2024 Volume 78 Issue 3 Pages 101-114
The damage to villages caused by the earthquake (M6.7) in northern Nagano on March 12, 2011 was concentrated along the Chikuma River on the southern slope of the Sekita Mountains. To clarify the damage, I investigated the geological survey and conducted a topographical interpretation by use of lidar images. Consequently, five gigantic landslide scars, 1-4 km in width, and three topographic depressions 1.5-3 km in width, were recognized on the southern foot of the Sekita Mountains. The reactivation of these gigantic landslides has dammed up the rivers twice on a large scale.
To consider the formation model of gigantic landslides distributed on the left bank of the Chikuma River and the Shinano River, I carried out an analysis of mineral composition of tephra beds and their stratigraphic position, to correlate them with the Shinanogawa Loam Formation.
Gigantic landslides on the southern slope of the Sekita Mountains is divided into eleven stratigraphic horizons from the Middle Pleistocene to the Holocene. I make clear the history of formation modele of the gigantic landslides.
