[S-VC32] Active Volcanism

[SVC32-P19] Three-dimensional resistivity structure of Hakone volcano, Japan.

*Hiromichi Shigmatsu¹, Koki Aizawa¹, Tomohiro Inoue¹, Harutou Tanabe¹, Keita Matsunaga¹, Taiki Kono¹, Kensuke Nakamura¹, Issei Hirata¹, Rin Matsubara¹, Asumi Katsuyama¹, murofushi ryuma¹, Kazunari Uchida¹, Makoto Uyeshima², Takao Koyama², Tasuku M. Hashimoto², Taro Nishimoto², Takahiro Akiyama², Atsushi Watanabe², Hideji Abe², Dan Muramatsu², Yasuo Matsunaga², Oishi Tatsuto², Ryokei Yoshimura³, Shintaro Komatsu³, Itaru Yoneda³, Tsutomu Miura³, Yasuto Tatsuyama³, Nagaoka Airi³, Ayano Nada³, Nakagawa Jun³, Kazuhide Tomisaka³, Masahiro Yoshikawa³, Hiroshi Ichihara⁴, Yasuo Ogawa⁵, Shogo Masuda⁵, Norihiro Kitaoka⁵, Nagi Yamashita⁶, Miku Sakurai⁶, Kokoro Onaka⁶, Fumihisa Yoshizawa⁷, Ryutaro Semba⁸, Koki Sasaki⁸, Kazutaka Mannen⁹ (1.Kyushu University, 2.University of Tokyo, 3.Kyoto University, 4.Nagoya University, 5.Tokyo Institute of Technology, 6.University of Hyogo, 7.National Research Institute for Earth Science and Disaster Resilience, 8.Japan Meteorological Agency, 9.Hot Springs Research Institute of Kanagawa Prefecture)

Keywords:Hakone volcano, Magneto-telluric, 3-D resistivity structure

We present a preliminary three-dimensional resistivity structure of Hakone volcano, located approximately 100 km southwest of Tokyo. This volcano experienced volcanic unrest in 2015, including small phreatic eruptions, seismic swarm activity, and ground deformation. Previous studies have provided high-resolution seismic velocity structures to a depth of 30 km, but resistivity structures have only been estimated to a depth of 3km (Yoshimura et al., 2018 EPS). Recent studies highlighted discrepancies between low-resistivity and low-velocity anomalies at other volcanoes, indicating the challenges in interpreting volcano structures based solely on resistivity. Our ultimate goal is to better understand volcanic fluid pathways by integrating these complementary datasets.
We conducted a wide-band Magneto-telluric survey in and around Hakone volcano, deploying 41-observation points during September and October 2023. We recorded Magneto-telluric (MT) data continuously for over a month, with a sampling rate of 32 Hz during the day and 1024 Hz at night (Japan Standard Time 2:00-3:00) to estimate good quality MT response functions in an area of high EM noise due to dense population, high-voltage power lines, and Direct Current electric railways. In calculating MT response functions, we used BIRRP program (Chave and Thomson, 2004) with remote-reference processing using geomagnetic data collected 850 km west of Hakone volcano. As a result, we obtained moderate-quality MT response functions in the periods range of 0.005 s to 1000 s at 20 of 41 sites.
In this study, we estimated the preliminary 3-D resistivity structure using hexahedral mesh and the FEMTIC code (Usui, 2015 GJI; Usui et al., 2017 GJI, 2024 JGR). The input data consists of the full components of impedance tensor and tippers. In addition to the broad-band MT data collected at 41 sites in 2023, we also used AMT data from 65 sites used in Yoshimura et al., 2018 EPS. The imaged shallow resistivity structure is similar to that of Yoshimura et al., (2018), with a bell-shaped low resistivity layer beneath the Owakudani geothermal zone. At depth greater than 4 km, the resistivity structure is not fully constrained from the dataset. Therefore, we need to improve the quality of MT response function of broad-band MT sites acquired in 2023.