2:15 PM - 2:30 PM
[SVC35-03] Changes in the resistivity structure around the Kusatsu-Bandaiko Spring
Keywords:Kusatsu Onsen, Bandaiko, resistivity structure
An audio-frequency magnetotellurics (AMT) survey was conducted in October 2013 around the Bandaiko Spring, the main source of Kusatsu Onsen, in October 2013 to estimate the a three3-D dimensional (3-D) resistivity structure (Kanda et al., 2014). As a result, the area containing hydrothermal fluids was imaged as a low-resistivity region, and it was inferred that the fluids were supplied from the underlying basement rocks toward the gushing source. Subsequently, the fact that the amount of discharge from Bandaiko Spring decreased rapidly around 2022, and that a phreatic eruption occurred in 2018 at Mt. Motoshirane, located approximately 3.5 km west of Bandaiko, suggests that the state of the underground hydrothermal system around Bandaiko may be changing. Therefore, in this study, we conducted an AMT survey around Bandaiko Spring again in 2023 with the aim of clarifying the structure of the hydrothermal system by estimating the 3-D resistivity structure using the latest analytical methods and examining changes in the state of the hydrothermal system over the past 10 years.
The AMT survey was conducted from May 8 to May 16, 2023. In addition to the 19 stations for which data were obtained by Kanda et al. (2014), five new stations were added for a total of 24 stations. Five components of the electromagnetic field were measured at all stations. To reduce noise, measurements were also taken to the east of the Yugama crater, approximately 4.5 km away, and used as reference data for remote reference processing (Gamble et al., 1979). As a result, generally good MT response functions were estimated in the frequency range of 1~10,000 Hz.
Three-dimensional resistivity structure analysis was performed using the obtained MT response functions. The analysis code used was FEMTIC (Usui, 2015; Usui et al., 2017), in which an unstructured tetrahedral mesh is created. Eighteen frequencies (1–8,800 Hz) of the impedance tensor and tipper were used as input data. We also used the broadband MT data obtained by Matsunaga et al. (2022). As a result of the analysis, low-resistivity regions that appears to express the presence of hydrothermal fluids were identified at approximately 300 and 600 m from the tunnel entrance. It was also found that the low-resistivity areas presumed to be the source of these hydrothermal fluids are widely distributed at an elevation of approximately 1000 m. In addition, the Sessho lava, which forms the north and south ridges, has high resistivity, whereas the Aoba lava exposed along the valley has relatively low resistivity. Therefore, Aoba lava was interpreted to have been altered by volcanic gas and hydrothermal fluids.
To evaluate the structural changes occurring 10 years ago, we compared the AMT data using a misfit tensor (Heise et al., 2007). The analysis showed that there were relatively large misfits in the shallow parts at several stations. In future, we plan to conduct a specific analysis of this misfit and the structural changes that cause it.
The AMT survey was conducted from May 8 to May 16, 2023. In addition to the 19 stations for which data were obtained by Kanda et al. (2014), five new stations were added for a total of 24 stations. Five components of the electromagnetic field were measured at all stations. To reduce noise, measurements were also taken to the east of the Yugama crater, approximately 4.5 km away, and used as reference data for remote reference processing (Gamble et al., 1979). As a result, generally good MT response functions were estimated in the frequency range of 1~10,000 Hz.
Three-dimensional resistivity structure analysis was performed using the obtained MT response functions. The analysis code used was FEMTIC (Usui, 2015; Usui et al., 2017), in which an unstructured tetrahedral mesh is created. Eighteen frequencies (1–8,800 Hz) of the impedance tensor and tipper were used as input data. We also used the broadband MT data obtained by Matsunaga et al. (2022). As a result of the analysis, low-resistivity regions that appears to express the presence of hydrothermal fluids were identified at approximately 300 and 600 m from the tunnel entrance. It was also found that the low-resistivity areas presumed to be the source of these hydrothermal fluids are widely distributed at an elevation of approximately 1000 m. In addition, the Sessho lava, which forms the north and south ridges, has high resistivity, whereas the Aoba lava exposed along the valley has relatively low resistivity. Therefore, Aoba lava was interpreted to have been altered by volcanic gas and hydrothermal fluids.
To evaluate the structural changes occurring 10 years ago, we compared the AMT data using a misfit tensor (Heise et al., 2007). The analysis showed that there were relatively large misfits in the shallow parts at several stations. In future, we plan to conduct a specific analysis of this misfit and the structural changes that cause it.