[H-DS08] Landslides and related phenomena

[HDS08-P20] Age of gravitational depression at Mt. Yokin-zan, the Etsumi Mountains, central Japan: implication to the evolution of DGSDs in a humid and tectonically active region

*Keitaro Komura¹, Heitaro Kaneda², Yu Nakae³, Koichi Suzuki⁴, Kazuhiro Kato⁵, Toru Moriya⁶, Fuyuki Tokanai⁶, Mirei Takeyama⁶, Satoru Kojima⁷ (1.Central Research Institute of Electric Power Industry, 2.Department of Civil and Environmental Engineering, Chuo University, 3.Kiso-Jiban Consultants Co., Ltd., 4.JOGMEC, 5.Paleo Labo co., Ltd., 6.Center for Accelerator Mass Spectrometry, Yamagata University, 7.Emeritus Professor, Gifu University)

Keywords:Deep-seated gravitational slope deformation, Deep-seated landslide, Etsumi Mountains, Mt. Yokin-zan, Humid and tectonically active region, Knickpoint retreat

Geomorphic features associated with deep-seated gravitational slope deformation (DGSD), such as double ridges, uphill-facing scarps, and linear depressions, often develop on and around mountain ridges, and some of these DGSD features are considered to be precursors of large-scale deep-seated landslides. In humid and tectonically active regions, including Japan, the retreat of knickpoints along valleys associated with regional uplift and climatic changes over tens of thousands of years has been considered to influence the distribution and development process of DGSD features. However, studies have yet to determine and compare the formation ages of multiple DGSD features in a specific area to examine the development process of such features.
The authors have been conducting coring surveys in several gravitational depressions in the Etsumi Mountains, which straddle Gifu and Fukui Prefectures, central Japan. As a part of such an attempt in the Etsumi Mountains, we conducted core drilling at a gravitational depression near Mt. Yokin-zan in the southern part of the Etsumi Mountains using a portable percussion coring system. The results indicate that the depression dates back to as old as 77–60 ka, significantly older than DGSD features previously found in the Etsumi Mountains that were dated after post-glacial period. The difference in the onset age of the DGSD within the same mountains suggests that the river incision phases that correspond to the marine oxygen isotope stages (MIS), especially MIS4 and MIS2, may be responsible for the distribution and formation ages of the DGSDs in the Etsumi Mountains.