We show an example of application of the waveform synthesis by the empirical Green's function technique based on the scaling law of the source spectra, in order to study source process. First, the relation of the ratios of source parameters (such as crack radius and local stressdrop), between small and large earthquakes, with the spectral ratio of the observed ground motion, are derived from the idea of multicrack model, in which subevents are composed of circular cracks obeying the ω^-2-scaling law. Then, the accelerograms of the second (M=6.1) and the third (M=6.0) largest aftershocks of the 1983 Japan Sea earthquake are synthesized, from a smaller event (M=5.0, 3.9) occurring in the same source region, in order to obtain the optimum value of the parameters used in the synthesis. In the case of the second largest event, the synthetic waveform and its spectra obtained by the formula for the single crack event, are successfully fit to the observed ones. In contrast, it's necessary, for the synthesis of the third largest event, to employ the formula for the multi-crack event. It's suggested by the source parameters, such as stress drop and crack radius, obtained from the observed spectral ratio. The used values of the parameters in synthesis make us guess such a complicated source process as relatively large crack with low-stress drop associated by small crack with high-stress drop.