Origin of subsidence within the Aso caldera after the 2016 Kumamoto earthquake: contributions of afterslip, viscoelastic deformation and volcanic deformation

SUMMARY We investigated the origin of subsidence within the Aso caldera after the 2016 Kumamoto earthquake using global navigation satellite system (GNSS) data. Tectonic deformation was effectively removed via an approximation in terms of a low-order spatial polynomial. Viscoelastic deformation caused by the 2016 Kumamoto earthquake was calculated with the refined coseismic fault model and removed from the deformation; 3-D quasi-static deformation equations for the proposed viscoelastic structures were solved to achieve this. Subsequently, we jointly estimated the afterslips and volume changes of the volcanic deformation source beneath the Aso caldera, and found that the main subsidence contributor was the existence of afterslips on two faults within the caldera. The volume changes of the volcanic deformation source were in equilibrium during the analysis period (2015–2021); this trend did not seem to change before or after the 2016 Kumamoto earthquake. The GNSS baseline data suggested that the volcanic deformation source has been in equilibrium since approximately 2012.