| Strong ground motions are the main cause of damage and collapse of buildings. The research of near-field ground motions involves series of fundamental scientific problems in seismology including the knowledge and understanding of the source, seismic wave propagation in the medium, the impact of local site conditions. The impact of source is emphasis in these three aspects. Current consensus is that the stochastic synthesis method is the main approach for the high frequency ground motions simulation. Based on hybrid source model of finite fault, this paper researches one improvement for source spectral model in stochastic synthesis.This paper first introduces the improvement of the Masuda source spectra, and the corner frequency of sub-source is determined by its seismic moment assigned as the distribution of slip. This model can characterize the influence of heterogeneous distribution of slip to frequency spectrum characteristics, especially the barrier's dominant role in high frequency seismic wave. Meanwhile, it can express the variation of frequency component of seismic wave during rupture process. As rupture continues, the number of cracked sub-sources increases and two parameters in source spectra model is also changed. This improvement is equal to the effect of modified corner frequency of each sub-source proposed by Canadian scholars, which can eliminate the seismic amplitude's dependence on the dimensions of sub-sources. However, there is the little relationship between corner frequency and the slip of sub-sources in Canadian scholars'evolutionary model, and the latter cracked sub-sources always have lower corner frequency than former sub-sources so as to the last cracked sub-source achieves the lowest corner frequency on the whole fault plane. Our improvement can avoid these unreasonable situations above.This paper further makes researches on influence of our improvement to far-field radiation energy and indicates that the variation of the shape of far-field radiation energy curve under different sub-source sizes is also reasonable. That is to say, as the dimensions of sub-sources increase, the high frequency components part in radiation energy decreases while the low frequency components part increases. Thus there is no need to pursue the exact agreement of shape of radiation energy curve under different dimensions of sub-sources. The key is idea, this paper adds a frequency-independent scaling factor in source spectra formula in order to compensate the energy loss due to variations of the two parameters in rupture process. Based on the equation of radiation energy between unmodified source spectra model and modified one, the formula of the scaling factor can be deduced.1994 Northridge earthquake in the United States is taken as an example to test our improvement. This paper adopt pre-existing source parameters and regional parameters and three different dimensions of sub-sources, 1km×1km,2km×2km and 4km×4km, to compute ground motions at 24 stations on rock sites. This can be used to testify synthesis result's dependence on sub-sources, and accordance between synthetic ground motion and seismic recordings. We also calculate the far-field radiate energy curve, and testify whether or not radiation energy varies as sub-sources change.By comparison, this improved source spectra model can ameliorate synthetic ground motions'dependence on dimensions of sub-sources. After adding the scaling factor, the variation of far-field radiate energy under different dimensions of sub-sources decreases. Meanwhile, adding the scaling factor makes no difference to synthetic ground motions'dependence on dimensions of sub-sources. In conclusions, there is no harm but improvement for the accordance between synthetic ground motions and seismic recordings after adding scaling factor. |
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