[SSS09-P05] GNSS observation around the shear zone in southern Kyushu (Follow-up report)
Keywords:shear zone in southern Kyushu, GNSS dense observation, arctangent, visco-elastic relaxation, 2016 Kumamoto earthquakes, Sakurajima
At first, common mode bias included in the coordinate time series during the observation period (March 20, 2016 - September 30, 2017) was removed. The variation of daily coordinate was reduced to 4.7-7.3 mm from 7.3-9.6 mm for the horizontal component and to 1.2-4.2 mm from 2.1-4.9 mm for the vertical component. After removing common mode biases, annual and semi-annual variations were estimated. Then, we extracted site velocities. While, the visco-elastic deformation of the earthquakes extends to the wide area of Kyushu district (Sutio, 2017), we estimated the deformation using a calculation cord of Fukahata and Matsu’ura (2006). We applied the faults model of the 2016 Kumamoto earthquakes (GSI, 2016) and calculated the temporal behavior of visco-elastic deformation. In this analysis, the post-seismic deformation was calculated in an elastic surface layer with thickness of 25 km overlying a Maxwell visco-elastic half-space with viscosity of 2x1018 Pa.s, according to Suito (2017). Moreover, because southern GNSS sites of our observation network are located in the western area of Sakurajima, we calculated volcanic deformation due to the volume change of two magma sources under the Aira Caldera using Mogi model.
Savage and Burford (1973) proposed a model for fault movement of a vertical strike-slip fault in a homogenous elastic half-space. This explained that the fault is locked from the surface to a certain depth and is sliding at a constant velocity, which is relative block motion below the locking depth. In this case, surface velocity perpendicular to the fault strike is expressed as a function of arctangent across the fault. Comparing with the velocity profile normal to the shear zone before the 2016 Kumamoto earthquakes, the profile after the earthquakes is well agreed with one before the earthquakes by applying the above mentioned model. This result shows that a steady state deformation at the shear zone continues after the earthquakes. Angela and Sagiya (2016) investigated strain rate in central-northeastern Japan based on GPS data before and after the 2011 Tohoku earthquake and observed inelastic deformation associated with E-W shortening, which is independent of large earthquakes continues throughout the whole time periods. Our result also suggests that a similar phenomenon as shown by them occurs beneath a deep region of the active shear zone.
This study was carried out under contract with the Agency of Natural Resources and Energy (ANRE), part of the Ministry of Economy, Trade and Industry (METI) of Japan as part of its R&D program supporting development of technology for geological disposal of high-level radioactive waste.