Nineteen sixty-one was the year in which Onishiyama rock fall broke out and its debris blocked the Koshibu river. This fall of mylonite took place on the west-side of the Median Tectonic Line.
Creeping of crystalline schist or its soil, on the east-side, is essentially the major mechanism for very slow down-slope movements. We cannot find out here a rapid and chaostic disaster except for toe erosion or heavy stormy rain.
This paper deals with geology and mass movement at Kashio area, Oshika-mura, with a radio-isotope calculation of 222Rn. Its density in soil shows a 50-to-60 day period of repeated mass strain.