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

[SEM14-12] 阿蘇カルデラ地下のマグマシステム
- MT法データおよびNetwork-MT法データによる3次元比抵抗分布 (2) -

*畑 真紀¹、上嶋 誠²、宇津木 充³、松島 喜雄⁴ (1.京都大学防災研究所、2.東京大学地震研究所、3.京都大学大学院理学研究科、4.産業技術総合研究所地質調査総合センター)

キーワード：マグマ供給系、阿蘇カルデラ、3次元比抵抗分布、MT&Network-MTデータ

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.