[H-DS12] Geohazards in humid, tectonically active countries and their precursors

[HDS12-P04] Holocene gravitational rock slope deformation and large-scale landslide features in the western Kamikochi Valley, northern Japanese Alps

*Yoshihiko Kariya¹, Sadao Takaoka¹ (1.Department of Environmental Geography, Senshu University)

Keywords:Deep-seated gravitational slope deformation, Rock avalanche, Granodiorite, Geomorphic change in alpine environments

Deep-seated gravitational slope deformation (DSGSD) is a remarkable precursor of catastrophic landslide phenomena in mountain areas. Also, DSGSD-derived micro-scale topographies such as linear depressions, unti-scarplets, and hummocks have been thought to be important elements for their creational role forming complex natural environments (i.e., differences in soil moisture, temperature, geomorphic processes, microclimates etc.). However, few attempts have been made to clarify historical development of DSGSD and related landforms in the Japanese high mountains. We carried out drilling survey using a portable auger and could obtain two cores from two linear depressions on Mount Nishihotaka-dake, both of which are developed on periglaciated DSGSD slopes of granodiorite (core: KNG-2017 and NSH-2018). Both cores contain altered layers of peat, humic silt, sand and gravel as well as tephras from a nearby volcano. Based on soil facies, age-depth models with ¹⁴C and tephra dates, the onset of topographic formations and pedogenes would be back to the middle Holocene around 4000-5000 cal BP. During this period, according to previous authors, two events of large-scale catastrophic rock slide occurred just below the drilling sites, and paleoclimates had being recovered from minor unstable and cool periods. Formation and changes around the linear depressions were probably caused by coupling of local topographic changes and climate variability. Our data indicate long-term (> 10³ y) slope changes in the alpin-subalpine zones.