The dynamic rupture process of the 1993 Kushiro-oki earthquake

We have constructed a dynamic rupture model of the 1993 Kushiro-oki earthquake (M(w)=7.6) that is consistent with the observed Seismograms. This earthquake occurred beneath the city of Kushiro, Japan, at a depth of about 100 km, and the aftershocks are limited to a horizontal plane between a double seismic zone in the subducting slab. Excellent near-field strong-motion records are available. In our analysis, a kinematic inversion is followed by a dynamic crack calculation, and this pair of operations is repeated until convergence to the data is obtained. Two different conditions concerning the dynamic stress drop were considered in the dynamic calculation, and a dynamic model was constructed for each condition. The difference in these conditions has little effect on the results. The final dynamic model shows that this earthquake occurred with a localized high stress drop of about 40 MPa in the subducting slab, which indicates that the region between the double seismic zone is strong enough to sustain such a high stress. One of the two high strength regions in the final model lies near the upper plane of the double seismic zone and represents a barrier due to a substantial change of stress state in the slab. In the region where the initial breaks occurred 5 s before the main rupture, both the strength and the stress drop are smaller than in the adjacent region. This implies that before the main rupture the accumulated stress had already been released in the area of the initial breaks. The aftershocks tend to occur near the high strength area; in some of them, the P wave first-motions are opposite to those of the main shock.