Publishing type：Research paper (scientific journal)  

Because the maximum temperature reached in the slip zone is significant information for understanding slip behaviors during an earthquake, the maturity of carbonaceous material (CM) is widely used as a proxy for detecting frictional heat recorded by fault rocks. The degree of maturation of CM is controlled not only by maximum temperature but also by the heating rate. Nevertheless, maximum slip zone temperature has been estimated previously by comparing the maturity of CM in natural fault rocks with that of synthetic products heated at rates of about 1 °C s−1, even though this rate is much lower than the actual heating rate during an earthquake. In this study, we investigated the kinetic effect of the heating rate on the CM maturation process by performing organochemical analyses of CM heated at slow (1 °C s−1) and fast (100 °C s−1) rates. The results clearly showed that a higher heating rate can inhibit the maturation reactions of CM
for example, extinction of aliphatic hydrocarbon chains occurred at 600 °C at a heating rate of 1 °C s−1 and at 900 °C at a heating rate of 100 °C s−1. However, shear-enhanced mechanochemical effects can also promote CM maturation reactions and may offset the effect of a high heating rate. We should thus consider simultaneously the effects of both heating rate and mechanochemistry on CM maturation to establish CM as a more rigorous proxy for frictional heat recorded by fault rocks and for estimating slip behaviors during earthquake.[Figure not available: see fulltext.].

DOI： 10.1186/s40623-018-0868-7

Publishing type：Research paper (scientific journal)  

Understanding variations of slip distance along major thrust systems at convergent margins is an important issue for evaluation of near-trench slip and the potential generation of large tsunamis. We derived quantitative estimates of slip along ancient subduction fault systems by using the maturity of carbonaceous material (CM) of discrete slip zones as a proxy for temperature. We first obtained the Raman spectra of CM in ultracataclasite and pseudotachylyte layers in discrete slip zones at depths below the seafloor of 1–4 km and 2.5–5.5 km, respectively. By comparing the area-under-the-peak ratios of graphitic and disordered bands in those Raman spectra with spectra of experimentally heated CM from surrounding rocks, we determined that the ultracataclasite and pseudotachylyte layers had been heated to temperatures of up to 700 and 1300 °C, respectively. Numerical simulation of the thermal history of CM extracted from rocks near the two slip zones, taking into consideration these temperature constraints, indicated that slip distances in the ultracataclasite and pseudotachylyte layers were more than 3 and 7 m, respectively. Thus, potential distance of coseismic slip along the subduction-zone fault system could have regional variations even at shallow depth (≤ 5.5 km). The slip distances we determined probably represent minimum slips for subduction-zone thrusts and thus provide an important contribution to earthquake preparedness plans in coastal areas facing the Nankai and Sagami Troughs.[Figure not available: see fulltext.].

DOI： 10.1186/s40623-018-0810-z

Publishing type：Research paper (scientific journal)  

Because shear stress in a fault is directly related to the frictional heat generated during slip, the thermal maturity of carbonaceous material has been used as a proxy for fault-rock temperature. We used infrared and Raman spectroscopic analyses and friction and heating experiments to investigate enhancement of the maturation of lignite by shear damage (the mechanochemical effect) and explored the resultant implications for the use of the thermal maturity of lignite as a proxy for temperature. We showed that higher shear stress applied to lignite samples resulted in greater progression of the thermal destruction of aliphatic C─H chains and the survival of the stronger C─C bonds. Thus, we demonstrated that shear damage during earthquake slip causes mechanochemical enhancement of organochemical reactions related to the aromatization of lignite. Our results suggest that shear stress estimated from Raman spectroscopic analyses in previous studies might have been overestimated.

DOI： 10.1002/2017GL076791

Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)  

To understand the tectono-seismic evolution of faults in the shallow part of a subduction–accretion system, we examined major faults in a fossil accretionary prism, the Emi Group (Hota Group), Boso Peninsula, Japan, by performing multiple structural, geochemical, and mineralogical analyses. Because the strata are relatively shallow (burial depth, 1–4 km), early stage deformation related to subduction, accretion, and uplifting processes is well preserved in three dominant fault zones. On the basis of both previous findings and our geochemical and mineralogical results, we inferred that early stage faulting in a near-trench setting under high pore fluid pressure and second stage faulting at relatively deep along subduction corresponded to aseismic deformations, as shown by velocity strengthening characteristics
and during late stage faulting, probably in association with accretion and uplift processes, a high-temperature fluid, revealed by a geochemical temperature proxy, triggered fault weakening by a thermal pressurization mechanism, and potentially led to the generation of a tsunami.

DOI： 10.1016/j.tecto.2018.01.014

Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)  

The criteria for designating an "Active Fault" not only are important for understanding regional tectonics, but also are a paramount issue for assessing the earthquake risk of faults that are near important structures such as nuclear power plants. Here we propose a proxy, based on the preservation of amorphous ultrafine particles, to assess fault activity within the last millennium. X-ray diffraction data and electron microscope observations of samples from an active fault demonstrated the preservation of large amounts of amorphous ultrafine particles in two slip zones that last ruptured in 1596 and 1999, respectively. A chemical kinetic evaluation of the dissolution process indicated that such particles could survive for centuries, which is consistent with the observations. Thus, preservation of amorphous ultrafine particles in a fault may be valuable for assessing the fault's latest activity, aiding efforts to evaluate faults that may damage critical facilities in tectonically active zones.

DOI： 10.1038/srep36536

Publishing type：Research paper (scientific journal)  

To understand the correlation between the mesoscale structure and the frictional strength of an active fault, we performed a field investigation of the Atera fault at Tase, central Japan, and made laboratory-based determinations of its mineral assemblages and friction coefficients. The fault zone contains a light gray fault gouge, a brown fault gouge, and a black fault breccia. Samples of the two gouges contained large amounts of clay minerals such as smectite and had low friction coefficients of approximately 0.2-0.4 under the condition of 0.01 m s(-1) slip velocity and 0.5-2.5 MP confining pressure, whereas the breccia contained large amounts of angular quartz and feldspar and had a friction coefficient of 0.7 under the same condition. Because the fault breccia closely resembles the granitic rock of the hangingwall in composition, texture, and friction coefficient, we interpret the breccia as having originated from this protolith. If the mechanical incorporation of wall rocks of high friction coefficient into fault zones is widespread at the mesoscale, it causes the heterogeneity in friction strength of fault zones and might contribute to the evolution of fault-zone architectures.

DOI： 10.1186/s40623-016-0512-3

Publishing type：Research paper (scientific journal)  

Coseismic shear stress and slip distance affect subduction-related earthquake processes. They need to be understood to evaluate the earthquake's mechanism and the tsunami generation potential near trenches. The amount of frictional heat generated depends on the shear stress and slip distance, which are therefore able to be derived from the temperature recorded in the fault. Here we developed a new temperature proxy for carbonaceous materials by performing spectroscopic, thermogravimetric, and organic elemental analyses in conjunction with heating experiments. We found marked anomalies in the infrared and Raman spectra and atomic compositions of carbonaceous materials retrieved from the slip zone of an ancient megasplay fault in the Cretaceous Shimanto accretionary complex, Japan: the infrared spectra show extinction of aliphatic C-H bonding and very weak aromatic C=C bonding, and the Raman spectra show a slightly elevated ratio of disordered band intensity to graphitic band intensity and relatively low H/C and O/C ratios. These correlate well with the spectral and elemental features of host-rock carbonaceous materials after heating to 600 degrees C. Thus, we conclude that the slip zone experienced a temperature of 600 degrees C during a past earthquake event, indicating coseismic slip of 2-9 m, which could have generated a large tsunami if the ruptures propagated to the seafloor.

DOI： 10.1002/2016GC006368

Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)  

Near-trench slip during large megathrust earthquakes (megaquakes) is an important factor in the generation of destructive tsunamis. We proposed a new approach to assessing the near-trench slip potential quantitatively by integrating laboratory-derived properties of fault materials and simulations of fault weakening and rupture propagation. Although the permeability of the sandy Nankai Trough materials are higher than that of the clayey materials from the Japan Trench, dynamic weakening by thermally pressurized fluid is greater at the Nankai Trough owing to higher friction, although initially overpressured fluid at the Nankai Trough restrains the fault weakening. Dynamic rupture simulations reproduced the large slip near the trench observed in the 2011 Tohoku-oki earthquake and predicted the possibility of a large slip of over 30 m for the impending megaquake at the Nankai Trough. Our integrative approach is applicable globally to subduction zones as a novel tool for the prediction of extreme tsunami-producing near-trench slip.

DOI： 10.1038/srep28184

Publishing type：Research paper (scientific journal)  

Black fault gouges sometimes develop, mainly in sedimentary rocks, but the cause of the color transformation is not well understood. Here we demonstrated the blackening of synthetic mixtures of montmorillonite and bituminous coal and of montmorillonite and magnetite in milling, heating, and friction experiments. Mixed samples with a higher volume fraction of coal or magnetite before the experiments showed lower L* values (lightness index; lower values indicate darker blacks), because coal and magnetite are intrinsically black. The milling and heating experiments showed that the L* values of mixed samples of montmorillonite and coal drastically decreased with longer milling times and higher temperatures. The L* values of mixed samples of montmorillonite and magnetite also decreased with longer milling times, but no notable change was observed in the samples after the heating experiments. Because comminution by milling induces granulation of the constituent materials, blackening of the experimental samples was primarily caused by dispersal through the sample of fine black par tides such as coal and magnetite, but it could be strengthened by adsorption onto host particles of organic gases produced by pyrolysis of carbonaceous material at high temperature. The friction experiment with mixed samples of montmorillonite and coal produced the remarkably low L* values. Friction induces both comminution and heating of samples, so the blackening could be greater than after either milling or heating alone. Therefore, relatively black fault, gouges, compared with, the surrounding host rocks, might have experienced comminution and heating, probably related to earthquake slip. Thus, black coloration could be one of the important information on fieldwork. (C) 2016 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.tecto.2016.03.039

Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)  

To understand the mechanism of fault lubrication during the 1999 Taiwan Chi-Chi earthquake, we developed a new temperature proxy for carbonaceous materials by using infrared and Raman spectroscopies together with heating and friction experiments. We found marked anomalies in the infrared and Raman spectra of carbonaceous materials retrieved from the primary slip zone of the earthquake: the infrared spectra exhibited very weak aliphatic CH2 and CH3 peaks and aromatic C=C absorbance peaks, and the Raman spectra exhibited very weak disordered and graphitic bands and a high ratio of disordered band area to graphitic band area. Those weak peaks and bands and the band area ratio were reproduced by heating carbonaceous materials from the nearby host rock to 700 degrees C. These results suggest that the frictional heat in the slip zone reached approximately 700 degrees C. We characterized the host rock's carbonaceous materials by means of elemental analysis, pyrolysis-gas chromatography-mass spectrometry, and simultaneous thermogravimetry-differential scanning calorimetry and found that the H/C and O/C ratios were 1.29 and 0.30, respectively (which are close to the ratios for lignin) and that the volatile fraction was as high as 48 wt %. The pyrolysates obtained by heating from 100 to 400 degrees C were dominated by phenols, fatty alcohols, and n-alkanes. When the residue from pyrolysis at 100-400 degrees C was rapidly heated to 700 degrees C, the resulting pyrolysate was dominated by phenols, aromatic compounds, heterocyclic compounds, and n-alkenes. This information suggests that change in the infrared and Raman spectra with increasing temperature may have been due to decomposition and aromatization reactions during pyrolysis. Rapid heating during earthquake slip may promote reactions of carbonaceous materials that are different from the reactions that occur during long-term geological metamorphism.

DOI： 10.1002/2014GC005622

Publishing type：Research paper (scientific journal)  

Friction experiments at high velocities (0.1 to 0.4m/s) at room temperature under high pore fluid pressures (2 or 5MPa) and trace element analyses of the run products were carried out on simulated fault gouge derived from the Chelungpu fault, Taiwan. The friction coefficient decreased to similar to 0.1 during sliding, and the slip surface temperature reached 300 degrees C by 6m of slip displacement. The slip caused a small, but distinct decrease of Li in the deformed gouge when the slip surface temperature exceeded 300 degrees C. The observed Li depletion agreed with calculated results, indicating that it resulted from a hydrothermal reaction at a high temperature produced by short-duration (up to 40s) frictional heating. The geochemical data suggest a small fluid/rock ratio and low reaction temperature in the simulated fault, which is explained by higher normal stress or repeated earthquakes which elevated fluid-rock ratios in the natural fault.

DOI： 10.1002/2015GL063195

Publishing type：Research paper (scientific journal)  

<p>We investigated structural, mineralogical and geochemical characteristics of the fault rocks from the active Atera fault, Gifu Prefecture, Japan. We recognized three dominant structural zones at the Tase outcrop: light gray fault gouge, brown fault gouge and black fault breccia. The light gray fault gouge is mainly composed of clay minerals, and exhibited intense shear foliation. The brown fault gouge shows the clearest linear continuity at the outcrop, cutting other structural zones, but the degree of shearing in this zone is relatively weak. The black fault breccia is composed of rock fragments, originated from surrounding host rocks, and black matrix. Fault rocks in these structural zones show fluid-mobile trace element compositions essentially consistent with those of host rocks, indicating that no fluid–rock interaction at high temperatures above 250 ºC took place in these fault zones.</p><p>Based on these features of each zone and their cross-cutting relationship at the outcrop, we inferred the structural evolution of the fault zone. The light grey fault gouge and the black fault breccia formed since the Atera fault became active. Under oxidative condition, water might react with minerals in both zones to form smectite and halloysite. The brown fault gouge formed most recently, possibly by the 1586 Tensho earthquake. Mineralogical and geochemical analyses of the fault rocks used in this study could be useful for understanding the structural and chemical evolution of the active faults.</p>

DOI： 10.11462/afr.2015.43_1

CiNii Article

Publishing type：Research paper (scientific journal)  

We used a field analysis of rock deformation microstructures and mesostructures to reconstruct the long-term orientation of stresses around two major active fault systems in Japan, the Median Tectonic Line and the Rokko-Awaji Segment. Our study reveals that the dextral slip of the two fault systems, active since the Plio-Quaternary, was preceded by fault normal extension in the Miocene and sinistral wrenching in the Paleogene. The two fault systems deviated the regional stress field at the kilometer scale in their vicinity during each of the three tectonic regimes. The largest deviation, found in the Plio-Quaternary, is a more fault normal rotation of the maximum horizontal stress to an angle of 79 degrees with the fault strands, suggesting an extremely low shear stress on the Median Tectonic Line and the Rokko-Awaji Segment. Possible causes of this long-term stress perturbation include a nearly total release of shear stress during earthquakes, a low static friction coefficient, or low elastic properties of the fault zones compared with the country rock. Independently of the preferred interpretation, the nearly fault normal orientation of the direction of maximum compression suggests that the mechanical properties of the fault zones are inadequate for the buildup of a pore fluid pressure sufficiently elevated to activate slip. The long-term weakness of the Median Tectonic Line and the Rokko-Awaji Segment may reside in low-friction/low-elasticity materials or dynamic weakening rather than in preearthquake fluid overpressures.

DOI： 10.1002/2014TC003600

Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)  

Because a megasplay fault branching from the deep subduction boundary megathrust in the Nankai Trough is thought to be the source of large tsunamis associated with past Tonankai earthquakes, investigation of the heat signal due to frictional slip recorded in the fault is important for estimating the earthquake slip parameters. We performed X-ray diffraction and infrared spectroscopic analyses of a megasplay fault-rock sample and re-examined previously reported trace-element and isotope compositions, but observed no specific change related to high temperature (&gt;= 250 degrees C). In addition, although a qualitative increase of the illite content in illite/smectite mixed-layer minerals within the slip-zone sample was previously reported, our kinetic evaluation of illitization, taking into consideration the coseismic temperature change due to frictional heating and heat conduction, revealed that the illitization reaction hardly progresses at temperatures under 250 degrees C. Alternatively, we suggest that the illite content in mixed-layer minerals might increase progressively via a comminution-dissolution-recrystallization process during multiple past slips. Accurate assessment of the slip behavior of the megasplay fault could be efficiently obtained by drilling to penetrate the fault zone at a deeper depth of approximately 1.5 km, where records of high temperatures would be detectable. (C) 2014 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.tecto.2014.04.020

Publishing type：Research paper (scientific journal)  

We carried out geochemical and mineralogical analyses on fault-zone rocks from the Anko section of the Median Tectonic Line in Nagano Prefecture, Japan, to investigate coseismic physicochemical processes in the fault zone. The latest fault zone in the Anko section contains cataclasite, fault breccia, and fault gouge of granitic composition, and brecciated basic schist. Protoliths of the granitic composition are from the Ryoke metamorphic belt and those of the basic schist from the Sambagawa metamorphic belt. X-ray diffraction analyses show a selective decrease of clay minerals coupled with an increase of amorphous phase in an intensely deformed layer of black gouge (5- to 10-cm thick). SEM observation reveals that the black gouge is characterized by a drastic reduction of grain size and abundant ultrafine particles of submicrometer to several tens of nanometers with well-rounded spheroidal shapes. These observations for the black gouge are indicative of strong mineral lattice distortion and granulation associated with earthquake slip. Geochemically, the black gouge is characterized by distinctly higher Li content and Sr-87/Sr-86 isotope ratio than surrounding cataclasites, breccias, and gouges, which have similar major element compositions. Model analysis reveals that the trace element composition of the black gouge is consistent with high-temperature (up to 250A degrees C) coseismic fluid-rock interactions. Thermal and kinetic constraints indicate that there have been repeated slips on the fault at moderate depths (e.g., 600 m), although the tectonic process by which the fault zone has been uplifted and exposed in this area is not well understood.

DOI： 10.1186/1880-5981-66-36

Publishing type：Research paper (scientific journal)  

To estimate the slip parameters and understand the fault lubrication mechanism during the 1999 Taiwan Chi-Chi earthquake, we applied vitrinite reflectance geothermometry to samples retrieved from the Chelungpu fault. We found a marked reflectance anomaly of 1.30%+/- 0.21% in the primary slip zone of the earthquake, whereas the reflectances in the surrounding deformed and host rocks were 0.45% to 0.77%. By applying a kinetic model of vitrinite thermal maturation together with a one-dimensional heat and thermal diffusion equation, we determined the shear stress and peak temperature in the slip zone during the earthquake to be 1.00 +/- 0.04 MPa and 626 degrees C +/- 25 degrees C, respectively. Taking into account the probable overestimation of the temperature owing to a mechanochemically enhanced reaction or flash heating at grain contacts, this temperature should be considered an upper limit. The lower limit was previously constrained to 400 degrees C by studies of fluid-mobile trace-element concentrations and magnetic minerals. Therefore, we inferred that the peak temperature during the Chi-Chi earthquake was 400 degrees C to 626 degrees C, corresponding to an apparent friction coefficient of 0.01 to 0.06. Such low friction and the previous evidence of a high-temperature fluid suggest that thermal pressurization likely contributed to dynamic weakening during the Chi-Chi earthquake.

DOI： 10.1186/1880-5981-66-28

Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)  

We determined mineral assemblages of samples from the Taiwan Chelungpu fault and from milling and heating experiments by using X-ray diffraction and scanning and transmission electron microscopy. The fault system contains three dominant fault zones, the shallowest of which slipped during the 1999 Chi-Chi earthquake. The quartz and clay mineral contents of the primary slip zone were low, and it contained partly amorphous ultrafine particles (several tens of nanometers). Up to 30 weight percent of materials in that zone could not be fit to standard diffraction patterns, whereas nearly 100 weight percent of those in surrounding samples could be. The unfitted component could be attributed to the observed ultrafine particles produced by comminution during the earthquake, because weak diffraction intensities are caused from mineral lattice distortion, granulation, and amorphous coatings. Such particles are a potential proxy for identifying the slip zone of the most recent earthquake along a fault.

DOI： 10.1002/2014GL059805

Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)  

Recent analyses of natural faults and laboratory rock friction experiments have revealed that physical transformations and chemical reactions of minerals and fluids occur in faults during earthquakes. However, exactly how these physicochemical processes affect fault weakening and earthquake energetics is not well understood. We present quantitative experimental evidence of mechanochemical changes in a clay mineral (illite) during frictional slip and evaluate the effect of those changes on slip behavior. Friction distorts the crystal structure of illite and dramatically decreases the activation energy (E-a) of the dehydroxylation reaction. Numerical modeling shows that the lower E-a allows the dehydroxylation to occur at relatively low temperatures (200-300 degrees C), which in turn drastically reduces fault strength by pressurization of released water from the mineral. Thus, dynamic thermo-chemo-mechanical processes can strongly affect earthquake instability, especially along clay-rich faults such as those in the shallow part of subduction zone plate boundaries.

DOI： 10.1002/grl.50609

Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)  

DOI： 10.5026/jgeography.122.323

CiNii Article

J-GLOBAL

Publishing type：Research paper (scientific journal)  

In this paper, we review recent progress on an attempt at evaluating coseismic fluid-rock interactions in fault zones on the basis of trace element and isotope analyses of fault rocks. The slip zone rocks from Taiwan Chelungpu fault at 1 km depth exhibit marked decreases of lithium, rubidium, cesium and ⁸⁷Sr/⁸⁶Sr and an increase of strontium relative to adjacent host sedimentary rocks. Model calculations reveal that these trace element and isotope spectra were produced by coseismic fluid-rock interactions at >350℃, which may have caused a dynamic decrease of friction along the fault through thermal pressurization. The slip zone rocks from a major reverse fault in the Boso Emi accretionary complex at 1-2 km depth also show similar evidence for coseismic fluid-rock interactions at high temperatures. For the slip zone rocks from the Shimanto accretionary complex in Kure area, which represent rocks of ancient megaspray fault at 2.5-5.5 km depth, the signals derived from high-temperature fluids overlap with those from melting, indicating coseismic fluid-rock interactions followed by frictional melting. These results demonstrate that high-temperature fluid-rock interactions widely occur during seismic slip and geochemical characteristics of the fault rocks are useful indicators of such coseismic events.

DOI： 10.14934/chikyukagaku.46.217

CiNii Article

Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)  

Transient frictional heating during earthquake slip induces dehydroxylation of phyllosilicate minerals. As this reaction is endothermic and releases H2O, it affects dynamic fault weakening and the energetics of earthquakes. To quantitatively evaluate this effect, accurate determination of both the kinetic parameters of the reaction and the thermal properties of the minerals is needed. We chose an illite-muscovite sample as representative of the phyllosilicates found in active crustal faults. For this sample, we measured the specific heat capacity and thermal diffusivity, investigated their temperature dependencies, and determined the weight loss and enthalpy of the dehydroxylation reaction to be 5.22 wt.% and 0.2895 kJ g(-1), respectively. We applied Friedman analysis to the weight loss data from heating experiments and found that the dehydroxylation reactions were well fitted by two-step reactions of an n-dimensional nucleation mechanism according to the Avrami-Erofeev equation with n = 0.5 (first step) and two-dimensional diffusion (second step). On the basis of these experimental results, we performed numerical analyses of dynamic fault weakening, which demonstrated that the fluids released by dehydroxylation contribute to pressurization of pore fluids inducing a decrease in effective normal stress on faults, and that the dehydroxylation reaction absorbs heat from the energy released. We also performed a sensitivity analysis on the kinetic function and parameters and the thermal properties, which showed that the contribution of these to fault weakening is considerably smaller than those of frictional coefficient and slip-zone thickness. (c) 2011 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.epsl.2011.04.042

Publishing type：Research paper (scientific journal)  

To investigate the slip behavior of a megasplay fault branching from a subduction boundary megathrust, we investigated the geochemistry of an out-of-sequence thrust that formed at 2.5-5.5 km depth. Rocks from the slip zone show major-element and fluid-immobile trace-element compositions that are consistent with disequilibrium flash melting. Distinct depletions of Li, Rb, and Cs in the slip-zone rocks indicate fluid-rock interactions at high temperatures (&gt;350 degrees C). These findings suggest a slip process in which high-temperature pore fluids were generated by frictional slip, but the thermally-enhanced pressure might not have reached a sufficient level to cause thermal pressurization, and the temperature continued to increase to attain melting of mica minerals. Comparison with slip zone that formed at a shallower depth (1-2 km), where only thermal pressurization occurred, suggests a transition from melt lubrication at depth to thermal pressurization at shallower depths along a megasplay faults. Citation: Honda, G., T. Ishikawa, T. Hirono, and H. Mukoyoshi (2011), Geochemical signals for determining the slip-weakening mechanism of an ancient megasplay fault in the Shimanto accretionary complex, Geophys. Res. Lett., 38, L06310, doi:10.1029/2011GL046722.

DOI： 10.1029/2011GL046722

Publishing type：Research paper (scientific journal)  

To understand the relation between seismic events and the geological structure in a shallow accretionary prism, we investigated a major reverse fault in a fossil accretionary prism, the Emi Group (burial depth, 1-4 km), Boso Peninsula, Japan. We examined the slip zone rocks and the surrounding host rocks microscopically and analyzed their trace elements and isotopes. Using the fluid-mobile trace element spectrum, which is sensitive to fluid-rock interaction at high temperatures, we estimated that the slip zone experienced frictional heating of >350 degrees C caused by high-velocity sliding. By numerical analysis of the frictional heating and thermal diffusion using various values for slip displacement, the frictional coefficient, depth, and fault dip, we determined the conditions sufficient for producing the estimated high-temperature frictional heating at shallow depth (1-2 km). The results suggest that such faulting in a shallow accretionary prism could potentially generate huge tsunamis.

DOI： 10.1029/2010JB007730

Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)  

Determination of the specific heat capacity and thermal diffusivity of rock samples, together with their temperature dependencies, is crucial for estimation of frictional heating during an earthquake. We measure these properties of a rock sample taken adjacent to the Taiwan Chelungpu fault, which slipped during the 1999 Chi-Chi earthquake, by differential scanning calorimetry and by using a laser-flash apparatus. We also evaluate the temperature dependencies of these properties up to 1000 degrees C and 800 degrees C, respectively, and determine their fitting equations. The specific heat capacity of the sample peaks at around 565 degrees C, and the thermal diffusivity decreases with increasing temperature to 600 degrees C but is almost constant in the temperature range of 600 degrees C-800 degrees C. These changes at around 550 degrees C-600 degrees C probably result from the alpha-beta phase transition of quartz, of which the sample is dominantly composed. We then perform a numerical analysis, adopting these values of specific heat capacity and thermal diffusivity along with their temperature dependencies, to reestimate dynamic shear stress and earthquake energetics during the Chi-Chi earthquake. In this way, we determine the residual shear stress after the stress drop to be 9.0 MPa and the energy taken up by coseismic chemical reactions to be 10.4 MJ m(-2), corresponding to 12.1% of the given work on the fault and tending to counteract the frictional heating.

DOI： 10.1029/2009JB006816

Publishing type：Research paper (scientific journal)  

To reveal details of stress perturbations associated with faults and fractures, we investigated the faults and large fractures accompanied by stress-induced borehole breakouts or drilling-induced tensile fractures in hole B of the Taiwan Chelungpu-fault Drilling Project (TCDP). Then, we determined the relationship between the faults and fractures and stress orientation changes. We identified faults and fractures from electrical images of the borehole wall obtained by downhole logging but also from photographs and descriptions of retrieved core samples, and measured the variations in the principal horizontal stress orientation ascertained from borehole breakouts observed on the electrical images in the vicinity of the faults and fractures. Identification of geological structures (faults, fractures, and lithologic boundaries) by electrical images only is difficult and may sometimes yield incorrect results. In a novel approach, therefore, we used both the electrical images and core photographs to identify geological structures. We found four patterns of stress orientation change, or no change, in the vicinity of faults and fractures in TCDP hole B: (i) abrupt (discontinuous) rotation in the vicinity of faults or fractures; (ii) gradual rotation; (iii) suppression of breakouts at faults, fractures, or lithologic boundaries: and (iv) no change in the stress orientation. We recognized stress fluctuations, that is, heterogeneous mesoscale (&gt;= 10 cm) stress distributions with respect to both stress orientation and magnitude. In addition, we found that stress state changes occurred frequently in the vicinity of faults, fractures, and lithologic boundaries. (C) 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.tecto.2009.06.020

Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)  

A megasplay fault branching from the subduction boundary megathrust in the Nankai Trough is thought to have caused large tsunamis associated with past Tonankai earthquakes. Because the temperature recorded in the fault can constrain parameters of seismic slip, we evaluated several temperature proxies (fluid-mobile trace element concentrations, Sr isotopes, magnetic minerals, inorganic carbon content, and Raman spectra of carbonaceous material) in material from a localized slip zone, sampled at Site C0004 by Integrated Ocean Drilling Program Expedition 316. Our results showed that the shallow part of the megasplay fault has not experienced temperatures above 300 degrees C. We also found that in addition to shear stress and slip distance, slip rate and porosity are effective parameters for frictional heat, as demonstrated by numerical analysis incorporating thermal diffusion. The slip rate estimated through these proxies may indicate that the fault did not slip with high velocity near the seafloor. (C) 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.tecto.2009.08.001

Authorship：Corresponding author   Publishing type：Research paper (scientific journal)  

Abstract

We carried out magnetic mineral analyses of samples from the shallowest major fault zone within the Chelungpu fault system, which is the zone that previous researchers believe slipped during the 1999 Taiwan Chi-Chi earthquake. Our aim was to gain an understanding of the changes to magnetic minerals during the earthquake. Magnetic hysteresis and low-temperature thermal demagnetization measurements showed that high magnetic susceptibilities in the black gouge zone within the major fault zone could be attributed not to fining of ferrimagnetic minerals but, rather, to their abundance. Thermomagnetic analyses indicated that the strata in and around the fault zone originally contained thermally unstable iron-bearing paramagnetic minerals, such as pyrite, siderite, and chlorite. We therefore concluded that frictional heating (&gt;400°C) occurred in the black gouge zone in the major fault zone during the slip of the Chi-Chi earthquake and that the resultant high temperature induced thermal decomposition of paramagnetic minerals to form magnetite, resulting in the observed high magnetic susceptibilities.

DOI： 10.1186/bf03353185

Other URL： https://link.springer.com/article/10.1186/BF03353185/fulltext.html

Publishing type：Research paper (scientific journal)  

Diffusion is one of the main mechanisms of solute transport in pore water of geologic media. The effective diffusion coefficient of a solute in a rock is usually measured by the through-diffusion experiment. However, in this experiment, the effect of advection, induced by density difference between dense aqueous solution and pure water, has not been considered. To evaluate the effect of density-driven flow, a through-diffusion experiment using Fontainebleau sandstone was conducted for KCl and KI aqueous solutions with various densities. The measured effective diffusion coefficients were positively correlated with the density difference; the effective diffusion coefficient of a 1 M KI solution (density difference, 0.119 g/cm(3)) was one order of magnitude larger than that of a 0.1 M KCl solution (density difference, 0.005 g/cm(3)). The result is explained by a theoretical model using a diffusion-advection equation including Darcy&apos;s law. Based on the theory, a diagram to evaluate the condition at which the measured effective diffusion coefficient does not include the effect of advection is presented. (C) 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jconhyd.2009.02.006

Publishing type：Research paper (scientific journal)  

We performed isothermal heating experiments on samples from a breccia zone adjacent to the slip zone of the Taiwan Chelungpu fault that slipped during the 1999 Chi-Chi earthquake to determine the chemical kinetics function and parameters for thermal decomposition of carbonate minerals in the fault zone. We found that a zero-order reaction was the most likely reaction mechanism for thermal decomposition of these samples and determined an activation energy for the reaction of 137 kJ mol(-1) and a pre-exponential term of 1.15 x 10(4) s(-1). We applied these parameters in chemical kinetics calculations, taking into account the temperature change over time caused by frictional heating and heat conduction, energy conservation, and energy taken up by the endothermic reaction, to estimate the dynamic shear stress and maximum temperature reached in the slip zone, and found them to be 6.62 MPa and 1079 degrees C, respectively. (C) 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.tecto.2009.01.036

Publishing type：Research paper (scientific journal)  

The seismic slip behavior during the 1999 Chi-Chi, Taiwan, earthquake (M-w 7.6) was contrastive between the northern and southern segments of the activated Chelungpu fault; large, fast and smooth slips with large stress drop in the north, while smaller, slower and irregular slips with smaller stress drop in the south. We analyzed the pseudotachylyte samples recovered from 1194 m, 1243 m and 1314 m depths of Hole-B of Taiwan Chelungpu fault Drilling Project (TCDP) to reveal the spatial difference in friction mechanism. All pseudotachylyte layers are thin (0.7-2.8 cm), the volume fraction of protoliths is very large (more than 63%), and the estimated temperature distribution is very heterogeneous from ca. 750-1750 degrees C. These observations suggest that these pseudotachylyte melts were in the partial melting regime of Montgomery [Montgomery, R.S., 1976. Friction and wear at high sliding speeds. Wear 36, 275-298] where friction coefficient is abnormally large. Similar pseudotachylyte was found already in the core sample from 175 m depth of the Nanto borehole penetrating the southern fault. Since both pseudotachylyte samples from the two boreholes are older than the 1999 Chi-Chi event and have been uplifted from depths farther down-dip of their current locations, it is likely that recent seismic ruptures also would have encountered these mechanical barriers of viscous melt patches at deeper parts in the north than in the south. Elastohydrodynamic lubrication of clayey gouge worked effectively at the shallower parts of the northern segment, however there is no evidence that it played an important role in the south. These differences are the plausible causes of the contrastive local slip behaviors during the Chi-Chi earthquake. (C) 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.tecto.2009.01.008

Publishing type：Research paper (scientific journal)  

We measured transport properties at a confining pressure of 60 MPa in core samples from the fault zone in Hole B of the Taiwan Chelungpu Fault Drilling Project (TCDP). Permeability and specific storage of the fault gouge range from 3 x 10(-15) to 1 x 10(-17) m(2) and from 2 x 10(-10) to 7 x 10(-10) Pa-1, respectively, and the measured hydraulic diffusivity was 6 x 10(-5) m2/s, which is consistent with the data measured in situ. Numerical analysis of the thermal pressurization mechanism during the 1999 Chi-Chi earthquake using laboratory measured transport and frictional properties showed that pore pressure at the fault zone increased dramatically during slip, whereas temperature increased only moderately to 400 degrees C at the end of slip. The results indicate that dehydration of interlayer water in smectite is plausibly caused by frictional heating, although such dehydration does not influence fault weakening. The decomposition reactions of other minerals are difficult to explain for only one slip event. A magnetic susceptibility anomaly observed in the fault zone is consistent with the modeling results, although low contents of inorganic carbon and clay minerals are not. We concluded that these inconsistencies can possibly be explained by the combined effects of enhancement of chemical reactions by mechanochemical influences and periodic movement on the Chelungpu fault. High-temperature water-rock interactions are also a possible explanation for inconsistencies.

DOI： 10.1029/2008GC002269

Publishing type：Research paper (scientific journal)  

Frictional heat in a fault zone during earthquake slip transiently induces chemical reactions that may use energy released during the earthquake. We estimated the energy used by such reactions (EC) by a numerical analysis incorporating frictional heat, thermal diffusion, chemical kinetics, and energy conservation, and found that EC has an auto-feedback effect that inhibits temperature rise in fault zone. During the 1999 Taiwan Chi-Chi earthquake, estimated EC was 0.43 MJ/m(2), corresponding to 0.79% of the frictional heat generated. This low percentage probably reflects the low initial concentrations of reactive materials. However, in the case of a fault with abundant reactive materials, EC could reach > 50% of the frictional heat and the auto-feedback effect could be large. At this case EC is a nonnegligible component on earthquake energy budget and can affect fault mechanics. Citation: Hamada, Y., T. Hirono, W. Tanikawa, W. Soh, and S.-R. Song (2009), Energy taken up by co-seismic chemical reactions during a large earthquake: An example from the 1999 Taiwan Chi-Chi earthquake, Geophys. Res. Lett., 36, L06301, doi: 10.1029/2008GL036772.

DOI： 10.1029/2008GL036772

Publishing type：Research paper (scientific journal)  

DOI： 10.1111/j.1440-1738.2009.00670.x

Publishing type：Research paper (scientific journal)  

Aqueous fluids are thought to have an essential role in faulting and the dynamic propagation of earthquake rupture. Fluid overpressure can affect earthquake nucleation(1,2) and in a process termed thermal pressurization, pore fluid pressure produced by frictional heating can reduce the effective normal stress acting on the fault surface(3-5). This may lead to a marked reduction in fault strength during slip. However, the coseismic presence of fluids within slip zones and the role of fluids in dynamic fault weakening is still a matter of debate. Here we present compositions of major and trace elements as well as isotope ratios of core samples representing relatively undamaged as well as very fine-grained deformed material from three active zones of the Chelungpu fault, Taiwan. Depth profiles across the most intensely sheared bands that range in thickness from 2-15 cm exhibit sharp compositional peaks of fluid-mobile elements and of strontium isotopes. We suggest that high-temperature fluids (&gt;350 degrees C) derived from heating of sediment pore fluids during the earthquake interacted with material within the fault zone and mobilized the elements. The coseismic presence of high-temperature fluids under conditions of low hydraulic diffusivity(6) within the fault zone is favourable for thermal pressurization. This effect may have caused a dynamic decrease of friction along the Chelungpu fault during the 1999 magnitude 7.6 Chi-Chi earthquake.

DOI： 10.1038/ngeo308

Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)  

To understand the chemical reactions of clay minerals in a fault zone during an earthquake, we analyzed the clay minerals in the Chelungpu fault, which slipped during the 1999 Chi-Chi earthquake. X-ray diffraction spectroscopy showed that kaolinite and smectite contents were lower in the black gouge zone than in the surrounding gray gouge, breccia, or fracture-damaged zones. We applied a chemical kinetics approach to examine whether dehydroxylation of kaolinite and dehydration of interlayer water, dehydroxylation, and illitization of smectite occurred during coseismic frictional heating, and found that the first two reactions could complete under the temperature-time profile of the Chi-Chi earthquake, reconstructed by a previous study. Because dehydration of smectite interlayer water and dehydroxylation of kaolinite would have completed 3.6 and 8.2 s after the beginning of slip, the resulting release of water might have affected the frictional mechanism during the earthquake.

DOI： 10.1029/2008GL034476

Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)  

The Emi Group in the Boso accretionary complex is reported to have experienced a relatively shallow burial depth, approximately 1-4 km below the sea floor at low temperature (50-75 degrees C), indicating that the group passed through or around the upper portion of the seismogenic zone. Two representative thrusts in the group were recognized as products of deformations in the setting. Frictional experiments using the powder samples from the fault zones showed the frictional behavior of velocity hardening under the wet condition with 20 MPa and 40 MPa normal stresses. Microstructures within the thrusts exhibited pervasive deformation with composite planer fabrics. This results that the shallow portion of the thrust at the base of the accretionary prism is aseismic with stable sliding. (C) 2008 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.tecto.2008.04.015

Publishing type：Research paper (scientific journal)  

We carried out thermomagnetic susceptibility analyses of fault rocks from core samples from Hole B of the Taiwan Chelungpu Fault Drilling Project (TCDP) to investigate the cause of high magnetic susceptibilities in the fault core. Test samples were thermally and mechanically treated by heating to different maximum temperatures of up to 900 degrees C and by high-velocity frictional tests before magnetic analyses. Thermomagnetic susceptibility analyses of natural fault rocks revealed that magnetization increased at maximum heating temperatures above 400 degrees C in the heating cycle, and showed three step increases, at 600 to 550 degrees C and at 300 degrees C during the cooling cycle. These behaviors are consistent with the presence of pyrite, siderite and chlorite, suggesting that TCDP gouge originally included these minerals, which contributed to the generation the magnetic susceptibility by thermomechanical reactions. The change in magnetic Susceptibility due to heating of siderite was 20 times that obtained by heating pyrite and chlorite, so that only a small fraction of siderite decomposition is enough to cause the slight increase of the susceptibility observed in the fault core. Color measurement results indicate that thermal decomposition by frictional heating took place under low-oxygen conditions at depth, which prevented the minerals from oxidizing to reddish hematite. This finding supports the inference that a mechanically driven chemical reaction partly accounts for the high magnetic susceptibility. A kinetic model analysis confirmed that frictional heating can cause thermal decomposition of siderite and pyrite. Our results show that decomposition of pyrite to pyrrhotite, siderite and, to some extent, chlorite to magnetite is the probable mechanism explaining the magnetic anomaly within the Chelungpu fault zone. (C) 2008 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.epsl.2008.05.002

Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)  

To characterize the fault-related rocks within the Chelungpu fault, we performed X-ray computed tomography (CT) image analyses and microstructural observations of Hole B core samples from the Taiwan Chelungpu-fault Drilling Project. We identified the slip zone associated with the 1999 Chi-Chi earthquake, within the black gouge zone in the shallowest major fault zone, by comparison with previous reports. The slip zone was characterized by low CT number, cataclastic (or ultracataclastic) texture, and high possibility to have experienced a mechanically fluidized state. Taking these characteristics and previous reports of frictional heating in the slip zone into consideration, we suggested that thermal pressurization was the most likely dynamic weakening mechanism during the earthquake. (C) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.tecto.2007.12.002

Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.ijrmms.2007.07.008

Publishing type：Research paper (scientific journal)  

DOI： 10.1029/2007GL032512

Publishing type：Research paper (scientific journal)  

Fourier transform infrared (FTIR) microanalysis of pseudotachylytes (i.e. friction-induced melts produced by seismic slip) from the Nojima fault (Japan) reveals that earthquakes almost instantaneously expel 99 wt.% of the wall rock CO2 content. Carbon is exsolved because it is supersaturated in the friction melts. By extrapolation to a crustal-scale fault rupture, large events such as the M7.2 Kobe earthquake (1995) may yield a total production of 1.8 to 3.4 x 10(3) tons CO2 within a few seconds. This extraordinary release of CO2 can cause a flash fluid pressure increase in the fault plane, and therefore enhance earthquake slip or trigger aftershocks; it may also explain the anomalous discharge of carbon monitored in nearby fault springs after large earthquakes. Because carbon saturation in silicate melts is pressure-dependent, FTIR can be used as a new tool to constrain the maximum depth of pseudotachylyte formation in exhumed faults. (C) 2007 Elsevier B.V. All fights reserved.

DOI： 10.1016/j.epsl.2007.10.041

Publishing type：Research paper (scientific journal)  

To determine the distribution pattern of water content in the three major fault zones penetrated by the Taiwan Chelungpu-fault Drilling Project (TCDP) hole B, and to assess a rapid, nondestructive water content measurement technique, time domain reflectometry (TDR), we determined the volumetric water content of sequential core samples and found that water content increased toward the center of each of the three fault zones, except in the disk-shaped black material. We observed distinct anomalies in the water content and resistivity profiles, particularly in the shallowest major fault zone (FZB1136), supporting the hypothesis that FZB1136 ruptured during the 1999 Chi-Chi earthquake. This study, the first successful application of the TDR technique to determine water content of core samples, including fault zone samples, collected by an active-fault drilling project, showed that this technique is suitable for measuring water content of fault core samples.

DOI： 10.1029/2007GL032158

Publishing type：Research paper (scientific journal)  

DOI： 10.5194/eed-2-85-2007

Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)  

[1] Estimation of the dynamic shear stress on a fault during an earthquake is important for understanding the earthquake itself. Using a chemical kinetic approach, we examined the thermal decomposition of carbonate minerals to estimate the shear stress on the Taiwan Chelungpu fault, which slipped during the 1999 Chi-Chi earthquake. The reaction rate of the decomposition was related to temperature by using the Arrhenius equation, and the chemical kinetics, taking into account the temperature change over time caused by frictional heating and heat conduction, was solved by the finite difference method. The dynamic shear stress during the Chi-Chi earthquake was deduced to be 1.31 MPa, and the frictional coefficient to be 0.04-0.05. This estimated value agrees with the hypothesis that friction along the Chelungpu fault was low.

DOI： 10.1029/2007GL030743

Publishing type：Research paper (scientific journal)  

We carried out various stress measurements in boreholes penetrating the northern segment of the Chelungpu fault, drilled about 5 years after the 1999 ChiChi, Taiwan, earthquake. In the possible depth range of the Chelungpu fault, three major fault zones were encountered. Clearly recognizable principal stress rotations in the vicinity of the shallowest major fault zone, at 1133 m depth in hole B, suggest that this fault zone ruptured during the 1999 earthquake. Moreover, the fault's rupturing altered the stress state in the area surrounding this fault zone. In this paper, we constrain the possible magnitudes of the current principal horizontal stresses around the fault zone and show that the current stress state belongs to a normal or strike-slip fault regime. Therefore, the stress state was changed from that of a reverse fault regime by the rupturing.

DOI： 10.1029/2007GL030515

Publishing type：Research paper (scientific journal)  

We carried out high-velocity frictional tests on crushed fault gouge from core samples from Hole B of the Taiwan Chelungpu-fault Drilling Project to investigate the cause of high magnetic susceptibilities in the fault core. Black ultracataclasite resembling that observed in Hole B formed during the experiments, even under low axial stress of 0.5 to 1.5 MPa. The bulk magnetic susceptibility of the tested samples was proportional to the frictional work applied and increased as slip increased. Thermomagnetic analysis of the samples before frictional testing revealed that magnetization increased at temperatures above 400 degrees C, probably because of thermal decomposition of paramagnetic minerals. Both the thermally and mechanically induced formation of ferrimagnetic minerals by high-velocity friction might have caused a magnetic susceptibility anomaly. Our experimental results support the assumption that heat generation of short duration, even if it is below the melting point, can increase magnetic susceptibility.

DOI： 10.1029/2007GL030783

Publishing type：Research paper (scientific journal)  

[1] The Taiwan Chelungpu-Fault Drilling Project was undertaken in 2002 to investigate the faulting mechanism of the 1999 M-w 7.6 Taiwan Chi-Chi earthquake. Hole B penetrated the Chelungpu fault, and core samples were recovered from between 948.42- and 1352.60-m depth. Three major zones, designated FZB1136 (fault zone at 1136-m depth in hole B), FZB1194, and FZB1243, were recognized in the core samples as active fault zones within the Chelungpu fault. Nondestructive continuous physical property measurements, conducted on all core samples, revealed that the three major fault zones were characterized by low gamma ray attenuation (GRA) densities and high magnetic susceptibilities. Extensive fracturing and cracks within the fault zones and/or loss of atoms with high atomic number, but not a measurement artifact, might have caused the low GRA densities, whereas the high magnetic susceptibility values might have resulted from the formation of magnetic minerals from paramagnetic minerals by frictional heating. Minor fault zones were characterized by low GRA densities and no change in magnetic susceptibility, and the latter may indicate that these minor zones experienced relatively low frictional heating. Magnetic susceptibility in a fault zone may be key to the determination that frictional heating occurred during an earthquake on the fault.

DOI： 10.1029/2006JB004738

Publishing type：Research paper (scientific journal)  

Taiwan Chelungpu-fault Drilling Project was conducted in drill site Dakeng, Taichung City of central western Taiwan during 2004 - 2005 principally to investigate the rupture mechanism in the northern segment of the Chi-Chi earthquake of 21 September 1999, and also to examine regional stratigraphy and tectonics. Core examination (500 - 1800 m) of Hole-A gave profound results aiding in illustrating the lithologic column, deformation structure, and architectural pattern of fault zones along the borehole.
Lithology column of Hole-A was identified downward as the Cholan Formation (500 - 1027 m), Chinshui Shale (1027 - 1268 m), Kueichulin Formation (1268 - 1712 m), and back to the Cholan Formation (1712 - 2003 m) again. A dramatic change is observed regarding sedimentation age and deformation structure around 1712 m. Along the core, most bedding dips 30 degrees toward N105 degrees. Around 1785 m, bedding dip jumps up to 70 degrees until the bottom of borehole. Five structure groups of different orientations (dip direction/dip) are observed throughout the core. Based on the orientation and sense of shear, they are categorized as thrust (105/30), left-lateral fault (015/30 - 80), right-lateral fault (195/30 - 80), normal fault (105/5 - 10), and backthrust (285/40 - 50). Ten fault zones have been recognized between 500 and 2003 m. We interpret the fault zone located at around 1111 m as being the most likely candidate for rupture deformation during Chi-Chi earthquake. The fault zone seated around 1712 m is recognized as the Sanyi fault zone which is 600 m beneath the Chelungpu fault zone. Ten fault zones including thrust faults, strike-slip faults and backthrust are classified as the Chelungpu Fault System (&lt; 1250 m) and the Sanyi Fault System (&gt; 1500 m). According to the deformation textures within fault zones, the fault zones can be categorized as three types of deformation: distinct fracture deformation, clayey-gouge deformation, and soft-rock deformation. Fracture deformation is dominant within the Chelungpu Fault System and abother two architectures prevail in the Sanyi Fault System. The fracture deformation pattern is asymmetric, which depended the shear sense of fault zone. From the core examination of TCDP Hole-A, the lithology plays an important role in controlling the location and deformation of fault zones.

DOI： 10.3319/TAO.2007.18.2.327

Publishing type：Research paper (scientific journal)  

In order to understand the feature of rock stress change at different depths above, within, and beneath the Chelungpu fault after the Chi-Chi earthquake, we employed a core-based stress measurement method, anelastic strain recovery (ASR) technique to determine both the orientations and magnitudes of present three-dimensional principal rock stresses using drill core samples retrieved from Taiwan Chelungpu-fault Drilling Project (TCDP) main Hole-A. The core samples used were taken from three depths; and their lithology were sandstone at depths of 592 and 1755 in and silt-stone at 1112 in. The anelastic strains of the specimens in nine directions, including six independent directions, were measured after its in-situ stress was released. Acquired anelastic strains were of high quality and reached several hundred microstrains, which is sufficiently high for the accuracy of the measurement system used. Thus, the strain data could be used for three-dimensional analysis resulting in the determination of orientations and the estimation of magnitudes of the principal in-situ stresses. Preliminary stress measurement results showed that the orientations of principal stresses changed between the shallower depth above the fault and the deeper depth beneath it, that is, the present stress distribution in the TCDP hole might be influenced by the Chelungpu fault rupture. At the same time, anelastic strain recovery measurement is well suited for the task of directly determining the orientations of principal in-situ stresses and to estimate the magnitude of stresses at large/great depth.

DOI： 10.3319/TAO.2007.18.2.379

Publishing type：Research paper (scientific journal)  

To elucidate fluid-rock interaction in a seismogenic zone along a plate-subduction boundary, we investigated the occurrence of mineral veins within the major thrusts in the Shimanto accretionary complex and examined their microstructures using a cathodoluminescence technique. We found discriminative structures, for example, a jigsaw-puzzle structure, within the quartz veins in the thrusts, which could indicate that hydraulic fracturing occurred by abnormal pore-fluid pressure during thrusting. Pore pressure values, estimated quantitatively by fluid inclusion analyses, were 3-27 MPa higher than the surrounding parts, which may be direct evidence of abnormal pore-fluid pressure. High pore-fluid pressures and subsequent hydraulic fracturing may play an important role within major thrusts along a plate-subduction boundary.

DOI： 10.1186/BF03352032

Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)  

We determined chloride and sulfate concentrations and (delta D-H2O and delta O-18(H2O) isotope ratios of interstitial fluids extracted from cores from two fault zones within the Taiwan Chelungpu fault. The fluids had higher concentrations of chloride and sulfate and higher delta D-H2O and delta O-18(H2O) values than the local meteoric water. We concluded that the higher (delta D-H2O) and delta O-18(H2O) values were the result of either entry of surface meteoric water at low altitude via fault zones, or isotopic fractionation by depletion at 50 degrees C in a shallower fault zone, and at 125 degrees C in the deeper fault zone. The high temperature in the deeper fault zone might have been earthquake-related frictional heating, which led to isotopic fractionation. However, neither of the above hypotheses explains the high concentrations of Cl- and SO42-. We concluded that the high concentrations of Cl- and SO42- might have resulted from mixing with seawater and dissolution of pyrite, respectively.

DOI： 10.2343/geochemj.41.97

Publishing type：Research paper (scientific journal)  

We presented the total and inorganic carbon contents of core samples recovered from the Taiwan Chelungpu fault system, which slipped at the 1999 Chi-Chi earthquake, and reported lower contents of inorganic carbon within the black gouge zone in FZB1136 (fault zone at depth 1136 m in Hole B) and in the black-material disks in FZB1194 and FZB1243.

DOI： 10.2343/geochemj.41.391

Publishing type：Research paper (scientific journal)  

As an example of core analysis carried out in active fault drilling programs, we report the procedures of core handlings, on the drilling site and non-destructive characterization in the laboratory. This analysis was employed on the core samples taken from Hole B of the Taiwan Chelungpu-fault Drilling Project (TCDP), which penetrated through the active fault that slipped during the 1999 Chi-Chi, Taiwan earthquake. We show results of the non-destructive physicale property measurements carried out at the Kochi Core Center (KCC), Japan. Distinct anomalies of lower bulk density andistinct higher magnetic susceptibility were recognized in all threey fault zones encountered in Hole B. To keep the core samples in good condition before they are used for various analyses is crucial. In addition, careful planning for core handling and core analyses is necessary for successful investigations.

DOI： 10.2204/iodp.sd.s01.35.2007

Publishing type：Research paper (scientific journal)  

To investigate the cause of high magnetic susceptibility in disk-shaped black materials (BM disks) within core samples of Hole B, the Taiwan Chelungpu-fault Drilling Project, we carried out magnetic analyses of samples from the BM disks and surrounding gouge zones. Frequency dependence of low-temperature magnetic susceptibility and thermal demagnetization of low-temperature remanent magnetization revealed lower amounts of superparamagnetic grains in the BM disks than in the surrounding gouges. Magnetic grain-size reduction by shearing is therefore not a plausible explanation for the high magnetic susceptibility of the BM disks. Thermomagnetic analyses of the gouge samples showed that above 400 degrees C magnetization could be increased by thermal decomposition of paramagnetic minerals, whereas analyses of the BM disks showed a much smaller increase in magnetization. Thus, formation of ferrimagnetic magnetite or maghemite by thermal decomposition of paramagnetic minerals was inferred to be the cause of the high magnetic susceptibility of the BM disks, indicating that the BM disks, but not the gouges, have experienced temperatures of at least 400 degrees C.

DOI： 10.1029/2006GL028088

Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)  

The Taiwan Chelungpu-fault Drilling Project penetrated three fault zones as the Chelungpu fault system, which slipped during the 1999 Chi-Chi earthquake, discovering disk-shaped black material ( BM disk) within the middle and lower fault zones in Hole B. The microscopic features of the BM disks indicated that they were pseudotachylytes, and they showed high magnetic susceptibility, possibly the result of intense shearing or high temperature conditions. Inorganic carbon content of the BM disks was low, possibly because of thermal decomposition of carbonate minerals. The high temperatures might be related to frictional heating during the earthquake, implying that the BM disks were produced under intense shearing with frictional heating that reached melting temperature. Because the disks, which provide the only evidence of melting, pre-date the 1999 earthquake, we concluded that frictional melting did not occur during the earthquake.

DOI： 10.1029/2006GL027329

Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)  

The Taiwan Chelungpu-fault Drilling Project (TCDP) was undertaken in 2002 to investigate the faulting mechanism of the 1999 Taiwan Chi-Chi earthquake. Hole B penetrated the Chelungpu fault, and recovered core samples from between 948.42 m and 1352.60 m depth. Three zones, marked 1136mFZ, 1194mFZ and 1243mFZ, were recognized in the core samples as active fault-zones within the Chelungpu fault. Multi-Sensor Core Logger measurements revealed lower densities and higher magnetic susceptibilities within the black gouge zones in all three fault zones. Even though the fault zone that slipped during the 1999 earthquake has not been identified, higher magnetic susceptibilities indicate that frictional heating has taken place in the Chelungpu fault.

DOI： 10.1029/2006GL026133

Publishing type：Research paper (scientific journal)  

Out-of-sequence thrusts (OST) play an important role in the thickening of accretionary prisms, formation of forearc basins, and tsunami generation in subduction zones. By detailed investigations of geological and paleogeothermal structures, an ancient seismogenic OST is identified in the Shimanto accretionary complex, southwest Japan. This OST consists of a mappable en echelon fault system developed at a late stage of the accretionary process. The estimated accumulated displacement of 2.5-8.5 km and formation depth of 2.5-5.5 km, based on a thermal analysis, indicates much higher fault activities sense of slip with a sinistral slip, and the large displacement was distributed among multiple small faults each with a small offset. Most of the faults in the en echelon fault system recorded repeated brittle failure, and one fault preserves pseudotachylyte, a fault rock indicative of seismic faulting. This is the first report of pseudotachylyte along an OST in an accretionary prism. Repeated fracturing in the same narrow shear zones (each only a few millimeters thick) suggests the OSTs perform as major shear localized zone in the accretionary prism. Shallow estimated formation depth and consequent low normal stress also support the hypothesis by a weak coupling along this fault zone. These findings are in accord with a thrust activity in the shallow portion of the accretionary prism being associated with rupture propagation from the deep seismogenic region. This OST showed high activity and repeated faulting in the shallow portion of the accretionary prism, comparable to that of the submarine tsunamigenic OST in the present Nankai trough. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.epsl.2006.02.039

Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.ijrmms.2005.06.006

Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)  

Geological investigation of the deformation structures and sedimentary setting of the Emi Group, a Miocene sand-rich accretionary complex, central Japan,. revealed a six stage-structural evolution during shallow level accretion in a subduction zone. The early deformation (stage 1) is characterized by independent particulate flow in layer parallel faults, scaly cleavages and web structures, and upward dewatering in dish-and-pillar structures and breccia injections, while later deformation (stages 2-6) involve mappable scale folding, meso- to macro-scopic thrusts and web structures with cataclastic flow. Based on microscopic analyses of these structures, the early faulting with independent particulate flow (stage I deformation) is associated with dilatancy and preferred orientation of void space, whereas the later faulting with cataclastic flow (stage 2 deformation) occurs with compaction and crude preferred orientation. The former features imply more permeable fluid migration pathways, supported by the permeability measurements and direct imaging of fluid flow by X-ray CT. On the other hand, the later fault zone has lower permeability and porosity than intact rock, and plays as fluid sealing. Thus, in the early stage (stages 1), fluid flow occurs as focused flow through dilatant fault zones with independent particulate flow or fluid migration by upward dewatering forming dish-and-pillar structures and breccia injections, whereas no evidence of fluid flow is recognized at the later stages (stages 2-6). Namely the fault zones focus fluid flow during primary accretion in shallow levels, and the fluid flow is strongly controlled by the deformation mechanism. Furthermore, the change of the deformation mechanism could be effected by progressive increment of the confining pressure, accompanied with accretion and lithification in the accretionary prism. In the shallow, dilatant-faulting regime where the deformation mechanism is independent particulate flow, focused flow dominates, whereas in the deep, cataclastic regime distributed flow may play a main conduit rather than the focused flow. (c) 2004 Elsevier B.V. All rigths reserved.

DOI： 10.1016/j.tecto.2004.12.006

Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)  

We applied an X-ray computed tomography (CT) imaging method to study fluid advection in a laboratory permeameter in order to visualize the advecting fluid image during permeability testing under atmospheric pressure and to elucidate the relationship between fluid flow properties within fault-related rocks and their deformation mechanism. The X-ray CT images show high-resolution, three-dimensional fluid flow distribution in the fault-related rocks. A fault zone without grain cataclasis enables fluid flow, whereas cataclastic fault zones act as barriers. The fluid flow properties of the deformation bands are strongly dependent on the cataclastic fabrics. In sheared rock, including some cracks, only connected cracks can function as fluid conduits. The localized permeabilities along permeable fault zones and fractures, calculated using fluid front rates, are two orders higher than averaged bulk permeability derived from the pressure or volume difference between inflow and outflow in the permeameter. (C) 2003 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0013-7952(03)00074-7

Publishing type：Research paper (scientific journal)  

[1] The hydrologic evolution of oceanic crust, from vigorous hydrothermal circulation in young, permeable volcanic crust to reduced circulation in old, cooler crust, causes a corresponding evolution of geophysical properties. Ocean Drilling Program (ODP) Hole 801C, which obtained the world's oldest section of in situ, normal oceanic crust, provides the opportunity to examine relationships among hydrologic properties ( porosity, permeability, fluid flow), crustal alteration, and geophysical properties, at both core plug and downhole log scales. Within these upper crustal basalts, fluid flux in zones with high porosity and associated high permeability fosters alteration, particularly hydration. Consequently, porosity is correlated with both permeability and a variety of hydration indicators. Porosity-dependent alteration is also seen at the log scale: potassium enrichment is strongly proportional to porosity. We extend the crustal alteration patterns observed at Hole 801C to a global examination of how physical properties of upper oceanic crust change as a function of age based on global data sets of Deep Sea Drilling Project and ODP core physical properties and downhole logs. Increasing crustal age entails macroporosity reduction and large-scale velocity increase, despite intergranular velocity decrease with little microporosity change. The changes in macroporosity and velocity are significant for pillows but minor for flows. Matrix densities provide the strongest demonstration of systematic age-dependent alteration. On the basis of observed decreases in matrix density that are proportional to the logarithm of age, approximately half of all intergranular-scale crustal alteration occurs after the first 10-15 Myr. Apparently, crustal alteration continues, at a decreasing rate, throughout the lifetime of oceanic crust.

DOI： 10.1029/2001JB001727

Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)  

The Tokai district in central Japan is located close to the convergent boundary between the Philippine Sea and Eurasian plates, and has experienced not only repeated large interplate earthquakes but also intense aseismic movement. In this paper, the spatial and temporal tectonic evolution of the Tokai district, particularly around the Omaezaki area, is discussed to assess whether the district has been and will be active or inactive. According to a geological survey, the horizontal crustal shortening strain can imply the hypothetical tectonic model that the area has been getting less active and the strain rate since the Neogene can be calculated as 12% and 2 × 10 - 6%/year, respectively. The present interseismic horizontal crustal strain and strain rate around the Omaezaki area are approximately 4 × 10- 7% and 4 × 10- 9%/year. By comparing these rates, the decrease since Neogene can imply the hypothetical tectonic model that the area has been getting less active influenced by the strain partitioning between the Suruga Trough and the Zenisu Thrust. © 2002 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0040-1951(02)00590-5

Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)  

Faults profoundly affect patterns and rates of fluid flow and solute transport in the geological environment. They may act as conduits, barriers or combined conduit-barrier systems. In order to elucidate the relationship between fluid flow properties and deformation mechanisms of fault-related rocks, we applied X-ray CT during laboratory permeameter measurements for direct imaging of fluid flow during permeability testing. A KI solution, which has high X-ray attenuation values, was used as a contrast medium for the advection imaging. Three-dimensional fluid flow distributions were measured for the studied fault-related rocks. Fault zones characterized by independent particulate flow as deformation mechanism act as conduits for fluid flow, whereas cataclastic fault zones act as barriers.

DOI： 10.1144/GSL.SP.2003.215.01.10

Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/S0191-8141(99)00173-X

Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)  

Part of the Emi Group, a Miocene accretionary prism in the Bose Peninsula, Japan, display's an extensive west-trending duplex array. The intense faulting there is divided into five stages. The first stage, involving layer-parallel to subparallel faulting, is primarily responsible for thickening and horizontal shortening of the strata, This stage thickened the sequence by a factor of 3.5. Duplex formation is here interpreted to be essential to thicken trench turbidites at a relatively early stage of deformation just after sedimentation, even in shallow levels of accretionary prisms. The east to west vergence of duplex formation might be attributed to the unique tectonics around the Bose trench-trench-trench-type triple junction.

DOI： 10.1130/0091-7613(1998)026<0779:DAATOA>2.3.CO;2