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使用北俄罗斯深地震测深剖面上记录到的和平利用核爆炸的地震波场的短周期、三分量记录(Quartz和Ruby-I,分别在1984和1988收集),来约束可以被3000km以外的传感器观测到的高频远震Pn震相的特征。这个震相是由上地幔内的速度波动造成的,这里的速度波动被看作是散射体。为了检验这个假设和确定上地幔散射体的特征,用弹性反射率法(一维各向同性模型)模拟高频远震Pn震相的尾波。通过细查远震Pn震相在5Hz时的尾波衰减率,分析了观测数据和合成记录。对一些不同的模型都计算了合成地震波。这些不同的模型是以恰好在壳幔边界下面具有一个随机分布的速度波动的附加带(薄层)的全球模型IASP91为基础的。这些模型也许可以用散射层厚度L、垂直不均匀性相关长度α和不均匀标准偏差σ来表征。这些模型中波传播的数字化模拟产生了具有明显长尾波的高频远震Pn震相。模拟与观测结果的比较清晰地显示出,在上地幔中包含速度波动的模型可以方便地解释高频远震Pn震相的出现。我们试图通过比较合成与观测地震图的衰减率,来约束上地幔速度波动性质L,α和σ。在超出1300km的非干涉高频远震Pn震相的距离范围内,我们拟合的最佳模型的尾波衰减率和观测结果是相似的。这个最佳模型在莫霍面下有75km厚的散射体带,包含一个平均2km厚的速度波动带和5％的RMS速度扰动。模拟结果还显示出两个其他可能的模型,即沿着壳幔边界的低音走廊(低速带)震相和下地壳的散射现象,单取其一不能解释高频远震Pn震相的尾波性质。
The short-period, three-component recording (Quartz and Ruby-I, collected in 1984 and 1988, respectively) of the seismic wavefield of the peaceful exploitation of nuclear explosions recorded on deep ruptured deep seismic profiles in North Russia was used to constrain the observations of sensors beyond 3000 km The characteristics of the high-frequency teleseismic Pn phase. This phase is caused by the velocity fluctuations in the upper mantle, where the velocity fluctuations are considered as scatterers. To test this hypothesis and determine the characteristics of upper mantle scatterers, the coda of the high-frequency teleseismic Pn was simulated using the elastic reflectivity method (one-dimensional isotropic model). By examining the coda attenuation rate at 5 Hz for the teleseismic Pn phase, the observed data and synthetic records were analyzed. Synthetic seismic waves have been calculated for some different models. These different models are based on the global model IASP91, which has an additional band (thin layer) of velocity fluctuations randomly distributed beneath the crust and mantle boundaries. These models may be characterized by scattering layer thickness L, vertical non-uniformity related length α, and non-uniform standard deviation σ. The digital simulation of wave propagation in these models resulted in a high-frequency teleseismic Pn seismic phase with a distinct long tail wave. A comparison of the simulation results with the observations clearly shows that the inclusion of velocity fluctuations in the upper mantle can easily explain the occurrence of the Pn seismic facies in the high frequency teleseismic area. We attempt to constrain the velocity fluctuations of the upper mantle, L, α and σ, by comparing the decay rates of synthetic and observed seismograms. Within a distance of more than 1300 km of the non-interfering high-frequency teleseismic Pn facies, the coda attenuation rate of our best fit model is similar to observations. The best model has a 75-km-thick scatterer band beneath the Moho with an average velocity fluctuation band of 2 km and a RMS velocity perturbation of 5%. The simulation results also show two other possible models, that is, the scattering of the seismic facies and the lower crust in the bass corridor (low velocity) along the crust and mantle boundary, and taking one of them can not explain the coda property of the high frequency teleseismic Pn facies.