Method Of S-wave Identification And Its Influence On Coherence Function

With the continuous economic growth and social progress of our country,many structures have been constructed.The common feature of these structures is the spatial distribution and extension.When earthquakes occur,the seismic effects of different parts of these structures will be different.Therefore,the spatial variation of ground motion must be considered in the design of these structures.The spatial variation of the ground motion represents the difference in amplitude and phase of the ground motion recorded by each station due to the position of the station during the propagation of the seismic wave.The best way to describe the spatial variation of ground motion is the coherence function.Through the research on the coherence function of ground motion,the artificial ground motion suitable for the engineering site can be better determined in the engineering design of ground motion input.The research on the spatial variation of ground motion is based on the strong earthquake records of the arrays.The seismic records contain different types of seismic waves,which will inevitably affect the final coherence coefficient.First,based on the 5th and 45 th seismic records of SMART-1 array in Taiwan,this paper studied the influence of interception of S-wave time window on the spatial coherence coefficient of ground motion.Calculate the coherence coefficient based on the entire seismic record and calculate the coherence coefficient corresponding to the S-wave time window intercepted from the entire seismic record,and perform parameter fitting.Comparing the finally obtained fitting curve,it is found that the two calculation results of selecting different waveforms are quite different: the statistical coherence coefficient of the entire record is smaller than the coherence coefficient of the S-wave,which reduces the spatial correlation of the ground motion.Second,this paper analyzes the influence of the cut-off frequency on the coherence function.When calculating the lagged coherency of the spatial variation of seismic ground motion,the cut-off frequency of different coherence function models varies greatly.In this paper,the SMART-1 array 5th and 45 th earthquake acceleration records' North-South component uesd as research object,first calculate the lagged coherency in nine conditions which the cut-off frequency were 8Hz,16 Hz,24Hz and the station spacing were 200 m,1000m,2000 m,and analyzed by curve fitting;after compared the calculated lagged coherency according to 9 fitting parameters,the results showed that:(1)the cut-off frequency values of the coherence function statistical model have great influence on the lagged coherency curve;(2)the lagged coherency calculated based on fitting parameters which obtained by smaller cut-off frequency value,the lagged coherency decreases rapidly with increasing frequency,on the other hand,which calculated based on greater cut-off frequency value decreases slowly with increasing frequency;(3)use the lagged coherency fitting parameters which obtained by the determined cut-off frequency calculates the lagged coherency beyond the cut-off frequency range,great errors will be produced.