The procedure for source modeling according to a characterized source model has been proposed and established in prevenient researches on the estimation of strong ground motions from scenario earthquakes along active faults. In recent researches, such procedure is also applied to the strong ground motion prediction for some great subduction earthquakes. In these cases, it seems that short-period motions are underestimated while long-period ground motions explain the observed records well. We examine the effects of complexity of fault rupture process on strong ground motions in period range form 5 to 10 s by handling source parameters of the 2003 Tokachi-Oki earthquake. For that purpose, we referred to the source model obtained by inversion analysis of near-field strong motion records in the antecedent research. First, we have made several simplified models from original source model. In the models, we gradually simplified the source parameters, such as slip amount, rupture time, rake angle, and the form of slip velocity time function of fault elements. Contribution of each source parameter to the waveform complexity has been evaluated by comparing spectral levels of synthetic waveforms from different source models in three frequency ranges and by comparing synthetic waveforms themselves. As a result, the complexity of rupture propagation is most important to correctly simulate ground motions in the period of several seconds for M 8 earthquake. We have also examined relation between the rupture propagation complexity and the slip distribution. The result shows that acceleration of rupture propagation is seen in the areas where either slip or spatial gradient of slip is large.
