3:30 PM - 5:00 PM
[SVC35-P05] What we clarified from the intensive observation of the Kusatsu-Shirane Volcano
Keywords:Kusatsu-Shirane Volcano, intensive observation, human resource development
In the B-4 project, we conducted intensive observations of the Kusatsu-Shirane Volcano (KSV). In this presentation, we will report on the main results obtained so far. The intensive observation was originally scheduled for two years from FY2019, but due to the phreatic eruption of Mt. Motoshirane in January 2018, it was partially carried out ahead of schedule in FY2018. The observation methods were seismic observation, electromagnetic structure survey, and repeated GNSS observation, which are still ongoing. Incidentally, a total of three PDs were employed for this intensive observation, and all of them are currently active as researchers.
As for seismic observation, three seismometers were installed on the south side of Mt. Motoshirane as part of the emergency observations conducted immediately after the 2018 eruption, which resulted in the expansion of the observation network to the south of Mt. Motoshirane. In addition, a total of 4 seismic stations were set up on the Nagano Prefecture side, and by processing the data of those stations together with the conventional 6 permanent stations around the Yugama crater, it became possible to determine the hypocenters over a wide area of approximately 10 km from KSV. As a result, we clarified a belt-like seismic activity leading to the Shiga Volcano north of KSV, and seismic activity accompanied by underground rumblings in the west (Suzaka City and Takayama Village).
Regarding electromagnetic observations, broadband MT surveys were conducted from 2019 to 2020 at 50 sites within a radius of ~10 km from KSV. A three-dimensional resistivity structure model of the entire KSV was estimated using the MT response functions of a total of 73 sites including the data of 23 sites acquired around Mt. Motoshirane from 2015 to 2016. As a result, a low-resistivity region was estimated in the deep northwestern part of KSV, suggesting the presence of magma. In addition, a highly conductive region was inferred above it, which was interpreted as a region of high-concentration saline fluids.
As for GNSS observation, since the observation network before the start of this project was limited to within an area of ~2.5 km in radius centered on the Yugama crater, there was a lack of information on the deep deformation source. Therefore, we expanded the observation network to ~15 km in radius and set up eight repeat measurement sites in FY2017. Immediately after the 2018 eruption, repeated observations were made only at accessible sites, but they were all far from Mt. Motoshirane and no deformation was observed. After that, two sites were converted to continuous observation sites, and currently, we have conducted annual repeat measurements at six sites. So far, no crustal deformation originating in the depth has been obtained.
As for seismic observation, three seismometers were installed on the south side of Mt. Motoshirane as part of the emergency observations conducted immediately after the 2018 eruption, which resulted in the expansion of the observation network to the south of Mt. Motoshirane. In addition, a total of 4 seismic stations were set up on the Nagano Prefecture side, and by processing the data of those stations together with the conventional 6 permanent stations around the Yugama crater, it became possible to determine the hypocenters over a wide area of approximately 10 km from KSV. As a result, we clarified a belt-like seismic activity leading to the Shiga Volcano north of KSV, and seismic activity accompanied by underground rumblings in the west (Suzaka City and Takayama Village).
Regarding electromagnetic observations, broadband MT surveys were conducted from 2019 to 2020 at 50 sites within a radius of ~10 km from KSV. A three-dimensional resistivity structure model of the entire KSV was estimated using the MT response functions of a total of 73 sites including the data of 23 sites acquired around Mt. Motoshirane from 2015 to 2016. As a result, a low-resistivity region was estimated in the deep northwestern part of KSV, suggesting the presence of magma. In addition, a highly conductive region was inferred above it, which was interpreted as a region of high-concentration saline fluids.
As for GNSS observation, since the observation network before the start of this project was limited to within an area of ~2.5 km in radius centered on the Yugama crater, there was a lack of information on the deep deformation source. Therefore, we expanded the observation network to ~15 km in radius and set up eight repeat measurement sites in FY2017. Immediately after the 2018 eruption, repeated observations were made only at accessible sites, but they were all far from Mt. Motoshirane and no deformation was observed. After that, two sites were converted to continuous observation sites, and currently, we have conducted annual repeat measurements at six sites. So far, no crustal deformation originating in the depth has been obtained.