[H-DS10] Geohazards in humid, tectonically active countries and their precursors

[HDS10-05] Was the Oya-kuzure landslide an abrupt event caused by the CE1707 Hoei earthquake?

*Yoshihiko Kariya¹, Keiju Kimura², Ryuji Yamada³ (1.Department of Environmental Geography, Senshu University, 2.Hoyu-Gakuin High School, 3.National Research Institute for Earth Science and Disaster Resilience)

Keywords:Debris flow, Large-scale landslide, Dammed-lake deposits , River-bed aggradation, Abe River

The Oya-kuzure landside has produced a large volume of debris (1.2 x 10⁸ m³; Shizuoka Prefecture in central Japan). Thick debris forms a valley-fill high terrace surface (H) in the upper Abe River. Some authors suggest that the landslide had occurred during the period between 1530 and 1702 CE, while others believe that the 1707 CE Hoei Earthquake caused a sudden massive rockslide. In 2022, we reported a dammed lake deposit in the Tachisawa River, a tributary of the Abe River, stratigraphically lower than the H-surface along the main stream, as evidence that the tributary was dammed during the early period of riverbed aggradation of the main stream due to channel filling. We thus concluded that the Oya-kuzure landslide continued for more than a several decades or centuries, and was not a single sudden phenomenon. In this presentation, we report on the different dammed lake deposits in another tributary as a new source of information to support this idea.

Topography and geology near the Konya-Abe River confluence. The topographic section is based on an 1 m DEM by MLIT. The geologic section is based both on Machida (1959) and our field checks. There was another dammed lake deposit in the Konya River basin (Machida 1959), which seems to be deposited in the late period of river-bed aggradation of the main channel forming the H surface. The inset map is based on GSI Maps.