[S-VC26] Active Volcanism

[SVC26-02] Seismo-acoustic activities and surface hydrothermal phenomena at the South crater of Kirishima Iwo-Yama volcano

*Dan Muramatsu¹, Mie Ichihara¹, Yohei Yukutake¹, Takeshi Matsushima², Yasuhisa Tajima³, Takeshi Nishimura⁴, Tsutomu Nagatsuma⁵, Ken T. Murata⁵ (1.Earthquake Reserch Institute, The University of Tokyo, 2.Institute of Seismology and Volcanology, Faculty of Science, Kyushu University, 3.Research and Development Center, Nippon Koei Co., Ltd., 4.Department of Geophysics, Graduate School of Science, Tohoku University, 5.National Institute of Information and Communications Technology)

Keywords:Kirishima Iwo-Yama volcano, Earthquake, Infrasound

Active hydrothermal activity has continued in the South crater of Kirishima Iwo-Yama volcano since the 2018 small phreatic eruption, and three vents (Y2a, Y2b, and Y3) are currently active. In particular, the vents Y2a and Y2b have repeated small emissions of ash and muddy deposits after 2022. To investigate the long-term changes in the surface hydrothermal activity, we analyzed seismo-acoustic data obtained at three temporary stations around the crater and referred to monitoring camera data operated by NICT. We examined 1) temporal changes in spectral content and RMS amplitude (power) of seismo-acoustic data and 2) distribution and temporal evolution of volcano-tectonic (VT) earthquakes beneath Iwo-Yama. In the former, we evaluated seismic power caused by air-to-ground coupling of acoustic waves to distinguish true seismic power (Ichihara et al., 2021, EPS). In the latter, we referred to the hypocenter catalog of the Kirishima volcanic group created by the method of Yukutake et al. (2023, EPS). We found a significant increase in the seismic tremor amplitude accompanying the emission events in November 2022 and March 2023, in which seismic power intensified over 3–20 Hz and especially dominated at 5–10 Hz. The number of VT earthquakes in the cluster at a depth of ~500–1500 m beneath Iwo-Yama increased before these emission events. In contrast, the October 2023 emission event showed no significant increase in the seismic tremor amplitude, which was already at a high level, and occurred after the number of VT earthquakes decreased. It has been suggested that the seismic tremor originates from the very shallow hydrothermal system at a depth of ~100 m beneath the South crater (Hata et al., VSJ Fall Meeting 2023). The VT earthquakes distribute around a spherical pressure source (hydrothermal chamber) beneath the clay layer (Tsukamoto et al., 2018, GRL) and the VT earthquake swarm suggests an increase in the supply of volcanic fluid to the hydrothermal chamber. We discuss the relationship between the differences observed among the emission events, the very shallow hydrothermal system, and the VT earthquake swarms.