Estimation of total discharged mass from the phreatic eruption of Ontake Volcano, central Japan, on September 27, 2014

Abstract

The total mass discharged by the phreatic eruption of Ontake Volcano, central Japan, on September 27, 2014, was estimated using several methods. The estimated discharged mass was 1.2 × 10 6 t (segment integration method), 8.9 × 10 5 t (Pyle's exponential method), and varied from 8.6 × 10 3 to 2.5 × 10 6 t (Hayakawa's single isopach method). The segment integration and Pyle's exponential methods gave similar values. The single isopach method, however, gave a wide range of results depending on which contour was used. Therefore, the total discharged mass of the 2014 eruption is estimated at between 8.9 × 10 5 and 1.2 × 10 6 t. More than 90 % of the total mass accumulated within the proximal area. This shows how important it is to include a proximal area field survey for the total mass estimation of phreatic eruptions. A detailed isopleth mass distribution map was prepared covering as far as 85 km from the source. The main ash-fall dispersal was ENE in the proximal and medial areas and E in the distal area. The secondary distribution lobes also extended to the S and NW proximally, reflecting the effects of elutriation ash and surge deposits from pyroclastic density currents during the phreatic eruption. The total discharged mass of the 1979 phreatic eruption was also calculated for comparison. The resulting volume of 1.9 × 10 6 t (using the segment integration method) indicates that it was about 1.6-2.1 times larger than the 2014 eruption. The estimated average discharged mass flux rate of the 2014 eruption was 1.7 × 10 8 kg/h and for the 1979 eruption was 1.0 × 10 8 kg/h. One of the possible reasons for the higher flux rate of the 2014 eruption is the occurrence of pyroclastic density currents at the summit area.

Infrasonic wave accompanying a crack opening during the 2015 Hakone eruption

“…The discharged mass was less than 1% of that discharged during the 2014 phreatic eruption of Mt. Ontake (Takarada et al 2016). An abrupt increase in VT earthquakes was observed after 07:32 on June 29 (JST = UTC + 8 h) .…”

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

“…Ontake eruption tephra was ejected as PDC, ballistics, and air fall. Takarada et al (2016) and Maeno et al (2016) document the total volume of tephra and the size of the eruption and Sasaki et al (2016) describe the resulting lahar. and Tsunematsu et al (2016) analyze the distribution of ballistic ejecta and Tsunematsu et al (2016) simulate ballistic trajectory and calculate a landing energy of 10 4 J, essential data for future shelter designs.…”

“…and Tsunematsu et al (2016) analyze the distribution of ballistic ejecta and Tsunematsu et al (2016) simulate ballistic trajectory and calculate a landing energy of 10 4 J, essential data for future shelter designs. Takarada et al (2016) estimate discharged mass of the fallout deposit by a segment-integration method and find a total mass of around 1 × 10 6 tons, more than 90% of which occurs proximal to the craters. The discharged mass of the 2014 eruption is about half that of the 1979 eruption.…”

The Volcanic Hazards Assessment Support System for the Online Hazard Assessment and Risk Mitigation of Quaternary Volcanoes in the World

“…The Tephra2 simulation is based on advection-diffusion model, hence the maximum isomass contour area is shifted far away from the source, especially in case of strong winds. In the real eruption case, much thicker pyroclastic materials are accumulated in and around the volcano due to ballistics, partial collapse of eruption column, pyroclastic density current deposit, and direct fallout from the side of the eruption column, placing the maximum isomass area (thickest part) near the source area (Takarada et al, 2016b). The May 10, 2016 simulation result indicates that the tephra fall deposit is distributed toward the east and the 20 kg/m 2 isomass contour area reaches as far as 87 km from the source (near Yokohama).…”