Study on variation of soil site amplification with depth：A case at Treasure Island geotechnical array, San Francisco bay
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摘要: 为了探究土层场地放大作用的机制，利用金银岛岩土台阵在四次地震中记录的26组弱震动的三分量加速度时程，采用考虑上行波场与下行波场相消干涉作用的传统谱比法，研究了土层场地的放大作用随深度的变化规律.结果表明：（1）地震波从基岩传播到土层中时，土层的多个振型被激励，放大作用随深度的变化是按照不同振型的特征而有规律地变化.自下而上直至地表，一阶振型的场地放大作用是逐渐增大的；二阶振型的场地放大作用经历了逐渐减小和逐渐增大两个过程；三阶振型的场地放大作用经历了逐渐增大、逐渐减小和逐渐增大三个过程；四阶振型的场地放大作用经历了逐渐减小、逐渐增大、逐渐减小和逐渐增大四个过程；更高阶振型的场地放大作用可以据此类推.（2）四次地震中同一振型的NS和EW两个水平分量的自振频率相差很小；二阶到六阶5个振型与一阶振型的自振频率之比小于相应的理论模型之比.（3）四次地震中均存在一些高阶振型的放大系数大于其一阶振型相应分量的放大系数的现象.目前的场地反应分析中，往往将最大放大系数对应的频率作为场地的卓越频率，这么做的结果，很有可能将不同振型的自振频率作为场地的卓越频率，导致同一地震中不同水平分量的卓越频率相差较大，或者在不同地震中同一水平分量的卓越频率差别较大，难以描述场地的固有特性；而按照不同振型分析场地放大作用的特征，可以从本质上揭示场地的固有特性.Abstract: To gain insight into the mechanism of soil site amplification, utilizing three-component acceleration time histories of 26 set weak motions recorded in Treasure Island geotechnical array during four earthquakes, and employing the traditional spectral ratio method with considering the destructive interference between upgoing and downgoing wave fields, the variations of the soil site amplification with depth in the array are studied. The results are as follows. (1) The different vibration modes in soil layer are excited while seismic wave propagates into the soil layer from the bedrock, and variations of the soil site amplification with depth are in accordance with characteristics of different vibration modes. From the bottom up to surface in the soil layer, the amplification effects are gradually increased for the first mode, and that for the second mode can be divided into two processes: decreasing and increasing gradually, and that for the third mode can be divided into three processes: increasing, decreasing and increasing gradually, and that for the fourth mode can be divided into four processes: decreasing, increasing, decreasing and increasing gradually. The amplification effects of other higher modes can be analogized in the same manner. (2) There are very small differences between the natural frequencies of NS and EW components in six modes during four earthquakes. The ratios between the natural frequencies of five modes from second to sixth and first mode are smaller than corresponding ratios of the theoretical models. (3) There is a phenomenon that the amplification factors of some higher modes are greater than that of corresponding components of the first mode during four earthquakes. In present site response analyses, the frequency corresponding to the maximum amplification factor is often cited as the predominant frequency of the soil site. As a consequence, the natural frequencies of some different modes may be likely taken as the predominant frequency of the soil site, and this would cause that the predominant frequencies between NS and EW components of the soil site during an earthquake, and between the same components of the soil site during different earthquakes, have a greater difference, and make it difficult to describe the intrinsic property of the soil site based on the predominant frequency. By analyzing the characteristics of site amplification according to different vibration modes, the intrinsic property of the soil site can be essentially revealed.
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