Course: Seminar II (for second-year MSc students)

Recognition of nonlinear site response applying the Time-Frequency Analysis Method

Chih-Wei Chang

Abstract

The predominant frequency decrease of a soil site and deamplification of strong motion are nonlinear site effects. In this study, the strong motion and weak motion events recorded by the LSST borehole array in Taiwan, and we use the time-frequency analysis method to analyze the predominant frequency varies with time. In this area, we knew the predominant frequency of the strong motion and the week motion respectively 1-2 Hz and 2-3 Hz by the past paper result. Then we estimate this method that the soil linear to nonlinear response and nonlinear to linear response by the strong motion.

In the result, we normalize the spectral ratios of every time windows to display the predominant frequency vary with time. We can see the predominant frequency of the strong motion is 3 Hz the same with the weak motion at the initial seismic wave using the time-frequency analysis and Fourier spectral ratio method and it’s soil linear response at this time. Then the predominant frequency of the shear wave is decrease to 2 Hz and its soil nonlinear response at this time. At the coda wave of the shear wave, the predominant frequency returns to 2.5-3 Hz immediately and its soil linear response at this time again. On the other situation, the predominant frequency varies with time from beginning to end of the week motion is about 3 Hz obviously using this method and it demonstrated that the soil response is linear on the weak motion.

In this work, the STFT and the Morlet transform method can be used to observe the different variations of predominant frequency with time on the strong motion event. Much remains to be done, then, but we anticipate that the same results will generate the HHT method to improve the resolution of the time and frequency domain.

Reference

Wen, K. L., Beresnev, I. A. and Yeh, Y. T., “Investigation of non-linear site amplication at two downhole strong ground motion arrays in Taiwan,” Earthq. Eng. Struct. Dyn., 24, 313-324, 1995.

(Abstract) (Full text)