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The frontal part of the Japan Trench plate‐boundary fault is enriched in low‐strength smectite, and this material characteristic is thought to be a factor for the large fault slip during the 2011 Mw 9.0 Tohoku‐oki earthquake. In this study, we evaluated the surface physicochemical properties of the particles that constitute the fault zone and examined their effect on the rheological properties of the fault. Measurements of the zeta potential, surface free energy and pore size distribution indicate that the interparticle interaction in the in situ state is very weak. This means that the viscous resistance is low, which may have contributed to the large fault slip during the earthquake. The interparticle distance appears be longer than the energetically stable state, which may reflect a transitory state after the earthquake. Changes in microscopic interparticle interactions during coseismic and interseismic periods may influence the rheological behaviors of the Japan Trench plate‐boundary fault. Plain Language Summary: Faults in the shallow crust commonly contain abundant clay minerals, and if these clayey faults are saturated with water, then the faults may behave like viscous fluid. The behavior of such fluid is also strongly dependent on interactions between particles in the fluid. In this study, we examine the physicochemical properties of the surfaces of clay particles that constitute the Japan Trench plate‐boundary fault to investigate the interactions between these particles. Our results suggest that such microscopic forces may be one of the factors that control the slip resistance of this fault and its strength recovery process. Key Points: The zeta potential and surface free energy of smectite‐rich fault gouge samples were determined for the Japan Trench plate‐boundary faultThese energies enabled the calculation of interparticle potential under high‐salinity conditions (such as beneath the seafloor)Such microscopic interactions may be the dominant factor in the sliding resistance and strength recovery of the fault [ABSTRACT FROM AUTHOR]

The frontal part of the Japan Trench plate boundary fault, which slipped during the 2011 Tohoku‐Oki earthquake (Mw 9.0), is enriched in smectite and is intrinsically weak owing to the high swelling pressure of this mineral. Our rheometric experiments using analog fault materials demonstrate that "cohesional slip," rather than frictional slip, is a realistic faulting process in the studied fault zone. The cohesional slip can be described by the Bingham plastic model. Consequently, the coseismic shear stress is described by the yield stress and plastic viscosity. Comparison of the results with the average shear stress derived from a previous study suggests that the slip zone thickness for cohesional slip is ~1.3 cm or less. This finding is consistent with textures characterized by discrete slip planes of comparable thickness observed in the recovered fault rocks, suggesting that these slip planes were activated during the Tohoku‐Oki earthquake. Plain Language Summary: Fault behavior is commonly understood to be governed by rock frictional properties. However, this does not hold for fault zones rich in sticky clay minerals. One such example is the frontal part of the Japan Trench plate boundary fault, which slipped during the 2011 Tohoku‐Oki earthquake (Mw 9.0) and is composed of 80% smectite (a clay mineral). Our experiments indicate that this fault material can behave like a fluid during faulting. The present results, together with previous findings, suggest that faulting process in the studied fault zone is governed mostly by cohesional properties of the fault material rather than frictional. Key Points: The frontal part of Japan Trench plate boundary fault, which slipped during the 2011 Tohoku‐Oki earthquake, is composed of 80% smectiteOur experiments indicate that this material behaves like a fluid during faulting, whereby the shear stress is governed mostly by cohesionCohesional slip, rather than frictional slip, may be a realistic faulting process in this fault zone [ABSTRACT FROM AUTHOR]

An analysis of clay minerals in core samples around the megasplay fault of the Nankai Trough, Japan, recovered during the Integrated Ocean Drilling Program Nankai Trough Seismogenic Zone Experiment reveals a local progression of the smectite-to-illite conversion (S–I) reaction in the slip zone, possibly associated with local thermogenesis caused by seismic slip on the fault. Our semiquantitative X-ray diffraction analysis demonstrates that the illite content in mixed-layer illite/smectite is ∼15% higher in the gouge than in the host sediments. The thermal history of the fault, constrained in a previous study, requires a reduction of the apparent activation energy by 20–30% compared with the literature value to explain the observed illitization. The Shirako Fault in the Miura–Boso accretionary complex, which is thought to have been activated in a tectonic setting similar to that of the megasplay fault of the Nankai Trough, also shows a 20–30% reduction in the apparent activation energy. These results suggest that the kinetic barrier of the smectite–illite reaction is lowered as a result of mechanochemical processes in seismogenic fault zones. • Local progression of clay-mineral reactions in fault zones was assessed kinetically. • The reactions can be reproduced by lower activation energy than in the static state. • The kinetic barrier may be mechanochemically lowered owing to fault activity. [ABSTRACT FROM AUTHOR]

• A consistent thermodynamic dataset for an illite-smectite (I/S) is acquired by calorimetry. • The enthalpies of mixing for a series of I/S minerals are also acquired and results slightly negative. • Although small, mixing energies are necessary for the stability of I/S. The stability of illite-smectite interstratified with respect to discrete illite and smectite minerals was investigated by measuring their thermodynamic properties. The standard thermodynamic properties of the illite-smectite ISCz-1 mineral (G , H , S , C p , and V) were determined between 298.15 K and 375 K by using calorimetric methods. Moreover, the enthalpies of mixing between the illite and smectite layers were measured at 298.15 K by acid solution calorimetry from a complete series of illite-smectite interstratified minerals (Shinzan area, Japan). The measured values were slightly negative, with a minimum value of −3.7 kJ.mol−1. This contributed to the stability of the mixed-layer with respect to a mechanical mixture of illite and smectite. In addition, the model from Blanc et al. (2015) was implemented to estimate the thermodynamic properties of the interstratified illite-smectite ISCz-1 mineral. The predicted values were consistent with the experimental results. However, the estimates were slightly improved by considering the thermodynamic properties of the mixture of the illite and smectite components and then adding the terms of energies of mixing. This could be confirmed by establishing the stability domains of ISCz-1 and those of the corresponding illite and smectite end-members according to Meunier and Velde's determination of the smectite to illite reaction pathways (Meunier and Velde, 1989). [ABSTRACT FROM AUTHOR]

Clay-rich fault gouge in the principal slip zones of faults stores abundant water within pores and between clay interlayers. During the preparation of thin-sections and rock chips for microstructural observations, fault-gouge samples are commonly air-dried at room temperature or in an oven. However, during the drying process, remnant liquid water between gouge grains produces an inter-particle adhesion force (liquid-bridge force), which rearranges the grain-to-grain structure, likely resulting in a disturbance of the original fabric. For this study, we prepared gouge samples from the Itozawa fault, northeastern Japan, using a t -butyl alcohol freeze-drying method that mitigates drying-induced fabric disturbance. We then compared the microstructure of our samples with those prepared using the conventional air-drying method. The freeze-dried samples preserve a smooth fault plane, clearly defined nanoparticles, and well-developed shear-sense indicators, including slickenlines and Riedel shear planes. In contrast, the air-dried samples underwent shrinkage during drying, which distorted the geometry of the fault plane. These air-dried samples lack nanoparticles and display only a weak shear fabric. We conclude that microstructural observations on samples prepared using the t -butyl alcohol freeze-drying method, compared with conventional air-drying, could preserve more evidence for the retrieval of fault information, including the kinematics, slip stability, and dynamic weakening mechanism of a fault. • We compare microstructures of air- and freeze-dried gouge of the Itozawa fault, Japan. • Air-drying distorts fault planes and destroys microstructural shear indicators. • Freeze-dried samples preserve clear shear indicators. • Shear indicators include slickenlines and Riedel shear planes. • Sample preparation methods can greatly affect microstructural faulting signatures. [ABSTRACT FROM AUTHOR]

Chlorite (C)-corrensite (Co)-smectite (S) seriesminerals occur as vein constituents in the two epithermal ore veins, the Chuetsu and Shuetsu veins of the Todoroki Au-Ag deposit. The characteristics of the C-Co-S seriesminerals indicate that the clays may be products of direct precipitation from hydrothermal fluids and subsequent mineralogical transformations during and/or after vein formation. The minerals from the Chuetsu vein are characterized by 'monomineralic' corrensite showing an extensive distribution throughout the vein, and trioctahedral smectite occurring locally. The Shuetsu vein minerals are characterized by C-Co series minerals which can be divided into three different types: a I type including discrete chlorite with minor amounts of S layers, a II type comprising interstratified C/Co and discrete chlorite, and a III type characterized by segregation structures of C and Co layers. The C-Co series minerals show slightly different spatial distributions in the Shuetsu vein. Different epithermal environments during the vein formations and possible kinetic effects may have played a role in the formation and conversion of Co-C series at the Shuetsu vein and S-Co series at the Chuetsu vein. [ABSTRACT FROM AUTHOR]

The sensitivity of smectite during brief and protracted heating intervals can provide crucial information about the temperature history of faults during seismogenic slip and creep. Pelagic-sourced smectite is the most abundant clay mineral that is incorporated into the slip zone that was drilled during the Japan Trench Fast Drilling Project (JFAST) Expedition 343 in the Japan Trench, located ~820 m below seafloor. This study investigates the potential for abundant smectite to preserve a record of coseismic frictional heating associated with the Tohoku earthquake in May 2011 by laboratory examination of mineral hydration and dehydration in JFAST drill core samples during brief (maximum of 5 min) and protracted (5 h) heating sequences. Using a real-time heating stage that is connected to an X-ray diffractometer, we observe that (1) both brief and protracted heating causes reduction of water interlayers in smectite, (2) smectite recovers faster to the original hydration state after quick heating than long heating, and (3) nonrecoverable collapse of all smectite occurs at >200 °C for brief and protracted heating rates. Because hydrated, smectite clays are widely present in the fault rocks, we conclude that frictional heating within the slip zone of the Tohoku fault zone cannot have reached significantly elevated temperatures. [ABSTRACT FROM AUTHOR]

The very large slip on the shallow portion of the subduction interface during the 2011 Tohoku-oki earthquake (Mw 9.0) caused a huge tsunami along the northeast coast of Honshu, Japan. In order to elucidate the mechanics of such tsunamigenic slip, the Integrated Ocean Drilling Program Expedition 343 (Japan Trench Fast Drilling Project, JFAST), was carried out one year after the earthquake and succeeded in recovering rocks constituting the active plate boundary fault. Our mineralogical analyses using X-ray diffraction reveal that the shallow portion of the fault zone that caused the earthquake is significantly enriched in smectite compared to the surrounding sediments, which may be intimately linked to the tsunamigenic shallow faulting. For comparison, we also analyzed mineralogical features of incoming sediments just prior to subduction, recovered on the outer rise of the Japan Trench (Site 436, Deep Sea Drilling Project Leg 56), and found a characteristic smectite-rich horizon in the uppermost ~5 m of the pelagic clay layer. This horizon should be mechanically weak and will become the future plate boundary fault, as observed in the JFAST cores. The smectite-rich deposits are broadly distributed in the northwestern Pacific Ocean, and may therefore potentially enhance conditions for large shallow slip during earthquakes that occur over a broad area of the Japan Trench plate boundary, which would result in large tsunamis for this region. [ABSTRACT FROM AUTHOR]

Integrated Ocean Drilling Program (IODP) Expedition 343, named the Japan Trench Fast Drilling Project (JFAST), drilled ocean floor boreholes through the fault zone of the 2011 Tohoku-Oki earthquake (M9.0) to enhance understanding of the rupture process and tsunami generation. This project investigated the very large fault slip that caused the devastating tsunami by making borehole stress measurements, sampling the plate boundary fault zone, and taking temperature measurements across the fault zone. The results show that the earthquake rupture occurred in a narrow fault zone (< 5 m). Based on both laboratory experiments on fault zone material and temperature monitoring of the fault zone, we show that the fault had very low friction during the earthquake. The low friction properties of the fault zone might be attributed to high smectite clay content of the sediment. These physical and frictional characteristics contributed to the unusually large fault slip that occurred during the earthquake and generated the tsunami. [ABSTRACT FROM AUTHOR]

Highlights: [•] The radionuclides sorb to minerals by the uptake on the sorption sites. [•] The sorption site capacity was estimated from the Cs sorption isotherms. [•] The isotherms were obtained on sedimentary rock sampled in Horonobe area in Japan. [•] Illite content was estimated by the alkylammonium treatment. [ABSTRACT FROM AUTHOR]

Abstract: A fault gouge sample, recovered from 389m depth of the Disaster Prevention Research Institute (DPRI) 500m bore hole of the Nojima fault, which triggered the 1995 M7.3 Kobe earthquake, was examined with transmission electron microscopy (TEM). We observed different types of amorphous material containing tiny flakes of crystalline phases. TEM-EDX analysis of the matrix indicates a pure smectite composition suggesting that matrix and crystalline flakes have the same composition. The detection of tiny crystalline flakes in the amorphous material may confirm previous assumption from the alteration of pseudotachylyte into smectite in fault rocks. [Copyright &y& Elsevier]

The Iwami epithermal silver deposit consists of Ag-Cu veins in a dacitic intrusive body at the deep portion of the Eikyu area, and veinlets with disseminated Ag mineralization in dacitic tuff breccia at a shallow portion of the Fukuishi area. Hydrothermal alteration associated with the silver mineralization is characterized by intense potassium metasomatism with oxidizing conditions. An illite zone occurs around the pathways of uprising fluids in both the Eikyu and Fukuishi areas. It grades laterally into the illite/smectite zone, which is surrounded by a broad smectite zone. Because of boiling, abundant adularia associated with silver mineralization overlaps on the altered tuff breccia in the Fukuishi area. The alteration zoning suggests that the western Eikyu area and the eastern Fukuishi area belong to a single hydrothermal system. The data of fluid inclusion microthermometry indicate that the temperatures range 220–270°C, and salinities range 5–7 wt percent NaCl equivalent for the silver mineralization at the upper portion in the Eikyu area and the lower portion in the Fukuishi area. Radiometric ages for volcanic rocks in the area range from 2.19 to 1.64 Ma, and the dacitic intrusion formed at approximately 1.6 Ma. The silver-dominant mineralizing hydrothermal fluids system was active around 1.44 to 1.07 Ma, which formed the Eikyu Ag-Cu veins at depth, and the Fukuishi Ag ores at the shallower portion. [ABSTRACT FROM AUTHOR]

Abstract: Magnetotelluric (MT) measurements were conducted at Iwate volcano, across the entirety of the mountain, in 1997, 1999, 2003, 2006, and 2007. The survey line was 18km in length and oriented E–W, comprising 38 measurements sites. Following 2D inversion, we obtained the resistivity structure to a depth of 4km. The surface resistive layer (~several hundreds of meters thick) is underlain by extensive highly conductive zones. Based on drilling data, the bottom of the highly conductive zone is interpreted to represent the 200°C isotherm, below which (i.e., at higher temperatures) conductive clay minerals (smectite) are rare. The high conductivity is therefore mainly attributed to the presence of hydrothermally altered clay. The focus of this study is a resistive body beneath the Onigajo (West-Iwate) caldera at depths of 0.5–3km. This body appears to have impeded magmatic fluid ascent during the 1998 volcanic unrest, as inferred from geodetic data. Both tectonic and low-frequency earthquakes are sparsely distributed throughout this resistive body. We interpret this resistive body as a zone of old, solidified intrusive magma with temperatures in excess of 200°C. Given that a similar relationship between a resistive body and subsurface volcanic activity has been suggested for Asama volcano, structural controls on subsurface magmatic fluid movement may be a common phenomenon at shallow levels beneath volcanoes. [Copyright &y& Elsevier]

The sediment formation mechanisms of a newly constructed reservoir in Ehime, Japan were evaluated by characterizing the soil particles (SP) and particulate phosphorus (PP) in the runoff and reservoir sediments. The SP and PP loads from the runoffs of the main river in the watershed considerably increased, when the specific discharge rates were over 300 I/s/km² (high flow conditions). When the specific discharge rates exceeded over 300 l/s/km², 19% of the watershed generated over 80% of the SP and PP loads. When the specific discharge rates were under 300 l/s/km² (low flow conditions), the contributions of the previously mentioned 19% area to the SP and PP loads were smaller. Significant amounts of smectite were found in the sediments in the reservoir and in the soil samples obtained at the forest exposed area in this 19% area while it was negligible in citrus orchards and paddy fields that constituted the remaining land surfaces. The forest area exposed by recent landslides was significant for the SP and PP in the reservoir. Judging from the outcomes, land use information alone may not be sufficient to detect critical sources of SP and PP in the runoffs and reservoirs. To identify and confirm crucial areas for the SP and PP in the runoffs, the investigations should be conducted under high flow conditions and the composition of clay minerals in the sediments should be checked against the clay mineral distributions of soils in the watershed. [ABSTRACT FROM AUTHOR]

We show that both fault mineralogy and regional excess fluid pressure contribute to low resolved shear stresses on the Nankai subduction plate boundary off southwest Japan. Ring and direct shear tests indicate that saturated clay minerals in the fault possess intrinsically low residual friction coefficients (μr) at stress levels between 1.0 and 40 MPa. The direct shear μr values for purified smectite are ∼0.14±0.02, for illite ∼0.25±0.01, and for chlorite 0.26±0.02 (for point load velocities of 0.0001 mm/s). These clay minerals dominate the Nankai subduction de´collement zone. Illite (plus quartz) is mechanically important in the altered incoming Muroto section and the predicted de´collement μr should lie between 0.2 and 0.32. This low residual strength, together with elevated fluid pressure, limits shear stresses to below ∼4 MPa within the frontal ∼50 km of the subduction system, consistent with the low wedge taper in this region. A higher wedge taper off the Ashizuri peninsula indicates basal shear stresses rise slightly along strike towards this region. Our analysis indicates lower fluid pressures must predominantly be responsible because only small second order along strike variations in μr are predicted to occur as a result of variations in smectite and total clay content. These variations should be further reduced at depth under the wedge as smectite is diagenetically altered to illite. However, our data suggest the low μr values of the clay-rich de´collement still limit shear stresses to between ∼17 and 29 MPa within the frontal ∼50 km of the wedge, consistent with other estimates of plate boundary weakness. Indeed, we propose that it should be expected that subduction plate boundaries like Nankai will be weak because of the intrinsic presence of clay-rich faults and moderate fluid overpressures. Our data do not support the hypothesis that the smectite-to-illite reaction directly controls the onset of seismogenic behavior deep in the Nankai system because there is already a mechanical dominance of illite (rather than smectite) in the shallow de´collement zone, and we find all the clay phases tend to velocity strengthen. However, temperature-activated clay diagenesis and dehydration may cause secondary changes in the fault properties and state of stress across the up-dip limit of the seismogenic zone. [Copyright &y& Elsevier]

Safety functions for the clay buffer in a repository for high-level radioactive waste (HLW) are fulfilled if the presence of montmorillonite with high swelling capacity and low permeability is maintained in the long-term. The transformation of montmorillonite to the non-swelling mineral illite is addressed in most safety assessments by using simple semi-empirical kinetic models, but this approach contrasts with all other near-field geochemical modelling activities that employ a full description of thermodynamic and kinetic mineral-fluid processes. The consistency of these two modelling approaches has been studied by simulating the montmorillonite to illite transformation in the marine sediment profile penetrated by the Ocean Drilling Program (ODP) Site 1174, offshore Japan. Illite in mixed-layer smectite-illite increases from 20% at <700 m below seafloor (mbsf) to 89% at 1100 mbsf. Illitization of smectite at Site 1174 using the semi-empirical approach has been shown by previous authors to provide a satisfactory match to the gradual change of illite content with depth, albeit with significant differences between model variants. In comparison, the approach used in the current study was to simulate the mineralogical and fluid chemical evolution of a 'packet' of a typical fluid-saturated sediment using a model involving full kinetic and thermodynamic treatment of mineral dissolution-precipitation reactions, along a temperature-time burial curve defined by published thermal conductivity data for Site 1174. The results of these simulations showed that the onset of illitization at a depth of 700 mbsf could be matched, but that the overall rate of conversion was significantly more rapid than observed, or as modelled by the simple semi-empirical kinetic approach. The onset and rate of illite growth was strongly linked to the rate of transformation of amorphous silica to quartz. Geochemical model simulations therefore err on the side of conservatism, but may produce unrealistic estimates of illitization. This comparison demonstrates that models involving full kinetic and thermodynamic treatment of mineral dissolution-precipitation reactions must be carefully applied to simulate other transformation reactions of montmorillonite relevant to the geological disposal of HLW, such as those arising from the interaction of montmorillonite with iron/steel, cement and/or groundwater. • Smectite to illite is usually addressed by semi-empirical models, contrasting with the use of geochemical simulations. • The consistency of these approaches has been studied by simulating smectite illitization at ODP Site 1174, Japan. • Illitization at Site 1174 using the semi-empirical approach gives a good match to the change of illite with depth. • Geochemical simulations showed the onset of illitization was matched, but the rate of conversion was quicker than observed. • This comparison shows that geochemical models must be carefully applied to simulate transformation reactions of smectite. [ABSTRACT FROM AUTHOR]

Slaking is a well-known process, in which the surface of rock masses consisting of rocks such as mudstone, shale, etc. show rapid and complete disintegration when they are subjected to drying and then wetting with fresh water. On the other hand, dried sandstones even if they contain clay minerals such as smectite, etc., usually do not exhibit complete disintegration when they are soaked in fresh water. Instead, they usually show a strength decrease. Their slaking durability is tested by rotating them in a drum which is partially immersed in fresh water and eventually measuring the decrease in weight. On the other hand, it was found that some of the Paleogene sandstones from Kushiro Coal Mine in Japan, were completely disintegrated when they were immersed in fresh water followed by drying, but they were not collapsed when saline water was used instead of the fresh water. It was also shown that sandstones containing calcite did not show the complete breakdown even in fresh water. To understand the complete collapse of sandstones in fresh water and prevention of the collapse by salt water, sandstones sampled from Neogene dikes and Cretaceous at the mine were immersed in pure and salt water with Shikotsu welded tuff for comparison. Only dike sandstones showed a complete collapse in pure water and 3% salt water. Dike sandstone further showed severe time-dependent damage in salt water whose salinity was less than 7%. The results were explained by the dissolution of halite, swelling pressure of montmorillonite, osmotic pressure, etc. [ABSTRACT FROM AUTHOR]