[S-SS13] Active faults and paleoseismology

[SSS13-06] Coseismic and postseismic deformation associated with the 17th-century megathrust earthquake along the Kuril Trench estimated by diatom analysis

*Yuichi Nishimura¹, Takashi Chiba² (1.Graduate School of Science, Hokkaido University, 2.Department of Environmental and Symbiotic Science, Rakuno Gakuen University)

Keywords:paleoseismology, crustal deformation, Kuril Trench

A method to construct the history of past crustal deformation based on diatom assemblage 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. In the marsh, tsunami deposits accompanied by the earthquake are distributed more than 2 km inland from the coast. We took cores including the tsunami deposits at an altitude of 2.5-3.0 m (TP) and investigated the diatom assemblage. As a result, it is inferred that the subside trend continued before the earthquake and it turned to the uplift trend after the event. As both of the trends are gradual, the coseismic deformation does not seem to be significant. On the other hand, Pseudopodosira kosugii, which inhabits the intertidal to supratidal zone, is most abundant on the horizon of the tsunami deposits. A detailed study of the relationship between the habitat distribution of Pseudopodosira kosugii and the relationship between altitude and tide level at three sites in the same marsh revealed that the upper distribution limit and mode were 0.8 to 1.3m (TP) and -0.2 to 0.3m (TP), respectively. This variation is considered to reflect the difference in the environment. Postseismic deformation can be estimated by comparing the upper distribution limit and mode of this diatom in the core with the present elevation. In other words, the distribution limit that appears in the core indicates the amount of further uplift after exceeding the distribution limit during the process of uplift. Therefore, the maximum amount of uplift is the height from the altitude of the distribution mode of this diatom. On the other hand, the mode indicates the time when subsidence changed to uplift at the time of the earthquake occurrence, so the maximum amount of uplift is the height of the altitude of this diatom distribution mode of the present day. The amount of uplift can be constrained by using both the distribution limit and the mode. As a result of examination using cores from multiple survey points, the amount of uplift was estimated to be 1.5m. Although this uplift amount is the sum of coseismic and post-seismic uplift, it is known from the diatom analysis that the uplift did not occur suddenly before and after the earthquake, so the contribution of the co-seismic uplift is minor. Detailed studies of prehistoric crustal deformation are valuable, and are important for constructing the source models and evaluating future tsunami assessments.