to supratidal zone, is most abundant on the horizon of the tsunami deposits. Present habitat distribution of Pseudopodosira kosugii in the same marsh revealed that the upper distribution limit and mode were 0.8 to 1.3 m (TP) and -0.2 to 0.3 m (TP), respectively. Prehistorical uplift can be estimated from the current elevation of the limit and the mode of this diatom in the core. The amount of the uplift in Toberi march after the 17th Century event was ca. 1.5 m. That was constrained by using both the distribution limit and the mode of cores from five survey points in the marsh. Information about the crustal deformation after the 17th Century event is useful for the evaluation of source models of the earthquakes that is used for tsunami hazard assessment along the Pacific coast of Hokkaido.