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[SEM14-05] Electromagnetic field fluctuations by infrasound during the eruption of Kirishima Shinmoe-dake in 2018
Keywords:electromagnetic signal, explosive eruption, infrasound, seismic motions, electrokinetic phenomena, aquifer
Shinmoe-dake is an active volcano located in the center of Kirishima volcanic area in Kyushu. In January 2011, magmatic eruption occurred for the first time in about 300 years and erupted again in October 2017 and March to June 2018. In this study, we discuss the relationship between electromagnetic signals obtained by broadband MT (magnetotelluric) observation and infrasound and seismic data close to the Shinmoe-dake volcano.
Twenty eruptions during March to June 2018 were selected for an analysis. We use the MT data recorded at two sites ((1): 3.2km WSW of the Shinmoe-dake crater, and (2) 5.0km NW of the Shinmoe-dake crater) and infrasound and seismic observations data recorded at 30m away from (1) MT site and 1km away from (2) MT site. We obtained MT time series data with physical unit (mV/km, nT) by incorporating the frequency responses of instruments. We also got time series data of physical units (Pa, μm/s) from infrasound and seismic data, and recalculated seismic data to the correct displacement waveform (μm) considering the frequency characteristics.
The electromagnetic signals of explosive eruptions well synchronized with the infrasound and seismic vertical motions. It is seemed that infrasound and seismic vertical motion associated with the eruption arrived at the observation point and generated electromagnetic field fluctuations in the very near vicinity of the observation point (Aniya, 2020, JpGU). As in the case in Popocatépetl Volcano (Matoza et al., 2019), the infrasound waveforms are associated with clear air-ground-coupled arrivals on seismometers, and it indicates that the ground is being pushed by infrasound. We conclude that the infrasound is moving the fluid in the aquifer just below the observation point and exciting the electromagnetic field fluctuation. In this study, we focused on the electromagnetic field fluctuation caused by infrasound, which has not been reported so far, and summarized the differences in the amount and particle motion of fluctuation among the stations and the possible mechanism of electrokinetic phenomena at present.
Using the resistivity ρ(ohm-m) and phase (deg) estimated from the impedance tensor Z and the geomagnetic transformation function T at each MT station, we calculated the 1D structural analysis from Ssq impedance (Rung-Arunwan et al., 2017). The result showed that the subsurface structure in the very shallow area just below the station is different, so it might be that the depth and interconnection of groundwater are related to the electromagnetic field variation.
Twenty eruptions during March to June 2018 were selected for an analysis. We use the MT data recorded at two sites ((1): 3.2km WSW of the Shinmoe-dake crater, and (2) 5.0km NW of the Shinmoe-dake crater) and infrasound and seismic observations data recorded at 30m away from (1) MT site and 1km away from (2) MT site. We obtained MT time series data with physical unit (mV/km, nT) by incorporating the frequency responses of instruments. We also got time series data of physical units (Pa, μm/s) from infrasound and seismic data, and recalculated seismic data to the correct displacement waveform (μm) considering the frequency characteristics.
The electromagnetic signals of explosive eruptions well synchronized with the infrasound and seismic vertical motions. It is seemed that infrasound and seismic vertical motion associated with the eruption arrived at the observation point and generated electromagnetic field fluctuations in the very near vicinity of the observation point (Aniya, 2020, JpGU). As in the case in Popocatépetl Volcano (Matoza et al., 2019), the infrasound waveforms are associated with clear air-ground-coupled arrivals on seismometers, and it indicates that the ground is being pushed by infrasound. We conclude that the infrasound is moving the fluid in the aquifer just below the observation point and exciting the electromagnetic field fluctuation. In this study, we focused on the electromagnetic field fluctuation caused by infrasound, which has not been reported so far, and summarized the differences in the amount and particle motion of fluctuation among the stations and the possible mechanism of electrokinetic phenomena at present.
Using the resistivity ρ(ohm-m) and phase (deg) estimated from the impedance tensor Z and the geomagnetic transformation function T at each MT station, we calculated the 1D structural analysis from Ssq impedance (Rung-Arunwan et al., 2017). The result showed that the subsurface structure in the very shallow area just below the station is different, so it might be that the depth and interconnection of groundwater are related to the electromagnetic field variation.