Simulation Of Near-field Ground Motions Associated With Fault Model And Investigation Of Topography Effect

Aiming at the lack of near-field seismic records,ground motion simulation could generate seismic motion at a site for a scenario earthquake,in which the effects of source,path and site condition are comprehensively considered.The simulation results could provide reliable input seismic motion for the dynamic time-history analysis of engineering structures.In ground motion simulation,source spectrum,fault model and topography characteristics have significant influence on the amplitude and frequency content of ground motions.Therefore,it is necessary to carry out thorough and systematic studies on these factors.These investigations are of great significance for improving the accuracy of ground-motion simulation results and the engineering value of simulation methods.Based on the above background,this thesis focuses on the source spectrum in stochastic finite-fault method,application of complex fault model in ground-motion simulation,and ground-motion amplification effect of slope topography.Systematic analysis and research are carried out for near-field ground motion simulation and topographic effect.The research contents and conclusions of the thesis are presented as follows:(1)An empirical source spectrum model is proposed for the stochastic finite-fault method.Based on Masuda source spectrum,the empirical relation between model parameters and magnitude is proposed for earthquakes with different fault types through fitting the response spectra of synthetic and recorded ground motions.On this basis,the empirical expression of model parameters with respect to the seismic moment of ruptured area is derived.The source spectrum model proposed in this study could express the influence of non-uniform slip distribution on source spectrum amplitude.The comparison of the proposed model and Brune source spectrum based on dynamic corner frequency(MA05 method)shows that the average errors of ground-motion parameters(PGA,PGV and response spectra)obtained using the model in this study are smaller than those from MA05 method.These results indicate that the method in this study has better simulation effect for realistic seismic records.Finally,after selecting reliable source models and input paramters,ground motions from the 2013 Lushan Mw 6.6 earthquake and 2017 Jiuzhaigou Mw 6.5 earthquake are simulated by applying the proposed source spectrum and stochastic finite-fault method.(2)Aiming at complex fault model,this thesis proposes a method to determine the rupture propagation path and associated propagation time on fault surface.On this basis,the near-field ground motions from the 1999 Hector Mine earthquake are simulated using fault models with different geometrical complexity and a hybrid deterministic-stochastic approach.The highfrequency and low-frequency ground motions are simulated using stochastic finite-fault method and discrete wavenumber method,respectively.The comparison between the simulation results of different fault models indicates that the broadband synthetic motions obtained using complex fault model are more consistent with near-fault records in terms of wavefofrm,amplitude and spectral characteristics.By comparing the peak parameters(PGA and PGV)of simulation results with GMPE,it is found that the peak parameters of synthetic motions obtained using complex fault model are consistent with regional attenuation characteristics of ground motions.(3)In order to comprehensively consider soil amplification and topography effects in ground motion simulation,the near-field ground motions from the 1995 Aigion earthquake are simulated using a hybrid deterministic-stochastic approach.Pertaining to the slope topography where the near-fault station AEG is located,2D seismic response analysis is carried out using the finite element method,in which nonlinear soil property is simulated by means of equivalent linear method.The comparison between synthetic ground motions and actual records leads to the following conclusions:When considering free-field site condition,the synthetic velocity and displacement at AEG are consistent with seismic records.Nonetheless,the acceleration amplitude and short-period response spectra of seismic record are underestimated in the simulation.After performing site response analysis,the acceleration amplitude and response spectra at AEG are in good agreement with recorded motion,indicating that the simulation results are significantly improved compared with the case of free-field condition.Therefore,for irregular topography,it is necessary to fully consider topographic ans soil amplification in ground motion simulation.In this way,the results could be consistent with actual ground motions in terms of peak amplitude and spectral characteristics.(4)Based on the borehold Vs data from realistic sites,typical 2D slope models with horizontally stratified soil medium are built.To quantitatively investigate the effect of nearsurface soil properties on topographic amplification under specific slope condition,the seismic response of slope is simulated using the finite-difference software FLAC,and the indicator parameters of topographic amplification are extracted from numerical results.Finally,a group of 2D slope models with different soil formation are built based on borehole data.The relation between topographic amplification and site VS30 is further analyzed.The numerical results of specific slope topography indicate that slope topography primarily amplifies the short-period component(T<0.5 s)of ground motions with respect to free-field response.On the surface of slope,topographic amplification effect is the most significant within a distance of 1/2 slope height from the crest,and gradually weakens with increasing distance.The intensity of topographic amplification gradually decrease with increasing VS30.