Study on medium heterogeneity beneath island arcs based on the analysis of seismic wave scattering

微震震源持续时间短；但由于介质不均匀性引起的散射，S波包络的视持续时间随着传播距离的增加而增加。明确了火山锋弧后侧介质不均匀性强于日本弧前侧。在本研究中，我们扩展了这一想法，仔细测量了s波峰值延迟与射线路径之间的关系。我们还开发了随机弹性介质中的包络综合作为这些分析的数学基础。研究发现，在日本北部第四纪火山下传播的射线路径比在第四纪火山之间传播的射线路径的峰值延迟更大。第四纪火山下介质不均匀性较强，而火山间介质均匀。将远震P波和P尾波能量划分为横向分量作为介质非均质性的度量，对世界岩石圈非均质性进行了分类：板块碰撞带、板块俯冲带和板块碰撞带的介质非均质性较大；然而，它在大陆上很小。在马尔可夫近似的基础上，我们发展了随机弹性介质中矢量波的直接包络合成。我们成功地建立了二维和三维情况下平面小波入射和点源辐射的包络综合。特别是在二维情况下，我们通过有限差分仿真验证了该方法的有效性。

Source duration times of microearthquakes are short; however, apparent duration times of S wave envelopes increase with travel distance increasing caused by scattering due to medium inhomogeneities. We clarified that the medium inhomogeneity is stronger in the back-arc side of the volcanic front compared with that in the fore-arc side in Japan arc. In this research, extending this idea, we carefully measured the relation between the S-wave peak delay from the onset and the ray path. We also developed the envelope synthesis in random elastic media as the mathematical foundation for these analyses. We find that the peak delays are larger for ray paths traveling beneath Quaternary volcanoes than those for ray paths traveling between Quaternary volcanoes in northern Japan. That is, the medium inhomogeneity is stronger beneath Quaternary volcanoes, but the medium is homogeneous between them. Interpreting the partition of energy of teleseismic P and P coda waves into the transverse component as a measure of medium heterogeneity, we classified the lithospheric heterogeneity in the world : the medium heterogeneity is large at plate collision, zones and subduction zones; however, it is small in continents. On the basis of the Markov approximation we have developed the direct envelope synthesis of vector waves in random elastic media. We have successfully formulated the envelope syntheses for both the incidence of a plane wavelet and the radiation from a point source in 2-D and 3-D cases. Especially in 2-D, we have confirmed the validity by using the finite difference simulations.