本研究根據最大振動值與地震頻譜的變化，探討台北盆地地盤振動的放大情形。內容分為二部分，首先分析幾次強震陣列的地表與下孔記錄，藉以瞭解地振動在單一場址之變化特性以及地表土層之放大效應；再進一步研究台北盆地在近年六次強震作用下的地表地盤振動特性。 下孔記錄的分析結果顯示，尖峰地動加速度(PGA)多由地底深處向上逐漸放大，至地表附近常達最大值；此外，比較地表的PGA發現，約半徑60m範圍內的PGA值可相差2倍以上。地震頻譜方面，規模愈大、震央距離愈遠的地震，其反應譜圖形在長週期的部分將愈顯著。本文比較二種頻譜比法的分析結果，反應譜比RRS與富氏振幅譜比RFAS的曲線趨勢相當接近，RRS尖峰值與RFAS的第一個尖峰值也幾乎在同一頻率下發生。 另一方面，利用近年六次規模大於6的強震記錄，研究台北盆地地盤放大特性。觀察6次地震PGA的分布情形，較大PGA值常出現在盆地的東部與南部，可能與該區較厚的淺層黏土層有關。依6次地震的反應譜放大特性，將測站分成以下四類：第一類測站反應譜有明顯的放大尖峰；第二類測站分布在盆地邊緣，反應譜無明顯的放大情形；第三類測站主要位在盆地南部較硬的土層上，反應譜放大幅度不似第一類測站明顯，RRS在週期1.0秒後約維持定值。部分測站之放大趨勢在二個水平方向上有相當的差異，無法描述其特性，暫將其歸類為第四類測站，有待進一步研究。 In this research, we study the site amplification in Taipei basin based on the variation of PGA and earthquake spectrum. The analysis includes two parts. First, data from several downhole arrays have been investigated to understand the variation of ground motion and site amplification at surface. Second, we study the characteristics of ground motion at surface in Taipei basin during recent 6 strong earthquakes. The results of downhole data show that, in general, peak ground acceleration (PGA) decreases with depth and can reach maximum at surface. Besides, the records of surface accelerometers are compared and found that PGA-value could greater than two times even within a radius of 60 meters. The shape of response spectrum is more significant with larger magnitude and longer epicenter distance. The results of two spectral ratio methods indicate that the trends of ratio of response spectrum (RRS) and ratio of Fourier amplitude spectrum (RFAS) are similar and both RRSmax and the first peak of RFAS occur at about the same frequency. On the other hand, six seismic records of magnitude greater than 6 are selected for study site amplification in Taipei Basin. The distributions of PGA are observed that larger PGA-value are usually found at east and south corner of the basin. It could be associated with thickness of soft clay in the shallow soil layers at these areas. Based on the amplification of response spectrum, we classified the Taipei basin strong motion stations into four types. Type one : the noticeable amplification peak of acceleration response spectrum can be observed. Type two : the stations locate at the Taipei basin edges, and the acceleration response spectra of this type are similar to that of our selected reference-site. Type three : most stations locate at the south part of the basin, the amplification of response spectra of this type are not obvious as that of type one and RRS-value hold constant when period is longer than 1.0sec. In some stations, the trends of amplification of the two horizontal components are quite different and difficult to describe the characteristics. For this reason, we temporarily classified those stations into type four and suggested to be further studied.
