[S-EM15] Electric, magnetic and electromagnetic survey technologies and scientific achievements

[SEM15-P07] Broadband Magnetotelluric Survey to Elucidate the Subsurface Resistivity Structure of Sakurajima Volcano

*Cinantya Nirmala Dewi¹, Ryokei Yoshimura², Maki Hata², Rintaro Miyamachi², Shintaro Komatsu², Ken'ichi Yamazaki², Tadaomi Sonoda², Yuusuke Takenaka² (1.Graduate School of Science, Kyoto University, 2.Disaster Prevention Research Institute, Kyoto University)

Keywords:Sakurajima volcano, Electromagnetic soundings, Resistivity model

Analysis of the magmatic and plumbing systems in volcanic areas is critical to understanding volcanic activity. One of the most active volcanoes in Japan is Sakurajima. It is a stratovolcano located in the Aira caldera, southern part of Kyushu with two craters: Kitadake in the north (1,117 m) and Minamidake in the south (1,040 m). The Showa Crater was formed in 1946 by the Showa eruption in the east of the Minamidake Crater. Ground deformation studies revealed that Sakurajima's magma plumbing system consists of a deep (10 km) magma chamber beneath the Aira caldera, a shallow (3-6 km) magma chamber beneath the summit, and a conduit connecting the shallow magma chamber to the Minamidake Crater (Iguchi, 2013). Meanwhile, the tilt record analysis at Amidagawa Station from 2014 to 2020 shows that Minamidake and Showa craters have the same conduit system, but its vertical extent cannot be determined (Nishimura et al., 2024). The shallow resistivity structure of Sakurajima volcano has been modeled based on audio magnetotelluric (AMT) data and the results show that the shallow structure (about 1.5 km) of Sakurajima consists of a lava layer with high resistivity values and a thick seawater or groundwater layer with low resistivity values underneath (Kanda et al., 2011). To elucidate the deep subsurface resistivity structure beneath Sakurajima Volcano, we conducted a broadband magnetotelluric (MT) survey at 35 sites. Our sites covered almost all accessible areas of Sakurajima Island consisting of 17 full-component MT sites and 18 two-component (electric field) MT sites. After carefully checking the time series data, we decided to use the nighttime data with less artificial noise from urban area to calculate the MT responses. The MT responses from 33 sites (exclude the 2 full-component MT sites that still recording) show that 12 sites have good quality and 12 sites have fair quality. Meanwhile, 9 sites show bad MT response quality and we are trying to make some improvements. This study aims to obtain a high-resolution subsurface structure of Sakurajima volcano based on an electrical resistivity model and evaluate its magmatic system.