Synthesis Of Three-Component Broadband Strong Ground Motion Field Based On Fk Approach

To meet the input demand of seismic analysis of important engineering structures,the research of ground motion synthesis has made great progress in recent years.Now,broadband ground motion synthesis is often performed using the hybrid method,in which the three-component low-frequency motion by numerical simulation and the single-component high-frequency motion by stochastic simulation are filtered and superposed.The superposition of two parts of ground motions is not strict and inharmonious,and thus may cause large error.This is a bottleneck for further development of broadband ground motion synthesis method.For this problem,a solution presented in this study is simulating the multi-dimensional broadband ground motion by means of frequency-wavenumber Green's function.The ground motion synthesis approach based on the frequency-wavenumber Green's function(hereafter called FK approach)has strict theoretical basis,considers the regional crustal structure,and can produce three-component ground motion with high calculation efficiency.This thesis analyzed the overall framework,calculation characteristics and controlling factors of the FK approach,and proposed a kinematic source modeling approach;the FK approach and the stochastic method were compared systematically;the generation mechanism and prediction model of near-fault velocity pulse were analyzed based on the ground motion field synthesized by FK approach;and the two-step ground motion synthesis considering the topographic effect was analyzed.The main research works are as follows:Based on the analysis of the overall framework,the key role of Green's function in ground motion synthesis and the advantages of FK approach over other methods were reviewed.The calculation process and characteristics of frequency-wavenumber Green's function and the expression of horizontally layered crustal structure and seismic source were summarized.It was found that the FK approach had the ability to simulate high-and low-frequency ground motions,and the Green's function and source model had controlling effects on the synthetic ground motion.This thesis pointed out that the bandwidth of ground motion by FK approach mainly depended on the source model,and thus proposed a kinematic source modeling approach.The size of the rupture plane and the slip distribution were estimated using the scaling laws,the correlations between rise time,rupture velocity and slip were considered,the criterion of joint selection of source parameters was proposed based on the radiated seismic energy,and the mean source model was adopted to reduce the uncertainty in source modeling.The Mw 5.89 Whittier Narrows and the Mw 6.94 Loma Prieta dip-slip earthquakes were selected from the SCEC broadband platform,and the time history and PSA of this study were compared with those of the observed records and the results of three SCEC methods.These showed the advantages of the proposed approach.The ground motion synthesized by the source model of this study was independent of the subsource size and was affected by the mean value and randomness of the rupture velocity.The spatial distribution and temporal evolution of slip controlled the lowand high-frequency ground motion amplitude,respectively.By comparing the calculation theory and technical way,this thesis illustrated the advantages of the FK approach over the stochastic method in expressing the regional crustal structure and ground motion phase.This improved the rationality and reliability of ground motion synthesis.The FK approach can express the difference between two horizontal components and the influence of near-fault effect and source rupture process,and the ability to simulate high-frequency ground motion was equal to or even better than the stochastic method.Based on the link between source models of two methods,this study pointed out that the constraint on subsource spectra in the stochastic method made the dynamic corner frequency meaningless,and proposed the concept and the determination method of the slip-correlated corner frequency.This corner frequency was negative correlated with slip and related to the subsource size,and no longer needed scaling factor.The case of the Mw 6.7 Lushan earthquake showed that this corner frequency was suitable for stochastic synthesis of high-frequency ground motion and eliminated the near-field overestimation caused by dynamic corner frequency.Using the cases of the Mw 6.58 Imperial Valley strike-slip earthquake and the Mw 6.7 Northridge dip-slip earthquake with rich pulse records,the ability of the FK approach to generate large velocity pulse was demonstrated.This avoided the defect of constructing pulse time history based on the equivalent pulse model in engineering field.The synthetic ground motion fields of different scenario earthquakes were used to identify the pulse area and extract the pulse period,and the isochrone theory was used to explain the generation mechanism of the directivity and nondirectivity pulses.The influences of source parameters and crustal structure on pulse were investigated.The spatial variation of pulse features of two earthquakes and the influence of slip distribution on pulse area were analyzed.According to these characteristics,the prediction equation of pulse period and the pulse occurrence model in the pulse prediction model of PSHA were improved.The two-step ground motion synthesis considering the topographic effect was analyzed.The FK approach and the explicit finite element method were used for calculation in the deep layered half space containing the seismic source and the shallow overlayer with the surface topography,respectively.This greatly reduced the computational cost when compared to the integrated numerical model.The basic model and the calculation process of two-step method were described,the combined measure of separate models was proposed,and the error of separate modeling and the ability of two-step calculation were investigated.The stability and feasibility of two-step method to simulate the topographic effect were analyzed using the case of the Lushan earthquake.