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Earthquake faults are stationary in a critical state where they either slip or not.The fluid pressure fluctuation in the fault zone at this critical state is thought to be one of the factors that cause a variety of earthquakes that progress from slow to fast. This process of earthquake generation has been studied using the conventional "slip displacement - rate control" method, however, this does not reproduce the process of initiation and acceleration of slip as natural earthquake generation. In this study, we conducted experiments to induce spontaneous fault slip using a rotating friction apparatus that can control torque and fluid pressure rather than slip rate.First, as a simple experiment, a sample (standard SiO2) whose frictional behavior was measured at low and constant velocity was used to continuously and gradually increase torque (0.3 N-m/s) under normal stress of 5 MPa. Slip started when the torque reached 200 N-m (corresponding to a shear stress of 4 MPa and a coefficient of friction of 0.8) and gradually accelerated to 170 microns/sec, at an acceleration of 20 micron/s/s. We also conducted an experiment in which the fluid pressure was increased in steps after the axial pressure and torque were applied at fixed values in advance, and similar acceleration behavior was obtained here as well. For both experiments, the termination velocity was 170 microns/sec and did not accelerate to the cm/sec order, which was set as the limit. These could have reproduced the onset of the slow earthquake, but it is also possible that the result may have been due to experimental problems that the torque was not controlled and decreased with the onset of the slip. The presentation will include this and other experimental results, as well as the interpretation of the torque-controlled experimental results and initiation of slow earthquake.

Knowledge of the strength of sediments overlying sub-seafloor gas-hydrate deposits is crucial for predicting borehole and seafloor stability during hydrate extraction. Ideally, sediment strength should be determined along a continuous downhole profile from the seafloor to the hydrate reservoir, but few such profiles have been obtained. In this study we used cores retrieved at Site NGHP-02-23 in the Krishna–Godavari Basin in unconfined penetration tests on split cores and in triaxial deformation experiments on hydrate-free sediment samples. Although penetrometer tests identified relatively low strength (70–250 kPa) likely due to hydrate dissociation in the hydrate-bearing interval 90–300 m below seafloor (mbsf), sediment strength exceeded 350 kPa in the intervals 140–170 and 250–270 mbsf, each of which lies just above a zone of high gas-hydrate concentration. The average stress ratio of triaxial strength at 4% axial strain to effective mean stress (qe=4%/p’) of hydrate-free silts was about 0.67 throughout the Hole. An exception to this trend was in fine sands from 280 mbsf in the deeper gas-hydrate zone, where the stress ratios were greater than 1.0. The stress ratios of hydrate-bearing sediments in the deeper gas-hydrate zone that were reported from the pressure-core measurements were far greater than those of any hydrate-free sediments for a given effective mean stress. The high-strength intervals in silty sediments we identified by the penetration tests could be associated with zones of high hydrate concentration. Because high-strength layers in fine-grained silty sediments commonly exhibit lower permeability than sandy layers (potential hydrate-host), they may act as seals that assist the precipitation of hydrate below them.

We estimate the rate of upward pore fluid flow and chlorinity (Cl−) concentrations at depth through a joint analysis of Cl− concentration and temperature versus depth profiles obtained from gas hydrate related test sites drilled and cored in the Krishna-Godavari Basin off the eastern coast of India. Cl−, measured on conventional core samples, decreases with depth at all sites but some of the Cl−profiles show a prominent convex shape, whereas in situ temperature profiles, obtained by the Advanced Piston Coring Temperature probe, are mostly linear at all sites established during the National Gas Hydrate Program 02 Expedition (NGHP-02). Assuming a one-dimensional, time-dependent model for the advection of pore fluid including the sedimentation effect and depth-dependent diffusivity, we estimate darcy velocity and the Cl− concentrations at depth. The best-fit darcy velocity of 1.2–1.6 × 10−11 m/s was estimated for the sites along the crest of the regional anticlinal structure in the NGHP-02 Area B, which was significantly faster than those on the flanks of the anticline. Because the thermal diffusion coefficient is much larger than the chloride ion diffusion coefficient, estimated darcy velocities are not great enough to generate nonlinear temperature profiles with depth, which is consistent with observed linear thermal profiles.

Physical properties like porosity and density are important parameters for size estimations of fossil fuel reservoir in host sediments and rocks. X-ray CT based imaging and subsequent analysis of the images by using various techniques for estimations of these properties of host rocks and sediments has been attempted. However, most of those techniques tend to be biased and subjective due to complications in image processing. Here we describe a new procedure for estimating porosity and density without processing the XCT images, but using the raw data that are used for producing the images. In this study, large-size cylindrical core samples (up to 150 cm long and 6.6 cm diameter) were imaged in a matter of minutes by using a medical XCT scanner soon after their recovery from seafloor onboard drillship in order to record as realistic as possible condition of hydrate-bearing sediments. Despite some uncertainty in relative volumes of solid, liquid and gas phases at voxel level, the results obtained by this rapid and non-destructive procedure are in good agreement with those obtained by time- and sample-consuming conventional methods of physical property measurements.

Gas hydrate-bearing layers are normally identified by a seismic imaged bottom simulating reflectors (BSR) or by downhole log responses because of their high acoustic velocity and electric resistivity compared to surrounding formations. These gas hydrate characteristics can also result in contrasting in-situ formation compressive strengths. Here, we describe gas hydrate-bearing layers based on equivalent strength (EST), which relates to in-situ compressive strength, in five exploration boreholes drilled during the Indian National Gas Hydrate Program Expedition 02 (NGHP-02). For Site NGHP-02-23, a representative site for those that were established during NGHP-02, the EST evaluated from drilling parameters shows a constant trend of ∼2 MPa, with some strong peak values in the 0–271.4 m-below-seafloor (mbsf) interval, and a sudden increase up to 4 MPa above the BSR depth (271.4–290.0 mbsf). Below the BSR, the EST stays at ∼2 MPa to the bottom of the hole (378 mbsf). Comparing the EST with logging data and a core sample description suggests that the EST depth profiles reflect the formation lithology and gas hydrate content. The EST increases in sand-rich and gas hydrate-bearing zone. In the lower gas hydrate layers in particular, the EST curve shows the same approximate trend with that of P-wave velocity and resistivity measured during downhole logging. Similar relationships between EST, hydrate layer, and log responses are confirmed in other four sites drilled nearby in NGHP-02 Area B. These results suggest that the EST, as a proxy for in-situ formation strength, can indicate the location and extent of the gas hydrate as well as borehole logging. Although the EST was calculated after drilling, utilizing the recorded surface drilling parameters (weight on bit, top drive torque, RPM and rate of penetration) in this study, the EST can be acquired during drilling using real-time drilling parameters. In addition, the EST only requires drilling performance data without any additional tools or measurements, making it a simple and economical tool for the exploration of gas hydrates.

The existence of overpressure in shallow sediments, which is often constrained by hydraulic properties, influences the gas-hydrate formation process and gas production. Porosity, permeability, and grain size measurements in laboratory experiments were conducted on core samples from gas-hydrate-bearing regions offshore from the Krishna-Godavari Basin, eastern India. Porosity was found to decrease with increasing effective stress; this is explained by the exponential decay curve along which porosity decreases from 65 % at 0 MPa to 40 % at 10 MPa. Permeability and the corresponding hydraulic diffusivity decrease from 10−17 to less than 10−18 m2 and from 10−7 to 10−8 m2/s at 0.5 and 5 MPa, respectively. Grain sizes were larger and the sand fraction more scattered in channel-filled sediment sites compared to slope sediment sites. The preconsolidation stresses evaluated from consolidation curves indicate the absence of overpressure at shallow depths. In contrast, a comparison of ship-board measurements and standard compaction curves suggested that measured porosity was higher than the predicted porosity at greater depths. These porosity anomalies are interpreted as a sign of overpressure that approaches near lithostatic values at greater depths in slope sediment sites. A one-dimensional sedimentation model recreated overpressure profiles similar to those predicted by porosity gaps, under the assumption of large basial fluid influx or lower permeability than that derived from laboratory data. The modeling results suggest that near-hydrostatic pressure at shallow depths and significant overpressure at greater depths proposed by the porosity gap method is explained by the non-linearity of transport properties. The relatively small overpressure generation in channel-filled sites compared with slope sites can be explained by the higher permeability due to coarser grain size and larger sand fraction and by the smaller basal influx. On the contrary, considerably large basal influx associated with clay mineral dehydration and methane gas supply from deep sediments was expected to promote overpressure at slope sites, which is confirmed by Cl− concentration-depth profiles.

金沢大学理工研究域地球社会基盤学系, Detailed reconstruction of Indian summer monsoons is necessary to better understand the late Quaternary climate history of the Bay of Bengal and Indian peninsula. We established a chronostratigraphy for a sediment core from Hole 19B in the western Bay of Bengal, extending to approximately 80kyr BP and examined major and trace element compositions and clay mineral components of the sediments. Higher δ 18 O values, lower TiO 2 contents, and weaker weathering in the sediment source area during marine isotope stages (MIS) 2 and 4 compared to MIS 1, 3, and 5 are explained by increased Indian summer monsoonal precipitation and river discharge around the western Bay of Bengal. Clay mineral and chemical components indicate a felsic sediment source, suggesting the Precambrian gneissic complex of the eastern Indian peninsula as the dominant sediment source at this site since 80kyr. Trace element ratios (Cr/Th, Th/Sc, Th/Co, La/Cr, and Eu/Eu*) indicate increased sediment contributions from mafic rocks during MIS 2 and 4. We interpret these results as reflecting the changing influences of the eastern and western branches of the Indian summer monsoon and a greater decrease in rainfall in the eastern and northeastern parts of the Indian peninsula than in the western part during MIS 2 and 4. © 2018. American Geophysical Union. All Rights Reserved.

Subduction of the buoyant Cocos Ridge offshore the Osa Peninsula, Costa Rica substantially affects the upper plate structure through a variety of processes, including outer forearc uplift, erosion, and focused fluid flow. To investigate the nature of a major seismic reflector (MSR) developed between slope sediments (late Pliocene-late Pleistocene silty clay) and underlying higher velocity upper plate materials (late Pliocene-early Pleistocene clayey siltstone), we infer possible mechanisms of sediment removal by examining the consolidation state, microstructure, and zeolite assemblages of sediments recovered from Integrated Ocean Drilling Program Expedition 344 Site U1380. Formation of Ca-type zeolites, laumontite and heulandite, inferred to form in the presence of Ca-rich fluids, has caused porosity reduction. We adjust measured porosity values for these pore-filling zeolites and evaluated the new porosity profile to estimate how much material was removed at the MSR. Based on the composite porosity-depth curve, we infer the past burial depth of the sediments directly below the MSR. The corrected and uncorrected porosity-depth curves yield values of 800 ± 70 m and 900 ± 70 m, respectively. We argue that deposition and removal of this entire estimated thickness in 0.49 Ma would require unrealistically large sedimentation rates and suggest that normal faulting at the MSR must contribute. The porosity offset could be explained with maximum 250 ± 70 m of normal fault throw, or 350 ± 70 m if the porosity were not corrected. The porosity correction significantly reduces the amount of sediment removal needed for the combination of mass movement and normal faulting that characterize the slope in this margin.

Pore pressure plays a key role in the generation of earthquakes in subduction zones. However, quantitative constraints for its determination are quite limited. Here, we estimate the subsurface pore pressure by analyzing the transient upwelling flow of drilling mud from borehole C0023A of the International Ocean Discovery Program (IODP) Expedition 370, in the Nankai Trough off Cape Muroto. This upward flow provided the first direct evidence of an overpressured aquifer in the underthrust sediments off Cape Muroto. To estimate the pre-drilling pore pressure in the overpressured aquifer around a depth of 950–1, 050 m below sea floor, we examined the measured porosities of core samples retrieved from nearby IODP wells; we then proceeded to explain the observed time evolution of the flow rate of the upwelling flow by modeling various sized aquifers through solving a radial diffusion equation. It was observed that for a permeability of 10⁻¹³ m², the aquifer possessed an initial excess pore pressure of ∼5–10 MPa above the hydrostatic pressure, with a lateral dimension of several hundred meters and thickness of several tens of meters. The overpressure estimates from the porosity-depth profile at Site C0023 differ from those at other drill sites in the region, suggesting the possible existence of multiple overpressured aquifers with a patchy distribution in the underthrust sediments of the Nankai Trough. As pore pressure is relevant in maintaining fault stability, the overpressured aquifers may be the source of slow earthquakes that have been observed around the drilling site., 南海トラフのスロー地震震源域近傍に高圧の間隙水帯を確認 --スロー地震発生のメカニズム解明へ前進--. 京都大学プレスリリース. 2021-06-17.

The Integrated Ocean Drilling Program (IODP) Expedition 337 was the first expedition dedicated to subseafloor microbiology that used riser-drilling technology with the drilling vessel Chikyu. The drilling Site C0020 is located in a forearc basin formed by the subduction of the Pacific Plate off the Shimokita Peninsula, Japan, at a water depth of 1180 m. Primary scientific objectives during Expedition 337 were to study the relationship between the deep microbial biosphere and a series of ∼ 2 km deep subseafloor coalbeds and to explore the limits of life in the deepest horizons ever probed by scientific ocean drilling. To address these scientific objectives, we penetrated a 2.466 km deep sedimentary sequence with a series of lignite layers buried around 2 km below the seafloor. The cored sediments, as well as cuttings and logging data, showed a record of dynamically changing depositional environments in the former forearc basin off the Shimokita Peninsula during the late Oligocene and Miocene, ranging from warm-temperate coastal backswamps to a cool water continental shelf. The occurrence of small microbial populations and their methanogenic activity were confirmed down to the bottom of the hole by microbiological and biogeochemical analyses. The factors controlling the size and viability of ultra-deep microbial communities in those warm sedimentary habitats could be the increase in demand of energy and water expended on the enzymatic repair of biomolecules as a function of the burial depth. Expedition 337 provided a test ground for the use of riser-drilling technology to address geobiological and biogeochemical objectives and was therefore a crucial step toward the next phase of deep scientific ocean drilling.

Heat transport properties are significant parameters that affect geothermal processes in deep sedimentary basins. We measured the thermal properties of deep sediment core samples to a depth of approximately 2500 m below sea floor (mbsf), whereas most previous studies have only presented data −1 K −1 , and calcite-cemented sandstone/siltstone exhibits the highest conductivity of approximately 3 Wm −1 K −1 . The other thermal properties are also influenced by the porosity and lithological contrast. The correlation between thermal conductivity and porosity is explained by the mixed-law model of the geometric mean. The one-dimensional temperature-depth profile at site C0020 estimated from measured thermal conductivity reveals a regression of the thermal gradient with depth, and the surface heat flow of 30 mWm −2 , which is consistent with the heat flow data near this site, was evaluated. The obtained results suggest that the linear geothermal gradient estimated from the average thermal conductivity value at shallow sediment materials could overestimate the temperature at greater depths and underestimate the heat flow.

Gas hydrates have been reported to exist in marine sediments from various parts of the world ocean. The hydrates start decomposing soon after recovery of the sediments through coring operations due...

Microbial life inhabiting subseafloor sediments plays an important role in Earth’s carbon cycle. However, the impact of geodynamic processes on the distributions and carbon-cycling activities of subseafloor life remains poorly constrained. We explore a submarine mud volcano of the Nankai accretionary complex by drilling down to 200 m below the summit. Stable isotopic compositions of water and carbon compounds, including clumped methane isotopologues, suggest that ~90% of methane is microbially produced at 16° to 30°C and 300 to 900 m below seafloor, corresponding to the basin bottom, where fluids in the accretionary prism are supplied via megasplay faults. Radiotracer experiments showed that relatively small microbial populations in deep mud volcano sediments (10 2 to 10 3 cells cm −3 ) include highly active hydrogenotrophic methanogens and acetogens. Our findings indicate that subduction-associated fluid migration has stimulated microbial activity in the mud reservoir and that mud volcanoes may contribute more substantially to the methane budget than previously estimated., Postprints der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe, 802

Friction experiments at high velocities (0.1 to 0.4 m/s) at room temperature under high pore fluid pressures (2 or 5 MPa) 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 ~0.1 during sliding, and the slip surface temperature reached 300°C by 6 m of slip displacement. The slip caused a small, but distinct decrease of Li in the deformed gouge when the slip surface temperature exceeded 300°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 40 s) 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.

The hydrous clay mineral smectite, which is pervasive in sediments on subducting oceanic plates, is thought to weaken and stabilize subduction thrust faults. However, these frictional properties of smectite alone cannot explain the large coseismic slip in the vicinity of a trench. Here we performed friction experiments to demonstrate the rate dependence of friction at slip rates from 30 µm/s to 1.3 m/s for water-saturated smectite-quartz mixtures with various smectite contents, so as to shed light on the frictional response of smectite-bearing faults at intermediate to high slip rates. At slip rates of 30 to 150 µm/s, the friction coefficients decreased gradually from 0.5–0.6 to 0.1 with an increase in smectite content from 20 to 50 wt %. In contrast, at slip rates higher than 1.3 mm/s, friction exhibited marked slip weakening, resulting in low friction coefficients of 0.1–0.05, even for low smectite contents (roughly

Summary Resistivity and induced polarization (IP) rock properties are effectively used in the exploration of massive sulphide deposits, formed by ancient seafloor hydrothermal activity. The present study has investigated these properties using sulphide-bearing drill core samples taken from the Ihaya North Knoll and the Iheya-Small Ridge, the location of modern seafloor hydrothermal deposits in Japan. Overall, the results showed that resistivity of sulphide-bearing sediments correlates with porosity, rather than sulphide content. Furthermore, the intensity of spectral IP (phase) was found to depend on the amount of the sulphide minerals, which has a peak at a high frequency range (several kHz). These features are similar to that of typical fine-grained disseminated sulphides. In contrast to the sediments, a massive sulphide sample was found to have unique IP property, possibly due to its large sulphide mineral particles.

Thermal–hydrological–mechanical coupling processes suggest that fault permeability should undergo dynamic change as a result of seismic slip. In igneous rocks, a fault's slip surface may have much higher permeability than the surrounding rock matrix and therefore operate as a conduit for fluids. We conducted laboratory experiments to investigate changes in fracture permeability (or transmissivity) of a fault in granite due to shear slip and cyclic heating and cooling. Our experiments showed that high initial fracture transmissivity (>10−18 m3) was associated with a high friction coefficient and that transmissivity decreased during slip. We propose that this reduction in transmissivity reflects the presence of gouge in fracture voids, increasing the area of contact in the fault plane and reducing the hydraulic aperture. In contrast, when initial fracture transmissivity was low (

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.

Human-made structures and shaped stones on the seafloor near coasts, possibly submerged by tsunami waves, coseismic subsidence, or flooding during typhoons, can often be linked to folk traditions of ancient natural disasters. Stone pillars that appear to have been artificially shaped are submerged in shallow waters inshore Tosashimizu city, southeast Japan, an area subject to repeated Nankai Trough earthquakes. We performed geophysical and geochemical analyses of samples from these pillars, building stones in use in villages in the Tsumajiro area, and nearby natural rock outcrops. In porosity and grain density, the pillars are similar to sandstone from the Tatsukushi Formation and to foundation stones used in the villages; this similarity was confirmed by a principal component analysis of chemical composition data. The shape and surface morphology of the pillars are also similar to those of the foundation stones and stone steps at nearby temples. Therefore, we inferred that the submerged stone pillars were made from Tatsukushi sandstone and had been utilized as building stones for structures such as steps and foundations in Tsumajiro village possibly >100 years ago. Because radiocarbon ages obtained on calcareous fragments adhering to the pillars were inconclusive, when, and by what process, the stone pillars were transported from the land and submerged in the sea is still uncertain.

Fluid transport properties such as permeability and porosity are significant parameters that affect earthquake generation. We measured the transport properties of shallow sediments sampled around the plate boundary near the Japan Trench during IODP Expedition 343 at confining pressures up to 40 MPa. The permeabilities of samples from the shallow plate boundary fault were very low at 10 − 20 m 2 , equivalent to a hydraulic diffusivity of 10 − 10 m 2 s − 1 . Permeability and porosity in the core of the fault zone at the plate boundary were lower than those in the immediately overlying sediments and the surrounding intact sediment, suggesting that the plate boundary fault can act as a barrier for fluid flow. Low permeability and high pore compressibility in the shallow plate boundary fault create a strong potential for dynamic fault weakening due to fluid pressurization with frictional heating, even when the initial shear stress is low. Our investigation supports the hypothesis that thermal pressurization on the fault plane helped facilitate the extremely large slip in the shallow part of the subduction zone during the Tohoku earthquake. As the fault zone has a lower permeability than the surrounding sediments and a higher clay content, pore pressure generation at depth by dehydration of clay minerals can explain formation of the shallow strong patch on the fault more reasonably than continuous fluid influx from the subducting oceanic crust, which does not affect pore pressure at depth in the fault zone. Although there are many possible mechanisms of fault weakening, thermal pressurization can act relatively efficiently as slip begins, even at shallow depths. Therefore our results support the role of thermal pressurization in shallow slip during the Tohoku earthquake.

[1] 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 (Ea) of the dehydroxylation reaction. Numerical modeling shows that the lower Ea allows the dehydroxylation to occur at relatively low temperatures (200–300°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.

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., 「ちきゅう」の断層掘削試料の分析と動力学解析による南海トラフ地震での断層すべり量の定量的評価. 京都大学プレスリリース. 2016-06-27.

Slow earthquakes in subduction zones have been linked to high pore-fluid pressures. Laboratory measurements reveal a high permeability contrast between crust and mantle rocks, implying that water released from the subducting slab could accumulate at the crust–mantle boundary of the overlying plate, raising pore-fluid pressures and generating slow earthquakes.

We used a rotary shear apparatus to investigate changes of fluid transport properties in a fault zone by real-time measurement of gas flow rates during and after shearing of hollow sandstone cylinders at various slip rates. Our apparatus measures permeability parallel to the slip plane in both the slip zone and wall rocks. In all cases, permeability decreased rapidly with increasing friction, but recovered soon after slip, reaching a steady state within several tens of minutes. The rate of reduction of permeability increased with increasing slip velocity. Permeability did not recover to pre-slip levels after low-velocity (ca. 0.0019 m/s) tests but recovered to exceed them after high-velocity (ca. 0.29 m/s) tests. Frictional heating of gases at the slip surface increased gas viscosity, which increased gas flow rate to produce an apparent permeability increase. The irreversible permeability changes of the low-velocity tests were caused by gouge formation due to wearing and smoothing of the slip surface. The increase of permeability after high-velocity tests was caused by mesoscale fracturing in response to rapid temperature rise. Changes of pore fluid viscosity contributed more to changes of flow rate than did permeability changes caused by shear deformation, although test results from different rocks and pore fluids might be different.

Transient frictional heating during earthquake slip induces dehydroxylation of phyllosilicate minerals. As this reaction is endothermic and releases H 2 O, 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.

Generation and maintenance of overpressure can prevent sediments from compaction and weaken sedimentary rocks in deep basins. Excess fluid pressure is one of the key factors to explain the disastrous mud eruption that took place in Sidoarjo, East Java, on 29 May 2006, though the mechanism by which it developed is not well known. We measured permeability and specific storage at a confining pressure of 100 MPa in outcrop samples from the East Java Basin. Both permeability and specific storage in our samples showed large stratigraphic variations. The mudstone of the Upper Kalibeng Formation that is thought to be the source of mud at Lusi had the lowest permeability of our samples at around 10 − 19 –10 − 20 m 2 , and the permeability of the Upper Kujung Formation limestone was 10 − 16 m 2 , which is two orders of magnitude larger than that of the Lower Kujung Formation limestone. In addition, the permeability and porosity of cemented sedimentary rocks showed low sensitivity to effective pressure. From numerical basin analysis of the Lusi site together with laboratory data, we evaluated the evolution of pore pressure and porosity histories and their present distributions. Our results show that high overpressure was generated below the mudstone of the Upper Kalibeng Formation and almost reached lithostatic levels. The modeled fluid pressure variation is consistent with the observed data. The long-lived overpressure at depth is mainly caused by the existence of thick low-permeability sediments and a high sedimentation rate. Undercompaction of the Upper Kalibeng Formation because of overpressurization may have caused the mud to lose strength and cause liquefaction (and hydrofracturing) as a result of small stress fluctuations induced by the Yogyakarta earthquake, which may have ended up causing the mud eruption.

Quick measurements of permeability can be made by using gas as the pore fluid. To apply gas permeability data to the evaluation of water permeability, the difference between gas and water permeabilities needs to be assessed. We measured intrinsic permeability of sedimentary rocks from the western foothills of Taiwan by using nitrogen gas and distilled water as pore fluids in effective-pressure cycling tests at room temperature. The observed difference in gas and water permeabilities was analyzed in view of the Klinkenberg effect. This effect is due to the slip flow of gases at pore walls, which enhances gas flow when pore sizes are very small. Our experimental results showed that (1) gas permeability was larger than water permeability by several times to one order of magnitude, (2) gas permeability increased with increasing pore pressure, and (3) water permeability increased slightly as the pore-pressure gradient across the specimen increased. Results (1) and (2) can be explained quantitatively by an empirical power law in relation to the Klinkenberg constant b that is applicable in low permeable range. This correlation enables us to estimate water permeability from gas permeability. The Klinkenberg effect is important when permeability is lower than 10-18 m2 and at low pore-pressure differentials, and correction for the effect is essential to estimate water permeability from gas permeability measurement data. A simplified Bingham flow model for water can partially explain the trend of result (3), though non-Darcy flow behavior or inertial forces of water-rock interaction are needed to account for the observed deviation from Darcy's law.

1999年に起きた台湾集集（Chi-Chi）地震（Mw7.6）を引き起こしたチェルンプ断層は，その南部と北部では，地震時に生じた断層の挙動に明瞭な違いが認められた．この挙動の違いは，地震時の断層の強度を減少させる“場”の違いや，強度低下を引き起こす機構の違いに起因すると考えられ，これらを明らかにする目的で，2003年に地震断層を貫く台湾チェルンプ断層掘削計画（TCDP）が開始された．その結果，Hole A（掘削深度2000 m）およびHole B（同1350 m）より，孔内検層と併せ地震断層を貫くコア試料が採取され，これまでに種々の面から分析が行なわれた．本総説では，この断層の動的強度低下機構と地震直後の断層のヒーリング過程を知る上で重要な断層帯内部の物理・化学的環境変化についてのコア試料の物性計測や化学分析等の研究成果についてレビューし，その結果を踏まえ現段階で考えられる作業仮説について論述した．

Three fault zones (FZB1136, FZB1194 and FZB1243) are found in core samples in Hole B borehole penetrating through the Chelungpu fault, central Taiwan that activated at the 1999 Taiwan Chi-Chi earthquake. Each fault zone is composed of a black gouge zone (BGZ) and a gray gouge zone (GGZ) surrounded by a fault breccia zone (BZ) and/or fracture-damaged zone (FZ). We conducted tri-axial friction experiments on samples collected from subzones of the above three fault zones under the 1 km depth ambient condition with axial shortening rates of 0.1 to 10 µm/s to investigate frictional strength and its velocity dependence. We found that the coefficient of friction is 0.3 for the BGZ at FZB1136 (1136BGZ), which is lower than those at other fault zones. This result indicates that fault slip at the Chi-Chi earthquake occurred at the 1136BGZ. The estimated frictional strength profile across FZB1136 was that the frictional strength was the lowest (the coefficient of friction was 0.3) for the BGZ and increased up to 0.5 for undeformed host rock with increasing distance from the BGZ. Presence of the intermediate frictional strength zone consisting of the GGZ and the BZ suggests a development of off-fault damage zone where deformation occurred not only in the BGZ but also in the intermediate zone. This feature should be important to estimate total fracture energy released during earthquakes. We also suggest that coefficients of friction and the velocity dependences of the samples are closely related to localization of shear deformation and clay content, respectively.

Chemical and isotopic variations in crushed material along the Chelungpu fault in Taiwan are suggestive of interactions at high temperatures with coseismically generated aqueous fluids. High fluid pressures along the fault zone during the magnitude 7.6 Chi-Chi earthquake in 1999 could have reduced friction and fault strength. Aqueous fluids are thought to have an essential role in faulting and the dynamic propagation of earthquake rupture. Fluid overpressure can affect earthquake nucleation1,2 and in a process termed thermal pressurization, pore fluid pressure produced by frictional heating can reduce the effective normal stress acting on the fault surface3,4,5. 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 (>350 ∘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 diffusivity6 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.

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 °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.