Scattering study and formation process of uppermost mantle at the island arc

在日本，爆炸地震学研究组（RGES）做了大量的地震实验。在本研究中，我们利用RGES获得的波形建立了一个数据库。对5条长度超过130 km的测线进行了研究。大多数数据提示PmP波具有长持续时间的尾波。研究了该相的特性。几种可能的模型被认为是解释长持续时间的尾波。本文对三种可靠的模型进行了检验，它们是：（1）变形莫霍面边界模型，（2）反射型上地幔模型，（3）在上地幔中存在许多散射体的模型。位于上地幔的散射体模型是解释长时尾波的最佳模型。我们估计的散射系数的上地幔和地壳使用模拟。上地幔的散射系数（0.025km/s^<-1>）略大于上地壳（0.01km/s^<-1>）和下地壳（0.02km/s^<-1>），具有反射性。这项研究表明，上地幔是不透明的，其特点是许多散射体的位置。日本地区的上地幔是不透明的，这与大陆地区的上地幔不一致。上地幔中的许多散射体可能与日本的岩浆活动有关。这对了解岛弧区Mono边界和上地幔的形成过程具有重要意义。

In Japan, many seismic experiments had been done by RGES (Research Group for Explosion Seismology). In this study, we made a database by the use of waveforms obtained by RGES. The five survey lines, with the length were longer than 130 km, were researched. Most of the data suggested PmP waves with long-duration coda wave. The characteristics of the phase are investigated. Several possible models are considered to explain the long duration coda-wave. Three reliable models, which are 1)distorted Moho boundary model, 2)reflective uppermost mantle model, and 3)a model in which many scatterers are located in the uppermost mantle, are examined. The scatterer located in the uppermost mantle model is the best model for explaining the long-duration coda-wave. We estimate the scattering coefficient of the uppermost mantle and crust using simulations. The scattering coefficient of the uppermost mantle (0.025km/s^<-1>) is slightly larger than those of upper crust (0.01km/s^<-1>) and lower crust (0.02km/s^<-1>) which is characterized as being reflective. This study revealed that the uppermost mantle is not transparent, and is characterized that many scatterers are located. The uppermost mantle at the Japan is nontransparent which is not consistent with that of continental area. The many scatterers in the uppermost mantle might be related to magmatism in Japan. This will be one of the important for understanding formation process of the Mono boundary and uppermost mantle at the island-arc region.