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由于地球的衰减作用,远震P波以频率较低(<1 Hz)的信号为主;因此,通常假定接收点的场地效应与频率无关。我们对由地表沉积层引起的场地振幅效应进行了研究,结果表明远震与频率相关的场地效应可能是重要的。一维的模拟表明,阻抗差异引起的放大效应,结合沉积层中混响造成的振幅衰减,能产生显著与频率相关的场地效应。与频率相关的程度取决于P波在沉积层中的双程走时及P波的主周期。采用CRUST2．0模型,我们计算了2°×2°的P波“参考”周期图,在这种图上可以忽略这种频率相关性。我们认为,这种通常将台站校正用到震级确定的做法将会造成大震震级被定得偏低。为了表明这些效应在实际中是可以观测得到的．我们分析了在一对台站KURK和MAKZ所记录的7次远震事件及另一对台站KMI和CHTO所记录的6次远震事件的振幅比。振幅比对其他的影响效应较为敏感,并且与预测的地表沉积层的频率相关效应相一致。
Teleseismic P-waves dominate due to Earth’s decay; however, it is generally assumed that the site effect at the receiving point is independent of frequency. We have studied the site amplitude effects caused by surface sediments and found that the effects of teleseismic and frequency-related sites may be important. One-dimensional simulations show that amplification effects due to impedance differences, combined with attenuation of amplitude due to reverberation in the sediment, can produce significant frequency-dependent site effects. The frequency-dependent degree depends on the two-way travel time of the P wave in the sedimentary layer and the main period of the P wave. Using the CRUST2.0 model, we calculated a 2 ° × 2 ° P-wave “reference” periodogram on which frequency correlations are negligible. We think this kind of practice, which usually uses magnitude correction for station calibration, will result in the earthquake magnitude being set low. To show that these effects are observable in practice. We analyzed the amplitude ratio of seven teleseismic events recorded by KURK and MAKZ at one pair of stations and the six teleseismic events recorded by the other pair of stations KMI and CHTO. The amplitude ratio is more sensitive to other effects and consistent with the predicted frequency-dependent effects of surface sediments.