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

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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.