摘要：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 × 106 t (segment integration method), 8.9 × 105 t (Pyle’s exponential method), and varied from 8.6 × 103 to 2.5 × 106 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 × 105 and 1.2 × 106 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 × 106 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 × 108 kg/h and for the 1979 eruption was 1.0 × 108 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.
