Slope failure commonly triggered off the debris flows in Rokko Mountains. These debris flows were streamed down along the steep channels involving the bed sediments developed in the neighbours. In this paper, Rokko district was divided into four types based on topographical and geological characteristics. Geographical shapes of the bed sediments of the respective type are controlled by the same characteristics, i. e., topography and geology.
On September 14, 1984, an earthquake of magnitude 6.8 (Richter) occurred at a depth of 2-3km near an active volcano, Mt. Ontake (3063m), in the mountainous area of central Japan. The geology of the mountain is Quaternary pyroclastic rocks intercalated with andesite lavas and lacustrine beds on the basement of Paleozoic formations and Mesozoic acidic volcanic rocks. Four large slope failures, Matsukoshi, Kiyotaki, Ontake, and Takigoshi failures, were associated with the earthquake shock causing the loss of 29 lives. The failures were concerned with the weathered volcanic ash, pumice, scoria and/or tuffeceous lacustrine layers. Matsukoshi failure occurred on a village terrace and its scale was about 150 m wide, about 230m long, the maximum depth of slide was about 40 m, and the total failure volume was 290, 000 m3. The sliding material ran up on the other side slope carrying with it a ready mixed concrete plant up to 30 m from the river bed. Kiyotaki failures consisted of five failures, which occurred on the foot slope of the mountain. Each failure was on a scale of about 50-100 m wide, 30-50 m long and 5-10 m in depth. The slip planes were in the weathered pumice and/or ash layers. Ontake failure was inferred to have occurred in five blocks consecutively on the southern flank of Mt. Ontake at an altitude between 2550 m and 1900 m. The scale of failure was about 0.4-0.6 km wide, about 1.5 km long, the maximum depth of slide was 150 m, and the total volume was about 36, 000, 000 m3. The slip plane was in the highly weathered pumice and scoria layers. The failure material turned into a gigantic debris flow and traveled the valley with a thickness of about 40-90 m at the average rate of about 20 m/s. The material flowed down about 8.5 km along the valley and flowed into the main stream of the area, forming a vast sedimentary plain of 100-400 m wide and 30-40 m thick over a distance of about 3 km. Part of the earth and rocks spilled over several terraced ridges on the opposite side of the failure slope and at the corners of the valley, and flowed into other valleys. The debris flow consisted of crushed pyroclastics and lava blocks and its surface was undulated during the flow. Takigoshi failure occurred in the lacusrine sandy soils intercalated with volcanic ash and pumice layers. The scale of failure was about 120 m wide and about 50 m long, the maximum depth of slide was 50 m, and the total volume was 500, 000 m3. The material flowed about 1 km and formed a fan-like sedimentary plain on the artificial lake. The failure was inferred to have occurred by liquefaction of the sandy soil. The failures indicate the importance of geological distribution of highly weathered volcanic ash, pumice, scoria, and/or tuffeceous lacustrine layers for the occurrence of earthquake-induced slope failures and successive flows.