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

微震的震源持续时间较短，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传播距离的增加而增加。阐明了日本弧火山前缘弧后侧介质不均匀性比弧前侧强。在本研究中，我们扩展了这一思想，仔细地测量了S波峰值延迟与射线路径之间的关系，并发展了随机弹性介质中的包络综合作为这些分析的数学基础。我们发现，在第四纪火山下旅行的射线路径的峰值延迟大于在日本北方第四纪火山之间旅行的射线路径。即第四纪火山岩下介质的不均匀性较强，而火山岩之间介质是均匀的。本文将地壳P波和P波尾波能量在横向分量中的分配作为介质不均匀性的一种度量，对世界岩石圈介质不均匀性进行了分类：在板块碰撞带和俯冲带介质不均匀性较大，而在大陆介质不均匀性较小。本文在马尔可夫近似的基础上，发展了随机弹性介质中矢量波的直接包络合成。我们已经成功地制定了二维和三维情况下的平面子波的入射和从一个点源的辐射的包络综合。特别是在2-D，我们已经证实了使用有限差分模拟的有效性。

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.