11:00 AM - 11:15 AM
[SVC31-08] Estimation of the deformation source of the long-term subsidence observed in and around Aso caldera
Keywords:Aso caldera, GPS, crustal deformation
In and around Aso caldera in Kyushu Japan, the long-term subsidence at a rate of several millimeters per year has been reported (e.g., Murakami and Ozawa, 2004). However, the mechanism of this phenomenon has not been quantitatively evaluated, with some qualitative conjectures presented such as some kind of volcanic subsidence (Murakami and Ozawa, 2004), or pumping of groundwater or tectonic movement (Tanaka, 2000). In addition, the deformation source of the subsidence phenomenon remains to be revealed. On the other hand, an intermittent uplift event was observed in 2003, of which Geospatial Information Authority of Japan (GSI) (2004) reported the deformation source to be a sill at a depth of 15.5 km just below the central cones of the caldera. Understanding the mechanism of such crustal deformation in terms of source location or deformation amount is very important, which leads to clarify the unrevealed magma plumbing system of Aso volcano.
In this study, we estimated the deformation source of long-term subsidence in Aso caldera based on GPS data (from GEONET stations by GSI and from stations by Kyoto University and National Research Institute for Earth Science and Disaster Resilience (NIED) in the approximate range of 32.5ºN~34ºN, 130ºE~132ºE) and levelling data around the central cones of Aso volcano or along Route 57 (surveyed by GSI, Kyoto University, etc.).
Annual and semi-annual fluctuation was subtracted from GPS coordinate data which was calculated using Gipsy Oasis v6.4. Since the regional tectonic influence is superimposed on the long-term horizontal GPS time series, we assumed that crustal deformation can be represented by the block fault model (Mochizuki and Mitsui, 2016) or quadratic functions in three-dimensional space. Then volcanic deformation was extracted for each tectonic model. Using Mogi's spherical deflation source or sill, or both (Mogi, 1958; Okada, 1992), we searched for the optimal source parameters by grid search applying modified Hotta et al. (2016) 's evaluation function.
As a result, the deformation source of the period from October 2004 to October 2008, when levelling survey was conducted and no slow slip events occurred in Bungo channel or Hyuga-nada, was estimated at a depth of 5~7 km b.s.l. beneath Kusasenri, regardless of tectonic model or source varieties. The location is close to the magma chamber estimated by Sudo and Kong (2001). Therefore, at least for this period, there is a high possibility that the subsidence of Aso caldera can be explained by the contraction of the magma chamber beneath Kusasenri.
Acknowledgements: In this study, we use GEONET data and levelling data by GSI, V-NET data by NIED, and levelling data by Kyoto University and others. We would like to thank for their cooperation.
In this study, we estimated the deformation source of long-term subsidence in Aso caldera based on GPS data (from GEONET stations by GSI and from stations by Kyoto University and National Research Institute for Earth Science and Disaster Resilience (NIED) in the approximate range of 32.5ºN~34ºN, 130ºE~132ºE) and levelling data around the central cones of Aso volcano or along Route 57 (surveyed by GSI, Kyoto University, etc.).
Annual and semi-annual fluctuation was subtracted from GPS coordinate data which was calculated using Gipsy Oasis v6.4. Since the regional tectonic influence is superimposed on the long-term horizontal GPS time series, we assumed that crustal deformation can be represented by the block fault model (Mochizuki and Mitsui, 2016) or quadratic functions in three-dimensional space. Then volcanic deformation was extracted for each tectonic model. Using Mogi's spherical deflation source or sill, or both (Mogi, 1958; Okada, 1992), we searched for the optimal source parameters by grid search applying modified Hotta et al. (2016) 's evaluation function.
As a result, the deformation source of the period from October 2004 to October 2008, when levelling survey was conducted and no slow slip events occurred in Bungo channel or Hyuga-nada, was estimated at a depth of 5~7 km b.s.l. beneath Kusasenri, regardless of tectonic model or source varieties. The location is close to the magma chamber estimated by Sudo and Kong (2001). Therefore, at least for this period, there is a high possibility that the subsidence of Aso caldera can be explained by the contraction of the magma chamber beneath Kusasenri.
Acknowledgements: In this study, we use GEONET data and levelling data by GSI, V-NET data by NIED, and levelling data by Kyoto University and others. We would like to thank for their cooperation.