Estimating the ashfall distribution for a small eruption using ellipse-approximated isopach analysis: How many seeking points are required to determine a suitable axis?

A volcanic ash eruption began on March 1, 2018, and a lava flow was observed on March 6 at Shinmoedake volcano in the Kirishima Volcano Group. The small ash eruptions continued until June 27, 2018. The amounts of volcanic ashfall around Shinmoedake volcano from these eruptions were observed, and the volcanic ashfall distributions for each eruptions were analyzed. However, many cases of small eruptions were insufficient observation points to determine the ashfall volumes. Therefore, the ellipse-approximated isopach (EAI) method was used. However, the EAI method requires verification seeking points to determine an angle of a calculation axis including two calculation points to perform the analysis. Fortunately, adequate wind conditions enabled the observation of ashfall amount values at some locations after one eruption on June 27, 2018. And the EAI analysis was conducted to determine this ashfall distribution using 1–4 seeking points. As a result, it is considered that a suitable axis requires three seeking points to determine the EAI distribution.