著者

河合 研志 内出 崇彦

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.128, no.3, pp.465-475, 2019-06-25 (Released:2019-07-11)

参考文献数

23

被引用文献数

1

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Teaching materials on determining epicenters using P-wave lateral polarity data were produced. These materials are intended for use in secondary and advanced education and only require inexpensive tools such as a ruler and a protractor; they do not require specialized software or devices. The materials consist of PDF files containing three-component waveform data before and after P-wave arrivals for 929 small events recorded at Hi-net stations in the Hida region, central Japan, from February 1 to April 30, 2011. A P-wave lateral polarity analysis using the two horizontal components provides information on seismicity in the Hida region before and after the March 11, 2011 off the Pacific coast of Tohoku Earthquake. Students can discuss temporal changes in the underground stress state based on their analyses. Depending on the grade and science education level of students, the time-series waveform data allow them to conduct additional exercises such as error analyses and epicenter determinations using S–P differential travel time, as well as geophysical interpretations of results.

著者

丸山 茂徳 大森 聡一 千秋 博紀 河合 研志 B.F. WINDLEY

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.120, no.1, pp.115-223, 2011-02-25 (Released:2011-05-20)

参考文献数

217

被引用文献数

26 42 22

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Pacific-type orogeny (PTO) has long been recognized as a contrasting accretionary alternative to continent-continent collisional orogeny. However, since the original concept was proposed, there have many new developments, which make it timely to produce a new re-evaluated model, in which we emphasize the following new aspects. First, substantial growth of Tonarite–Trondhjemite–Granite (TTG) crust, and second the reductive effect of tectonic erosion. The modern analog of a Pacific-type orogen developed through six stages of growth exemplified by specific regions; initial stage 1: the southern end of the Andes; stage 2: exhumation to the mid-crustal level at Indonesia outer arc; stage 3: the Barrovian hydration stage at Kii Peninsula, SW Japan; stage 4: the initial stage of surface exposure of the high-P/T regional metamorphic belt at Olympic Peninsula, south of Seattle, USA; stage 5: exposure of the orogenic core at the surface at the Shimanto metamorphic belt, SW Japan; and stage 6: post-orogenic processes including tectonic erosion at the Mariana and Japan trench and the Nankai trough. The fundamental framework of a Pacific-type orogen is an accretionary complex, which includes limited ocean floor material, much terrigenous trench sediment, plus island arc, oceanic plateau, and intra-oceanic basaltic material from the ocean. The classic concept of a PTO stresses the importance of the addition within accreted rocks of new subduction-generated arcs and TTGs, which were added along the continental margins particularly during the Cretaceous. Besides the above additional or positive aspects of a PTO, here we emphasize the negative effects of previously little-considered tectonic erosion caused by subduction over time. The evaluation of such extensive tectonic erosion leads a prospect of the presence of huge quantities of TTG material in the lower transition zone, where many subducted slabs have ponded, as illustrated by mantle tomography. This is confirmed by density profiles of the mantle, which show that TTGs are abundant only along the bottom of the upper mantle accompanied by slab peridotite, lherzolite, and MORB. The major velocity anomaly in the lower transition zone is best explained by the predominance of SiO2 phases, hence TTG, and not by MORB or ultramafic rocks. Reasonable calculations indicate that at a depth range of 520-660 km TTG material amounts to 6-7 times more than the total mass of the surface continental crust. The traditional view is that the Japanese islands evolved since 520 Ma through five Pacific-type orogenies, which grew oceanward, thus creating a continuous accretionary complex ca. 400-500 km wide, with TTG growth at the continental side of each orogen. However, the subducting oceanic lithosphere has produced five times more TTG crust compared with the present TTG crust in the Japan islands. This is explained by the fact that over time tectonic erosion has dominated the increasing arc-TTG crust. Accordingly, Japan has lost four arc-TTG crusts to tectonic erosion. TTG material, such as trench sediment, arc crust, and continental margin crust, was fragmented by tectonic erosion and transported into the bottom of the upper mantle at depths of 520-660 km. Worldwide data suggest that tectonic erosion destroyed and fragmented most of the Pacific-type orogens.(View PDF for the rest of the abstract.)

著者

奥田 花也 片山 郁夫 佐久間 博 河合 研志

雑誌

JpGU-AGU Joint Meeting 2020

巻号頁・発行日

2020-03-13

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Brucite (水酸化マグネシウム)は蛇紋岩の主構成鉱物の一つであり、超苦鉄質岩の水和反応によって形成される。これまでbruciteは粒径が非常に小さく天然環境で観察されにくいことから注目されてこなかったが、近年の研究では含水下マントルウェッジにおいてantigoriteと安定に共存し(Kawahara et al., 2016)、さらにマントルウェッジでの長期スロースリップがbruciteの形成に伴う高有効法線応力によって説明される可能性も示唆されている(Mizukami et al., 2014)。さらに、bruciteの存在はマントルウェッジ中の岩石の摩擦の安定性を変える可能性がある。このようにbruciteは水和した超苦鉄質岩帯における地震活動に影響する可能性があるが、bruciteの摩擦特性はこれまであまり調べられていなかった。本研究では一連の摩擦実験によりbruciteの基礎的な摩擦特性を報告する。摩擦実験は粒径70 nmの合成試薬を用いて広島大学の二軸摩擦試験機により行った。大気乾燥下と含水条件下の両方で、様々な垂直応力下(10, 20, 40, 60 MPa)で実験を行った。最大の剪断変位は20 mmであり、実験初期の剪断速度は3 μm/secとした。33 μm/secでのvelocity step testを数回行い、それぞれのstepから速度状態依存摩擦構成則(RSF)を用いて定量的に摩擦の不安定性を解析した。乾燥下において、定常状態の摩擦係数はおよそ0.40であり、不安定滑り(velocity-weakeningまたはstick-slip)が全ての垂直応力で観察された。剪断変位が2 mm程度において摩擦係数に明瞭なピークが観察され、このピークの摩擦係数は垂直応力に反比例した。含水下においては、ピークの摩擦係数は乾燥下と同様垂直応力に反比例したが、摩擦係数自体は乾燥下の場合より低かった。垂直応力が10と20 MPaの場合はvelocity-weakeningが観察されたが、40と60 MPaの高い垂直応力の場合はvelocity-strengtheningに変化した。こう垂直応力での安定滑りは100 MPaの垂直応力での先行研究と調和的である(Moore & Lockner, 2007)。RSF則のaとbの値は乾燥下の場合の方が含水下の場合より小さく、臨界すべり距離dcも乾燥下の場合の方が含水下の場合より短かった。Antigoriteはbruciteよりも高い摩擦係数を示すため、bruciteの不安定な摩擦挙動は含水した超苦鉄質岩帯において地震を引き起こす可能性がある。なお本研究では温度依存性については調べていない。発表では、実験したガウジの微細構造観察を通して摩擦特性のメカニズムについて考察を行い、実験結果と先行研究でのモデルから天然の超苦鉄質岩帯における地震活動について議論する予定である。

著者

ゲラー ロバート 河合 研志

出版者

東京大学

雑誌

基盤研究(C)

巻号頁・発行日

2013-04-01

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マントルの地震学的構造とその地球化学的性質を明らかにすること、特に、核-マントルの相互作用と共進化の理解に貢献することを目指し、我々はより高解像度の地球内部構造を推定するための独自の手法(波形インバージョン)を開発してきた。波形インハージョンは走時などの2次データではなく、地震波形そのものをデータとして用いて情報を最大限抽出し、これまでになく高い解像度でD”領域の構造を推定する手法である。本研究ではその手法を三次元不均質構造に適用できるように拡張し、同手法を使用して中米下及び西太平洋下のD”領域の局所的3次元S波速度構造を推定した。

著者

丸山 茂徳 大森 聡一 千秋 博紀 河合 研志 WINDLEY B. F.

出版者

Tokyo Geographical Society

雑誌

地學雜誌 (ISSN:0022135X)

巻号頁・発行日

vol.120, no.1, pp.115-223, 2011-02-25

被引用文献数

5 42

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Pacific-type orogeny (PTO) has long been recognized as a contrasting accretionary alternative to continent-continent collisional orogeny. However, since the original concept was proposed, there have many new developments, which make it timely to produce a new re-evaluated model, in which we emphasize the following new aspects. First, substantial growth of Tonarite–Trondhjemite–Granite (TTG) crust, and second the reductive effect of tectonic erosion. The modern analog of a Pacific-type orogen developed through six stages of growth exemplified by specific regions; initial stage 1: the southern end of the Andes; stage 2: exhumation to the mid-crustal level at Indonesia outer arc; stage 3: the Barrovian hydration stage at Kii Peninsula, SW Japan; stage 4: the initial stage of surface exposure of the high-P/T regional metamorphic belt at Olympic Peninsula, south of Seattle, USA; stage 5: exposure of the orogenic core at the surface at the Shimanto metamorphic belt, SW Japan; and stage 6: post-orogenic processes including tectonic erosion at the Mariana and Japan trench and the Nankai trough.<br> The fundamental framework of a Pacific-type orogen is an accretionary complex, which includes limited ocean floor material, much terrigenous trench sediment, plus island arc, oceanic plateau, and intra-oceanic basaltic material from the ocean. The classic concept of a PTO stresses the importance of the addition within accreted rocks of new subduction-generated arcs and TTGs, which were added along the continental margins particularly during the Cretaceous. Besides the above additional or positive aspects of a PTO, here we emphasize the negative effects of previously little-considered tectonic erosion caused by subduction over time. The evaluation of such extensive tectonic erosion leads a prospect of the presence of huge quantities of TTG material in the lower transition zone, where many subducted slabs have ponded, as illustrated by mantle tomography. This is confirmed by density profiles of the mantle, which show that TTGs are abundant only along the bottom of the upper mantle accompanied by slab peridotite, lherzolite, and MORB. The major velocity anomaly in the lower transition zone is best explained by the predominance of SiO₂ phases, hence TTG, and not by MORB or ultramafic rocks. Reasonable calculations indicate that at a depth range of 520-660 km TTG material amounts to 6-7 times more than the total mass of the surface continental crust.<br> The traditional view is that the Japanese islands evolved since 520 Ma through five Pacific-type orogenies, which grew oceanward, thus creating a continuous accretionary complex <i>ca.</i> 400-500 km wide, with TTG growth at the continental side of each orogen. However, the subducting oceanic lithosphere has produced five times more TTG crust compared with the present TTG crust in the Japan islands. This is explained by the fact that over time tectonic erosion has dominated the increasing arc-TTG crust. Accordingly, Japan has lost four arc-TTG crusts to tectonic erosion. TTG material, such as trench sediment, arc crust, and continental margin crust, was fragmented by tectonic erosion and transported into the bottom of the upper mantle at depths of 520-660 km. Worldwide data suggest that tectonic erosion destroyed and fragmented most of the Pacific-type orogens.<br>(View PDF for the rest of the abstract.)