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[SVC31-11] Is the subsidence in the Aso caldera following the 2016 Kumamoto earthquake of volcanic origin? -Evaluation of viscoelastic deformation, afterslip, and volcanic deformation-.
Keywords:crustal deformation, 2016 Kumamoto earthquake, Mt. Aso
Is the subsidence in the Aso caldera following the 2016 Kumamoto earthquake of volcanic origin?
-Evaluation of viscoelastic deformation, afterslip, and volcanic deformation-.
Hiroshi Munekane and Tomokadu Kobayashi (Geospatial Information Authority of Japan)
1. Introduction
Mt. Aso is an active volcano, with eruption as recent as in October 2021. To predict its long-term activity, it is effective to continuously monitor volume changes of the magma chamber using crustal deformation. However, after the 2016 Kumamoto earthquake, post-seismic deformation by viscoelastic flow and afterslip make it difficult to assess crustal deformation of volcanic origin.
After the 2016 Kumamoto earthquake, significant subsidence continued within the Aso caldera. Does this subsidence imply contraction of the magma chamber? Or is the subsidence mainly caused by post-seismic deformation, with no significant change in the magma chamber? To clarify this question, this study will quantitatively evaluate the post-seismic deformation of the 2016 Kumamoto earthquake and the volume change of the magma chamber simultaneously and identify the cause of subsidence.
2. Methodology
We used the crustal deformation observed at GNSS stations (GSI, NIED, JMA) for input data. First, we corrected for tectonic deformation by removing linear trend estimated for the period between January 2012 to March 2016. For the stations around Mt. Aso, a fifth order function was used to interpolate the linear trends at surrounding stations. Then viscoelastic deformation was corrected for using the two-dimensional model of Pollitz (2017). Visco2.5D (Pollitz et al., 2014) was used to calculate viscoelastic deformation. Then, we used the corrected crustal deformation to infer afterslip and the volume changes of magma chamber simultaneously. Regarding afterslip, we selected five faults from the coseismic faults of 2016 Kumamoto earthquake (Kobayashi et al., 2023) with large slip (including two in the Aso caldera), discretized them by small sub-faults, and estimated the afterslip on each subfault. We set the magma chamber under the northern part of Kusasenrigahama, about 4 km deep, following a previous study (e.g. Nobile et al., 2017), and estimated the volumetric change together.
3. Results.
Figure 1 shows the distribution of afterslip on two fault planes in the Aso caldera. In both cases, normal fault slip is estimated around 4 km to 8 km in depth. Figure 2 shows the crustal deformation time series at Furubochu station in the Aso caldera. One can see that the subsidence after the earthquake is comparable to that from the afterslip. Figure 3 shows the volumetric change of magma chamber, which indicates that there is no significant difference in the trend of magma chamber volume change before and after the 2016 Kumamoto earthquake, and the volume of magma chamber increases in response to the increase in volcanic activity in 2015-2016, 2019, and 2021.
4. Conclusion
To clarify the cause of the subsidence observed in the Aso caldera after the 2016 Kumamoto earthquake, we performed simultaneous estimation of afterslip and magma chamber volume change after correcting for earthquake-induced viscoelastic deformation. As a result, we found that the subsidence was mainly caused by afterslip on the fault planes in the Aso caldera, and that there was no significant difference in the volume change of magma chamber before and after the earthquakes.
Reference:
Nobile, A. et al (2017) Bull. Volcanol., 79, 32, https://doi.org/doi:10.1007/s00445-017-1112-1
Kobayashi et al. (2023), under review
Pollitz (2014), F., Post-earthquake relaxation using a spectral element method: 2.5-D case, Geophys. J. Int., doi: 10.1093/gji/ggu114 .
Pollitz, F. et al. (2017) Geophys. Res. Lett., 44. 8795--8803, https://doi.org/10.1002/2017GL074783
-Evaluation of viscoelastic deformation, afterslip, and volcanic deformation-.
Hiroshi Munekane and Tomokadu Kobayashi (Geospatial Information Authority of Japan)
1. Introduction
Mt. Aso is an active volcano, with eruption as recent as in October 2021. To predict its long-term activity, it is effective to continuously monitor volume changes of the magma chamber using crustal deformation. However, after the 2016 Kumamoto earthquake, post-seismic deformation by viscoelastic flow and afterslip make it difficult to assess crustal deformation of volcanic origin.
After the 2016 Kumamoto earthquake, significant subsidence continued within the Aso caldera. Does this subsidence imply contraction of the magma chamber? Or is the subsidence mainly caused by post-seismic deformation, with no significant change in the magma chamber? To clarify this question, this study will quantitatively evaluate the post-seismic deformation of the 2016 Kumamoto earthquake and the volume change of the magma chamber simultaneously and identify the cause of subsidence.
2. Methodology
We used the crustal deformation observed at GNSS stations (GSI, NIED, JMA) for input data. First, we corrected for tectonic deformation by removing linear trend estimated for the period between January 2012 to March 2016. For the stations around Mt. Aso, a fifth order function was used to interpolate the linear trends at surrounding stations. Then viscoelastic deformation was corrected for using the two-dimensional model of Pollitz (2017). Visco2.5D (Pollitz et al., 2014) was used to calculate viscoelastic deformation. Then, we used the corrected crustal deformation to infer afterslip and the volume changes of magma chamber simultaneously. Regarding afterslip, we selected five faults from the coseismic faults of 2016 Kumamoto earthquake (Kobayashi et al., 2023) with large slip (including two in the Aso caldera), discretized them by small sub-faults, and estimated the afterslip on each subfault. We set the magma chamber under the northern part of Kusasenrigahama, about 4 km deep, following a previous study (e.g. Nobile et al., 2017), and estimated the volumetric change together.
3. Results.
Figure 1 shows the distribution of afterslip on two fault planes in the Aso caldera. In both cases, normal fault slip is estimated around 4 km to 8 km in depth. Figure 2 shows the crustal deformation time series at Furubochu station in the Aso caldera. One can see that the subsidence after the earthquake is comparable to that from the afterslip. Figure 3 shows the volumetric change of magma chamber, which indicates that there is no significant difference in the trend of magma chamber volume change before and after the 2016 Kumamoto earthquake, and the volume of magma chamber increases in response to the increase in volcanic activity in 2015-2016, 2019, and 2021.
4. Conclusion
To clarify the cause of the subsidence observed in the Aso caldera after the 2016 Kumamoto earthquake, we performed simultaneous estimation of afterslip and magma chamber volume change after correcting for earthquake-induced viscoelastic deformation. As a result, we found that the subsidence was mainly caused by afterslip on the fault planes in the Aso caldera, and that there was no significant difference in the volume change of magma chamber before and after the earthquakes.
Reference:
Nobile, A. et al (2017) Bull. Volcanol., 79, 32, https://doi.org/doi:10.1007/s00445-017-1112-1
Kobayashi et al. (2023), under review
Pollitz (2014), F., Post-earthquake relaxation using a spectral element method: 2.5-D case, Geophys. J. Int., doi: 10.1093/gji/ggu114 .
Pollitz, F. et al. (2017) Geophys. Res. Lett., 44. 8795--8803, https://doi.org/10.1002/2017GL074783