17:15 〜 19:15
[SVC32-P08] 稠密多点GNSS観測網とひずみ計から推定した2024年岩手山膨張イベントの力源推定
キーワード：岩手山、GNSS、ソフトバンク
Iwate volcano is one of the active volcanoes in northeastern Honshu, Japan, with histories of magmatic eruptions in 1686 and 1732. At the beginning of 1998, the number of shallow earthquakes increased, and dilatational crustal deformation became apparent. After several months of intense seismic activity, a moderate tectonic earthquake with magnitude 6.1 occurred on September 3, 1998, in the region adjacent to the volcano; after this event, seismic activity in the volcano declined. Since then, the volcano has been generally quiescent. However, in early 2024, seismicity and ground deformation began to increase again.
In this presentation, we present the results of estimating the deformation source of the 2024 unrest based on dense GNSS sites and strainmeter observations. Data from a total of 51 GNSS sites located within a circle of 40 km radius centered on the summit of the Iwate volcano-comprising 11 GSI GEONET sites, 31 SoftBank original GNSS sites, 2 GNSS sites from the National Research Institute for Earth Science and Disaster Prevention, 2 GNSS sites from the Japan Meteorological Agency, and 5 GNSS sites from Tohoku University-were used for analysis. Daily coordinate values were estimated using the GipsyX Ver.2.2 precise point positioning method. The common mode noise was removed after visually excluding sites of low quality from the obtained daily coordinate time series. The steady component was estimated for the period from May 2, 2021 to January 31, 2024, and removed. For the three volumetric strainmeters operated by Tohoku University, a 1-hour sampled time series of strain changes was computed and used as input for force source estimation after subtracting the effects of tides, atmospheric pressure, precipitation, a linear trend, step changes, and annual cyclic variations.
We used RUNE (Real-time automatic uncertainty estimation of source models using crustal deformation data) (Ohno et al., EPS, 2022, 2024), an MCMC-based source estimation method. The three components (horizontal and vertical) of the GNSS data were used for the estimation. The varying depth method (Williams and Wadge, 1998), which provides a simple correction for the site elevations, was applied during the estimation. A total of 1x107 iterations were performed with a burn-in of 1%, and the posterior probability distribution was obtained from the samples after excluding the burn-in period. For the period from July 26, 2024, to December 15, 2024, when crustal deformation became pronounced at stations on the west side of the edifice, estimation was performed by assuming a spheroid and a vertical tensile fault on the western side of Iwate Volcano, regarding the crustal deformation source of 1998 unrest.
The estimated source explained 71.4% of the variance reduction (VR) in the horizontal component of the GNSS data and 99.1% of the VR in the volumetric strainmeter data. A spheroid with a major axis oriented in the north-south direction was estimated to be located at a top depth of approximately 3.6 km, with a volume change of 6.11 [1.88–9.61]x106 m³ (median and 95% confidence interval). Additionally, the dyke exhibited a volume change of 5.52 [0.68–11.35]x106 m³, clearly indicating significant uncertainty in the dyke’s volume change. In the presentation, we will discuss the temporal evolution of the source, along with data analysis covering the most recent period.
Acknowledgments: The SoftBank's GNSS observation data used in this study was provided by SoftBank Corp. and ALES Corp. through the framework of the "Consortium to utilize the SoftBank original reference sites for Earth and Space Science". We used the RINEX data from the GEONET sites of the Geospatial Information Authority of Japan (GSI). We also utilized GNSS data from the Fundamental Volcano Observation Network (V-net) operated by the NIED, as well as GNSS data from GNSS sites operated by the JMA.
In this presentation, we present the results of estimating the deformation source of the 2024 unrest based on dense GNSS sites and strainmeter observations. Data from a total of 51 GNSS sites located within a circle of 40 km radius centered on the summit of the Iwate volcano-comprising 11 GSI GEONET sites, 31 SoftBank original GNSS sites, 2 GNSS sites from the National Research Institute for Earth Science and Disaster Prevention, 2 GNSS sites from the Japan Meteorological Agency, and 5 GNSS sites from Tohoku University-were used for analysis. Daily coordinate values were estimated using the GipsyX Ver.2.2 precise point positioning method. The common mode noise was removed after visually excluding sites of low quality from the obtained daily coordinate time series. The steady component was estimated for the period from May 2, 2021 to January 31, 2024, and removed. For the three volumetric strainmeters operated by Tohoku University, a 1-hour sampled time series of strain changes was computed and used as input for force source estimation after subtracting the effects of tides, atmospheric pressure, precipitation, a linear trend, step changes, and annual cyclic variations.
We used RUNE (Real-time automatic uncertainty estimation of source models using crustal deformation data) (Ohno et al., EPS, 2022, 2024), an MCMC-based source estimation method. The three components (horizontal and vertical) of the GNSS data were used for the estimation. The varying depth method (Williams and Wadge, 1998), which provides a simple correction for the site elevations, was applied during the estimation. A total of 1x107 iterations were performed with a burn-in of 1%, and the posterior probability distribution was obtained from the samples after excluding the burn-in period. For the period from July 26, 2024, to December 15, 2024, when crustal deformation became pronounced at stations on the west side of the edifice, estimation was performed by assuming a spheroid and a vertical tensile fault on the western side of Iwate Volcano, regarding the crustal deformation source of 1998 unrest.
The estimated source explained 71.4% of the variance reduction (VR) in the horizontal component of the GNSS data and 99.1% of the VR in the volumetric strainmeter data. A spheroid with a major axis oriented in the north-south direction was estimated to be located at a top depth of approximately 3.6 km, with a volume change of 6.11 [1.88–9.61]x106 m³ (median and 95% confidence interval). Additionally, the dyke exhibited a volume change of 5.52 [0.68–11.35]x106 m³, clearly indicating significant uncertainty in the dyke’s volume change. In the presentation, we will discuss the temporal evolution of the source, along with data analysis covering the most recent period.
Acknowledgments: The SoftBank's GNSS observation data used in this study was provided by SoftBank Corp. and ALES Corp. through the framework of the "Consortium to utilize the SoftBank original reference sites for Earth and Space Science". We used the RINEX data from the GEONET sites of the Geospatial Information Authority of Japan (GSI). We also utilized GNSS data from the Fundamental Volcano Observation Network (V-net) operated by the NIED, as well as GNSS data from GNSS sites operated by the JMA.