[S-VC29] Dynamics of volcanic eruptions and their physical and chemical processes

[SVC29-01] Long-term change in surface hydrothermal activities at the South craters of Kirishima Iwo-Yama volcano

*Dan Muramatsu¹, Takeshi Matsushima², Tsutomu Nagatsuma³, Ken T. Murata³, Takeshi Nishimura⁴, Masato Hata⁴, Harutou Tanabe⁵ (1.Earthquake Reserch Institute, The University of Tokyo, 2.Institute of Seismology and Volcanology, Faculty of Science, Kyushu University, 3.Resilient ICT Research Center, National Institute of Information and Communications Technology, 4.Department of Geophysics, Graduate School of Science, Tohoku University, 5.Department of Earth and Planetary Sciences, Graduate School of Science, Kyushu University)

Keywords:Kirishima Iwo-Yama volcano, Infrasound, Hydrothermal activity

Active hydrothermal activity has continued in the South craters of Kirishima Iwo-Yama volcano since the 2018 small phreatic eruption, and three main vents (Y2a, Y2b, and Y3) are currently active. At vent Y2a, the most active one, the formation of a hydrothermal pool and its drying-up have been repeated over 5 years. When the vent has a hydrothermal pool, it splashes hot water in a fountain-like manner (sometimes called ‘boiling spring’), and when the pool dries up, the activity shifts to energetic fumarole. To investigate the long-term changes in the surface activity observed at the South craters, we conducted acoustic, seismic, and video observations. Acoustic and seismic data were obtained at temporary stations operated by Kyushu University, and video data at a monitoring camera site operated by the National Institute of Information and Communications Technology (NICT) since September 2022. First, a comparison of the acoustic spectrogram and the video images confirmed a ~2 Hz acoustic signal associated with the hydrothermal pool activity. The 2 Hz acoustic signal has been intermittent since mid-October 2022, possibly reflecting temporal formation and disappearance of the hydrothermal pool at the vent. In addition, we observed a gliding of the spectral peak before the disappearance of the hydrothermal pool, which is an important piece of information to consider the source mechanisms. Next, we compared temporal changes of acoustic and seismic RMS amplitude with spectrograms and found a significant increase in seismic power at the end of November 2022. This activity may correspond to the formation of a new vent in Y2b. Based on these observations, we consider 1) relationships between the long-term change in the surface hydrothermal phenomena and volcanic activity and 2) possible mechanism of the ~2 Hz acoustic signal from the hydrothermal pool activity.