研究テーマ/Research Theme

断層、地震、そして地球の複雑さに挑む!

地震や地殻変動は、エネルギーが保存しない非平衡なシステムで起こる地球(特に地殻)の複雑な変形活動です。私は、室内での岩石変形実験、野外での地質調査、および計算機を使ったシミュレを有機的に組み合わせることで、岩石・鉱物の変形素過程から、露頭規模での断層・剪断帯の形成機構を結びつけ、さらには列島での地殻変形活動(地殻変動及び地震活動)の議論を通して、観察スケールを超えた地球の変形活動の包括的な理解を目指しています。

上記の問題を解決するために、具体的には下記の3点を主な研究対象としています。

  1. 岩石の流動変形・破壊・摩擦実験と各種分析計測に基づく地殻や岩石変形の素過程の究明
  2. 野外調査および観測に基づく地殻や岩石の変形挙動の把握
  3. 数理物理的知識(連続体力学、フラクタル幾何学など)を用いた地殻や岩石変形挙動のモデル化

特に2011年の東北沖地震以降は、岩石の変形という素過程がM9の巨大地震とそれに引き続く地殻変動にどのように絡んでいるのかという点に非常に興味を持っています。

下記はこれまでの研究をまとめたものです。


 

地質学的観察および測地学的観測から知る 下部地殻・上部マントルの流動

  • 武藤 潤・木戸正紀(2019)地学雑誌, 128(5)731-745, doi:10.5026/jgeography.128.731 (Japanese with English abstract).Flow of Lower Crust and Upper Mantle Inferred from Geological and Geophysical Observations, Jun Muto, Masaaki Kido, Journal of Geography(Chigaku Zasshi).

下部地殻および上部マントルの流動特性は、地震サイクルや島弧のジオダイナミクスを定量的に理解するために重要なパラメーターです。特に、地震後の余効変動など、遷移的な地殻変動に関連する地殻の変形過程は、定常的な変形過程に比べ、その信号強度が強いことから、地下のレオロジー特性を定量的に評価するために非常に重要なイベントです。この論文は、新学術領域「地殻ダイナミクス」の特集号として、近年の下部地殻と上部マントルのレオロジー特性を明らかにする地質学的および地球物理学的研究をまとめました。特に、巨大地震後に遷移的に現れる地殻変動(余効変動)は、下部地殻・上部マントルのレオロジー特性や震源断層の摩擦特性に強い制約を与えることに成功した最新の研究結果を紹介しました。さらに、地表に露出した断層岩の微細組織的観察からも、下部地殻と上部マントルの応力レベルを制約することが可能です。地震後の余効変動観測から、上部マントル内の局所的な変形過程を知ることができ、岩石のさまざまなレオロジー特性を考慮に入れることで、地下の温度構造および含水量の不均一性も明らかにすることが可能です。さらに、この論文では、近年進んでいる測地学的観測の逆解析から、地下のレオロジーの不均一性を推測する最近の活動も紹介しています。まだまだこのような研究は緒 についたばかりですが、様々な制約の下で、測地学的観測は室内での岩石変形実験と同様に、下部地殻および上部マントルの非線形流動特性を推定することが可能です。 この論文は、H31に修了した木戸正紀博士との共著で発表しました。

Flow properties of the lower crust and upper mantle are important parameters for better understanding geodynamics. In particular, they are crucial for quantitatively evaluating the process of stress accumulation and relaxation of seismic faults associated with transient crustal deformation, such as post-seismic deformation. Recent activities to illuminate the rheological properties of the lower crust and upper mantle are reviewed based on geological and geophysical observations. In particular, large, transient crustal deformation places robust constraints on the rheological properties. Careful observations of exhumed fault rocks limit the range of stress levels in the lower crust and upper mantle. Localized flow in the upper mantle from post-seismic observations can also be constrained. Combining detailed geodetic observations and numerical simulations, taking into account various rheological properties of rocks, can constrain the heterogeneity of viscosities possibly related to the heterogeneities of temperature and water content distribution. Furthermore, recent activities to infer rheological heterogeneity from an inversion analysis are also introduced. Under well-constrained conditions, nonlinear flow properties of the upper mantle can be estimated. These activities clearly indicate that various aspects of the rheological(flow) properties of the lower crust and upper mantle can be extracted from the analyses.


ゲルマニウムオリビンを使った変形実験

  • Sawa, S., Miyajima, N., Muto, J., Nagahama, H. (2021) Strain-induced partial serpentinization of germanate olivine with a small amount of water. American Mineralogist.
  • Sawa, S., Muto, J., Miyajima, N., Shiraishi, R., Kido, M., Nagahama, H. (2021) Strain localization bands in fine-grained aggregates of germanate olivine and pyroxene deformed by a Griggs type apparatus. International Journal of Rock Mechanics and Mining Sciences, 104812, doi:10.1016/j.ijrmms.2021.104812.

Strain-induced partial serpentinization of germanate olivine with a small amount of water

海洋プレートに存在する蛇紋岩の主要構成物質である蛇紋石は含水鉱物であり、地下60-300 kmで発生するやや深発地震を引き起こす鉱物の1つと考えられています。しかしながら、高密度の海洋プレートの内部にどれだけの水が浸透し、かんらん石と反応して蛇紋石を作るかは明らかにされていません。そこで、限られた水の量でも蛇紋石が形成できるかどうかを調べるために、通常のシリケイトかんらん石のアナログ物質であるゲルマニウムかんらん石を用いて変形実験を行いました。結果、サンプルに地震を引き起こさない安定したすべりが発生し、透過型電子顕微鏡分析によって、断層に沿ってゲルマニウム蛇紋石の細粒な板状粒子が観察されました。フーリエ変換赤外分光法分析では、蛇紋石に由来するピークがサンプルの広範囲に見られました。この結果は、海洋プレート内部のかんらん石が差応力下で極少量の水により蛇紋岩化することを示唆しています。この論文は、博士課程院生 澤 燦道(学振特別研究員)くんの研究で、環境地球科学国際共同大学院プログラムを通じて、ドイツ バイエルン地球科学研究所(Bayerischen Geoinstitut)の宮島 延吉博士との共著で発表しました。

Antigorite, a high-pressure polymorph of serpentine, is considered to be the most abundant hydrous mineral in the subduction zone. Although the antigorite is presumed as one of the origins of intermediate-depth earthquakes in the subduction zone, the amount of antigorite is uncertain because the amount of water infiltrated into the oceanic lithosphere is still debated. To investigate whether antigorite can be formed even under the limited availability of water, we conducted the axial deformation experiments of magnesium germanate at 1.2 GPa and T = 500-800 ℃ using a Griggs-type deformation apparatus. Magnesium germanate is an analog material of magnesium silicate, and the absorbed water in the starting material was eliminated preliminary. Nevertheless, the samples had initially high porosity, and hence the small amount of water (about 200 ppm wt H2O) was retained in the samples. In the samples deformed at 600 ℃, the stable slip occurred, and TEM analysis revealed that fine-grained platelets of germanate antigorite existed along the faults. Sharp absorption band assigned to the OH stretching vibration of antigorite in Fourier transform infrared spectroscopic (FT-IR) analysis also implies that antigorite widely was formed in the samples deformed at temperature lower than 600 ℃. Our results indicate that strain-induced hydration of germanate olivine results in antigorite formation even under a small amount of water. Thus, partly hydrated peridotite in the oceanic lithosphere can be formed under slight water infiltration due to high strain accumulated by the subduction.

Strain Localization Bands in Fine-grained Aggregates of Germanate Olivine Deformed by a Griggs Type Apparatus

歪集中帯は周囲の岩石と空隙率や透水率が異なるため、岩盤へのCO2貯留や地下水浸透を理解する上で重要です。歪集中帯は天然や実験において、脆性塑性遷移領域に相当する圧力で変形した空隙率の高い砂岩によく見られます。しかしながら、非常に細粒な実験試料で歪集中帯が観察された例はありませんでした。そこで、数ミクロンサイズという細粒なゲルマニウムかんらん石を用いて実験を行ったところ、数百ナノサイズの粒子で充填された数多くの歪集中帯が見られました。透過電子顕微鏡で歪集中帯を観察した結果、変形によって生じた転位(結晶の面欠陥)に沿って粒子が破壊し、歪集中帯が形成されたことが明らかになりました。歪集中帯の形成には粒径は特に関係がなく、岩石の空隙率と変形圧力が重要であることを示唆しています。この論文は、博士課程院生 澤 燦道(学振特別研究員)くんの研究で、環境地球科学国際共同大学院プログラムを通じて、ドイツ バイエルン地球科学研究所(Bayerischen Geoinstitut)の宮島 延吉博士との共著で発表しました。

Strain localization bands are often observed in natural and experimentally deformed porous rocks. Samples used in the majority of the previous studies are natural porous coarse sandstones. Meanwhile, fine-grained aggregates originating from powders have been used as an initial sample of high pressure coaxial deformation experiments. Generally, the compaction for sintering these aggregates is performed before the experiments. No coaxial experiments have been done the compaction of powder under high pressure and low temperature during experiments (cold pressing). This stress state during sintering is close to the transitional regime from brittle fault- ing to cataclastic flow, so there is a possibility that compaction bands which are a type of strain localization bands may form at the cold-pressing stage of deformation in laboratory experiments. We conducted cold pressing experiments of the powder of fine-grained germanate olivine and pyroxene having about 30 % porosity with a Griggs type apparatus. The experiments reveal that compaction bands form in the compacted and sintered sample. As a result of microstructural analysis, grains with high dislocation density rotate and crush with shear strain concentration during compaction. At the end of the compaction, the grain size becomes small, and bands made of fine-grained material form.


東北日本弧の強度断面と東北沖地震の余効変動解析

  • Jun Muto (2011) Rheological structure of northeastern Japan lithosphere based on geophysical observations and rock mechanics. Tectonophysics, 503, 201-206.
  • 武藤 潤・大園 真子 (2012) 東日本太平洋沖地震後の余効変動解析に向けた東北日本弧レオロジー断面. 地質学雑誌 特集号(東北地方太平洋沖地震 -統合的理解に向けて-), 118, 323-333.
  • Jun Muto, Bunichiro Shibazaki, Yoshihiro Ito, Takeshi Iinuma, Mako Ohzono, Takumi Matsumoto, Tomomi Okada (2013) Two-dimensional viscosity structure of the northeastern Japan island arc-trench system. Geophys. Res. Lett., 40, 1–5, doi:10.1002/grl.50906.
  • Jun Muto, James D. P. Moore, Sylvain Barbot, Takeshi Iinuma, Yusaku Ohta, Hikaru Iwamori (2019) Coupled afterslip and transient mantle flow after the 2011 Tohoku earthquake. Science Advances, 5, eaaw1164, doi: 10.1126/sciadv.aaw1164.

岩石鉱物のレオロジー特性は,地殻の非弾性変形,地震断層への応力集中や地震後の余効変動などの様々な地殻変動に大きく影響を及ぼす.東北日本弧の現在の地殻変形活動を理解するために,地球物理学的観測と近年得られた岩石力学実験の結果を使い,東北日本弧のレオロジー強度断面を作成した.得られた断面を用いて,現在の東北日本弧で起こっている地殻変形活動(地震後の余効変動及び測地学的歪場など)や既存断層の再活動原因に関して議論した.さらに,巨大地震後の余効変動において,粘性緩和を詳細に評価するために,得られた強度断面を使い,地震後の応力降下量を仮定することで,粘性構造を計算した.通常の測地学的な手法で仮定される1次元成層粘弾性構造に比べ,東北日本弧は島弧に直交する方向に著しい粘性率の不均質性を持つことを示した.東北沖のプレートカップリングを詳細に評価するための東北沖地震余効変動の定量的解析において,レオロジー不均質構造を考慮する必要があることを指摘した.現在,得られた不均質構造を考慮した東北沖地震後の余効変動解析に挑戦中!本研究は、建築研究所 芝崎文一郎上席研究員、山形大学 大園真子講師、Jamstec飯沼卓史博士、防災科研 松本拓巳博士、東北大学岡田知己准教授との共同研究です。

Two-dimensional viscosity profiles were constructed for the northeastern Japan islands arc-trench system covering the source area of the 2011 Tohoku-Oki earthquake. From seismologically determined models of lithospheric structure, experimentally derived constitutive laws of various rocks, and densely measured geothermal gradient data, we have predicted the steady-state effective viscosity across the subduction zone. The profile reveals strong lateral viscosity gradients both parallel and normal to the trench axis. The detailed viscosity structures presented here contribute to accurate evaluation of viscoelastic relaxation components when modeling geodetically measured postseismic deformation at high spatial and temporal resolution.


Jun Muto, Bunichiro Shibazaki,  Takeshi Iinuma,Yoshihiro Ito, Yusaku Ohta, Satoshi Miura, Yoshihiro Nakai (2016) Heterogeneous rheology controlled postseismic deformation of the 2011 Tohoku-Oki earthquake, Geophys. Res. Lett., 43, doi:10.1002/2016GL068113.

2次元有限要素モデリングに岩石のレオロジーを考慮した東北日本弧のレオロジーモデルを適用することで、2011年東北地方太平洋沖地震後の余効変動のモデル化を行いました。東北日本島弧−海溝系のレオロジー不均質として、火山直下の低粘性帯やリソスフェアーアセノスフェア境界などの含んだ我々のモデルは、第四紀火山である鳴子火山周辺で観測された局所的(空間スケール20 km以下)な沈下などの余効変動をうまく再現することに成功しました。また粘弾性モデルと逆解析により推定された余効すべりは、本震の破壊領域の下限付近にピークを持ち、微小繰り返し地震で推定されたすべり量と調和的です。これらの結果は、岩石のレオロジーを考慮することで、非常に局所的な余効変動観測からレオロジーの不均質性を評価できる可能性を示唆するとともに、プレートカップリングの回復を詳細に評価するためには東北日本弧のレオロジー不均質を考慮する必要性を示しました

Using two-dimensional finite element modeling, we reproduced the observed postseismic deformation of the 2011 Tohoku-Oki earthquake. Our model, which accounts for the lithosphere-asthenosphere boundary and weak zones beneath volcanoes, was able to reproduce small-scale (<20 km) perturbations in postseismic deformation observed by the dense geodetic network, such as local subsidence around Quaternary volcanoes. The inverted afterslip has a peak at the downdip limit of the main rupture region on the subducting plate interface, consistent with physical predictions. The combination of afterslip and viscoelastic relaxation in a heterogeneous rheology model explains the observations well, even on small scales.


Jun Muto, James D. P. Moore, Sylvain Barbot, Takeshi Iinuma, Yusaku Ohta, Hikaru Iwamori (2019) Coupled afterslip and transient mantle flow after the 2011 Tohoku earthquake. Science Advances, 5, eaaw1164, doi: 10.1126/sciadv.aaw1164.

これまでの大地震後に起こる地殻変動(余効変動)の測地学的観測とモデリングにより、マントルの粘性構造と地震断層の摩擦特性が明らかにされています。しかし、沈み込み帯でおこる巨大地震では、粘弾性流動と余効すべりは地震後の期間に互いに力学的に相互作用し、地表変動に影響を及ぼすことが知られています。そこで、我々は、Mw9.0の東北地方太平洋沖地震後の余効変動を対象に、岩石力学に基づく非線形レオロジー(べき乗流動則および速度状態依存摩擦校正速)によるモデリングから、余効すべりと粘弾性緩和の非線形相互作用(カップリング効果)を検証しました。岩石力学に基づく複雑なモデリングは、水平・垂直変動だけでなくその時系列も復元することが可能であり、粘弾性緩和と余効すべりの力学的相互作用により地表変動は影響を受けます。特に、地震後6年が経過し、現在活発に隆起を続ける太平洋沿岸部は、震源域深部での余効すべりによって引き起こされていることが明らかになりました。粘弾性緩和と余効すべりの力学的相互作用は、地震直後5年程度では地表変動に対してそれほど大きくな影響を及ぼしませんが、今後、沿岸部の隆起過程など、長期の余効変動観測に関して無視できない効果を持っています。本研究を通じて、特に東北日本弧の地震サイクルやジオダイナミクスにおいて、粘弾性緩和と余効すべりの力学的相互作用の定量的評価の重要性を強調することに成功しました。

Modeling of postseismic deformation following great earthquakes has revealed the viscous structure of the mantle and the frictional properties of the fault interface. However, for giant megathrust events, viscoelastic flow and afterslip mechanically interplay with each other during the postseismic period. We explore the role of afterslip and viscoelastic relaxation and their interaction in the aftermath of the 2011 Mw (moment magnitude) 9.0 Tohoku earthquake based on a detailed model analysis of the postseismic deformation with laterally varying, experimentally constrained, rock rheology. Mechanical coupling between viscoelastic relaxation and afterslip notably modifies both the afterslip distribution and surface deformation. Thus, we highlight the importance of addressing mechanical coupling for long-term studies of postseismic relaxation, especially in the context of the geodynamics of the Japan trench across the seismic cycle.

この研究は、日経新聞電子版(2019年9月27日)、朝日新聞科学の扉(2020年4月20日)などで取り上げられました。


低速摩擦試験による石英ガウジのESR信号強度変化

  • Kiriha Tanaka, Jun Muto, Yasuo Yabe, Toshitaka Oka, Hiroyuki Nagahama (2020) Effect of fracture on ESR intensity using a low-velocity rotary shear apparatus. Geochronometria (In press).

電子スピン共鳴(ESR)法を用いた断層年代推定法は「地震時に起こる断層すべりが、断層岩に含まれる石英中のESR信号の信号強度(欠陥が捕獲した不対電子数)を0にする」という仮定が成り立つことを前提としています。ESR信号のゼロセットと呼ばれるこの仮定の要因の1つに地震性すべりによる岩石破砕があります。過去に行われた模擬石英ガウジの低速摩擦実験から、岩石破砕はESR信号強度を増加またはゼロセットすると報告されてきました。しかし、これらの結果は摩擦実験中にガウジに混入した変形治具の摩耗物がESR測定の感度を低下させた影響を受けている可能性があることが報告されています。そこで、地震性すべりによる岩石破砕とESR信号強度の関係を明らかにするため、ガウジに混入した摩耗物がESR測定に及ぼす影響を調べる実験と回転式低速摩擦試験機を用いた模擬石英ガウジの低速摩擦実験を行いました。結果として、剪断変位量の増加と共に、石英中に見られるE1’ 中心(≡Si・)は増加、Peroxy 中心(-Si-O-O・)、OH 中心(O3-)は変化しないことが明らかになりました。ガウジの微細構造観察、変形治具の摩耗物が混入したガウジの模擬試料のESR測定、ガウジの粒子間や摩擦面での温度上昇の推定を行うと、ESR信号強度変化は、主に破砕が原因で起きたことが示唆されます。本研究は、地殻浅部での摩擦発熱の小さい地震性すべりではESR信号のゼロセットを引き起こすことができないことを示唆しています。この研究は、地震観測センター矢部先生、JAEA 岡先生らの協力によるD2 田中桐葉君の博士課程での研究成果です。

We sheared simulated-quartz gouges using a low-velocity rotary shear apparatus and evaluated the relationship between electron spin resonance (ESR) intensity and displacement quantitatively considering problems of contaminants. ESR intensity of E1’ center increased while OHC and peroxy center kept constant with the increasing displacement up to 1.4 m. Microstructural analysis showed grain size reduction and fracture of starting gouges, hence, the fracture can affect the change in ESR intensity. ESR measurements were also conducted for starting gouges with variable amounts of contaminants and it was confirmed that the effect of contaminants on the change in ESR intensity was negligible. Moreover, we estimated the temperature rise by the frictional heating on the surface and between particles and it was shown that the effect of frictional heating on ESR intensity was also negligible in our experimental condition. Therefore, we could clarify the relationship between ESR intensity and fracturing with various displacements separately from contaminants and frictional heating. The results imply that the zero-setting of ESR signals can not occur by the fracture with low frictional heating at the shallow depth.


亀裂を含んだ岩石の弾性波速度に関する研究

  • Eranga Gayanath Jayawickrama, Hayata Tamai, Jun Muto, Hiroyuki Nagahama (2019) Pressure dependence of elastic deformation at multiple contacts and estimation of contact state of thin cracks.  Geophysics 84 (4): WA153–60.

本研究では、弾性波速度の変化により、薄い亀裂の複数の接触における弾性変形の圧力依存性を調査し、圧力の変化による薄い亀裂の開放性を定量化することができました。このプロセスでは、最初に、以前に導入したいくつかの単一接点の荷重-変位関係を複数の接点の関係に拡張し、次に接点での変位に関する圧力の一次導関数を取得しました。この形状の、接点の弾性変形の圧力依存性の関係。圧力依存性は、「薄い亀裂の多重接触状態」と呼ばれる圧力指数μによって与えられます。これは、小林・古住(1977)による経験的に導出された弾性波速度関係における同様の圧力指数と相関し、弾性波速度の圧力依存性によって岩石の薄い亀裂の多重接触状態が得られることを発見した。推定された多重接触状態は、円錐接触と平坦接触でそれぞれ0.67と0.5の2つの理想値を持ち、理想状態間のμ値は、2つの理想状態とその母集団の混合によって形成される接触の弾性変形の圧力依存性を示しました。 0.5より小さい接触状態値と0.67より大きい値は、それぞれ亀裂の閉じた状態と開いた状態を示します。したがって、亀裂の開放性は、この関係から定量化できました。さらに、さまざまな岩石の弾性波速度に関する以前に公開されたデータを利用して、この関係の適用性をテストし、複数の接触の弾性変形の圧力依存性が、の圧力依存性から推定された複数の接触状態によって定量的に取得できることを確認しました。

In this study we investigated the pressure dependence of elastic deformation at multiple contacts in thin cracks through the variation of elastic wave velocity and were able to quantify the openness of thin cracks with the variations in pressure. In the process, first, we extended several previously introduced single contact load-displacement relationships to those of multiple contacts and then obtained the first derivative of pressure with respect to the displacement at contacts. This shaped, relationships for the pressure dependence of elastic deformation of the contacts where the pressure dependence was given by a pressure exponent μ which we termed “the multiple contact state of thin cracks”. This was correlated with a similar pressure exponent in an empirically derived elastic wave velocity relationship by Kobayashi and Furuzumi (1977) and found that the multiple contact state of thin cracks in a rock can be obtained by the pressure dependence of elastic wave velocity. The estimated multiple contact state has two ideal values of 0.67 and 0.5 for conical and flat contacts, respectively and μ values between the ideal states gave the pressure dependence of elastic deformation of contacts, formed by a mixture of two ideal states and their population. The contact state values smaller than 0.5 and values greater than 0.67 show closed and open state of cracks, respectively, hence the openness of the cracks were quantifiable from this relationship. We further tested the applicability of this relationship by utilizing previously published data on elastic wave velocities of various rocks, and it was verified that the pressure dependence of elastic deformation of multiple contacts can be quantitatively obtained by the estimated multiple contact states from the pressure dependence of elastic wave velocity.


模擬断層ガウジのゆっくりとした破壊伝播に伴う応力状態の効果

  • Ken-ichi Hirauchi,  Jun Muto, (2015) Observations of the slow rupture process in synthetic fault gouges. Earth Planets Space, 67:25, doi:10.1186/s40623-015-0199-x.

沈み込み帯でのスロースリップイベント(SSE)は、ゆっくりとした破壊によりおこり検出可能な地震波を発生しないこと知られています。さらに、特に破壊は通常の地震よりも4桁以上も遅い速度で伝播します。しかし、沈み込み帯深部にせまる条件での実験結果が少ないことなどから、スロースリップ発生機構は未だ理解されていません。我々は、沈み込み帯を構成する岩石群(低温型・高温型蛇紋岩とカンラン岩)の模擬断層ガウジを用いて、不安定すべりの実験を行いました。単一の不安定すべりのイベントにおいて、多くの場合、不安定な高速破壊の前にゆっくりとした破壊が先行することを明らかにしました。特に、模擬断層ガウジの摩擦係数が低下(0.7から0.5まで)するのに従い、遅い破壊伝播モードが現れ、破壊伝播速度は実際の観測から得られる短期的SSEの伝播速度に相当するような値にまで低下します(0.07 to 5.43 m/s)。我々の実験結果は、SSESの発生が、低垂直応力と低い断層帯強度という2つの条件によって促進されることを示唆しています。これらはSSEの発生は、プレート境界の応力状態、たとえば含水相(蛇紋岩やタルク等)の生成や流体の脱水に伴う間隙圧の増加、を強く反映している可能性を示唆しています。静岡大学平内健一博士との共同研究です。


摩擦に伴う石英の非晶質化と速度弱化に関する研究

  • Yu Nakamura, Jun Muto, Hiroyuki Nagahama, Ichiko Shimizu, Takashi Miura & Ichiro Arakawa (2012), Amorphization of quartz by friction: Implication to silica-gel lubrication of fault surfaces. Geophys. Res. Lett., 39, L21303, doi:10.1029/2012GL053228

摩擦に伴う岩石鉱物のエネルギー散逸過程として,岩石鉱物の非晶質化が知られている.特に石英岩の非晶質化は,大気中の水分を吸着し含水したゲルの生成によりすべり・速度弱化を引き起こすことから,摩擦特性にも直接影響を及ぼす.しかし、その非晶質化の素過程や含水過程は未だよくわかっていない.摩擦に伴う石英の非晶質化を調べるために,Pin-on-disk型の摩擦試験機を用いて,摩擦すべりさせた後の石英トラックのラマン分光分析を行った.摩擦に伴い,SiO4四面体6員環からなる石英構造中に,3−4員環という歪んだネットワーク構造が形成されていることが明らかになった.非晶質化は,高速すべりを必要としないことから,真実接触点での高応力下で,SiO4 中距離構造がより歪んだ平面 3 および 4 員環となり,歪んだ Si-O 結合部から選択的に水和反応が進むことで,摩擦表面に非晶質シリカの水和物を生成したことを示唆する.非晶質シリカ水和物は,強度の低い粘性物質として振舞うことで,速度弱化を引き起こしたことを考えられる.本研究は、H24年修了 中村さんの修士論文の一部で、東京大学清水以知子博士、学習院大学荒川一郎教授、(独) 労働安全衛生総合研究所 三浦崇博士との共同研究です。

To understand physico‐chemical processes at real contacts (asperities) on fault surfaces, we conducted pin‐on‐disk friction experiments at room temperature, using single crystalline quartz disks and quartz pins. Velocity weakening from friction coefficientμ ∼ 0.6 to 0.4 was observed under apparent normal stresses of 8–19 MPa, when the slip rate was increased from 0.003 to 2.6 m/s. Frictional surfaces revealed ductile deformation of wear materials. The Raman spectra of frictional tracks showed blue shifts and broadening of quartz main bands, and appearance of new peaks at 490–520 and 610 cm−1. All these features are indicative of pressure‐ and strain‐induced amorphization of quartz. The mapping analyses of Fourier transform infrared (FT‐IR) spectroscopy at room dry conditions suggest selective hydration of wear materials. It is possible that the strained Si‐O‐Si bridges in amorphous silica preferentially react with water to form silica‐gel. In natural fault systems, amorphous materials would be produced at real fault contacts and accumulate over the fault surfaces with displacements. Subsequent hydration would lead to significant reduction of fault strength during slip.


石英の格子定向配列に関する研究

  • Jun Muto, Greg Hirth, Renee Heilbronner & Jan Tullis (2011) Plastic anisotropy and fabric evolution in sheared and recrystallized quartz single crystals. J. Geophys. Res., B02206, doi:10.1029/2010JB007891.

塑性変形した岩石の格子定向配列(LPO)は,変形時の物理条件(温度,ひずみ速度)を反映することが知られている.累進変形時のP, T条件が熱力学的に規制できる剪断帯においては,石英のc軸LPOは,変形時の温度上昇により優先すべり系がbasal <a>からprism <a>へ転移することで,type I crossed girdle からY 集中へとLPOが転移することが知られている.一方,天然の変形岩や実験から,一定温度下でも,歪の増加に伴い,LPOが変化する事例が報告されたが,転移のメカニズムは不明であった.一定温度下で剪断歪の増加に伴うLPO転移を明らかにするために,結晶方位が既知の人工水晶単結晶を用いて,様々な結晶方位を持つ試料の高温高圧変形実験を行った.人工水晶は,剪断歪の増加および動的再結晶により,結晶方位を容易に90度再配置し,初期方位によらず,最終的にprism <a> の方位を持つ再結晶粒子が卓越する(Y集中LPO).本研究により,LPOの発達機構に関して,動的再結晶の重要性が明確になり,温度の影響だけでなく,歪や変形履歴の効果を考慮する必要があることを示した.これにより,地質学的にしか測定出来なかった“歪”を地震波異方性などの地球物理学的観測から測定できる可能性を指摘した.この研究は、Brown大学 Jan Tullis教授、Greg Hirth 教授、Tromso 大学 Renee Heilbronner教授との共同研究です。

The effect of a lattice preferred orientation on the flow strength of quartz aggregates dynamically recrystallized from single crystals of synthetic quartz was investigated using general shear experiments in a Griggs apparatus. Experiments were conducted at shear strains (γ) up to 5 at a temperature of 900°C, confining pressure of 1.5 GPa,and shear strain rate of 10<sup>-5</sup> s<sup>-1</sup>. Three starting orientations of crystal were used, to activate three slip systems: basal<a>, prism [c], and prism<a>, although slip‐induced rotation of the crystal axes in the first two orientations led to the activation of additional slip systems. For crystals with higher water contents, basal<a> and prism <a> orientations are relatively weak and prism [c] orientations are stronger. All three initial crystal orientations undergo dynamic recrystallization with increasing shear strain, although the strain required for 100% recrystallization varies: gamma ≈ 2 for prism [c] slip, gamma ≈ 3.8 for basal <a> slip, and gamma ≈ 5 for prism <a> slip. For all three starting orientations, distinct domains of recrystallized grains develop with c axes parallel to Y of the strain ellipsoid (Ymax), replacing recrystallized grains of other orientations; the Ymax domains increase in size with increasing strain. In addition, strain markers show that strain is highly localized within the Ymax domains, indicating geometrical softening of up to an order of magnitude in effective viscosity.


摩擦に伴う岩石の帯電と気体放電プラズマ発生に関する研究

  • Jun Muto, Hiroyuki Nagahama, Takashi Miura & Ichiro Arakawa (2006) Frictional discharge plasma from natural semiconductor/insulator junctions: origin of seismo- electromagnetic radiation. Phys. Chem. Earth, 31, 346-351.
  • Jun Muto, Hiroyuki Nagahama, Takashi Miura & Ichiro Arakawa (2006) Frictional discharge from sheared asperity: implication for fractal seismo-electromagnetic radiation. Trans. IEE Jpn., 126, 242-243.
  • Jun Muto, Hiroyuki Nagahama, Takashi Miura & Ichiro Arakawa (2007) Frictional discharge at fault asperities: Origin of fractal seismo-electromagnetic radiation. Tectonophysics, 431, 113-122.
  • Jun Muto, Hiroyuki Nagahama, Takashi Miura & Ichiro Arakawa (2008) Frictional discharge plasma and seismo-electromagnetic phenomena. Phys. Earth Planet. Inter., 168, 1-5.
  • Kumi Onuma, Jun Muto, Hiroyuki Nagahama & Kenshiro Otsuki (2011) Electric potential changes associated with nucleation of stick-slip of simulated gouges. Tectonophysics, 502, 308-314.

地震に先行して,電磁放射や発光などの電磁気現象が知られている。一方,岩石実験から,岩石破壊時に電磁波(光),荷電粒子やプラズマの発生が報告されているが,地震に先行する電磁放射機構の決定的な解明には至っていない.断層面上でのアスペリティの動きを模擬したすべり摩擦実験から,石英等の天然鉱物間での摩擦帯電による発光を顕微鏡下にて直接観察した.発光は低応力(4MPa),低すべり速度(9.5mm/s)で発生し,分光測定から,摩擦帯電による気体の絶縁破壊(プラズマ発光)に起因することが明確になった.さらに,模擬断層ガウジを用いた摩擦実験から,高速すべりの直前のゆっくりすべり時に断層表面が局所的に帯電する可能性を明らかにした.これらの発見により,地震直前のゆっくりすべり時(地震核形成)に長波長電磁異常が伴われる可能性があることを摩擦実験から明らかにし,地震先行電磁現象を断層力学と地球電磁気学のフレームワークで理解することに成功した.

Although several mechanisms of precursory electromagnetic emissions of earthquakes have been pro- posed, there has been no in situ observation of electromagnetic phenomena occurring at the frictional contacts during rock frictions. From an experiment simulating the motion of an asperity on a fault surface, we report the generation of photon emissions around frictional contacts between natural rock minerals. Spectroscopic analysis clarified that the photon emissions are caused by dielectric breakdown of ambient gases (electric-discharge plasma) due to frictional electrification. The plasmas were found to occur under normal stress of 4 MPa and sliding speed of 10-2 m/s. This indicates that plasma generations do not require high-normal stresses needed to fracture rocks and sliding speeds as high as seismic rates (∼1 m/s). Thus, frictional discharges could occur at microscopic asperities at the onset of the slips of earthquakes, and might be one of the sources of the seismo-electromagnetic emission.

Our friction experiments using simulated gouges detected premonitory changes in electric potential before stick-slip events. These precursor electric signals have been detected both in piezoelectric quartz and non- piezoelectric gabbroic gouges. The changes in the potential were locally detected by electrodes set very close to the fault surface associated with slow slip prior to stick-slip events. The magnitude of electrification is proportional to fault slip associated with gradual stress releases, indicating that the electrification is slip- dependent process. From the detailed microstructural analyses on a representative sample, Riedel (R1) shears known to be formed during stable sliding were the most densely developed around the electrode pair which detected the precursory voltage changes. This indicates that local increases in the voltages were likely caused by frictional electrification due to slow slip on R1 shears in nucleation phases of stick-slip events. Our experimental results imply that natural faults with thicker gouge zones would require greater precursory slips, resulting in larger electric signals in the nucleation phase of earthquakes.


上部地殻岩石の水・歪軟化に関する研究

  • Jun Muto, Hiroyuki Nagahama & Tetsuo Hashimoto (2004) Micro-IR reflection spectroscopic mapping: application to detection of hydrogen-related species in natural quartz. J. Microscop., 216, 222-228.
  • Jun Muto, Hiroyuki Nagahama & Tetsuo Hashimoto (2005) Water distribution in dynamically recrystallized quartz grains: cathodoluminescence and micro-infrared spectroscopic mappings. In Burlini L. and Bruhn D. (eds.) Geol. Soc. London Spec. Publ., 245, 397-407.

かつての内陸地震断層深部である畑川破砕帯の野外地質調査を行い,採取した断層変形岩の解析から,地殻の主要鉱物である石英多結晶体中に微量の水(数百ppm H/Si程度のH2O, OH)が加わることで,石英の塑性強度が大きく減少する可能性を指摘した.岩石力学実験から,石英の塑性変形強度が含水条件下で著しく減少すること(水軟化)が知られている.しかし,従来の赤外分光計による計測では,水の空間分布を解明することは困難だった.そのため,顕微鏡スケールでの変形組織(結晶方位,再結晶粒径等)と,含水量の比較は困難であり, 水軟化の機構解明には至っていない.本研究では,畑川破砕帯に産するマイロナイト中の再結晶石英多結晶体の赤外マッピング測定から水欠陥の分布・濃度を解明できる可能性を指摘した.再結晶石英粒の細粒化(塑性歪の増加)に伴い,H2O(3600~3300cm-1)およびSiOH(970~880cm-1)の濃度が増加する.上述の変化は,地殻中には顕微鏡スケールで,水の分布が不均質であり,石英粒の細粒化に伴い,粒界に存在する水欠陥(H2O,OH)濃度が増加することを示唆している.粒界に存在する水は,石英の粒界移動再結晶を促し,内陸地震断層の深部延長において,歪局所化の一因となると考えられる.


Effect of stress state on slow rupture propagation in synthetic fault gouges

  • Ken-ichi Hirauchi,  Jun Muto, (2015) Observations of the slow rupture process in synthetic fault gouges. Earth Planets Space, 67:25, doi:10.1186/s40623-015-0199-x.

Slow slip events (SSEs) in subduction zones are known to proceed so sluggishly that the associated slow ruptures do not generate any detectable radiating seismic waves. Moreover, they propagate at speeds at least four orders of magnitude slower than regular earthquakes. However, the underlying physics of slow slip generation has yet to be understood. Here, we carry out laboratory studies of unstable slip along simulated fault zones of lizardite/chrysotile (liz/ctl) and antigorite (i.e., low- and high-temperature serpentine phases, respectively) and olivine, under varying conditions of normal stress, with the aim of better understanding the influence of stress state on the process of slow rupture along the plate interface. During a single unstable slip, we clearly observe a slow rupture phase that is often followed by an unstable, high-speed rupture. We find that lower fault-zone friction coefficients (μ values from 0.7 down to 0.5) lead to increasing degree of the slow rupture mode, and also that the slow rupture velocities (Vr = 0.07 to 5.43 m/s) are largely consistent with those of short-term SSEs observed in nature. Our findings suggest that the generation of SSEs is facilitated by conditions of low normal stress and low fault-zone strength along the plate interface, which may be weakened by metamorphic reactions that result in the production of hydrous phases (e.g., serpentine) and/or the direct involvement of fluid itself, leading to a reduction in effective normal stress.

Amorphization of quartz by friction: Implication to silica-gel lubrication of fault surfaces 

  • Yu Nakamura, Jun Muto, Hiroyuki Nagahama, Ichiko Shimizu, Takashi Miura & Ichiro Arakawa (2012), Amorphization of quartz by friction: Implication to silica-gel lubrication of fault surfaces. Geophys. Res. Lett., 39, L21303, doi:10.1029/2012GL053228

To understand physico-chemical processes at real contacts (asperities) on fault surfaces, we conducted pin-on-disk friction experiments at room temperature, using single crystalline quartz disks and quartz pins. Velocity weakening from friction coefficient μ 0.6 to 0.4 was observed under apparent normal stresses of 8–19 (18 > 19) MPa, when the slip rate was increased from 0.003 to 2.6 m/s. Frictional surfaces revealed ductile deformation of wear materials. The Raman spectra of frictional tracks showed blue shifts and broadening of quartz main bands, and appearance of new peaks at 490–520 and 610 cm-1. All these features are indicative of pressure and strain-induced amorphization of quartz. The mapping analyses of Fourier transform infrared (FT-IR) spectroscopy at room dry conditions suggest selective hydration of wear materials. It is possible that the strained Si-O-Si bridges in amorphous silica preferentially react with water to form silica-gel. In natural fault systems, amorphous materials would be produced at real fault contacts and accumulate over the fault surfaces with displacements. Subsequent hydration would lead to significant reduction of fault strength during slip.

Rheological profiles of the NE Japan island arc-trench system and viscous relaxation after the 2011 Tohoku Oki Earthquake

  • Jun Muto (2011) Rheological structure of northeastern Japan lithosphere based on geophysical observations and rock mechanics. Tectonophysics, 503, 201-206.
  • 武藤 潤・大園 真子 (2012) 東日本太平洋沖地震後の余効変動解析に向けた東北日本弧レオロジー断面. 地質学雑誌 特集号(東北地方太平洋沖地震 -統合的理解に向けて-), 118, 323-333.
  • Jun Muto, Bunichiro Shibazaki, Yoshihiro Ito, Takeshi Iinuma, Mako Ohzono, Takumi Matsumoto, Tomomi Okada (2013) Two-dimensional viscosity structure of the northeastern Japan island arc-trench system. Geophys. Res. Lett., 40, 1–5, doi:10.1002/grl.50906.
  • Jun Muto, Bunichiro Shibazaki,  Takeshi Iinuma,Yoshihiro Ito, Yusaku Ohta, Satoshi Miura, Yoshihiro Nakai (2016) Heterogeneous rheology controlled postseismic deformation of the 2011 Tohoku-Oki earthquake, Geophys. Res. Lett., 43, doi:10.1002/2016GL068113.

A two-dimensional rheological profile crossing the northeastern Japan arc was created to evaluate and distinguish various styles of post-seismic deformation relating to the 2011 Tohoku Oki earthquake (Mw 9.0). The calculated profile is based on recent rock mechanics studies and geophysical observations that explain observed patterns of geodetic strain accumulation and shallow seismicity prior to the earthquake. Viscosities were then calculated assuming stress-change magnitudes commonly associated with interplate earthquakes. Recently derived flow laws for various rocks and minerals predict the presence of weak zones that developed via processes likely operating in the lithosphere (e.g., partial melting and shear zone development). Strain localization into weak zones explains low viscosity estimates (1018–1020 Pa s) from post-seismic creep after inland earthquake events. Our calculations reveal significant lateral variations in strengths and viscosities across the northeastern Japan arc. These viscoelastic lithospheric structures should be taken into account in order to differentiate viscous relaxation from other post-seismic deformation processes. Now we’re trying to analyze the post-seismic deformation of the Tohoku Oki Earthquake by the rheological profiles taking lithosphere heterogeneity into account!

Using two-dimensional finite element modeling, we reproduced the observed postseismic deformation of the 2011 Tohoku-Oki earthquake. Our model, which accounts for the lithosphere-asthenosphere boundary and weak zones beneath volcanoes, was able to reproduce small-scale (<20 km) perturbations in postseismic deformation observed by the dense geodetic network, such as local subsidence around Quaternary volcanoes. The inverted afterslip has a peak at the downdip limit of the main rupture region on the subducting plate interface, consistent with physical predictions. The combination of afterslip and viscoelastic relaxation in a heterogeneous rheology model explains the observations well, even on small scales.

Effect of dynamic recrystallization on the development of lattice preferred orientation of quartz

  • Jun Muto, Greg Hirth, Renee Heilbronner & Jan Tullis (2011) Plastic anisotropy and fabric evolution in sheared and recrystallized quartz single crystals. J. Geophys. Res., B02206, doi:10.1029/2010JB007891.

The effect of a lattice preferred orientation on the flow strength of quartz aggregates dynamically recrystallized from single crystals of synthetic quartz was investigated using general shear experiments in a Griggs apparatus. Experiments were conducted at shear strains (γ) up to 5 at a temperature of 900°C, confining pressure of 1.5 GPa,and shear strain rate of 10<sup>-5</sup> s<sup>-1</sup>. Three starting orientations of crystal were used, to activate three slip systems: basal<a>, prism [c], and prism<a>, although slip‐induced rotation of the crystal axes in the first two orientations led to the activation of additional slip systems. For crystals with higher water contents, basal<a> and prism <a> orientations are relatively weak and prism [c] orientations are stronger. All three initial crystal orientations undergo dynamic recrystallization with increasing shear strain, although the strain required for 100% recrystallization varies: gamma ≈ 2 for prism [c] slip, gamma ≈ 3.8 for basal <a> slip, and gamma ≈ 5 for prism <a> slip. For all three starting orientations, distinct domains of recrystallized grains develop with c axes parallel to Y of the strain ellipsoid (Ymax), replacing recrystallized grains of other orientations; the Ymax domains increase in size with increasing strain. In addition, strain markers show that strain is highly localized within the Ymax domains, indicating geometrical softening of up to an order of magnitude in effective viscosity.

Electric signals related to frictional instability and seismoelectromagnetic phenomena

  • Jun Muto, Hiroyuki Nagahama, Takashi Miura & Ichiro Arakawa (2006) Frictional discharge plasma from natural semiconductor/insulator junctions: origin of seismo- electromagnetic radiation. Phys. Chem. Earth, 31, 346-351.
  • Jun Muto, Hiroyuki Nagahama, Takashi Miura & Ichiro Arakawa (2006) Frictional discharge from sheared asperity: implication for fractal seismo-electromagnetic radiation. Trans. IEE Jpn. 126, 242-243.
  • Jun Muto, Hiroyuki Nagahama, Takashi Miura & Ichiro Arakawa (2007) Frictional discharge at fault asperities: Origin of fractal seismo-electromagnetic radiation. Tectonophysics, 431, 113-122.
  • Jun Muto, Hiroyuki Nagahama, Takashi Miura & Ichiro Arakawa (2008) Frictional discharge plasma and seismo-electromagnetic phenomena. Phys. Earth Planet. Inter., 168, 1-5.
  • Kumi Onuma, Jun Muto, Hiroyuki Nagahama & Kenshiro Otsuki (2011) Electric potential changes associated with nucleation of stick-slip of simulated gouges. Tectonophysics, 502, 308-314.

Although several mechanisms of precursory electromagnetic emissions of earthquakes have been pro- posed, there has been no in situ observation of electromagnetic phenomena occurring at the frictional contacts during rock frictions. From an experiment simulating the motion of an asperity on a fault surface, we report the generation of photon emissions around frictional contacts between natural rock minerals. Spectroscopic analysis clarified that the photon emissions are caused by dielectric breakdown of ambient gases (electric-discharge plasma) due to frictional electrification. The plasmas were found to occur under normal stress of 4 MPa and sliding speed of 10-2 m/s. This indicates that plasma generations do not require high-normal stresses needed to fracture rocks and sliding speeds as high as seismic rates (∼1 m/s). Thus, frictional discharges could occur at microscopic asperities at the onset of the slips of earthquakes, and might be one of the sources of the seismo-electromagnetic emission.

Our friction experiments using simulated gouges detected premonitory changes in electric potential before stick-slip events. These precursor electric signals have been detected both in piezoelectric quartz and non- piezoelectric gabbroic gouges. The changes in the potential were locally detected by electrodes set very close to the fault surface associated with slow slip prior to stick-slip events. The magnitude of electrification is proportional to fault slip associated with gradual stress releases, indicating that the electrification is slip- dependent process. From the detailed microstructural analyses on a representative sample, Riedel (R1) shears known to be formed during stable sliding were the most densely developed around the electrode pair which detected the precursory voltage changes. This indicates that local increases in the voltages were likely caused by frictional electrification due to slow slip on R1 shears in nucleation phases of stick-slip events. Our experimental results imply that natural faults with thicker gouge zones would require greater precursory slips, resulting in larger electric signals in the nucleation phase of earthquakes.