[S-EM14] Electric, magnetic and electromagnetic survey technologies and scientific achievements

[SEM14-12] Magma supply system beneath Aso caldera
- 3-D resistivity distribution by MT data and Network-MT data (2) -

*Maki Hata¹, Makoto Uyeshima², Mitsuru Utsugi³, Nobuo Matsushima⁴ (1.Disaster Prevention Research Institute, Kyoto University, 2.Earthquake Research Institute, The University of Tokyo, 3.Graduate School of Science, Kyoto University, 4.Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology)

Keywords:Magma supply system, Aso caldera, 3-D resistivity distribution, MT and Network-MT data

Mt. Aso, an active Quaternary volcano, with a large caldera lie at the Beppu-Shimabara graben in the island of Kyushu. Aso caldera was formed at the central part of the graben by a series of huge eruptions, with a volcanic explosivity index of 7, during 270–90 ka. A post-caldera cone of Naka-dake in Aso caldera is a quite active volcano, at which magmatic and phreatomagmatic eruptions occurred during 2014–2016, ash eruptions/emissions continued from July 2019 to the middle of 2020, and a phreatic eruption occurred in October 2021. At an area completely covering Aso caldera, we carried out magnetotelluric (MT) and network-MT surveys during 2015–2016 and 2019–2022, respectively. Network-MT surveys/method based on MT method, whereas the electric potential difference (the electric field) of the ground is measured by using long metallic wires/dipoles (~10 km) of communication lines of the commercial telephone company. In addition, the other network-MT surveys were previously carried out in Aso caldera during 1993–1998 [e.g., Hata et al., 2015]. For clarifying magma supply system beneath Aso caldera in a crustal depth scale, we determined three-dimensional (3-D) electrical resistivity models through inversion analysis of each MT and network-MT data set. Here, different period ranges of 0.005–2,380 s for the MT data and 30–20,480 s for the network-MT data were adopted, which means that each 3-D model has different resolution characteristics for depths and resolved-sizes. Moreover, we used data-space inversion codes, according to the same theory, for the MT and network-MT data in this study [e.g., Siripunvaraporn et al., 2004; 2005]. The code for network-MT data has an advantage which can deal with the length and direction of each dipole. Consequently, all 3-D electrical resistivity models similarly imaged the following magma supply system as a series of low resistivity anomalies beneath Aso caldera; a significant low resistivity anomaly of northward dipping in the upper crust and the absence of a large distinctive anomaly in the lower crust [e.g., Hata et al., 2016; 2018]. The northward dipping anomaly is considered a magma pathway/reservoir which feeds magma to Naka-dake eruptions. In this presentation, we introduce a detailed discussion for the magma supply system beneath Aso caldera which are inferred from the 3-D resistivity models.