It has been indicated that the inhomogeniety of faulting strongly influences the excitation of short-period seismic waves, while the excitation of long-period seismic waves can be described by the average displacement or the average stress drop over the fault plane. A semi-empirical method has been developed to synthesize seismic waves of large earthquakes in both the long and the short period ranges. According to this method, the displacement on the fault plane of large earthquake is divided into two parts. One is the average displacement over the fault plane and the other is the deviation from the average value. An element earthquake should be selected, of which hypocenter is in and around the focal region of the large earthquake. The size of fault area of the element earthquake should be consistent with the fault patch size of the large earthquake. By using the observed seismograms of the element earthquakes, a seismic wave S_L(t) due to the average displacement and a seismic wave S_S(t) due to the deviation from the average value are synthesized separately, and the synthesized seismic wave can be expressed as a sum of S_L(t) and S_S(t).
In this study herein, this method is applied to the strong ground motions of three Japanese earthquakes with magnitude 7.4 to 7.9, in the period range from 0.1 to 7sec. After comparing with observed and synthesized records, the applicability of this method is made clear for the estimation of the strong ground motions. The results of comparison with the spectral amplitudes S_S(t) and S_L(t) in the present study and in other studies indicate that S_S(t) is larger than S_L(t) in the short period range, while S_L(t) is larger than S_S(t) in the long period range. The boundary of the period ranges is almost consistent with the rise time of the average displacement over the fault plane of large earthquake.