[H-QR03] Quaternary, Diachronic dynamics of human-environment interactions

[HQR03-P11] Recent earthquakes and paleoenvironment reconstruction based on diatom analysis in Lake Harutori, Kushiro, Hokkaido

*Yuka Matsuno¹, Kota Katsuki², Futoshi Nanayama³, Nakanishi Toshimichi³, Keta Fukkazu⁴, Keisuke Sakai⁵, Naoto Hukuyo⁶, Hirokuni Oda⁶ (1.Sci., Shimane Univ., 2.EsReC, Shimane Univ., 3.Museum of Natural and Environmental History, Shizuoka, 4.INE, 5.Kbe Univ., 6.AIST)

Keywords:Lake Harutori, tsunami, diatoms

Large earthquakes and tsunamis are generated periodically along the Kuril-Kamchatka trench, which lies along the southeast coast of Hokkaido, the south coast of the Kuril Islands, and the south coast of Kamchatka. The coastal areas are often damaged by these earthquakes and tsunamis. There are many marshes and lagoons in southeast area of Hokkaido, which formed after the Holocene glacial retreat. They have been affected by artificial modifications like dredging, but lacustrine sediments remain undisturbed. In this study, we investigate how the environment in and around the coastal lake was affected and changed by recent tsunamis. Lake Harutori is a coastal lake located along the Pacific coast of Kushiro, southeast Hokkaido. The investigation was carried out in August 2022. The lake-bottom topography and water quality (temperature and salinity) distribution were measured in Lake Harutori. After that, four sediment cores (cores R4H1, R4H2, R4H3, R4H4) were corrected in the west area of the lake using a Mackereth piston corer. The sediments were mainly composed of laminated and massive silt. Three sediment cores included the sandy beds that were inferred tsunami sediments. According to the previous researches as Nanayama (2001) and Nanayama et al. (2021), there sand beds are inferred to form by twice magnitude 9 earthquakes, which occurred 13th and 17th century (GTS2 and 1), and once magnitude 8 earthquake, which occurred 1843 (1843 Tokachi-oki earthquake). Each 25 cm slab soft X-ray photographs were filmed to investigate the sedimentary structure of the tsunami deposits and the silt layers. The freshwater diatoms taxa increased after GTS2, magnitude 9 earthquakes. This suggested that the lake environment changed to low salinity after the huge earthquake. After that, the lake environment was alternated between marine and low salinity conditions. Especially the marine environment in 230-179cm depth of R4H3 is considered that there was caused by transgression from the end of the 14th to 16th century.