Japan Geoscience Union Meeting 2023

Presentation information

[J] Online Poster

S (Solid Earth Sciences ) » S-VC Volcanology

[S-VC31] Active Volcanism

Tue. May 23, 2023 10:45 AM - 12:15 PM Online Poster Zoom Room (16) (Online Poster)

convener:Yuta Maeda(Nagoya University), Takahiro Miwa(National research institute for earth science and disaster prevention), Takeshi Matsushima(Institute of Seismology and Volcanology, Faculty of Science, Kyushu University)

On-site poster schedule(2023/5/22 17:15-18:45)

10:45 AM - 12:15 PM

[SVC31-P14] Long-term observations of electric and magnetic fields and steam effusion activity at West Crater of Iwo-Yama, Kirishima Volcanic Complex, Japan

*Harutou Tanabe1, Takeshi Matsushima2, Koki Aizawa2, Dan Muramatsu3 (1.Deoartment of Earth and Planetary Sciences, Graduate School of Science, Kyushu University, 2.Institute of Seismology and Volcanology, Faculty of Science, Kyushu University, 3.Earthquake Research Institute, University of Tokyo)


Keywords:Steam effusion activity, electric and magnetic fiels, Kirishima

From December 2021 to April 2022, steam effusion activity was observed at West Crater, Iwo-Yama, Kirishima Volcano Complex, Japan. During this activity, steam effusion from the vent was observed intermittently. Steam effusion suddenly stopped coincident with rainfall and resumed 3 to 5 days later. This activity was completely stopped in April 2022.
We performed long-term observations (two electric fields, temperature, movie) around the vent (Fig. 1) from December 2021 to September 2022. In this study, we discuss relationship between steam effusion activity and resistivity structure estimated by electric and magnetic field data. In particular, by long-term (approximately 10 months) observation data, we expect how changes in subsurface structure affects the progress of steam effusion and volcanic activity. We will introduce the observation data in the following.
First, we could see how the steam effusion progressed based on the temperature and movie data. During the strong steam effusion from the vent, the temperature was maintained at approximately 96 ℃, which was the boiling point at this elevation (1233 m). The temperature suddenly dropped to 15 to 40 °C at the cessation of the steam effusion, but rose to 96 °C 3 to 5 days later. This intermittent steam effusion activity was observed from December 2021 to April 2022. In the electric field time series data, from December 2021 to May 2022, the temporal variation of both station 1 and station 2 (Fig. 1) were observed with an interval of approximately 27 hours. However, the variation of station 1 was more obvious than that of station 2. In addition, only in the steam effusion, the temporal change of electric field is accompanied by a small temperature change of 0.5 °C. The temporal change could be caused by cold groundwater flowing through porous media towards the underlying vent. In other words, the cyclical cold groundwater flow toward the vent occurs when steam effusion is active.
We calculated MT response functions from magnetic and electric field data. In this study, we calculated the sum of squared element impedances under the 1-D subsurface structure (Rung-Arunwan et al., 2016). In the 8 Hz and 20 Hz data of the observed impedance, we confirmed that the apparent resistivity gradually increased and the phase gradually decreased from December 2021 to September 2022 at station 1. In other words, the subsurface structure underlying the vent is low resistivity during the active steam effusion period (from December 2021 to April 2022) and high resistivity during the quiescent period (after April 2022). In the case of Station 2, the apparent resistivity changed but the phase did not. Station 2 may be affected by static-shift.
Our observation data suggest that the resistivity underlying the vent became high resistivity and the cyclic groundwater flow disappeared as the steam effusion activity became quiescent. We will consider the cause of these changes.