御岳山南麓伝上川ぞいの地質と斜面崩壊  [in Japanese] Quaternary Geology and Land-slide of Denjo Valley on the Southern Slope of Mt. Ontake, Central Japan  [in Japanese]

The Quaternary system on the southern slope of Mt. Ontake is classified into the Suzugasawa, Tarusawa and Ohtaki Formations in ascending order. The former two are Middle Pleistocene and the later is Upper Pleistocene. The large land-slide occurred in the strata of the Upper Pleistocene on the southern slope of Mt. Ontake by the Naganoken-seibu Earthquake on the 14th of September, 1984. It occurred in the rock sequence of the Ohtaki Formation under the following conditions. (1) The slope where land-slide occurred was formed by thick beds of Senbonmatsu Pumice and S-O Scoria of the upper part of the Ohtaki Formation, overlain by inclined beds of lava flows and tuff-breccias. (2) The lowermost part of the slope was cut deeply by the Denjo River. (3) The precipitation of the preceding six days had attained about 150 mm, which seeped into the beds of the Ohtaki Formation. It raised probably the ground-water table, and also increased pore-water pressure near the base of the Upper Ohtaki Formation. (4) First and small land-slide occurred at the lower part of the slope by a shock of the earthquake. Second and large land-slide occurred uninterruptedly at the upper part of the slope. The first played an important role as a trigger of the second.