[S-VC30] Mitigation of volcanic disasters - basic and applied researches

[SVC30-07] Geothermal activities around Ioyama since 2014 and the associated disaster incident for the Kirishima volcano in Kyushu, Japan

*Yasuhisa Tajima¹, Setsuya Nakada², Masashi Nagai², Fukashi Maeno³, Takeshi Matsushima⁴ (1.Nippon Koei, 2.NIED, 3.ERI, The University of Tokyo, 4.Kyushu University)

Keywords:Kirishima volcano, Ioyama, Steam driven eruption

Historically, Japan has experienced many disasters caused by steam-driven eruptions. In this study, we present the results of a study on the effects of steam-driven eruptions in Ebinokogen-Ioyama. A low-resistivity caprock layer developed beneath Ioyama (Tsukamoto et al., 2018), and magmatic liquids and ground water mixed in the very shallow region above this caprock layer prior to the 2018 eruption (Tajima et al., 2020a). Tajima et al. (2020b) considered that the two processes of capturing magmatic hydrothermal fluids just under the caprock layer and at the groundwater layer caused the 2018 Ioyama eruption. However, based on the results of Kato et al. (2015), it is thought that these processes might not explain the 2014 eruption of the Ontake volcano. For ease of disaster prevention discussions, we refer to these incidents as the Ontake- and Ioyama-type incidents. In Ebinokogen, various disaster prevention measures were undertaken in order to respond to Ontake-type incidents. For example, measures were taken at the Ebino Eco Museum Center (EEMC) to reinforce the facility in response to ballistics disasters, including the use of aramid fiber (Yamada et al., 2018) to repair the roof (Kyushu Regional Environment Office, Ministry of the Environment, http://kyushu.env.go.jp/pre_2016/post_36.html). The facility manager of the EEMC emphasized that private evacuation drills were a highly effective tool between the public and private facilities in Ebinokogen. On the other hand, in the Ioyama type incident a gentle increase in geothermal activity began during the summer of 2014. Firstly, the geothermal area with fumaroles in this area gradually expanded from December 2015. Therefore, access to the fumarolic area was restricted to prevent volcanic gas exposure and accidents due to falls during the early stages of the activity (Miyazaki Prefecture, https://www.pref.miyazaki.lg.jp/kiki-kikikanri/kurashi/bosai/kirisima.html). As an example of our response, we discussed the replacement of the trail at the beginning of 2016 in the local community. We do not know if this discussion was utilized, but the trail was replaced in March 2016 and again in April 2017. Voluntary restrictions on such expanding activities might also have the effect of emphasizing the urgency between levels 1 and 2 of the volcanic alert levels of the Japan Meteorological Agency (JMA). Next, a jet fumarole started in fumarole H on April 22, 2017, and a jet fumarole formed the small vent of jet fumarole A on April 26 (Tajima et al., 2020a). When jet fumarole A began, the muddy ash diffused and spread up to 200 m southwest of the vent. The JMA defined an eruption as follows: "an eruption is recorded when eruptive material exceeds a horizontal or vertical distance of approximately 100-300 m from the crater/vent" (JMA, 2014). It is necessary to review the concept of such gray-zone events to avoid confusion in emergency communications. The most important point for disaster prevention was that the eruption started on April 19, 2018 at southward Ioyama, which was followed by a small venting from 21:05 on April 20, approximately 500 m west of the main crater (Tajima et al., 2020a; Muramatsu et al., 2021). This event occurred closer to tourist facilities, was serious, and should be recognized as a disaster. We are grateful to Kentaro Sakamoto, former director of the Ebino Eco Museum Center, for his assistance with disaster management. We also thank Toshio Huruzono and other members of the Kirishima Nature Guide Club for their help in the field.