Identification of Nonlinear Site Response from Time Variations of the Predominant Frequency

Abstract eng:
The predominant frequency decrease and de-amplification of spectra on strong motion at a soil site are recognized as occurring nonlinear site effects. In this study, the strong motion and weak motion events recorded by the LSST borehole array in Taiwan and the Port Island borehole array in Japan are analyzed by spectral ratio method with the Short Time Fourier Transform (STFT) and Wavelet analysis. The spectral ratios of surface to borehole sites are calculated to analyze the predominant frequency variations with time. On the weak motion events, the predominant frequency keeps at around one frequency and does not varies with time from beginning to end of the motions. It demonstrated that the soil response is linear on the weak motion events. Based on the analysis of the strong motion records, it shows the predominant frequency varies from the frequency that is the same as the weak motion event’ result at the window before the strong shaking part, then the predominant s frequency decrease during the strong shaking parts, after the strong shaking portion it move back to around the same as that of the weak motion part. So, according to this behavior we can identify the soil response is linear or nonlinear during the time history of the strong motion events through the variation of predominant frequency with time.

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