Reconstruction of the 2014 eruption sequence of Ontake Volcano from recorded images and interviews

Abstract

A phreatic eruption at Mount Ontake (3067 m) on September 27, 2014, led to 64 casualties, including missing people. In this paper, we clarify the eruption sequence of the 2014 eruption from recorded images (photographs and videos obtained by climbers) and interviews with mountain guides and workers in mountain huts. The onset of eruption was sudden, without any clear precursory surface phenomena (such as ground rumbling or strong smell of sulfide). Our data indicate that the eruption sequence can be divided into three phases. Phase 1: The eruption started with dry pyroclastic density currents (PDCs) caused by ash column collapse. The PDCs flowed down 2.5 km SW and 2 km NW from the craters. In addition, PDCs moved horizontally by approximately 1.5 km toward N and E beyond summit ridges. The temperature of PDCs at the summit area partially exceeded 100 °C, and an analysis of interview results suggested that the temperature of PDCs was mostly in the range of 30-100 °C. At the summit area, there were violent falling ballistic rocks. Phase 2: When the outflow of PDCs stopped, the altitude of the eruption column increased; tephra with muddy rain started to fall; and ambient air temperature decreased. Falling ballistic rocks were almost absent during this phase. Phase 3: Finally, muddy hot water flowed out from the craters. These models reconstructed from observations are consistent with the phreatic eruption models and typical eruption sequences recorded at similar volcanoes.

Special issue “The phreatic eruption of Mt. Ontake volcano in 2014”

“…These visual data are valuable for process studies of phreatic eruptions, which have rarely if ever been recorded in such detail. Two papers Oikawa et al 2016) are based mainly on the visual data; see also Maeno et al (2016) for eruption chronology based on combined analysis of visual images and the tephra sequence. describe the spatial characteristics of the eruption based in large part on aerial observations made the following day.…”

“…The interviewees included mountain guides and workers in mountain huts. Oikawa et al (2016) divide the eruption into three phases: PDC flow, pyroclastic fall with muddy rain, and muddy water overflowing from the crater. The eruption began with dry pyroclastic density currents and without any precursory surface phenomena.…”

“…Ballistics ejecta emerged tens of seconds after the beginning of the eruption, reached distances of up to 950 m from the vent, and attained maximum initial velocities estimated at 111 m/s. Oikawa et al (2016) describe the sequence of the eruption based on interviews and visual records such as still photographs and videos taken by hikers, which provide precise time records of the eruption. The interviewees included mountain guides and workers in mountain huts.…”

The Communication and Risk Management of Volcanic Ballistic Hazards

“…Additionally, ballistics may be accompanied by a surge as seen in the 2014 Mt. Ontake Oikawa et al 2016) and August 2012 Te Maari eruptions , inhibiting the ability to see ballistics until it is too late to act.…”

Geological study of phreatic eruptions

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