[SSS04-P07] Source inversion using empirical Green's functions for the 2019 Off Yamagata earthquake (Mj 6.7)
Keywords:2019 Off Yamagata earthquake, Source process, Inversion analysis
During the 2019 Off Yamagata earthquake on June 18, JMA seismic intensity scale of 6 upper was recorded at maximum on Niigata Yamagata border region. Further even at the strong-motion station on the bedrock, seismic intensity of 5 lower was observed, which is located on the epicentral distance of 16 km. Aftershocks distribute on the area north of the 1964 Niigata earthquake (Mj 7.5) fault region, so that the relation of the slip area of both events should be focused. In this study the inversion method using empirical Green's functions is adopted for the source modeling of the 2019 Off Yamagata earthquake and estimated model is compared with the slips during the 1964 Niigata earthquake.
From the aftershock distribution and CMT solution of the main shock estimated by F-net, we assumed the reverse fault plane model with the strike in the N40E direction and the south-east dipping with 30 degrees. This fault plane model has the conjugate relation with that of the 1964 Niigata event adopted by Shiba and Uetake (2011). Observed records from the aftershock of Mj 4.0 occurring near the hypocenter of the main shock were used for the empirical Green's functions. Velocity motions of two horizontal components for KiK-net subsurface stations, K-NET stations and one bedrock station operated by CRIEPI (RK-net) are used for the source inversion in the frequency range up to 2 Hz. The spatial distribution of the seismic moment density, effective stress, rise time and rupture time on the fault plane are estimated simultaneously by using the simulated annealing, which is one of the heuristic search techniques.
The obtained source model indicates large slips mainly in the north east area apart from the hypocenter, which corresponds to the region just outside the fault plane of the 1964 Niigata earthquake. The distribution of the effective stress roughly coincides with the slip distribution. Peak slip and effective stress reach 1.0 m and 14.2 MPa respectively. On the other hand, the rise time distribution does now show obvious correlation to other source parameters, which might be due to the low resolution at the low frequency range. The rupture front propagates almost concentrically, but slightly accelerates on the high effective stress area. It suggests the increase of surface energy with high stress drop contributes the acceleration of rupture velocity on asperity or SMGA.
From the aftershock distribution and CMT solution of the main shock estimated by F-net, we assumed the reverse fault plane model with the strike in the N40E direction and the south-east dipping with 30 degrees. This fault plane model has the conjugate relation with that of the 1964 Niigata event adopted by Shiba and Uetake (2011). Observed records from the aftershock of Mj 4.0 occurring near the hypocenter of the main shock were used for the empirical Green's functions. Velocity motions of two horizontal components for KiK-net subsurface stations, K-NET stations and one bedrock station operated by CRIEPI (RK-net) are used for the source inversion in the frequency range up to 2 Hz. The spatial distribution of the seismic moment density, effective stress, rise time and rupture time on the fault plane are estimated simultaneously by using the simulated annealing, which is one of the heuristic search techniques.
The obtained source model indicates large slips mainly in the north east area apart from the hypocenter, which corresponds to the region just outside the fault plane of the 1964 Niigata earthquake. The distribution of the effective stress roughly coincides with the slip distribution. Peak slip and effective stress reach 1.0 m and 14.2 MPa respectively. On the other hand, the rise time distribution does now show obvious correlation to other source parameters, which might be due to the low resolution at the low frequency range. The rupture front propagates almost concentrically, but slightly accelerates on the high effective stress area. It suggests the increase of surface energy with high stress drop contributes the acceleration of rupture velocity on asperity or SMGA.