11:45 〜 12:00
[SSS11-17] 強震波形記録を用いて推定された2021年福島県沖の地震の震源過程
キーワード：2021年福島県沖の地震、震源過程、強震動、スラブ内地震
The 2021 off Fukushima earthquake (Mjma 7.3) struck Tohoku and Kanto regions in Japan on February 13, 2021 (JST). This earthquake caused strong ground motions over a wide area with a maximum seismic intensity of 6+ on the JMA scale and a maximum PGA of approximately 1400 cm/s2. The moment tensor solution of F-net and the spatial distribution of the mainshock and its aftershocks indicate that this event was an intraslab reverse-fault-type earthquake. In this study, we estimate the source process of this earthquake using strong motion waveforms.
We use velocity waveforms at 12 stations of K-NET and KiK-net of NIED. The waveforms are band-pass filtered between 0.05 and 0.5 Hz, resampled to 5 Hz, and windowed from 2 s before S-wave arrival for 30 s.
We assume a 32 km × 24 km rectangular fault model that has a strike of 24° and a dip of 34° based on the F-net moment tensor solution. The rupture starting point is set at 37.729°N, 141.698°E, and a depth of 55.4 km, which is the hypocenter determined by JMA.
The rupture process is spatially and temporally discretized following the multiple-time-window linear waveform inversion scheme (Olson and Apsel 1982; Hartzell and Heaton 1983; Sekiguchi et al. 2000). For the spatial discretization, the fault plane is divided into 8 subfaults along the strike direction and 6 subfaults along the dip direction, with a size of 4 km × 4 km each. For temporal discretization, the moment rate function of each subfault is represented by 4 smoothed-ramp functions (time windows) progressively delayed by 0.8 s and having a duration of 1.6 s each. The triggering velocity of first time window is set to 2.4 km/s to minimize the data-fit residual. Two orthogonal slips of each time window at each subfault are derived by minimizing the difference between the observed and synthetic waveforms using the non-negative least-squares scheme (Lawson and Hanson 1974). The slip angle is allowed to vary centered at 108°, which is the rake angle of the F-net analysis. We impose a spatiotemporal smoothing constraint on slips (Sekiguchi et al. 2000).
Green's functions are calculated using the discrete wavenumber method (Bouchon 1981) and the reflection/transmission matrix method (Kennett and Kerry 1979) assuming 1-D velocity structure models. The structure models are obtained for each station from the 3-D structure model (Fujiwara et al. 2009). Logging information is also used for the KiK-net station. To consider the rupture propagation effect, 25 point-sources are uniformly distributed over each subfault in the calculation of Green’s functions.
In the estimated source model, large slips are found south-west of the hypocenter and partly overlap the active-aftershock area. In the first 5 s, the rupture slowly grew around the hypocenter with small slips. Then, the rupture developed with the large moment release between 5 s and 10 s in the large slip area.
We use velocity waveforms at 12 stations of K-NET and KiK-net of NIED. The waveforms are band-pass filtered between 0.05 and 0.5 Hz, resampled to 5 Hz, and windowed from 2 s before S-wave arrival for 30 s.
We assume a 32 km × 24 km rectangular fault model that has a strike of 24° and a dip of 34° based on the F-net moment tensor solution. The rupture starting point is set at 37.729°N, 141.698°E, and a depth of 55.4 km, which is the hypocenter determined by JMA.
The rupture process is spatially and temporally discretized following the multiple-time-window linear waveform inversion scheme (Olson and Apsel 1982; Hartzell and Heaton 1983; Sekiguchi et al. 2000). For the spatial discretization, the fault plane is divided into 8 subfaults along the strike direction and 6 subfaults along the dip direction, with a size of 4 km × 4 km each. For temporal discretization, the moment rate function of each subfault is represented by 4 smoothed-ramp functions (time windows) progressively delayed by 0.8 s and having a duration of 1.6 s each. The triggering velocity of first time window is set to 2.4 km/s to minimize the data-fit residual. Two orthogonal slips of each time window at each subfault are derived by minimizing the difference between the observed and synthetic waveforms using the non-negative least-squares scheme (Lawson and Hanson 1974). The slip angle is allowed to vary centered at 108°, which is the rake angle of the F-net analysis. We impose a spatiotemporal smoothing constraint on slips (Sekiguchi et al. 2000).
Green's functions are calculated using the discrete wavenumber method (Bouchon 1981) and the reflection/transmission matrix method (Kennett and Kerry 1979) assuming 1-D velocity structure models. The structure models are obtained for each station from the 3-D structure model (Fujiwara et al. 2009). Logging information is also used for the KiK-net station. To consider the rupture propagation effect, 25 point-sources are uniformly distributed over each subfault in the calculation of Green’s functions.
In the estimated source model, large slips are found south-west of the hypocenter and partly overlap the active-aftershock area. In the first 5 s, the rupture slowly grew around the hypocenter with small slips. Then, the rupture developed with the large moment release between 5 s and 10 s in the large slip area.