**2. Geological Setting**

The Kirishima Volcano Group is a compound group of volcanoes extending from the southeast to the northwest (Figure 1). This volcano has grown since the Kakuto ignimbrite eruption in 340 ka [38]. Shinmoedake, Nakadake, Ohachi and Takachihomine volcanoes produced frequent magmatic eruptions during the last 10 ky. Furthermore, Miike Volcano produced a Plinian eruption in 4.6 ka characterized a single massive eruption. Ebinokogen Ioyama and Ohatayama volcanoes produced dominant steam-driven eruptions during the last 10 ky e.g. [32]. The historical eruptions are recorded in Ebinokogen Ioyama, Shinmoedake and Ohachi volcanoes [29].

**Figure 1.** Geographic map of Kirishima Volcano Group. Dashed box shows the map of the study area in Figure 2. The base map with Kashimir3D [39]. In the upper right corner, the red circles denote the calderas while the gray areas indicate active volcanoes. VF: Volcanic front.

Around the Ebinokogen area, Karakunidake Volcano produced a Plinian eruption in 16.7 ka, forming the present Karakunidake cone [29,40]. Magmatic eruptions did not occur in this area between 16.7 and 9 ka. The small eruptions were repeated in the saddle place surrounded by older Karakunidake, Koshikidake and Ebinodake volcanoes (Figure 2a). The eruption of Fudoike lava flow with tephra occurred at the Fudoike Crater in 9 ka. The small amphitheater topography was formed by steam-driven eruptions with an avalanche (Karakunidake debris avalanche) in 4.3 ka. It was followed by a steam-driven eruption in 1.6 ka at Fudoike Crater, a lava flow between 0.3 and 0.4 ka at Ioyama and a steam-driven eruption in 1768 AD at Ioyama east crater. Ebinokogen Ioyama Volcano has formed from 9 ka [32].

**Figure 2.** Shaded relief maps showing geological and geothermal features around Ebinokogen Ioyama Volcano. (**a**) Distribution of lavas and craters over 10,000 years [32]. Kn: Karakunidake north crater, Ie: Ioyama east crater, Io: Ioyama crater, Fd: Fudoike Crater, Fs: Fudoike south crater. Red circular lines denote the crater, red curves with ticks denote the amphitheater. The relief map using Kashimir3D [39]; (**b**) The distribution of Ebino-fumarole and Ioyama-fumarole areas in the 1950s [41]. Red points: active fumaroles. Numerals: the temperature of isothermals.

Geothermal activity is known to have continued at Ebinokogen Ioyama since around the 1900s (Meiji Era). The geothermal area was divided into two areas in the east as the Ioyama-fumarole area and in the west as the Ebino-fumarole area in the 1950s (Figure 2b). The center of the Ebino-fumarole area was about 1 km west from the center of the Ioyama-fumarole area [41]. Sulfur mining was worked in the Ioyama-fumarole area in the 20th Century. A fumarole temperature in the Ioyama lava was over 80 ◦C on 12 August 1916 [42]. Akiko Yosano, a famous poet in Japan, described fumaroles in this area in one of her poems in 1929 [43]. The temperatures of the Ioyama-fumarole area were between 96 and 120 ◦C in 1954 [41]. The highest temperature recorded 247 ◦C in the Ioyama-fumarole area in March 1975 [34]. It gradually declined in the 1990s, and its activity had stopped by around 2008 [43].

Alteration identified kaolinite, alunite and montmorillonite zones at the geothermal or alteration areas in the western part of Kirishima Volcano Group. The kaolinite zone was distributed within small area around the Ioyama, while the alunite zone surrounded the kaolinite zone. The presence of the following minerals reported alunite, kaolinite, quartz, cristobalite, halloysite, montmorillonite, tridymite and jarosite in the alteration area [44].
