地震波トモグラフィでみたスラブの沈み込みと島弧マグマ活動

抄録

Since late 1990’s, a dense nationwide seismograph network has been constructed in the Japanese Islands with an average station separation of ∼ 20 km, which has produced the highest quality data in the world and contributed to enhance the understanding of seismotectonics and volcanotectonics there. Travel-time tomography using such high-quality data has provided two important constraints on water-transportation paths in subduction zones. One is the evidence for hydrous minerals in and immediately above the slab. The hydrated oceanic crust is imaged as a low-velocity layer to a depth of 40 km for the Philippine Sea slab and 70-130 km for the Pacific slab. Another low-velocity layer is revealed immediately above the Pacific slab down to a depth of ∼ 110 km, which might correspond to a hydrous layer through which water is carried to deeper depths. The other is seismological imaging of mantle upwelling. Mantle return flows induced by the subduction of the Pacific and Philippine Sea slabs are imaged in Tohoku and Kyushu, respectively, whereas that probably generated by the subduction of both the Pacific and Philippine Sea slabs is apparent in central Japan. A large upwelling in the upper mantle revealed in the Chugoku district might be the origin of Quaternary volcanism there.