Envelope synthesis of short-period seismograms in 3-D random media for a point shear dislocation source based on the forward scattering approximation : Application to small strike-slip earthquakes in southwest Japan
Envelope synthesis of short-period seismograms in 3-D random media for a point shear dislocation source based on the forward scattering approximation : Application to small strike-slip earthquakes in southwest Japan
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基于前向散射近似的点剪切位错源的 3-D 随机介质中短周期地震图的包络合成：在日本西南部小型走滑地震中的应用
DOI：
10.1029/2010jb008182
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发表时间：
2011
期刊：
影响因子：
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通讯作者：
Takeshi Nishimura
中科院分区：
文献类型：
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作者：
Kaoru Sawazaki;Haruo Sato;Takeshi Nishimura
尽管由于随机岩石圈非均质性引起的散射波的激发，局部地震的高频地震记录比较复杂，但在随机介质随机特征的基础上，前向散射近似很好地再现了它们的包络。利用抛物型波动方程的马尔可夫近似，用随机射线路径方法合成了点剪切位错源的三分量矢量波包。由于多次前向散射，合成包络的方向性与原始辐射方向图相比呈现平滑的象限图案；散射或绕射的能量甚至出现在零轴方向和节面上。随着震中距离的增加，象限模式变得平滑。为了检验合成包络的可用性，我们分析了日本西南部发生的两次小走滑地震的地震图。首先，在von Karman型随机介质的假设下，我们估计了表征随机性的统计参数和在2-4、4-8和8-16赫兹频率下的本征吸收，然后我们检查了包络的方向性。包络的最大能量密度对P波和S波都表现出45度平移的象限分布。方向性出现在最高16赫兹的频率和最远200公里的距离上。这种方向性也出现在S波包的持续时间上。合成包络很好地解释了观测到的方向性，但仍难以解释合成包络将能量分成三个分量的原因。
Even though high‐frequency seismograms of local earthquakes are complex because of the excitation of scattered waves caused by random lithospheric heterogeneities, their envelopes are well reproduced by the forward scattering approximation on the basis of stochastic characterization of random media. We present a method to synthesize three‐component vector wave envelopes for a point shear dislocation source by way of the stochastic raypath method using the Markov approximation for the parabolic wave equation. Because of multiple forward scattering, the directionality of the synthesized envelope shows a smoothed quadrant pattern compared to the original radiation pattern; the scattered or diffracted energy appears even in the direction of the null axis and on the nodal planes. The quadrant pattern becomes smooth as hypocentral distance increases. To examine the availability of the synthesized envelope, we analyze seismograms of two small strike‐slip earthquakes that occurred in southwestern Japan. First, we estimate the statistical parameters characterizing the randomness and the intrinsic absorptions in frequencies 2–4, 4–8, and 8–16 Hz assuming a von Karman‐type random medium, and then we examine the directionality of the envelope. The maximum energy density of the envelope shows a quadrant pattern for both P and S waves with a 45 degree shift. The directionality appears in the frequencies up to 16 Hz and at distances up to 200 km. The directionality also appears on the S wave envelope duration. The observed directionality is well explained by the synthesized envelope, but it is still difficult to explain the partition of energy into three components by the synthesized envelopes.