著者

片山 郁夫 平内 健一 中島 淳一

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.119, no.2, pp.205-223, 2010-04-25 (Released:2010-07-06)

参考文献数

70

被引用文献数

6 8 2

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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.

著者

東 真太郎 片山 郁夫 平内 健一 山下 茂

出版者

日本鉱物科学会

雑誌

日本鉱物科学会年会講演要旨集

巻号頁・発行日

vol.2010, pp.118-118, 2010

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本研究では、地殻・マントル境界でのレオロジー変化を流動則の外挿によるのではなく、高圧変形実験によって、含水条件下のモホ面付近における斜長石とカンラン石の強度を直接決定する実験を行った。実験条件は圧力1GPa、温度400~800℃の条件で斜長石とオリビンの出発物質を同時にアルミナピストンに挟み、相対的な強度を決定する。実験後の変形微細組織から、斜長石とオリビンはどちらも格子選択配向を示し、転位密度からも斜長石とオリビンは転位クリープに対応する塑性変形をしていることが確認された。実験結果としては、400℃においてオリビンは斜長石より強度が低いが、一方で800℃においては、オリビンより斜長石の方が柔らかくなった。結果として、斜長石とオリビンの強度比は温度に依存することが示された。そして、地球のモホ面に対応する温度圧力条件では斜長石よりオリビンの方が強度が低い、もしくは、ほとんど強度に差がないという結果が得られた。この結果から、地球の大陸リソスフェアの地殻とマントルのレオロジー層構造のモデルとして適切であるのは「クレームブリュレモデル」であることが考えられる。

著者

大橋 聖和 竹下 徹 平内 健一

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.129, no.4, pp.473-489, 2020-08-25 (Released:2020-10-02)

参考文献数

108

被引用文献数

2 3

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Knowledge of the strength of faults in the continental upper crust is critical to our understanding of crustal stress states, coseismic faulting, and lithospheric deformation. In this paper, we investigate time- and displacement-dependent fault-zone weakening (softening) over geological time caused by the hydrothermal alteration of rock, the development of faulting-related structure and fabric, and changes in the relevant deformation mechanisms. In the shallow portion of the continental seismogenic zone (< 5 km), hydrothermal alteration induced by comminution and fluid flow along fault zones progressively enriches weak phyllosilicates. The development of phyllosilicate-aligned fabric with increasing shear strain leads to an effective weakening with increasing cumulative fault displacement. In the deep portion of the seismogenic zone (> 5 km), frictional–viscous flow occurs in combination with friction contributed by phyllosilicates and the dissolution–precipitation of clasts after the introduction of water, phyllosilicates and anastomosing fabrics all increasing with greater fault displacement. In addition, the water weakening of quartz and feldspar is an important softening process in the deeper portion of the seismogenic zone (> 10 km). The smoothing of fault-zone topography by the shearing of irregularities and asperities, as well as the thickening of the fault zone, leads to a reduction over time in the bulk frictional resistance of a fault as displacement increases. These time- and displacement-dependent weakening processes of fault zones give rise to diverse strength and stress states of the crust depending on its maturity and may provide clues to reconciling the stress–heat flow paradox of crustal faults.

著者

平内 健一 片山 郁夫

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.124, no.3, pp.371-396, 2015-06-25 (Released:2015-07-10)

参考文献数

140

被引用文献数

3 5

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Serpentine minerals (lizardite, chrysotile, and antigorite) are a major group of hydrous phyllosilicates resulting from the hydrothermal alteration of mantle peridotite. Their distinct rheological properties mean that serpentine minerals have a strong influence on the mechanical and seismogenic behavior of faults and plate boundaries in both continental and oceanic settings. In this paper we review the results of laboratory experiments performed to understand the frictional and mechanical properties, and deformation mechanisms of serpentinite. Frictional sliding experiments at low slip rates show that antigorite exhibits velocity-strengthening behavior (a−b > 0) over a wide range of temperature (25-400°C) , while values of (a−b) for chrysotile become negative as temperature increases (25-281°C) . This indicates that the stability of slip along serpentinite-bearing faults depends on the serpentine species and fault depth. Frictional sliding of antigorite at seismic slip rates leads to weakening by flash heating. Axial compression experiments at confining pressures of up to 4 GPa show that antigorite is stronger than lizardite by at least a factor of two. The flow law for dislocation creep of antigorite based on stress values at 〜15% strain also predicts differential stresses that are substantially lower than those for the dislocation creep of olivine at natural strain rates (10−10 to 10−14 s−1) . This suggests that the viscosity of serpentinite promotes slab–mantle decoupling. However, the antigorite flow law should be used with caution because antigorite starts to deform by semi-brittle flow after 〜20% strain. Large-strain simple-shear deformation of antigorite aggregates at high pressure (1 GPa) results in a strong alignment of antigorite c-axes normal to the shear plane. This observation explains the trench-parallel anisotropy beneath the Ryukyu subduction zone. Although dehydration embrittlement is considered a primary cause of intermediate-depth earthquakes, recent high-pressure experiments on antigorite show stable sliding behavior or detect no acoustic emissions during dehydration reactions. We emphasize that the presence of talc derived from the metasomatic alteration of serpentine further weakens and stabilizes the slab–mantle interface and promotes long-lived ( > 1 Ma) detachment faulting.

著者

平内 健一

出版者

広島大学

雑誌

特別研究員奨励費

巻号頁・発行日

2009

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本年度は蛇紋岩を用いた摩擦実験を行い,沈み込み帯プレート境界における蛇紋岩の存在状態とその流動特性に関する研究をさらに進める目的で,以下のとおり実施した.1.実験はユトレヒト大学設置の熱水回転剪断装置を用いて行い,温度20^~500℃,有効法線応力200MPa,間隙流体圧200MPa,滑り速度0.1^~30μm/sの条件で行った.試料は厚さ約1mmの蛇紋石と石英の混合粉末試料を断層模擬物質(ガウジ)として用いた.2.結果:試料は,温度300^~500℃の条件において,初期の最大摩擦強度後(変位1^~2mm),定常状態に至るまでに著しい歪弱化を示した.弱化の程度は,温度の上昇および滑り速度の低下に伴い増加する傾向が見られた.また,薄片観察の結果,弱化は滑石を伴うboundary(B)shearの発達に起因して起こり,B shearの層厚が増加(最大70μm)するに従ってガウジ全体の摩擦強度が減少していくことがわかった.3.結論:本実験は,前弧マントルウェッジにおいてシリカに富んだスラブ由来の流体が供給されれば,蛇紋石とともに滑石が形成される可能性を示唆する.滑石の生成は不均質で断層境界に平行なB shearとして存在し,著しい歪弱化を引き起こす.定常摩擦強度値は温度と滑り速度に関係するが,これはそれぞれの実験中に溶解したシリカの生成量の違いによって説明できる.滑石は蛇紋石よりも摩擦強度が非常に弱いため,沈み込みプレート境界近傍に形成される薄い滑石層がプレート境界全体のレオロジーを支配すると思われる.