Keywords:phreatic eruption, Ontake, pyroclastic density current, pyroclastic fallout, grain-size distribution
The phreatic eruption at Ontake volcano on 27 September 2014, which caused the worst volcanic disaster (58 deaths and 5 missing persons) in Japan in the past half-century, was reconstructed based on observation of proximal pyroclastic density current (PDC) and fallout deposits. Witnesses’ observations were also used to clarify the eruption process. The deposits are divided into three major depositional units (Units A, B, and C) which are characterized by massive, extremely poor-sorted, and multimodal grain-size distribution with 30–50 wt.% of silt to clay component. The depositional condition was initially dry but eventually changed to wet. Unit A originated from gravity-driven turbulent PDCs in the relatively dry, vent-opening phase. Unit B was then produced mainly by fallout from a vigorous moist plume during vent development. Unit C was derived from wet ash fall in the declining stage. Ballistic ejecta continuously occurred during vent opening and development. As evidenced in the finest population of the grain-size distribution, aggregate particles were formed throughout the eruption, and the effect of water in the plume on the aggregation increased with time and distance. The lithofacies and grain-size characteristics of the poorly-sorted deposits observed in the proximal area are similar to those of mudflows or fallout tephra from past phreatic events. It is important to understand the similarity of the deposits when we interpret this type of poorly-sorted deposit solely based on geological records. Using geological records, witness observations, and a theoretical approach, the physical parameters of the Ontake eruption can be constrained. Based on the deposit thickness, duration, and grain-size data, the particle concentration and flow velocity for three PDC lobes in the initial phase were estimated to be 2 × 10−4 to 2 × 10−3 and 24–56 m/s, respectively, applying a scaling analysis using a depth-averaged model of turbulent gravity currents flowing down slopes. The tephra-thinning trend shows a steeper slope in the proximal area than on the trends of similar-sized magmatic eruptions, indicating a large tephra volume deposited over a short distance owing to the wet dispersal conditions. The Ontake eruption provided an opportunity to examine the deposits from a phreatic eruption with a complex eruption sequence that reflects the effect of external water on the eruption dynamics. Further studies may enable to quantitatively evaluate the major factors that caused the many casualties and severe damage to buildings near the eruption source.