A comparison of coda and S-wave spectral ratios as estimates of site response in the southern San Francisco Bay area
A comparison of coda and S-wave spectral ratios as estimates of site response in the southern San Francisco Bay area
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尾波和横波频谱比的比较作为旧金山湾南部地区场地响应的估计
DOI：
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发表时间：
1994
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影响因子：
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通讯作者：
J. Boatwright
中科院分区：
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作者：
L. Margheriti;L. Wennerberg;J. Boatwright
摘要：我们计算了 S 波和尾波的频谱比，以评估旧金山湾区南部 14 个地点相对于 Coyote Hills 附近基岩地点的放大情况。我们的数据是洛马普列塔余震记录的地震图：震中距和到台站的方位角实际上是相同的。研究中的所有位点均以 0.5 至 7 Hz 的频率相对于参考位点进行放大。横波和尾波谱比的形状相似，但尾波比大于横波比高达 4 倍。冲积层和海湾泥沉积物上的差异更大，特别是在 1 Hz 左右的频率下，表明存在被困在冲积盆地中的波。一般来说，分析窗口的长度影响冲积区的横波谱比。较长的窗口给出的比率与尾声比率相似，显然是因为这些窗口包括更多对尾声有贡献的相位。我们根据地质特征和表面剪切波速度对站点进行分类。对于S波比率，各类别之间的差异没有表现出系统性趋势；我们认为最软和最硬的土壤类别实际上具有相同的横波放大。随着土壤类别包括较慢和“较软”的材料，场地类别的平均尾声比率明显增加。在对参考地点的差异进行校正后，尾波放大与 Borcherdt 和 Glassmoyer (1992) 根据 1989 年洛马普列塔地震的强震记录估计的这些地点类别的相对放大非常相似。
Abstract We calculate spectral ratios for S waves and codas to evaluate the amplification of 14 sites in the southern San Francisco Bay area relative to a nearby bedrock site in the Coyote Hills. Our data are seismograms written by Loma Prieta aftershocks: the epicentral distances and azimuths to the stations are effectively the same. All the sites in the study are amplified with respect to the reference site at frequencies from 0.5 to 7 Hz. The shapes of the S -wave and coda spectral ratios are similar, but the coda ratios are greater than the S -wave ratios by as much as a factor of 4. The difference is larger for sites on alluvial and bay mud deposits, particularly at frequencies around 1 Hz, suggesting the presence of waves trapped in the alluvial basin. In general, the length of the analysis window affects the S -wave spectral ratios for alluvial sites. Longer windows give ratios similar to coda ratios, apparently because these windows include more of the phases that contribute to the coda. We classify the sites according to their geological characteristics and surficial shear-wave velocities. For the S -wave ratios, the differences between the classes showed no systematic trend; the softest and hardest soil classes we consider have practically identical S -wave amplifications. The average coda ratios for the site classes clearly increase as the soil classes include slower and “softer” materials. After correction for differences in reference sites, the coda amplifications are very similar to the relative amplifications for these site classes estimated by Borcherdt and Glassmoyer (1992) from the strong-motion recordings of the 1989 Loma Prieta earthquake.