11:45 AM - 12:00 PM
[SSS11-17] Source process of the 2021 off Fukushima, Japan, intraslab earthquake derived from strong motion data
Keywords:The 2021 off Fukushima earthquake, Source process, Strong motion, Intraslab earthquake
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.