11:00 〜 13:00
[SSS07-P22] Modeling the post-seismic deformation around the hypocentral area of the 2016 Kumamoto earthquake sequence, in central Kyushu, Japan
キーワード：非弾性歪み、余効変動、アフタースリップ
When a large earthquake occurs, the stress is released on the earthquake fault. In the crust, co-seismic stress change could load the stress to the surrounding area. The surrounding media of the large earthquake fault is expected to be loaded or relaxed the stress due to the co-seismic slip. After a large earthquake, slow deformation due to aseismic slip and viscoelastic deformation in the crust are often observed, providing with substantial constraint for investigating stress loading system of earthquake generation as well. For understanding tectonic evolution of the crust and the process of the earthquake occurrence in detail, we analyzed the GNSS data and aftershock focal mechanism data around Futagawa and Hinagu fault zones, the hypocentral area the 2016 Kumamoto earthquake sequence.
Aftershock activity can be considered as an inelastic response to stress loading by large co-seismic slip of a large earthquake. Inelastic strain rate by aftershock activity follows a power law (dε/dt∝t-P) [Matsumoto et al., 2020]. We defined the power -P in this relation as the P-value and investigated the spatial distribution of the P-value of the target region using focal mechanism data. The P-value around co-seismic faults [e.g., Asano and Iwata, 2016; Mitsuoka et al., 2020] is high (P~1.33), however, we found P<1 area at the southwestern extensions of the co-seismic faults. The lower P-value indicates the slow decay rate of the strain rate and the condition that the inelastic strain does not converge over time.
In addition to information of inelastic strain fields in the crust, we estimated the displacement field on the ground surface using GNSS data from GEONET. The displacements at the ground surface were estimated about several to 10 cm with the right-lateral slip directions during about 4.5 years after the mainshock. The steady movement at each GNSS station was estimated from 2006 – 2008 location time-series at the station.
The estimated inelastic strain field in the crust and displacement field on the ground surface imply possibility of activity by the other deformation sources other than co-seismic faults. We assumed the afterslip on the fault plane after the mainshock and estimated the slip distribution that can explain the displacements at GNSS stations by Bayesian inversion procedure. For explaining the deformation in the crust, we constrained the principal stress directions of the stress change due to afterslip to the directions of the inelastic strain at P<1 region. The large slip region was estimated at the southwestern edge of the co-seismic faults, where was affected the large co-seismic stress change significantly. In addition, the resistivity structure in the region suggests that the low resistivity facilitates the aseismic slip due to their low viscosity. The estimated after-slip could promote the earthquake occurrence effectively.
Aftershock activity can be considered as an inelastic response to stress loading by large co-seismic slip of a large earthquake. Inelastic strain rate by aftershock activity follows a power law (dε/dt∝t-P) [Matsumoto et al., 2020]. We defined the power -P in this relation as the P-value and investigated the spatial distribution of the P-value of the target region using focal mechanism data. The P-value around co-seismic faults [e.g., Asano and Iwata, 2016; Mitsuoka et al., 2020] is high (P~1.33), however, we found P<1 area at the southwestern extensions of the co-seismic faults. The lower P-value indicates the slow decay rate of the strain rate and the condition that the inelastic strain does not converge over time.
In addition to information of inelastic strain fields in the crust, we estimated the displacement field on the ground surface using GNSS data from GEONET. The displacements at the ground surface were estimated about several to 10 cm with the right-lateral slip directions during about 4.5 years after the mainshock. The steady movement at each GNSS station was estimated from 2006 – 2008 location time-series at the station.
The estimated inelastic strain field in the crust and displacement field on the ground surface imply possibility of activity by the other deformation sources other than co-seismic faults. We assumed the afterslip on the fault plane after the mainshock and estimated the slip distribution that can explain the displacements at GNSS stations by Bayesian inversion procedure. For explaining the deformation in the crust, we constrained the principal stress directions of the stress change due to afterslip to the directions of the inelastic strain at P<1 region. The large slip region was estimated at the southwestern edge of the co-seismic faults, where was affected the large co-seismic stress change significantly. In addition, the resistivity structure in the region suggests that the low resistivity facilitates the aseismic slip due to their low viscosity. The estimated after-slip could promote the earthquake occurrence effectively.