Recognition soil nonlinearity area by applying the time-frequency analysis method for Wenchuan earthquake
Speaker: Hsuan-Jui Hsu
Abstract
Using time-frequency analysis to determine the nonlinear soil response is a new method developed in the past few years. It could effectively observe the predominant frequency variation with time and amplitude de-amplification of seismic waves in the surface soil layer. In this study, data recorded during the May 12, 2008 China Wenchuan earthquake are used. After the baseline correction, the time-frequency analysis which combined single-station spectral ratio and the short time Fourier transform was used to calculate the time-frequency diagram. If the nonlinear soil response occurred, it clearly shows the predominant frequency shift to lower frequency in the strong wave portion. When amplitude change back to small, the predominant frequency will recover, but sometimes couldn’t return to the value before the strong motion portion. In the non-normalized time-frequency diagram it could observed the de-amplification when the nonlinear soil response occurred. The non-linearity response of soil station mostly located at PGA area larger than 80 gal for the case of May 12, 2008 China Wenchuan earthquake.
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