[H-DS07] Landslides and related phenomena

[HDS07-P02] History the Tsugauru-Juniko landslide in Northern Japan from historical documents and radiocarbon dating

*Ching-Ying Tsou¹, Hiroshi Yagi², Ryuji Yamada³, Mutumi S. Odagiri^3,4, Daisuke Higaki⁵, Rio Ogasawara¹ (1.Faculty of Agriculture and Life Science, Hirosaki University, 2.Faculty of Education, Art and Science, Yamagata University, 3.National Research Institute for Earth Science and Disaster Resilience, 4.Hanamaki City Museum, 5.Nippon Koei Co., Ltd.)

Keywords:landslide, historical dosument, radiocarbon dating, landslide age

The Tsugaru-Juniko landslide is located at the western side of Shirakami Mountains, northern Honshu Island, Japan. The landslide is 2 km long in an E–W direction and 2.5 km wide, where its block is characterized by hummocks, linear depressions, and a total of 33 lakes on and around the rim of the landslide. It is assumed that the trigger for the landslide is the 1704 Iwadate earthquake, according to a historical document, Hirosaki Clan Agency Diary in 1704 (Hirosaki-han Onkuni Nikki, Kan’ei gannen). However, the landslide scarp, oriented N–S, appears that its appearance is fresher on the northern than on the southern part and with associated debris blocks below the scarp, implying the possibility of episodic landslide events. To verify the landslide history, we interpreted historical documents, including the above mentioned Hirosaki Clan Agency Diary, historical geographic maps of “Ongunchu ezu” (belonging to the Hirosaki City Public Library) of Syoho Era (1648-1652), “Tsugaruryo genroku Kuniezu utsushi” (belonging to Hirosaki University Library) of Genroku-Era (1697-1700), and “Tenpo Kuniezu (Mutsunokuni Tsugaru-gun)” (belonging to the National Archives of Japan Digital Archive) of Tenpo Era (1831-1845), and a coastal map of “Tsugaru-gun kaiganzu nishihama” (belonging to the Hirosaki City Public Library) in Genji gan-nen (1864). We also collected 17 samples from outer preserved rings of drowned snags behind lakes and buried logs, and from buried wood fragments for AMS radiocarbon (14C) dating of landslide. The landslide’s occurrence in 1704 was confirmed in the Dairy and the lakes were indicated in the coastal map, while the landslide and the related geographic name were not shown in the historical geographic maps before and after 1704 (Tsou et al., 2021). Radiocarbon ages indicated that the landslides might have existed from 1440 to 1660 before it was later reactivated by the 1704 Iwadate earthquake. Besides, radiocarbon ages of some smaller landslides, western of the Tsugaru-Juniko landslide, showed their occurrence may be coeval with the above-mentioned events.