Rupture process of the Mj7.3, April 16, event of the 2016 Kumamoto earthquake derived from strong-motion data (Updated version with rectangular plane fault model)

Introduction

We derive the rupture process of the Mj 7.3 event of the 2016 Kumamoto earthquake at 1:25 on April 16 (JST) using the near-source strong-motion data. In this analysis, we use the rectangular plane fault model. Except the fault model, we use the same inversion settings as for the analysis with the curved fault model.

Data

Strong motion data recorded at 27 stations (13 K-NET stations, 9 KiK-net borehole stations, 2 KiK-net surface station, and 3 F-net stations) 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 0.05 and 1.0 Hz, resampled to 5 Hz, and windowed from 1 s before S-wave arrival for 30 s.

Fault model and discretization of the rupture process

We assume the 56 km x 24 km rectangular fault model that has a strike of 224 degree based on the F-net moment tensor solution. The dip angle of the fault model is set to 65 degree based on the hypocenter distribution of aftershocks and the surface-rupture distribution, and the static coseismic displacements inferred by InSAR and GNSS. The rupture starting point is set at 32.7557N, 130.7616E, and a depth of 13.58 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 28 subfaults along the strike and 12 subfaults along dip directions, with a size of approximately 2km x 2km each. For the temporal discretization, the moment rate function of each subfault is represented by 13 smoothed-ramp functions (time windows) progressively delayed by 0.8 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 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 -142 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). The weight of the smoothing constraint is determined based on ABIC (Akaike, 1980). Vftw is set to 2.6 km/s, which is same value as for the analysis with the curved fault model.

Results

Figure 2, Figure 3, and Figure 4 show the total slip distribution by map projection, perspective illustration, and projection on the fault, respectively. Figure 5 shows the comparison between the observed and the synthetic waveforms. Figure 6 shows the rupture progression. Figure 7 shows the moment rate function of each subfault. The maximum slip and the seismic moment are 4.4 m and 5.6×1019Nm (Mw7.1), respectively. Large slips (> 2.4 m) are found 10-30 km northeast of the rupture starting point and are distributed from the depth of approximately 15 km to the top of the fault model. The northeastern edge of this large slip region reaches the northwestern part of the caldera of Mt. Aso. These large slips were caused by the main rupture at 4-16 s after rupture initiation, which propagated toward the northeast shallow area. It is also found that another rupture propagated toward the surface from the rupture starting point at 2-6 s, and then propagated toward the northeast along the near surface at 6-10 s. The extent of the large near-surface slips in this source model is roughly consistent with the extent of the observed large surface ruptures.

*For details of this analysis, we refer the reader to Kubo et al. (2016)

The Japanese report using rectangular plane fault model was released on November 15th, 2016.
English page was created on November 15th, 2016.

The Japanese report using curved fault model was released on August 9th, 2016.
English page was created on August 9th, 2016.

The previous report in Japanese was released on May 12th, 2016.
English page was created on May 13th, 2016.

fig1

Figure 1: Station distribution and rectangular plane fault model on the map. A star denotes the rupture starting point.

fig2

Figure 2: Map projection of slip distribution. A star denotes the rupture starting point. Gray circles denote the hypocenters of aftershocks occurring during 1 month after the mainshock, which are determined by the NIED Hi-net. Black lines denote the surface traces of active faults.

fig3

Figure 3: Perspective illustration of total slip distribution (azimuth: 310 degree, elevation: 20 degree). A star denotes the rupture starting point. Black lines denote the surface traces of active faults.

fig4

Figure 4: 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.

fig5

Figure 5: Comparison between the observed and the synthetic waveforms. The maximum values are shown on the upper right of each waveform.

fig6

Figure 6: Rupture progression in terms of slip amount for each 2.0 s time window.

fig7

Figure 7: Slip-velocity time function of each subfault. A star denotes the rupture starting subfault.