Estimation of the shoreline in the 13th and 17th century at Kiritappu marsh, eastern Hokkaido, Japan

In this study, we reconstruct position of shorelines at the time of the 13th and 17th century tsunami events to reevaluate great earthquakes in southern Kuril trench. Deposits of prehistoric tsunamis generated from Kuril trench have been found at eastern Hokkaido along the Pacific coast. The youngest large event occurred in the 17th century and the second one occurred in the 13th century (Nanayama et al., 2007). Fault models of the 17th century large tsunami were estimated by comparing distribution of tsunami deposits and computed inundation area of tsunami (Satake et al., 2008; Ioki and Tanioka, 2016). However, in numerical simulation of tsunami inundation by previous studies, shoreline progradation during the last several hundred years were not considered. Kiritappu marsh, a beach ridge plain, is one of the most important area to evaluate prehistoric tsunami inundation. The 13th and 17th century tsunami deposits are distributed beneath marshes that formed between beach ridges. At Hamanaka and Biwase area in Kiritappu marsh, we traced the both tsunami deposits continuously along the leveled transect seaward from inland and identified where tsunami deposits vanished. Because inter-ridge marshes have been appended seaward as shoreline prograded, disappearance of tsunami deposits implies that the paleo-shore existed around the nearest landward beach ridge. The paleo-shore on the transect was then used to reconstruct shoreline based on geomorphological correlation by aerial photograph of the 1940s, map of the 1920s (Geospatial Information Authority of Japan, GSI), and the coastline about 200 years ago (digital Inou map). As a result, the shoreline of the 13th and 17th century, was located almost the same place, was estimated 200-300 m inland from the present. For estimation of fault models of the 13th and 17th century great earthquakes, in the future, we will compute inundation of these tsunamis using the estimated topography around the coast.