Abstract
The detailed rupture process of the 1975 central Oita earthquake is investi-gated by a non-linear waveform inversion technique using the near-field data of the strong-motion seismograms at JMA (Japan Meteorological Agency) stations. It is confirmed that the strike-slip mechanism is appropriate rather than the normal fault type, as was shown by Hatanaka and Shimazaki in 1988. The rupture process of this earthquake is characterized by two subevents: the first subevent at a deeper portion of the fault and the second subevent at shallower portion of the NW end of the fault. Total seismic moment of 3.8 x 1025 dyn cm is obtained, which is larger than that of 2.9 x 1025 dyn cm estimated by an analysis of far-field data. The seismic moment of 3.4 x 1025 dyn cm is obtained by averaging the above two values. The synthetic far-field seismograms based on the parameters obtained in this study are consistent with observed ones. The result suggests that the faulting at a deeper portion triggered or guided the shallower rupture.
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