Viscosity distribution beneath the Kyushu Island estimated by GNSS postseismic time series of the 2016 Kumamoto earthquake (M7.3)
不破, 智志大園, 真子
55 , 2018-03-19 , 北海道大学大学院理学研究院
In order to investigate the viscosity distribution beneath the Kyushu Island, we analyzed postseismic time series of Global Navigation Satellite System induced by the 2016 Kumamoto earthquake（M7.3） . For the ～1.5 years of daily coordinates at 135 GEONET stations, we fitted theoretical time evolutions with two approaches.（1） Assuming that the first 50-days is dominated by the afterslip, we applied both of logarithmic and exponential functions. For the estimation, we firstly found the appropriate common time decay constants for the afterslip（0.996 days） , then estimated other parameters.（2） Ignoring first rapid afterslip signals, we fitted only exponential function for the time series after 50-days. As a result, both approaches indicate 1017-1020 Pa s of viscosity in the Kyushu Island. Especially, low viscosity of ～1018 Pa s was obtained around the central region of Kyushu. This corresponds to the location of low viscosity in the lower crust. Therefore, the estimated viscosity distribution is probably affected by the beneath of the viscoelastic structure through the lower crust to the surface.