Evaluation of strength of heterogeneity in the lithosphere from peak amplitude analyses of teleseismic short-period vector P waves
Evaluation of strength of heterogeneity in the lithosphere from peak amplitude analyses of teleseismic short-period vector P waves
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DOI：
10.1111/j.1365-246x.2007.03544.x
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发表时间：
2007-10
影响因子：
2.8
通讯作者：
M. Kubanza;T. Nishimura;H. Sato
中科院分区：
文献类型：
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作者：
M. Kubanza;T. Nishimura;H. Sato
总结我们通过分析观测到的0.5 - 4 Hz频带内的超高压P波的地震图包络，定量地描述了地球仪岩石圈非均匀性强度的区域变化。我们应用一个理论散射模型的基础上的马尔可夫近似的平面P波传播通过随机介质的高斯自相关函数。由于该模型假定了脉冲平面子波的垂直入射，我们首先分析了发生在西太平洋地区沿着的深源地震的脉冲P波。我们测量了横向分量的峰强度与三个分量之和的峰强度的比值，并确定了随机性量e2 z/a，其中e，a和z分别是分数涨落，相关距离和非均匀结构的厚度。虽然浅源地震的震源时间函数过于复杂，不能直接应用散射模型，但峰值振幅与归一化横向振幅（P尾波能量分配到横向分量的平方根）之比之间的良好相关性使我们能够使用世界各地广泛发生的浅源地震。因此，在0.5 - 1 Hz时，e2 z/a的范围为1.15 × 10 −4至6.34 × 10 −2，在1 - 2 Hz时为2.02 × 10 −3至1.89 × 10 −1，在2 - 4 Hz时为1.49 × 10 −4至1.89 × 10 −1，这与以前使用不同方法的研究结果一致。随机性的空间分布与各种构造背景几乎一致，并与Lg尾波Q的横向变化和80 km深度处的剪切波速度扰动大致相关，表明横向非均匀性从地壳浅层延伸到上地幔。
SUMMARY We quantitatively characterize the regional variations in the strength of heterogeneity in the lithosphere of the globe by analysing the observed seismogram envelopes of teleseismic P waves in the frequency band of 0.5‐4 Hz. We apply a theoretical scattering model based on the Markov approximation for a plane P wave propagating through the random medium characterized by a Gaussian autocorrelation function. Since this model presumes the vertical incidence of an impulsive plane wavelet, we first analyse teleseismic P waves from deep earthquakes occurring along the western Pacific regions. We measure the ratios of peak intensity of transverse components to that of the sum of the three components, and determine the quantity of randomness e 2 z/a, where e, a and z are fractional fluctuation, correlation distance and thickness of a heterogeneous structure, respectively. Although source time functions of shallow earthquakes are too complex to directly apply the scattering model, a good correlation between the ratios of peak amplitude and the normalized transverse amplitude, which is the square root of the energy partition of the P-coda waves into the transverse component, enables us to use the shallow earthquakes that occur widely around the world. As a result, the quantity e 2 z/a extends from 1.15 × 10 −4 to 6.34 × 10 −2 at 0.5‐1 Hz, 2.02 × 10 −3 to 1.89 × 10 −1 at 1‐2 Hz and 1.49 × 10 −4 to 1.89 × 10 −1 at 2‐4 Hz, which are in agreement with the results of previous studies using different methods. The spatial distribution of randomness almost agrees with various tectonic settings and roughly correlates with lateral variations of Lg coda Q and shear wave velocity perturbations at 80 km depth, suggesting that lateral heterogeneity extends from the shallow crust to uppermost mantle.