地震波トモグラフイーから推定した九州地方の島弧マグマ分布とスラブ構造

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The Kyushu subduction zone in southwest Japan is characterized by active subduction of the young Philippine Sea slab beneath the Eurasian plate. We have determined the detailed three-dimensional P-wave velocity structure of the Kyushu subduction zone to a depth of 120 km. We used 9,041 P and S wave arrival times from 486 shallow and intermediate-depth earthquakes, which occurred in and around Kyushu. The arrival times were recorded by the Japan University Seismic Network, which covers the entire Japan Islands densely and uniformly. Our results show that the subducting Philippine Sea slab is imaged clearly as a high-velocity zone. It has a thickness of about 35 km and a P wave velocity 3-5% faster than the initial 1-D velocity model used in this study. Very slow velocity anomalies (3-5%) exist in the mantle wedge and extend to the forearc region down to the subducting Philippine Sea slab beneath northern Kyushu. These results indicate that regimes of melting and magmatism in a subduction zone with a young slab are different from those with old slabs. Dehydration and melting occur beneath the arc and forearc regions above a young slab, such as in northern Kyushu (age<26 Ma), while they occur beneath the volcanic front and back arc above old slabs, such as Northeast Honshu and South Kyushu. The formation of magmatism and volcanism in Kyushu is considered to be related to the convective circulation process in the mantle wedge and the dehydration of the subducting Philippine Sea slab. Large crustal earthquakes during the period 1886-1996 are found to occur in the vicinity of the low-velocity zones, indicating that volcanoes and magma chambers weaken the brittle seismogenic crust, and cause those weak areas to be subject to tectonic stress and prone to large earthquakes.

著者

浅森浩一
愛媛大学理工学研究科

地震波トモグラフイーから推定した九州地方の島弧マグマ分布とスラブ構造

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