| The ground motion attenuation relationship is a major issue in engineering earthquake research, which draws attention to specialists domestic and overseas. The strong motion observed data accumulated in our country is not rich enough compared with the empirical attenuation re lationship. The seismology based ground motion attenuation relationship, the ground motion of the surface of earth Fourier spectra formula caused by point source based on seismology theory and the regional parameters are provided for the inversion of small event records of the regional earthquake observation network. It is a promising and attractive way for research.1303 strong motion records with moment magnitude not lower than 4.5 in Sichuan and Yunnan are collected for further research of the seismology based ground motion attenuation relationship. The number of strong motion data of near field and strong earthquake is small through the pictures and graphs. The number of PGA with small value is big while PGA with big value is opposite. On the basis of this situation, it is not suitable to count empirical attenuation relationship. The comparison with empirical attenuation relationship is better. Totally 147 velocity time histories at digital observation stations of Sichuan in 82 small quakes with moment magnitude 3.5 to 4.5, and 863 recorded at stations of Yunnan in 154 small earthquakes which spread evenly with moment magnitude 3.6 to 4.3 and distance 50 km to 30 km.The measure this dissertation used in inversion is mGA, whose essence is optimizing searching. The ground motion of the surface of earth Fourier spectra formula in Seismology is used in forward modeling. 5 regional parameters of source spectrum and the crust medium, stress drop Δσ, quality factor parameters Q0 and η, geometric attenuation parameters R1 and R2 are not easily obtained and need inverse. Here comes the contradiction. Fourier spectra estimated in formula are smooth curves while recorded curves obtained through FFT are fluctuated and tend to be of considerable oscillation. The smooth ob ject curves extracted from Fourier spectra calculated through observed data who can reflect the whole trend of spectra curves and be close to average spectra amplitude are vital to the stability of the inversion. The measure that envelops Fourier spectra o f the small records has been taken to solve the problem but there still exists some trouble. The whole curves move up to keep safe, but it may cause overestimation of ground motion. Two types of curves which represent average features of Fourier curves, moving average curves and periodogram curves have been discussed. Moving average curves is selected to be the object curves for inversion through comparison. The value range of above-mentioned 5 parameters is confirmed through regional achievement after enlarging appropriately. Two groups of values of parameters in two areas are obtained via i nversed object function formated by object curves through moving average method. The values vary little compared with the parameters obtained from the inversion of the o bject curves via envelope line. It would make Geometric attenuation faster and inelastic attenuation slower and the value of estimated amplitude of near field in strong ground motion smaller.The PGA attenuation relationship of ground motion in Sichuan an d Yunnan established by inversed parameters and attenuation curves and partial values written in two dimensional table are given. Compared with the limited regional observed data, all of the PGA attenuation curves of this dissertation pass through the data point plexus, and the estimation of moment magnitude 7 is more suitable to the observed data. The crustal waveguide in Geometric attenuation is obvious weaken by the influence of the effect of slower inelastic attenuation. Compared with the empirical PGA attenuation relationship, the result of this dissertation can be involved in it. The value of quantitative error terms are large on the whole and would be larger by the increase of distance. The purpose of this dissertation is mainly reflected in the improvement of the estimation in strong ground motion. The average value and the standard deviation of the error terms of ground motion of moment magnitude 7 in Sichuan and Yunnan decrease largely.Compared with the seismology based measure of random vibration simplified theory, the underestimation of the simplification would expose and the gap would widen by the increase of moment magnitude, es pecially in moment magnitude 7 whose distance would be two to three times. The main reason may be that spectral moment and peak factor are both established on the basis of stationary random process which is not suitable for the intense unsteady of ground motion.Finally, three problems for the further study are mentioned from the progress of this paper. |
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