長野県北部の第三紀層地すべりの特性  [in Japanese] Characteristics of the Tertiary Landslide in the Northern Part of Nagano Prefecture  [in Japanese]

The Tertiary strata are extensively distributed in the northern part of Nagano Prerecture. In this region, there are 358 landslides designated by the government for control works in 1982. These landslides are densely populated in altitude from 600 t0 900 meters above sea level. Geologically, a large number of the landslides occur in the areas dominated by muddy rocks consisting of the Aoki-, Ogawa- and the Takafu Formations. From the tectonic point of view, they occur densely in the concentrate zones of strains. It should be emphasized that most of the present landslides are the reactivation of ancient landslide debris produced by the mass movements. The past mass movements took place three times during Pleistocene age, the oldest occurred about 40 ten-thousand years ago.