[SEM21-P01] メッシュ状に配置したNetwork-MTデータによる阿蘇カルデラと雲仙火山の地下の3次元比抵抗分布（２）
Mt. Aso with a large caldera and Mt. Unzen, which are composed by active Quaternary volcanoes, lie at the Beppu-Shimabara graben. The graben travels across the island of Kyushu almost in an E-W direction. 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 and ash eruptions (emissions) have continued since July 2019. On the other hand, Unzen, a back-arc volcano at the western edge of the graben showed a dome-forming eruption at Fugen-dake cone during 1990–1995, preceded by a significant eastward migration of volcano-tectonic seismicity from Tachibana bay to the summit. The total amount of extruded dacitic lava was 2×108 m3 DRE, and the repetitive partial collapses generated ~10,000 pyroclastic flows [e.g., Nakada et al., 1999; Yamamoto et al., 1993].
In and around Aso caldera and Unzen volcano, network-MT surveys for measuring the electric potential difference (the electric field) were carried out during 1993–1998 by using long metallic wires/dipoles of the commercial telephone company's networks [e.g., Uyeshima et al., 1995; Tanaka et al., 1998; Hashimoto et al., 1999; Hata et al., 2015]. 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. Three-dimensional (3-D) inversion analyses were performed for obtaining each electrical resistivity model beneath Aso caldera and Unzen volcano. In the inversion analyses, we used a data-space inversion code, which can be taken into account the length and direction of respective dipoles [e.g., Siripunvaraporn et al., 2004]. Moreover, two data sets in a period range from 480 to 20,480 s, which are distributed as to respectively cover the two target regions (Aso caldera and Unzen volcano) with reticular dipole arrangement, were used for obtaining well-resolved 3-D models. In this presentation, we show magma supply systems inferred from the 3-D resistivity models beneath Aso caldera and Unzen volcano.
In and around Aso caldera and Unzen volcano, network-MT surveys for measuring the electric potential difference (the electric field) were carried out during 1993–1998 by using long metallic wires/dipoles of the commercial telephone company's networks [e.g., Uyeshima et al., 1995; Tanaka et al., 1998; Hashimoto et al., 1999; Hata et al., 2015]. 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. Three-dimensional (3-D) inversion analyses were performed for obtaining each electrical resistivity model beneath Aso caldera and Unzen volcano. In the inversion analyses, we used a data-space inversion code, which can be taken into account the length and direction of respective dipoles [e.g., Siripunvaraporn et al., 2004]. Moreover, two data sets in a period range from 480 to 20,480 s, which are distributed as to respectively cover the two target regions (Aso caldera and Unzen volcano) with reticular dipole arrangement, were used for obtaining well-resolved 3-D models. In this presentation, we show magma supply systems inferred from the 3-D resistivity models beneath Aso caldera and Unzen volcano.