Analysis of the rupture process of the 1995 Kobe earthquake using a 3D velocity structure
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB. 2013
Received: 11 March 2013
Accepted: 19 July 2013
Published: 6 December 2013
A notable feature of the 1995 Kobe (Hyogo-ken Nanbu) earthquake is that violent ground motions occurred in a narrow zone. Previous studies have shown that the origin of such motions can be explained by the 3D velocity structure in this zone. This indicates not only that the 3D velocity structure significantly affects strong ground motions, but also that we should consider its effects in order to determine accurately the rupture process of the earthquake. Therefore, we have performed a joint source inversion of strong-motion, geodetic, and teleseismic data, where 3D Green’s functions were calculated for strong-motion and geodetic data in the Osaka basin. Our source model estimates the total seismic moment to be about 2.1 × 1019 N m and the maximum slip reaches 2.9 m near the hypocenter. Although the locations of large slips are similar to those reported by Yoshida et al. (1996), there are quantitative differences between our results and their results due to the differences between the 3D and 1D Green’s functions. We have also confirmed that our source model realized a better fit to the strong motion observations, and a similar fit as Yoshida et al. (1996) to the observed static displacements.
The Kobe (Hyogo-ken Nanbu) earthquake, with a JMA magnitude of 7.3, occurred at 5:46 on January 17, 1995 (JST), and claimed more than 6,000 lives. The damaging ground motions with a JMA intensity of 7 were generated in a narrow zone called the “damage belt” of the Kobe area. Kawase (1996) concluded that the “damage belt” resulted from the constructive interference of direct S-waves and basin-edge-induced diffracted/Rayleigh waves. Furumura and Koketsu (1998) showed that this zone arises from strong amplification and ray bending in the sedimentary basin below Kobe city in conjunction with the multipathing effects at a basin/bedrock boundary. These previous studies imply that the ground-motion amplification in the Kobe area was significantly affected by a 3D velocity structure such as a basin edge. Thus, we should inevitably consider its effects in precisely determining the rupture process of this earthquake.
In some previous studies (e.g., Hashimoto et al., 1996; Ide et al., 1996; Kikuchi and Kanamori, 1996; Sekiguchi et al., 2000) separate inversions of static displacement, teleseismic body waves, or strong motions were performed to analyze the rupture process of this earthquake. In these studies, half-space velocity structure models were used for calculating Green’s functions of static displacements, and 1D stratified velocity structure models were used for those of teleseismic body waves or strong motions. In addition to these separate inversions, joint inversions have been conducted using strong motions and static displacements by Horikawa et al. (1996), and using strong motions, teleseis-mic body waves, and static displacements by Wald (1996) and Yoshida et al. (1996). 1D stratified and half-space velocity structure models were also used in these studies.
The use of a 3D velocity structure model and the calculation of 3D Green’s functions are very useful methods for separating the effects of the source and site when the time duration of strong motions is longer than the duration of the rupture process. In addition, the use of 3D strong-motion Green’s functions can result in an improvement of the fitting between the observed and synthetic data, especially in later phases. The combined use of geodetic and strong-motion data can also provide a stabilizing constraint on the slip distribution (Wald and Graves, 2001). For these reasons, in this study, we refine a 3D velocity structure model and calculate 3D Green’s functions for the strong-motion and geodetic data in the Osaka basin in order to incorporate the effects of a 3D velocity structure. As in previous studies, we also use 1D stratified velocity structure models for teleseismic and strong-motion data outside the Osaka basin. A half-space velocity structure model is used for geodetic data outside the Osaka basin. We then performed a joint inversion of strong motions, static displacements, and teleseismic body waves.
According to Graves and Wald (2001), the use of well-calibrated 3D Green’s functions provides very good resolution of the slip distribution of a source model, and adequately separates source and 3D propagation effects. In contrast, using a set of inexact 3D Green’s functions allows only partial recovery of the slip distribution. This indicates that a 3D velocity structure model should be carefully validated before performing a source inversion. Therefore, we also perform this validity test and refine the 3D velocity structure through waveform modeling of ground motion data from aftershocks.
2. Refinement of the 3D Velocity Structure Model
Parameters of each layer in the 3D velocity structure model. The sediment-bedrock interface z varies station by station. r1 and r2 represent the common proportionality constants of Kagawa et al. (2004).
Depth of top surface (km)
Sedimentary layer 1
Sedimentary layer 2
Sedimentary layer 3
The sediment-bedrock interface z in Table 1 varies from station to station, and the depths of the top surfaces for sedimentary layers 2 and 3 are r1z and r2z, respectively, where r1 and r2 are common proportionality constants. In our refinement procedure, r1, r2, and z j (j = 1, 2,…), where j represents the number of stations, are treated as unknowns. We calibrated these unknowns using ground motion data from aftershocks.
Source parameters of the three aftershocks used in the refinement of the velocity structure model. These parameters were determined by Katao et al. (1997).
In modeling ground motions, we calculated synthetic velocity waveforms using the finite element method (FEM) with voxel meshes (Koketsu et al., 2004; Ikegami et al., 2008) with intervals of 40 m. The source time function was a smoothed ramp function with a rise time of 0.5 s. Next, we checked whether the synthetic waveforms agreed with the observed ground motions in the frequency range 0.3–1.0 Hz. If not, we calibrated the depth z beneath a station and the common proportionality constants r1 and r2 by trial and error, then three-dimensionally combined them with the depths surrounding the station using the interpolating method of Smith and Wessel (1990). Thereafter, we again performed waveform modeling. We repeated this successive procedure until the synthetic waveforms fit the observed waveforms well with regard to their specific phases and travel times and the sum of squares of the residual of their waveforms shows the smallest value. In our refined model, the common proportionality constants r1 and r2, which were 0.19 and 0.47 (Kagawa et al., 2004) in the initial model, were revised to 0.08 and 0.39, respectively.
3. Source Inversion
We introduced a positivity constraint to confine the slip angles within 180° ± 45°.
The fault model described above is the same as that of Yoshida et al. (1996). However, they used only 1D stratified layer velocity structure models to calculate Green’s functions for strong motions and teleseismic body waves, and only a half-space velocity structure model for static displacements. In our source inversion, we used a 3D velocity structure model refined as mentioned in the previous section for the strong-motion and geodetic data for the Osaka basin. We used the same Green’s functions as Yoshida et al. (1996) for the data outside the Osaka basin. The datasets shown in Fig. 2 and their time duration, weights, filtering and sampling were also the same as those in Yoshida et al. (1996). We note that the sensor orientation of the borehole seismometer installed at the strong-motion station KPI has been corrected.
For calculating 3D Green’s functions, we again used the FEM with voxel meshes. We then derived geodetic Green’s functions by averaging calculated ground motions from about 50 s to 70 s.
4. Conclusions and Discussion
In this section, we summarize and discuss our results mainly by comparing them with those of Yoshida et al. (1996).
Figure 4(B) shows two rupture propagations in our source model. One propagated toward the shallow part along the Nojima fault, and the other propagated from the deeper part beneath the Akashi Strait to the zone beneath the city center of Kobe. This figure also shows a large slip around the hypocenter in the early stage, and this consequently contributes to the large final slip in this area.
We thank CEORKA, GSI, JMA, JUNCO, the Kobe city government, Prof. Iwata (Kyoto Univ.), and Prof. Nagano (Tokyo Science Univ.) for providing data. We used GMT for drawing the figures. We also thank two reviewers Atsushi Nozu and Haruko Sekiguchi, and the Editor Tatsuhiko Hara for helpful comments.
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