Abstract

The source geometry and slip distribution at rupture termination of the 1995 Hyogo-ken Nanbu earthquake were investigated using waveform inversion on the assumption of fault branching in the northeastern part of the rupture model. Possible branching of the Okamoto fault is suggested both by the static-displacement distribution and damage extension east of Kobe (Nishinomiya area). To exclude data contaminated by the basin-edge-diffracted wave in the waveform-inversion process, we examined the spatio-temporal variation of its influence and from a comparison with a flat model, we determined windows appropriate for the data. Three subevents were identified, as was reported in previous works. The largest was in the shallow part on the Awaji side, whereas the smaller two occurred at great depths (>7km) on the Kobe side. We found a smaller subevent in the deep part of the branch. Total variance reduction was larger, and the ABIC value was smaller when we assumed the branch than when we did not, which shows the superiority of the branching fault model. Resolution checks showed that the slips on proposed branched portions are physical and not caused by random-data noise or systematic errors in Green's functions that arise from misestimation of velocity structures. Calculation of the relative static displacement between two leveling observation stations near the branching fault also showed the greater utility of the branching fault model. The effect of slip on the branched fault has been shown in near-source ground-motion simulation using the 3D finite-difference method (Iwata et al., 1999). The characteristic distribution of ground motion in the near-source region indicated by the damage distribution is well reproduced by the modeling of both the source process and wave propagation in the realistic 3D velocity structure. Slip on the branched fault affected the ground motion in eastern Kobe (Nada and Higashi-Nada wards), Ashiya, and Nishinomiya cities, but its contribution is not dominant even in those regions; about 30 to 50% maximum velocity in the frequency range of 0.1 to 1.0 Hz.

We conclude that branching rupture of the Okamoto fault during the Hyogo-ken Nanbu earthquake is the preferable interpretation on the source process of this earthquake. This supports the idea of a geometrical barrier suggested by geological observations.

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