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[SVC31-P04] Ground deformation from 2009 to 2020 at Mt. Usu analyzed with data of GNSS network
Keywords:Mt. Usu, GNSS observation
Mt. Usu is an active volcano located in the southwestern part of Hokkaido, Japan. It has erupted nine times since the beginning of the 20th century, and 22 years have passed since the last eruption in 2000. No significant volcanic activity has been observed after the 2000 eruption, and thermal contraction of lava domes has been observed from GNSS observations and InSAR analysis. However, considering the recent eruption cycle, there is a high possibility of an eruption soon, and it is essential to understand the current status of the volcano.
In this study, we analyzed the displacement and coordinate time series for the 12 years from 2009 to 2020, using a network of nine GNSS observation points around Mt. Usu maintained by the Institute of Seismology and Volcanology Faculty of Science, Hokkaido University and four electronic reference points maintained by the Geospatial Information Authority of Japan. The displacement and coordinate time series were analyzed with RTKLIB 2.4.3 (Takasu et al., 2007) by using the relative positioning with Otaki as the reference point. The coordinate time series shows artificial step-like displacement due to the replacement of antennas at the observation points. We applied a correction to the time series to remove such step-like displacements unrelated to ground deformation. In order to extract the volcanic ground deformation at Mt. Usu volcano, we also applied a correction to the coordinate time series to remove the step-like displacement caused by the 2011 Tohoku Earthquake. Other corrections were made as necessary, focusing on the characteristics of the data specific to each observation point, such as removing data for periods when the coordinates could not be determined and considering annual modulations.
A linear approximation was made to the corrected coordinate time series, and the total displacement over 12 years at each observation point was calculated from the slope of the approximate line. The displacements at each station include volcanic ground deformation, the difference in crustal deformation from the reference point Otaki, and local ground deformation. Both vertical and horizontal displacements are caused by the contraction of lava domes generated in past eruptions at points near the volcano. The horizontal displacement towards the southeast direction is estimated due to the regional deformation at the points far from the mountain.
We compared the ground deformation calculated using the Mogi model with the displacement observed from GNSS observation to examine whether ground deformation near the volcano can be explained by the thermal contraction of the lava dome. We assumed the location and depth of the spherical pressure source estimated by Wang and Aoki (2018), and the volume change estimated based on the information in Wang and Aoki (2018). In the vertical direction, the calculated and observed data showed that the most significant subsidence occurred at SWRM and SSB, in that order, but the amount of subsidence did not correspond to each other. In the horizontal direction, the displacement at SWRM, where the observed displacement was the largest, was well explained by the displacement calculated from the Mogi model, but that of SSB, SBT, and KIB did not match with the calculation. The possibility of isostatic landslides was suggested for the SSB, SBT, and KIB.
More precise correction of the coordinate time series, including the correction of the regional deformation, will be necessary to capture the inflation indicating pressure changes in the deep magma reservoir.
In this study, we analyzed the displacement and coordinate time series for the 12 years from 2009 to 2020, using a network of nine GNSS observation points around Mt. Usu maintained by the Institute of Seismology and Volcanology Faculty of Science, Hokkaido University and four electronic reference points maintained by the Geospatial Information Authority of Japan. The displacement and coordinate time series were analyzed with RTKLIB 2.4.3 (Takasu et al., 2007) by using the relative positioning with Otaki as the reference point. The coordinate time series shows artificial step-like displacement due to the replacement of antennas at the observation points. We applied a correction to the time series to remove such step-like displacements unrelated to ground deformation. In order to extract the volcanic ground deformation at Mt. Usu volcano, we also applied a correction to the coordinate time series to remove the step-like displacement caused by the 2011 Tohoku Earthquake. Other corrections were made as necessary, focusing on the characteristics of the data specific to each observation point, such as removing data for periods when the coordinates could not be determined and considering annual modulations.
A linear approximation was made to the corrected coordinate time series, and the total displacement over 12 years at each observation point was calculated from the slope of the approximate line. The displacements at each station include volcanic ground deformation, the difference in crustal deformation from the reference point Otaki, and local ground deformation. Both vertical and horizontal displacements are caused by the contraction of lava domes generated in past eruptions at points near the volcano. The horizontal displacement towards the southeast direction is estimated due to the regional deformation at the points far from the mountain.
We compared the ground deformation calculated using the Mogi model with the displacement observed from GNSS observation to examine whether ground deformation near the volcano can be explained by the thermal contraction of the lava dome. We assumed the location and depth of the spherical pressure source estimated by Wang and Aoki (2018), and the volume change estimated based on the information in Wang and Aoki (2018). In the vertical direction, the calculated and observed data showed that the most significant subsidence occurred at SWRM and SSB, in that order, but the amount of subsidence did not correspond to each other. In the horizontal direction, the displacement at SWRM, where the observed displacement was the largest, was well explained by the displacement calculated from the Mogi model, but that of SSB, SBT, and KIB did not match with the calculation. The possibility of isostatic landslides was suggested for the SSB, SBT, and KIB.
More precise correction of the coordinate time series, including the correction of the regional deformation, will be necessary to capture the inflation indicating pressure changes in the deep magma reservoir.