Estimating the ashfall volume 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.

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 3, 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 volumes for each eruption 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 the angle of the calculation axis including two calculation points to perform the analysis. In this study, the ashfall amount values at some locations from one eruption on June 27, 2018 were observed, and the EAI analysis was conducted to determine the ashfall distribution using one to four seeking points. It is considered that a suitable axis requires three seeking points to determine the EAI distribution.

Shallow Magmatic Hydrothermal Eruption in April 2018 on Ebinokogen Ioyama Volcano in Kirishima Volcano Group, Kyushu, Japan

The Kirishima Volcano Group is a volcanic field ideal for studying the mechanism of steam-driven eruptions because many eruptions of this type occurred in the historical era and geophysical observation networks have been installed in this volcano. We made regular geothermal observations to understand the hydrothermal activity in Ebinokogen Ioyama Volcano. Geothermal activity resumed around the Ioyama from December 2015. A steam blowout occurred in April 2017, and a hydrothermal eruption occurred in April 2018. Geothermal activity had gradually increased before these events, suggesting intrusion of the magmatic component fluids in the hydrothermal system under the volcano. The April 2018 eruption was a magmatic hydrothermal eruption caused by the injection of magmatic fluids into a very-shallow hydrothermal system as a bottom–up fluid pressurization, although juvenile materials were not identifiable. Additionally, the upwelling of mixed magma–meteoric fluids to the surface as a kick was observed just before the eruption to cause the top–down flashing of April 2018. A series of events was generated in the shallower hydrothermal regime consisting of multiple systems divided by conductive caprock layers.