11:00 AM - 11:15 AM
[SGD01-02] Evaluate the viscoelastic response due to the shallow pressure source at the Ontake Volcano using InSAR data
Keywords:The Ontake Volcano, InSAR, GNSS, Viscoelastic relaxation
The Ontake Volcano is the second-highest active volcano in Japan. In 1979, an eruption occurred for the first time in recorded human history, followed by phreatic eruptions in 1991, 2007, and 2014. We should observe this volcano constantly because plumes continue to be emitted from the vents of the eruption on September 27, 2014. In order to clarify pressure sources inside the Ontake Volcano, many previous studies have been conducted based on the Mogi model assuming an elastic half-space medium (e.g., Murase et al. 2016; Takagi and Onizawa 2016). On the other hand, Segall (2016) reported that the analytical surface deformation solution form due to a spherical pressure source with a viscoelastic shell in an elastic half-space body. Generally, when the source pressure increases, a surface uplift is expected. However, he also reported that, after the eruption, temporary surface subsidence occurred even though the source pressure increased. In this study, I evaluated how much effect the viscoelastic shell has on the surface displacement by adapting Segall’s model to the shallow source (at a depth of 500 m below the vents) estimated by Narita and Murakami (2018).
I obtained the crustal deformation time-series at the surface, analyzing SAR images of the Ontake Volcano observed by ALOS-2/PALSAR-2 between August 2014 and November 2021. For InSAR analysis, I used RINC (Ozawa et al. 2016) and its GUI tool, rinc_gui (Okuyama 2018). We set Takane (950281) GNSS observation site as deformation reference. Then, we used the F5 solution to correct the crustal deformation components. In this way, at 13 points around the summit, time-series of the Line-of-Site distance (dLOS) changes were obtained (Figure). Then, I searched for the model parameters such that the model dLOS time-series created from Segall's model matches with the observed dLOS time-series. At this time, fixing parameters other than those related to the viscoelastic relaxation (viscosity and thickness of the viscoelastic shell), I attempted to estimate the model parameters by using a forward calculation.
As a result of the InSAR time-series analysis, around the pressure source, I found the tendency of the mountain contraction starting from the 2014 eruption finally became stable after seven years from the eruption. This result was consistent with the result of the time-series analysis (from 2016 to 2021) regarding the campaign GNSS observation sites installed around the summit. Moreover, as a result of the assumption of Segall’s model parameters, we succeeded to explain the surface subsidence observed by InSAR analysis as the viscoelastic relaxation with the viscoelastic shell surrounds a spherical pressure source. As a result, viscosity and thickness are 3×1016 Pa・s and 66 m, respectively. At the shallow part, the result of this study suggested that the mountain still continues to contract slightly. However, at the deep part, such a contraction have already been stable since the beginning of 2021, according to the deformation time-series of the GNSS baseline (Ochiaikartani-Tanohara baseline: about 8.5 km long) owned by JMA. This suggested that the gas emission from the deep part became to stop now.
As for future work, I am planning to understand comprehensively the inside of the Ontake Volcano, assuming the model parameters by using inversion calculation toward the shallow pressure source, and by using deformation time-series of the GNSS observation sites toward the deep one.
PALSAR-2 SLC data are shared among PIXEL (PALSAR Interferometry Consortium to Study our Evolving Land surface), and provided from JAXA under a cooperative research contract with PIXEL. The ownership of PALSAR-2 data belongs to JAXA. This study was supported by ERI JURP 2021-B-03 in Earthquake Research Institute, the University of Tokyo.
I obtained the crustal deformation time-series at the surface, analyzing SAR images of the Ontake Volcano observed by ALOS-2/PALSAR-2 between August 2014 and November 2021. For InSAR analysis, I used RINC (Ozawa et al. 2016) and its GUI tool, rinc_gui (Okuyama 2018). We set Takane (950281) GNSS observation site as deformation reference. Then, we used the F5 solution to correct the crustal deformation components. In this way, at 13 points around the summit, time-series of the Line-of-Site distance (dLOS) changes were obtained (Figure). Then, I searched for the model parameters such that the model dLOS time-series created from Segall's model matches with the observed dLOS time-series. At this time, fixing parameters other than those related to the viscoelastic relaxation (viscosity and thickness of the viscoelastic shell), I attempted to estimate the model parameters by using a forward calculation.
As a result of the InSAR time-series analysis, around the pressure source, I found the tendency of the mountain contraction starting from the 2014 eruption finally became stable after seven years from the eruption. This result was consistent with the result of the time-series analysis (from 2016 to 2021) regarding the campaign GNSS observation sites installed around the summit. Moreover, as a result of the assumption of Segall’s model parameters, we succeeded to explain the surface subsidence observed by InSAR analysis as the viscoelastic relaxation with the viscoelastic shell surrounds a spherical pressure source. As a result, viscosity and thickness are 3×1016 Pa・s and 66 m, respectively. At the shallow part, the result of this study suggested that the mountain still continues to contract slightly. However, at the deep part, such a contraction have already been stable since the beginning of 2021, according to the deformation time-series of the GNSS baseline (Ochiaikartani-Tanohara baseline: about 8.5 km long) owned by JMA. This suggested that the gas emission from the deep part became to stop now.
As for future work, I am planning to understand comprehensively the inside of the Ontake Volcano, assuming the model parameters by using inversion calculation toward the shallow pressure source, and by using deformation time-series of the GNSS observation sites toward the deep one.
PALSAR-2 SLC data are shared among PIXEL (PALSAR Interferometry Consortium to Study our Evolving Land surface), and provided from JAXA under a cooperative research contract with PIXEL. The ownership of PALSAR-2 data belongs to JAXA. This study was supported by ERI JURP 2021-B-03 in Earthquake Research Institute, the University of Tokyo.