日本列島下での沈み込みプロセスの多様性

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Water is known to play important roles in earthquake generation and volcanic activity. Consequently, the presence of water and its heterogeneous distribution in subduction zones may contribute to the variability of subduction seismicity and arc magmatism. In this study, we infer water distribution, including aqueous fluids and hydrous minerals, based on the seismic tomography beneath Japan, and discuss subduction processes in terms of water circulation in subduction zones. Two distinct oceanic plates, the Pacific plate and Philippine Sea plate, are subducting beneath Japan. These plates have quite different characteristics. The Pacific plate is old (∼130 Ma) and is subducting beneath northeast Japan as rapidly as 10 cm/year. In contrast, the Philippine Sea plate is relatively young (∼20-50 Ma) and is subducting at ∼3-5 cm/year beneath southwest Japan. The subducting old Pacific plate results in cold environments beneath northeast Japan, whereas the thermal structure beneath southwest Japan is relatively warm as a result of the young Philippine Sea plate subduction. Most water is released by eclogite transformation in the subducting oceanic crust, and the expelled water infiltrates into the mantle wedge, forming hydrous minerals such as serpentine and chlorite. The seismic tomography beneath northeast Japan shows that eclogite transformation occurs at depths of ∼80-100 km, and above these depths, a low-velocity anomaly and high Vp/Vs are detected in the mantle wedge. In southwest Japan, eclogite transformation occurs at much shallower depths (50-60 km) due to a warm subduction geotherm. The down-dip limit of interplate seismicity is likely to be controlled by a brittle-ductile transition in southwest Japan, whereas such a limit beneath northeast Japan coincides with the low-velocity anomaly at depths of 60-70 km, suggesting that the presence of serpentine inhibits earthquake activity at the plate interface. The double plane of intraplate seismicity is probably caused by dehydration of eclogite forming reactions in the upper plane and serpentine/chlorite dehydration in the lower plane, although seismic activity is absent at the mantle wedge where water is released by serpentine breakdown. Low-frequency tremors above the Philippine Sea plate are mostly located at the interface between island arc Moho and subducting plate surface. Abundant aqueous fluids in this region due to permeability contrasts may trigger low-frequency tremors. A slab parallel low-velocity zone beneath northeast Japan is interpreted as a melt-filled upwelling flow in the mantle wedge. Such an anomaly is not detected in southwest Japan, and slab melting of the subducting Philippine Sea plate is probably the source of the arc magmatism in this region.

日本列島下での沈み込みプロセスの多様性

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