5:15 PM - 6:45 PM
[MTT37-P04] Detection of Volcanic Crustal Deformation by Dense GNSS Observations in Northern Kyushu
Keywords:GNSS, Volcanic crustal deformation, Aso Volcano
For the evaluation of the potential for volcanic eruption, it is important to accurately estimate magma accumulation in magma chambers that exist beneath volcanic edifices. For this purpose, it is necessary to detect crustal deformation associated with the increase or decrease of magma accumulation volume.
Unzen, Aso, Kuju, Yufu, and Tsurumi-Garan volcanoes are located in northern Kyushu, and numerous eruptions have occurred since the beginning of time, especially at the Aso volcano. These volcanoes are thought to exist within the so-called Beppu-Shimabara Graben This graben is characterized by east-west normal faults, a north-south extensional field, and negative Bouguer gravity anomalies, and is one of the most active volcanic regions in the Japanese Islands (Machida, et al., 2001). Furthermore, the subduction zone of the Philippine Sea Plate, including the Bungo Channel, is located offshore to the east of this area, and interplate coupling and periodic slow-slip events are superimposed on the crustal deformation in the graben. In addition, afterslip and viscoelastic relaxation caused by the 2016 Kumamoto earthquake have been observed. Therefore, the crustal deformation in this region is expected to be very complex, and a dense observation network is needed to accurately extract crustal deformation caused by volcanic activity.
In this region, GEONET and V-net permanent GNSS observation networks exist, and Kyushu University, Kagoshima University, and Kyoto University have also established their own GNSS stations. We have started a feasibility study to extract volcanic crustal deformations and evaluate their correspondence to volcanic activities using data from a dense network of GNSS stations in the region, which has recently been expanded with the addition of SOFTBANK's reference stations.
RINEX files obtained between January 1, 2021, and December 31, 2022, at GNSS stations of GEONET, V-net, various universities, and Softbank in Kumamoto-Oita prefectures were preliminarily analyzed using the PPP with Ambiguity Resolution in GIPSY OASYS 6.4 to obtain daily coordinate.
Although some of the SoftBank original reference points were not conducive to crustal deformation detection due to the large variation in the obtained coordinate values, the majority of the reference points had a coordinate reproduction rate that was comparable to that of nearby GEONET and V-net.
Within the period of data analysis, a phreatic eruption occurred at Aso volcano in October 2021. In addition, seismic activity increased on July 8, 2022, at Garandake in Oita Prefecture, and the volcanic alert level was raised to 2 from 1. This was the first time this level had been raised since the volcanic alert level began operating in July 2016.
There are three SOFTBANK original reference points in the Aso Caldera, all of which have obtained highly reproducible coordinate values. Using these coordinate values, we calculated the change in distance between the baselines across the central cones and found that the baselines elongated from pre-eruption to post-eruption, indicating the expansion of the mountain body. Although these elongations have been observed between existing observation points, we expect that the accuracy of estimating the location of the magma reservoir and the amount of change will be improved in the future due to the increased density of observation points.
On the other hand, no baseline length change corresponding to volcanic activity was obtained in the vicinity of Garandake, and since the seismic activity that became active in July 2022 soon subsided and the volcanic alert level was lowered to 1 on July 27, the degree of volcanic activity in the deep crust of the volcano is assumed to have been very small. The development of SOFTBANK's original reference points is considered to have contributed to the improvement of the accuracy of the assessment of volcanic activity since many baselines did not show any change.
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". The present study was financially supported by the Ministry of Education, Culture, Sports, and Technology of Japan under its Earthquake and Volcano Hazards Observation and Research Program.
Unzen, Aso, Kuju, Yufu, and Tsurumi-Garan volcanoes are located in northern Kyushu, and numerous eruptions have occurred since the beginning of time, especially at the Aso volcano. These volcanoes are thought to exist within the so-called Beppu-Shimabara Graben This graben is characterized by east-west normal faults, a north-south extensional field, and negative Bouguer gravity anomalies, and is one of the most active volcanic regions in the Japanese Islands (Machida, et al., 2001). Furthermore, the subduction zone of the Philippine Sea Plate, including the Bungo Channel, is located offshore to the east of this area, and interplate coupling and periodic slow-slip events are superimposed on the crustal deformation in the graben. In addition, afterslip and viscoelastic relaxation caused by the 2016 Kumamoto earthquake have been observed. Therefore, the crustal deformation in this region is expected to be very complex, and a dense observation network is needed to accurately extract crustal deformation caused by volcanic activity.
In this region, GEONET and V-net permanent GNSS observation networks exist, and Kyushu University, Kagoshima University, and Kyoto University have also established their own GNSS stations. We have started a feasibility study to extract volcanic crustal deformations and evaluate their correspondence to volcanic activities using data from a dense network of GNSS stations in the region, which has recently been expanded with the addition of SOFTBANK's reference stations.
RINEX files obtained between January 1, 2021, and December 31, 2022, at GNSS stations of GEONET, V-net, various universities, and Softbank in Kumamoto-Oita prefectures were preliminarily analyzed using the PPP with Ambiguity Resolution in GIPSY OASYS 6.4 to obtain daily coordinate.
Although some of the SoftBank original reference points were not conducive to crustal deformation detection due to the large variation in the obtained coordinate values, the majority of the reference points had a coordinate reproduction rate that was comparable to that of nearby GEONET and V-net.
Within the period of data analysis, a phreatic eruption occurred at Aso volcano in October 2021. In addition, seismic activity increased on July 8, 2022, at Garandake in Oita Prefecture, and the volcanic alert level was raised to 2 from 1. This was the first time this level had been raised since the volcanic alert level began operating in July 2016.
There are three SOFTBANK original reference points in the Aso Caldera, all of which have obtained highly reproducible coordinate values. Using these coordinate values, we calculated the change in distance between the baselines across the central cones and found that the baselines elongated from pre-eruption to post-eruption, indicating the expansion of the mountain body. Although these elongations have been observed between existing observation points, we expect that the accuracy of estimating the location of the magma reservoir and the amount of change will be improved in the future due to the increased density of observation points.
On the other hand, no baseline length change corresponding to volcanic activity was obtained in the vicinity of Garandake, and since the seismic activity that became active in July 2022 soon subsided and the volcanic alert level was lowered to 1 on July 27, the degree of volcanic activity in the deep crust of the volcano is assumed to have been very small. The development of SOFTBANK's original reference points is considered to have contributed to the improvement of the accuracy of the assessment of volcanic activity since many baselines did not show any change.
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". The present study was financially supported by the Ministry of Education, Culture, Sports, and Technology of Japan under its Earthquake and Volcano Hazards Observation and Research Program.