5:15 PM - 6:30 PM
[SSS11-P24] Strong Motion Generation Area for the 2021 off Fukushima, Japan, Earthquake Estimated based on the Empirical Green’s Function Method
Keywords:2021 off Fukushima earthquake, Strong motion generation area, Empirical Green’s function method
At 23:07 on 13 February 2021 (JST: UTC+9), an Mw 7.1 intra-slab earthquake with reverse faulting occurred off-shore Fukushima prefecture, northeastern Japan. The seismic intensity of 6+ and 6- in JMA scale were widely observed in the Fukushima and Miyagi prefectures. Within the nation-wide strong motion networks (K-NET and KiK-net) operated by NIED, strong ground motions with a peak acceleration 1426 cm/s2 were recorded at MYGH10 (surface record of KiK-net Yamamoto). For understanding the mechanisms of strong motion generation processes during this event, we estimate the source model composed of strong motion generation area (SMGA) to explain the observed strong motion records in broadband frequency range between 0.3 and 10 Hz.
In this study, we use the empirical Green’s function method to simulate the strong motion records at downhole sensors of 18 KiK-net stations located at Miyagi, Fukushima, and Ibaraki prefectures. The observed ground motion records of MJMA 4.7 event (element event) occurring at 23:36 on 13 February 2021, are used as the empirical Green’s functions. For an objective estimation of corner frequencies for the target and element events, we apply the source spectral ratio fitting method (Miyake et al., 1999). From the obtained corner frequencies, scaling parameters N and C, which required for the empirical Green’s function method of Irikura (1986), are determined. Then, the parameters of each SMGA (e.g., the size, relative rupture starting point in SMGA, rise time, and rupture velocity) are estimated by trial and error method. Here we assume a squared SMGA on the fault plane with strike angle of N45°E and dip angle of 45°, according to the distribution of aftershocks occurring within 24 h after the mainshock.
As a provisional result, we construct the source model with SMGA located including the hypocenter. The rupture within SMGA mainly propagates southwest direction from the hypocenter. The size of SMGA is 108.2 km2. The stress drop of SMGA is estimated to be 111.8 MPa. The short-period level of acceleration source spectrum calculated from the size and stress drop of SMGA is 1.2×1020 Nm/s2, which is comparable to that expected from the empirical scaling relationship for intra-slab earthquakes occurring in the Pacific plate (Arai et al., 2015). Overall, the simulated ground motion generated from the SMGA explain well the observed acceleration, velocity, and displacement waveforms, however, there is still room to improve the reproducibility of observed ground motions. The fault geometry and parameters of SMGA need to be examined objectively in more detail.
Acknowledgements: We use the strong motion data of K-NET and KiK-net operated by NIED.
In this study, we use the empirical Green’s function method to simulate the strong motion records at downhole sensors of 18 KiK-net stations located at Miyagi, Fukushima, and Ibaraki prefectures. The observed ground motion records of MJMA 4.7 event (element event) occurring at 23:36 on 13 February 2021, are used as the empirical Green’s functions. For an objective estimation of corner frequencies for the target and element events, we apply the source spectral ratio fitting method (Miyake et al., 1999). From the obtained corner frequencies, scaling parameters N and C, which required for the empirical Green’s function method of Irikura (1986), are determined. Then, the parameters of each SMGA (e.g., the size, relative rupture starting point in SMGA, rise time, and rupture velocity) are estimated by trial and error method. Here we assume a squared SMGA on the fault plane with strike angle of N45°E and dip angle of 45°, according to the distribution of aftershocks occurring within 24 h after the mainshock.
As a provisional result, we construct the source model with SMGA located including the hypocenter. The rupture within SMGA mainly propagates southwest direction from the hypocenter. The size of SMGA is 108.2 km2. The stress drop of SMGA is estimated to be 111.8 MPa. The short-period level of acceleration source spectrum calculated from the size and stress drop of SMGA is 1.2×1020 Nm/s2, which is comparable to that expected from the empirical scaling relationship for intra-slab earthquakes occurring in the Pacific plate (Arai et al., 2015). Overall, the simulated ground motion generated from the SMGA explain well the observed acceleration, velocity, and displacement waveforms, however, there is still room to improve the reproducibility of observed ground motions. The fault geometry and parameters of SMGA need to be examined objectively in more detail.
Acknowledgements: We use the strong motion data of K-NET and KiK-net operated by NIED.