**7. Conclusions**

Geothermal activity resumed at Ebinokogen Ioyama Volcano in December 2015. We observed the very shallow hydrothermal event of a blowout and jet fumaroles from March to April 2017. This event was accompanied by a thermal anomaly expansion and rapid increase in the water temperature at the spring K3. The transformation of a hydrothermal mud pot A to jet fumarole A over one month following the blowout steaming event in 2017.

The magmatic hydrothermal eruptions took place in the Ioyama–south craters on 19 April and in the Ioyama-west crater on the evening of 20 April 2018. The increasing of geothermal anomaly phenomena before the eruption indicated that the magmatic liquid may be stored in a very-shallow hydrothermal system. Upwelling of hydrothermal water was observed to pour out at the hydrothermal pond Y1 just before the eruption in April 2018. The relationship of the δD and δ18O ratio of this water in the Y1 indicated that the liquid was a mixture of meteoric and magmatic liquids. Lastly, the upwelling caused by a kick may have led to the eruption at about 60 m away from the upwelling center. Hydrothermal mineral analyses and geochemical observations sugges<sup>t</sup> that the mixture was spouted from the very-shallow hydrothermal system (less than 350 m depth) at the top of the conductive caprock layer. In addition, the inflation source at 600 to 700 m depth was likely the shallow hydrothermal system (reservoir). Thus, we propose an underground hydrothermal structure consisting of two systems. The secondary eruptive event on the evening of 20 April, 2018 occurred 500 m away from the eruption site on 19 April, and it is necessary to investigate the formation mechanism and evaluation method for this event in more detail.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2076-3263/10/5/183/s1, Figure S1: The distribution of geothermal anomaly area over 50 ◦C in Ioyama fumarole area; Figure S2: Digital elevation models and photographs; Figure S3: Photomicrographs of ashy deposit grains; Figure S4: XRD analyses; Table S1: The oxygen and hydrogen isotope analyses of hydrothermal water from the Ioyama.

**Author Contributions:** Y.T.; geological and geothermal investigation, XRD analyses, project administration and writing—Original draft preparation, S.N.; supervision, geological investigation and writing—Review and editing. F.M.; drone investigation and project administration, T.H.; temperature investigation and geothermal monitoring, M.T. and A.I.; oxygen and hydrogen isotope analyses, T.M.; review and project administration, M.N.; geological investigation, J.F.; water investigation. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** The authors thank K. Ishihara, M. Ichihara, A. Takagi for o ffering helpful comments. The authors thank H. Shinohara, J. Itoh, N. Morikawa of GSJ and H. Suzuki, J. Teruya, A. Hasegawa of Nippon Koei for helping the field or laboratories. The Kirishima Nature Guide Club member as K. Haraguchi, T. Higashi, H. Yoshinaga, M. Baba, Y. Matsumura, T. Miyagawa, T. Nagatomo, S. Takada, S. Watanabe are thanked for helping with our field observations. We contacted the Kagoshima Meteorological O ffice of JMA when we entered the restricted area. In addition, permissions were obtained from the Ebino city and the Kyusyu Regional Forest O ffice at Miyakonojo during the investigation involving drones in the restricted access area. We thank J. Vandemeulebrouck, anonymous reviewers and J.C. Eichelberger editor for grea<sup>t</sup> constructive suggestions and helpful comments on this manuscript. We extend our heartfelt condolence to H. Itoh.

**Conflicts of Interest:** The authors declare no conflict of interest.
