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Teleseismic shear wave tomography of the Japan subduction zone

遠地地震トモグラフィーによる日本列島下の三次元S波速度構造の推定

浅森 浩一; Zhao, D.*

Asamori, Koichi; Zhao, D.*

将来の地層処分システムに重大な影響を及ぼす可能性がある地震・火山活動の潜在的なリスクや超長期の地殻変動を適切に評価するためには、その背景にあるプレートの沈み込みやそれに起因する流体及びマグマの生成・移動等のダイナミクスに関する理解が必要である。本研究では、遠地地震波形データから読み取った多量のS波相対走時残差を地震波トモグラフィー法に適用することにより、日本列島下のマントル遷移層までを対象とした高分解能三次元S波速度構造を初めて推定した。その結果、西南日本下には非地震性のフィリピン海スラブがマントル遷移層まで沈み込んでいることや、太平洋スラブの深部脱水に伴う流体の上昇が西南日本の火成活動やフィリピン海スラブの複雑な構造の形成に関与していることが明らかになった。

We present the first high-resolution S-wave tomography of the Japan subduction zone down to a depth of 700 km, which is determined by inverting a large number of high-quality S-wave arrival-time data from local, regional and teleseismic events. The subducting Pacific and Philippine Sea (PHS) slabs are revealed clearly as high-velocity (high-V) zones, whereas low-velocity (low-V) anomalies are revealed in the mantle wedge above the two slabs. The PHS slab has subducted aseismically down to a depth of 480 km under the Japan Sea and to a depth of 540 km under the Tsushima Strait. A window is revealed within the aseismic PHS slab, being consistent with P-wave tomography. Prominent low-V anomalies exist below the PHS slab and above the Pacific slab, which reflect hot and wet mantle upwelling caused by the joint effect of deep dehydration of the Pacific slab and convective circulation process in the mantle wedge above the Pacific slab. The hot and wet mantle upwelling has caused the complex geometry and structure of the PHS slab in SW Japan, and contributed to the Quaternary volcanism along the Japan Sea coast. In eastern Japan, low-V zones are revealed at depths of 200-700 km below the Pacific slab, which may reflect hot upwelling from the lower mantle or even the core-mantle boundary.

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パーセンタイル:24.23

分野:Geochemistry & Geophysics

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