[S-EM16] 電気伝導度・地殻活動電磁気学

[SEM16-P05] メッシュ状に配置したNetwork-MTダイポールデータによる阿蘇カルデラ地下の3次元比抵抗分布

*畑 真紀¹、上嶋 誠¹、宇津木 充²、松島 喜雄³ (1.東京大学地震研究所、2.京都大学大学院理学研究科附属地球熱学研究施設火山研究センター 、3.産業技術総合研究所 )

キーワード：3次元電気比抵抗構造モデル、ネットワークMTデータ、阿蘇カルデラ

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. In and around Aso caldera, network-MT surveys, in which the electric potential difference (the electric field) of the ground are measured by using long metallic wires/dipoles of the commercial telephone company's networks, were carried out during 1993–1998 for the first time [e.g., Uyeshima et al., 1995; Tanaka et al., 1998; Hashimoto et al., 1999; Hata et al., 2015]. On the other hand, we newly performed network-MT surveys around Aso caldera during 2019–2021 in order to obtain data of electric potential differences at a high sampling rate compared to previous data. By using the new data, we determined two components of network-MT response functions between the potential differences for respective dipoles and the two horizontal components of the magnetic field at the Kanoya Geomagnetic Observatory. Then, we obtained three-dimensional (3-D) electrical resistivity models beneath Aso caldera in a crustal depth scale through inversion analysis of the network-MT response functions in two period ranges of 480–20,480 s and 30–20,480 s. In the inversion analysis, we used a data-space inversion code, which can be considered the length and direction of respective dipoles [e.g., Siripunvaraporn et al., 2004]. Moreover, the dipoles are distributed as to cover Aso caldera into reticular formation in order to obtain well-resolved 3-D models. In this presentation, we introduce the two 3-D resistivity models based on the respective network-MT data sets of 480–20,480 s and 30–20,480 s. Moreover, we discuss a magma supply system beneath Aso caldera which are inferred from the 3-D resistivity models.