Rupture process of the Mj6.5 April 14, foreshock of the 2016 Kumamoto earthquake derived from strong-motion data

Introduction

We derive the rupture process of the Mj 6.5 foreshock of the 2016 Kumamoto earthquake at 21:26 on April 14 (JST) using the near-source strong-motion data.

Data

Strong motion data recorded at 16 stations (5 K-NET stations, 10 KiK-net stations, and 1 F-net 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 10 Hz and windowed from 1 s before S-wave arrival for 10 s.

Fault model and discretization of the rupture process

We assume the 22 km x 14 km rectangular fault model that has a strike of 212 degrees and a dip of 89 degrees based on the F-net moment tensor solution. The rupture starting point is set at 32.7417N, 130.7994E, and a depth of 12.49 km, determined by the double-difference method.
The rupture process is spatially and temporally discretized following the multi-time-window linear waveform inversion scheme (Olson and Apsel, 1982; Hartzell and Heaton, 1983). For the spatial discretization, the fault plane is divided into 11 subfaults along the strike and 7 subfaults along dip directions, with a size of 2km x 2km 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 the Green's function.
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, 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 -164 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 the slip (Sekiguchi et al., 2000).

Results

Figure 2 shows the total slip distribution. Figure 3 shows the comparison between the observed and the synthetic waveforms. Figure 4 shows the rupture progression. Figure 5 shows the moment rate function of each subfault. Vftw, the maximum slip, and the seismic moment are 2.4 km/s, 0.7 m, and 1.8×1018Nm (Mw6.1), respectively. Two large slip areas are found in the region around the rupture staring point and the shallow region north-northeast of the rupture starting point.

Please note that this is the first analysis and will be modified after the further examination.

The latest report in Japanese was released on August 9th, 2016.
English page was created on May 13th, 2016.

The previous report in Japanese was released on April 15th, 2016.
English page was created on May 9th, 2016.

fig1

Figure 1: Station distribution and fault model on the map. A star denotes the rupture starting point. A broken rectangle denotes the fault model in the estimation of the rupture process of the Mj7.3 April 16, mainshock of the 2016 Kumamoto earthquake.

fig2

Figure 2: Total slip distribution on the fault. The vectors denote the direction and amount of the slip of the hanging wall side. A star denotes the rupture starting point.

fig3

Figure 3: Comparison between the observed and the synthetic waveforms. The maximum values are shown on the upper right of each waveform.

fig4

Figure 4: Rupture progression in terms of slip amount for each 1.0 s time window.

fig5

Figure 5: Moment rate function of each subfault. A star denotes the rupture starting subfault.