The influence of cut-off frequency on the statistical results of spatial coherency function of seismic ground motion

Based on the north-south component of the 45th seismic record on the SMART-1 station, in this paper, we firstly calculated the lagged coherencies under nine conditions which were combined with the three cut-off frequencies （8, 16, 24 Hz） and three separation distances （200, 1 000, 2 000 m）. Then, we fitted the curves. Finally, the coherencies received according to nine groups of fitting parameters were compared. The conclusion can be drawn as follows: ① The selection of the cut-off frequency f_cut has great effect on the parameters of coherence function model and lagged coherency fitting curves. ② The lagged coherency calculated by the fitting parameters which obtained by smaller cut-off frequency f_cut decays rapidly with increasing frequency; on the other hand, the lagged coherency calculated by the fitting parameters which obtained by larger cut-off frequency f_cut decays slowly with increasing frequency. ③ Great errors will be produced when using fitting parameters of the lagged coherency that got from the determined cut-off frequencyf_cut to calculate the lagged coherency beyond that frequency range.