地震觀測研究中，近地表土層所造成之土壤非線性反應之研究對於災害防制與評估非常重要。本研究使用單站頻譜比法與短時傅氏轉換結合之時頻分析技術檢視2008 年5 月12 日發生於中國四川省之汶川地震其土壤非線性反應區域。汶川地震造成中國四川省非常大的財產及人員之災害損失，其破裂長度超過310 Km，地表最大加速度更是大於950 gal。在次生地質災害上也有發現土壤液化。土壤非線性反應與土壤液化息息相關，故本研究將對於各測站之土壤非線性反應劃分強度區域。張志偉(2008)中使用雙站頻譜比法與短時傅氏轉換結合之時頻分析技術得以有效於時間序列上觀察出土壤非線性反應的發生，而本研究改為使用單站頻譜比法，對於土壤非線性反應特性之中，共振主頻會向低頻移動之特性得以有效觀察。對於強振動後在時間序列上，共振主頻逐漸回復之情形也有發現。但對於壓抑放大作用之特性並不明顯，推測可能原因來自於單站頻譜比法其比值無法有效的反應土壤放大因子之倍率有關。利用時間序列上A-Window(背景雜訊)、B-Window(強振動)與C-Window(強振動後時間序列上最弱振動)三個視窗其單站頻譜比特定頻段與交互相關值差量化出測站土壤非線性強度區域。B-Window 與C-Window 差異大致上主頻向低頻移動0.2 Hz∼0.6 Hz 為疑似土壤非線性反應區域，0.6 Hz 以上為明顯土壤非線性反應區域。比較B-Window與A-Window 差異，明顯土壤非線性反應測站大多位於頻率差值大於2.1 Hz。土壤非線性反應強度區域與水平向加速度峰值以及地表最大加速度較符合，與土壤非線性反應主要來自於剪應力所造成相符合，大致上當PGA 大於50 gal 有可能產生土壤非線性反應，PGA 大於200 gal 以上則有非常明顯之土壤非線性反應。且與現地調查之土壤液化點位比較結果。土壤液化區域主要有三區，皆為近斷層附近。正好位於本研究中所顯示之明顯土壤非線性反應之頻率差值較高(大於2.4 Hz)的區域。Using time-frequency analysis to determine the time variation soil response is the new development method in the past few years. It could effectively observe the predominant frequency variation with time and de-amplification when the strong seismic waves pass into the surface soft soil layer. In this study, the May 12, 2008 China Wenchuan earthquake data were used. After the baseline correction, using the time-frequency analysis combined single-station spectral ratio with the short time Fourier transform, the resulting time-frequency diagram clearly show the predominant frequency shift to low-frequency in the strong seismic wave part. When amplitude change to small, the predominant frequency will recover, but most couldn’t return to the value before the arrival of strong shaking. The soil stations classified to possible non-linear response as the predominant frequency shifted 0.2 Hz~0.6 Hz. The clear non-linear response stations which predominant frequency shift greater than 0.6 Hz. The possible non-linear response stations mostly located at PGA area larger than 50 gal. The clear non-linear response stations mostly located at PGA area larger than 200 gal. The observed soil liquefaction points and the non-linear response stations located at same areas in three regions along the fault.
