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基于美国加州Caltrans/CDMG的两个井下台阵加速度记录,从时域和频域分析了百米级易液化深厚覆盖层在不同地震下的地震动放大效应,揭示了地震动从基岩向覆盖层传播的4个重要特征:(1)加速度放大效应受土层深度影响,近地表20~30 m以内放大效应突出;(2)地震动三分量的放大效应具有方向性,水平向与竖向地震动放大效应差异显著;(3)基岩加速度呈现“小震放大、大震衰减”的规律;(4)加速度放大规律与频率相关,深厚覆盖层放大频带较宽。初步分析了造成上述放大效应的可能原因。在此基础上,基于平面波动假设提出了考虑层间波阻抗比放大和传播路径衰减的深厚覆盖层加速度放大效应简化函数,改进了1/4波长法的参数取值,并结合自由表面效应,对前述台阵记录的放大效应进行了估算,发现无论时域还是频域,预测结果与实际观测都较接近。本文研究成果可为深厚覆盖层液化判别和抗震设计的加速度选取提供理论依据和简化分析方法。
Based on the two downhole acceleration records of Caltrans / CDMG in California, USA, the ground motion amplification effects of deep liquefied mine over one hundred meters under different earthquakes are analyzed from time domain and frequency domain. There are four important characteristics of layer propagation: (1) The acceleration amplification effect is affected by soil depth, and the amplification effect is prominent within 20-30 m of the near surface. (2) The amplification effect of the three components of ground motion is directional, horizontal and vertical (3) The bedrock acceleration shows a law of “small earthquakes amplifying and large earthquakes decaying ”; (4) The law of acceleration amplification is related to frequency, and the deep cover widening band is wider. Preliminary analysis of the possible causes of the above amplification effect. Based on the above, based on the assumption of plane wave motion, a simplified function of amplifying acceleration effect of deep overburden considering the ratio of the impedance of the inter-layer wave and the attenuation of the propagation path is proposed. The parameters of the 1/4 wavelength method are improved. Combined with the free surface effect, The magnification effect of the aforementioned array recording was estimated and it was found that the prediction results were close to the actual observations in either time domain or frequency domain. The results of this paper can provide theoretical basis and simplified analytical method for the acceleration selection of liquefaction discrimination and seismic design of deep overburden.