2017 Volume 123 Issue 5 Pages 291-307
A phreatic eruption occurred from Ontake Volcano (3067 m a.s.l.) on September 27, 2014 (at 11:52 a.m.), resulting in the distribution of diluted volcanic ash over an area extending eastward. Details of ash fall distribution were determined using a questionnaire survey of observations in Nagano, Yamanashi, Gunma prefectures, and Tokyo Metropolis, together with data from other published research. Results indicate that trace amounts of ash (<1 g/m2) were deposited over a widespread area in mid-south Nagano Prefecture and northwestern Yamanashi Prefecture. The deposition pattern was particularly wide to the south of the ash fall axis, especially in the Ina basin (from Ina City to Iida City), located between the Kiso and Akaishi mountains. This indicates that the deposition of trace amounts of ash is affected not only by prevailing wind direction, but also by local topographic effects on surface winds. The effect of surface winds on the distribution of ash fall has previously been reported for magmatic eruptions from Asama and Sakurajima volcanoes, in addition to being a feature of the phreatic eruption from Ontake Volcano. Therefore, it can be assumed to be a factor commonly affecting fine-grained ash falls.
