Introduction
We derive the rupture process of the 2016 central Tottori earthquake (Mj 6.6) at 14:07 on October 21 (JST) using the near-source strong-motion data.
Data
Strong motion data recorded at 15 stations (7 K-NET stations, 5 KiK-net borehole stations, and 3 KiK-net surface station) shown in Figure 1 are used in the inversion analysis. The velocity waveforms (converted by integration of the original K-NET and KiK-net accelerations) are band-pass filtered between 0.1 and 1.0 Hz, resampled to 5 Hz and windowed from 1 s before S-wave arrival for 10 s.
Fault model and discretization of the rupture process
We assume the 16 km x 16 km rectangular fault model that has a strike of 162 degrees and a dip of 88 degrees based on the F-net moment tensor solution.
The rupture starting point is set at 35.3806N, 133.8545E, and a depth of 11.58 km, which is the hypocenter determined by Hi-net.
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).
For the spatial discretization, the fault plane is divided into 8 subfaults along the strike and 8 subfaults along dip directions, with a size of 2 km x 2 km each.
For the temporal discretization, the moment rate function of each subfault is represented by 5 smoothed-ramp functions (time windows) progressively delayed by 0.4 s and having a duration of 0.8 s each.
The first time window starting time is defined as the time prescribed by a circular rupture propagation with the constant speed of Vftw.
Thus, the rupture process and the strong-motion waveforms are linearly related via Green's functions.
The Green's functions between each subfault and each station are calculated using the discrete wavenumber method (Bouchon, 1981) and the reflection/transmission matrix method (Kennett and Kerry, 1979) assuming a 1-D layered velocity structure model.
The underground structure model is obtained for each station from the 3-D structure model (Fujiwara et al., 2009).
Logging data is also referred to for the KiK-net station.
To consider the rupture propagation effect inside each subfault, 25 point-sources are uniformly distributed over each subfault in the calculation of Green's functions.
Waveform inversion
Moment of each time window at each subfault is derived by minimizing the difference between the observed and the synthetic waveforms using the least-squares method. To stabilize the inversion, the slip angle is allowed to vary within ±45 centered at -11 degrees, which is the rake angle of the F-net moment tensor solution, using the non-negative least-squares scheme (Lawson and Hanson, 1974). In addition, we impose the spatiotemporal smoothing constraint on slips (Sekiguchi et al., 2000). The weight of the smoothing constraint is determined based on ABIC (Akaike, 1980). Vftw is selected to minimize data-fit residual.
Results
Figure 2 shows the total slip distribution on the fault.
Figure 3 shows the rupture progression.
Figure 4 shows the source time function of each subfault.
Figure 5 shows the comparison between the observed and the synthetic waveforms.
Vftw, the maximum slip, and the seismic moment are 3.3 km/s, 0.6 m, and 2.1×1018 Nm (Mw 6.1), respectively.
A large-slip region, with a maximum slip of 0.6 m, extends from the rupture starting point to the shallower part.
Another large-slip region, with a maximum slip of 0.5 m, is located to the NNW of the rupture starting point.
After the rupture initiation, the rupture mainly propagated from the rupture starting point in the upward direction, and continued for 3 s.
Then, the rupture propagated in the NNW direction at 3-5 s. The total rupture duration is approximately 5 s.
*For details of this analysis, we refer the reader to Kubo et al. (2019).
