[S-EM35] Electromagnetic Induction in the Earth and Planetary Interiors, and Tectono-Electromagnetism

[SEM35-07] Three-dimensional resistivity structure of Asama Volcano

*Yoshiya Usui^1,2, Yasuo Ogawa^1,2, Koki Aizawa³, Wataru Kanda^1,2, Takeshi Hashimoto⁴, Takao Koyama⁵, Yusuke Yamaya⁶, Masaaki Mishina, Tsuneomi Kagiyama⁷ (1.Graduate School of Science, Tokyo Institute of Technology, 2.Volcanic Fluid Research Center, Tokyo Institute of Technology, 3.Institute of Seismology and Volcanology, Faculty of Sciences, Kyushu University, 4.Institute of Seismology and Volcanology, Faculty of Science, Hokkaido University, 5.Volcano Research Center, Earthquake Research Institute, University of Tokyo, 6.Fukushima Renewable Energy Institute, National Institute of Advanced Industrial Science and Technology, 7.Aso Volcanological Laboratory Institute for Geothermal Sciences Graduate School of Science Kyoto University)

Keywords:magnetotelluric method, volcanic structure, 3-D topography, 3-D inversion, unstructured tetrahedral element

Asama volcano is an andesitic composite volcano located in central Japan. The present active crater locates at the eastern part of the complex. At the west of the crater, there is a horseshoe-shaped caldera, which was formed after the collapse of an old stratovolcano at around 24,000 years ago. In order to reveal the relationship between volcanic activities and subsurface structure, two-dimensional resistivity structure of Asama volcano has already been obtained by Aizawa et al. (2008) from the data of dense magnetotelluric survey. However, three-dimensional steep topography around Asama volcano can distort the observed response functions. Therefore, in this study, we performed three-dimensional inversion with the same data set as the previous study. In the inversion, we utilized the scheme proposed by Usui (2015), which enabled us to incorporate precise topography around the mountainous area into the computational mesh with the aid of the unstructured tetrahedral element.
The measurement stations used in the inversion consist of 36 magnetotelluric stations and 37 audio-magnetotelluric stations, and we used full components of the impedance tensor and the vertical magnetic transfer function. Though some stations of them measured only electric fields, the different locations of electric and magnetic fields were taken into account in the inversion algorithm. Galvanic distortion parameters were also estimated as model parameters in addition to subsurface resistivity values.
In the obtained resistivity structure, there is a spherical resistive body at the altitudes from 0.5 to 1.5 km under the collapse caldera. From impedance phases, Aizawa et al. (2008) inferred that the resistive body was isolated. By the three-dimensional inversion, we confirmed that the resistive body under the caldera was isolated. We found that hypocenter locates around the isolated resister under the caldera. Aizawa et al. (2008) suggested that this resistive body is old solidified magma and it impedes the ascending magma. The result of our analysis supports the suggestion.
In addition, at the depths deeper than 0 km below sea level, resistivity of the west of the summit was relatively higher than surrounding area. This higher resistivity area is elongated to WNW–ESE direction and locates over the location of dyke intrusions estimated from seismic and geodetic measurements (Takeo et al., 2006). This high resistivity area also corresponds to high P-wave velocity and high-density area revealed by Aoki et al. (2010). They suggested that the high velocity is due to the solidification of repeatedly intruded magma. Our result is consistent with this interpretation since the porosity of solidified magma is considered to be low and it can lead to high resistivity in that area. On the other hand, the surrounding conductive area may consist of higher porosity rocks with saline water.