The 1984 Collapse and Debris Avalanche Deposits of Ontake Volcano, Central Japan

Abstract

The Western Nagano Prefecture Earthquake (M = 6.8) triggered the collapse of the southern ridge of Ontake Volcano, central Japan, on 14 September 1984. The collapsed rock fragments, with a volume estimated as 3.4 × 10⁷ m³, became a debris avalanche which travelled a distance of 13 km, descending a total height of 1600 m. By means of geological observations and photo interpretations, three stratigraphic units of the debris avalanche/ debris flow deposits were distinguished: Flows I, II, and III. These consist of lithofacies A (breccia and its powder), B (less fluid mixture of breccia and mud), and C (very fluid mixture of breccia and mud). Flow I is the main debris avalanche, which left hummocky hills in the deposition area and longitudinal and transverse ridges on the lava plateaus that were overflowed. Lithofacies, preserved within Flow I on the plateaus overflowed, are characterized by facies A in the proximal parts, and by alternations of facies A and B in the more distal parts. The transformation in lithofacies of Flow I is due to incorporation of vegetation and water-bearing surface materials on the way to the deposition area. Flow II consists of the secondary flow deposits, composed of facies B, which overlie Flow I in the valley floor, and are attached cohesively to the steep valley slopes. Flow III, composed of facies C, consists of fluid debris flow deposits, found only in the valley floor. It eroded Flows I and II, creating flat surfaces. Wet and mobile materials, incorporated into the avalanche and debris flows during their passage to the deposition area, occupy 40% of the total volume of those deposits, which was estimated at 5.6 × 10⁷ m³.