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[SVC31-12] Detection of the lava dome deformation at Unzen volcano by ALOS and ALOS-2 interferometry to constrain a physical model
Keywords:Unzen, Lava dome, Viscoelasticity, InSAR, GNSS, Finite element method
Mt. Unzen is an active volcano located in the central part of Shimabara Peninsula, Japan. A lava dome was formed in May 1991 following an eruption that began in November 1990 and continued growing until February 1995. The lava dome consists of a domal high at the summit area and lave lobes on the eastern slope. The collapse of the lave dome has been concerned for years. Izutani and Takada (2022) detected the lava dome deformation since 2015 by InSAR time series analysis using ALOS-2 data, and proposed a finite element model to explain the deformation by viscous flow and thermal contraction of the lava dome. In this study, to quantitatively understand the effect of thermal evolution of the lava dome on its deformation, we performed InSAR time series analysis using SAR data acquired by ALOS (2006-2011) and ALOS-2 (2015-2022), by which we can grasp the deformation of the entire lava dome spanning a longer period than previous studies. The results were then integrated with the GNSS data at the summit area (1999-2022) to constrain a finite element model of the lava dome deformation in both space and time (Izutani and Takada, 2022).
We conducted a SBAS-based InSAR time series analysis (Berardino et al., 2002). The results showed that the mean velocity field in the LOS (satellite line of sight) direction showed motion away from the satellite for the SAR data taken from both ascending and descending orbits in both 2006-2011 and 2015-2022. The LOS velocities were 9.8 cm/yr (2006~2011) and 8.5 cm/yr (2015~2022) for the ascending orbit and 3.8 cm/yr (2006~2011) and 3.4 cm/yr (2015~2022) for the descending orbit. Thus, the velocities are higher in 2006~2011 than in 2015~2022. Next, we performed a 2.5D analysis using the mean velocity field in the LOS direction to obtain the quasi-upward and quasi-easterly velocities. The results quantitatively revealed that the lava dome has been moving eastward and subsiding over a long period of time. The subsidence rate at the summit area was 8.5 cm/yr from 2006 to 2011 and 7.5 cm/yr from 2015 to 2022, slowing with time, and these velocities were also consistent with the GNSS data there.
As described above, the results from the InSAR time series analysis, as well as GNSS data, indicate an eastward and downward (subsidence) motion of the lava dome, at the rates decreasing with time, which provides the following constraints on the finite element model proposed by Izutani and Takada (2022). (1) Viscous flow of the lava dome: Since the high-temperature lava dome was formed on a former mountain body steeply tilted to the east, it mainly flows eastward due to gravity (Matsushima and Takagi, 2000). The effective viscosity should be lower at higher temperatures, which is consistent with the fact that the velocity is higher in the ALOS age than in the ALOS-2 age. (2) Thermal elasticity: Thermal contraction occurs with decreasing temperature inside the lava dome. The cooling rate should decrease with time since the formation of the lava dome, which is consistent with the fact that the subsidence rate at the summit area decreases with time.
References:
Izutani, T., Takada, Y., Current deformation rates of the Unzen lava dome and its physical mechanism: Application of InSAR time series analysis, JPGU2022, SVC34-P07.
Matsushima, T., Takagi, A., GPS and EDM monitoring of Unzen volcano ground deformation, Earth Planet and Space, 52, 1015-1018, 2000.
Acknowledgments: This work was supported by a grant-in-aid (2021-B-03) from the Earthquake Research Institute, the University of Tokyo. The PALSAR-2 data used in this study are shared by PIXEL (PALSAR Interferometry Consortium to Study our Evolving Land surface). The PALSAR-2 data are the property of JAXA. Unzen Restoration Project Office provided us with the GNSS data.
We conducted a SBAS-based InSAR time series analysis (Berardino et al., 2002). The results showed that the mean velocity field in the LOS (satellite line of sight) direction showed motion away from the satellite for the SAR data taken from both ascending and descending orbits in both 2006-2011 and 2015-2022. The LOS velocities were 9.8 cm/yr (2006~2011) and 8.5 cm/yr (2015~2022) for the ascending orbit and 3.8 cm/yr (2006~2011) and 3.4 cm/yr (2015~2022) for the descending orbit. Thus, the velocities are higher in 2006~2011 than in 2015~2022. Next, we performed a 2.5D analysis using the mean velocity field in the LOS direction to obtain the quasi-upward and quasi-easterly velocities. The results quantitatively revealed that the lava dome has been moving eastward and subsiding over a long period of time. The subsidence rate at the summit area was 8.5 cm/yr from 2006 to 2011 and 7.5 cm/yr from 2015 to 2022, slowing with time, and these velocities were also consistent with the GNSS data there.
As described above, the results from the InSAR time series analysis, as well as GNSS data, indicate an eastward and downward (subsidence) motion of the lava dome, at the rates decreasing with time, which provides the following constraints on the finite element model proposed by Izutani and Takada (2022). (1) Viscous flow of the lava dome: Since the high-temperature lava dome was formed on a former mountain body steeply tilted to the east, it mainly flows eastward due to gravity (Matsushima and Takagi, 2000). The effective viscosity should be lower at higher temperatures, which is consistent with the fact that the velocity is higher in the ALOS age than in the ALOS-2 age. (2) Thermal elasticity: Thermal contraction occurs with decreasing temperature inside the lava dome. The cooling rate should decrease with time since the formation of the lava dome, which is consistent with the fact that the subsidence rate at the summit area decreases with time.
References:
Izutani, T., Takada, Y., Current deformation rates of the Unzen lava dome and its physical mechanism: Application of InSAR time series analysis, JPGU2022, SVC34-P07.
Matsushima, T., Takagi, A., GPS and EDM monitoring of Unzen volcano ground deformation, Earth Planet and Space, 52, 1015-1018, 2000.
Acknowledgments: This work was supported by a grant-in-aid (2021-B-03) from the Earthquake Research Institute, the University of Tokyo. The PALSAR-2 data used in this study are shared by PIXEL (PALSAR Interferometry Consortium to Study our Evolving Land surface). The PALSAR-2 data are the property of JAXA. Unzen Restoration Project Office provided us with the GNSS data.