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The main goal of active fault investigation is obtaining evidence of the Quaternary fault activity through trenching. To accomplish this, electrical resistivity surveys are widely utilized to accurately detect faults buried beneath alluvium. In this study, 2D and 3D electrical resistivity surveys were conducted at the Miho site of the southern Yangsan Fault, the Quaternary active fault area already well-studied via trench investigations. The efficacy of electrical resistivity surveys for fault detection was examined by comparing the resistivity distributions against data in the trench logs. At the Miho site, three lines were installed for the 2D electrical resistivity survey and 22 lines were set within a 27 × 27 m square area for the 3D electrical resistivity survey. The length of each survey line was 27 m with an electrode spacing of 1 m. A dipole-dipole array was used to measure the potential difference between each measurement electrode using the same transmitted current and voltage. To derive the 2D and 3D resistivity distributions from the data acquired in the field, the inversion programs DC_2DPRO and DC_3DPRO were employed. Comparison of the 2D resistivity distribution with trench log data shows that the zone of dacitic welded tuff west of the Quaternary fault plane has relatively high resistivity, while the zone of foliate gouge and breccia derived from sedimentary rocks east of the Quaternary fault plane has a low resistivity of less than 40 Ω·m. From these results, it is evident that the method is effective, particularly when different rock types are distributed on either side of the fault boundary or when highly conductive materials, such as clay-rich fault gouges, are present within the fault zone. The resistivity distribution in the 3D survey area can be depicted using both block diagrams and depth-specific slices, facilitating a spatial understanding of the continuity of fault (or fracture) zones. Consequently, this study demonstrates that 3D surveys offer numerous advantages over 2D surveys by accurately capturing planar structures and enabling spatial interpretation based on 3D resistivity distribution. [ABSTRACT FROM AUTHOR]

Optically stimulated luminescence (OSL) dating utilises the detection of trapped charge in minerals, and has an ultralow closure temperature. There is the potential for direct dating of fault movement using fault gouges, because frictional heating caused by large earthquakes can reduce the OSL signal intensity of minerals within gouges. In this study, we conducted quartz OSL dating on four fault gouge and breccia samples from a surface outcrop of the Atotsugawa Fault, one of the most active dextral strike-slip faults in central Japan, where the last large earthquake occurred in AD1858, with an estimated magnitude of 7. The natural OSL signal intensity of fine-grained quartz was clearly below the signal saturation level, with the fraction of saturation (n/N) between 0.30 ± 0.02 and 0.39 ± 0.03, indicating there was signal resetting by past earthquakes. However, the apparent OSL ages ranged from 22 ± 1 to 72 ± 4 ka, two orders of magnitude older than the age of the last earthquake. To explain the significant age overestimation, we measured the thermal stability of the OSL signal, and used a thermal model with punctuated episodic losses to constrain the average shear heating temperature experienced during an individual faulting event. For an independently known recurrence interval of 2.5 ka and a presumed shear heating duration of 1 s, the observed n/N and the measured thermal stability of the OSL signals correspond to a resetting temperature of ~ 300 °C during a single earthquake event. [ABSTRACT FROM AUTHOR]

Hazards created by the landslide damming of rivers have become common in tectonically active mountainous areas. However, it remains unclear how tectonic effects may influence the formation and characteristics of landslide dams. The purpose of this paper is to explore how tectonic effects impact the drivers, geomorphic features, and activity characteristics of landslide dams along a fault zone. We investigated 83 landslide dams clustered along a fault zone in the Bailong River Basin. Most of the dams are located in areas of high tectonic stress, resulting from the rapid river incision and destruction of slope structure caused by intense tectonic activities in these areas. Statistical analysis, InSAR monitoring, and field investigation revealed that different tectonic effects were associated with significant differences in the geomorphic features, activity characteristics, and controlling factors of the landslide dams. Thus, we identified three distinct patterns of landslide dams in tectonically active mountainous areas: (1) Topography-driven landslide dams are caused by rapid rock uplift and river incision. Here, the steep terrain enhances the development of small landslides, the narrowness of the channels favors river damming, and the residual deposits on the hillslope remain active. (2) Tectonic activities promote the development of structural planes in the rock mass and reduce its strength, ultimately forming structural plane-controlled landslide dams. Although their volumes are not very large, the strong erosion resistance of rockslides can cause river damming and maintain the stability of deposits. (3) Fractured rock mass-controlled landslide dams are composed of broken rock and fault gouge. The extremely low strength of these materials allows them to form very large landslides that can easily dam the river, and maintain a slow-moving state. Through a geomorphological and geological model, our study offers new insights and enhances the understanding of the formation and characteristics of landslide dams induced by tectonic activity in mountainous regions. [ABSTRACT FROM AUTHOR]

The fault gouge, a crushed fault rock resulting from seismic slip, exhibits variations in crystallinity and particle size contingent on the magnitude of deformation. Therefore, the fabrication of fault gouge analogues with carefully controlled properties such as crystallinity, grain size, and mineral composition is necessary to systematically study their role in a range of fault slips. While the single mineral fault gouge analogues have been reproduced using a milling method, the fabrication methods for multi-mineralic fault gouge analogues with varying particle size and crystallinity are rarely reported, yet. In this study, we propose two methodologies for fabricating multi-mineralic fault gouges controlling the degree of amorphization and the particle size using a high-energy ball mill: mixing-grinding (MG) and grinding-mixing (GM) types. The MG type was designed to simulate the fault gouge that the same grinding energy is applied to all the constituent minerals, while the GM type can be controlled the degree of deformation of each mineral according to the research objective. To investigate the effects of these methodologies on the reduction in crystallinity and particle size, we compared the characteristics of MG type_6h and GM type_6h samples, both ground for 6 h. Consequently, despite undergoing the same grinding duration, the GM type_6h sample exhibited more significant reduction in crystallinity and more heterogeneous particle size. Additionally, we fabricated the GM type_Xc50 sample, where the crystallinity of all constituent minerals was reduced to < 50%. For this, each mineral was ground for an optimized duration that reduced its crystallinity to < 50%, after which the samples were mixed. Consequently, the GM type_Xc50 sample demonstrated the greatest reduction in crystallinity and the most uniform particle size distribution. To fabricate the generation process of natural fault gouge, it is appropriate to use the MG type to apply the same energy to constituent mineral, resulting the deformation reflecting its hardness. The GM type is recommended for use in fabricating the characteristics of the fault gouge that has undergone intense deformation at the slip surface, which allows for controlling the uniform crystallinity and particle size reduction of each constituent mineral. Our results suggest that the diverse nature of naturally occurring fault gouges can be fabricated in laboratory settings by adjusting the grinding conditions. This study offers effective methods for fabricating fault gouge analogue for frictional experiments for fault slip and potential applications in simulating geochemical reactions in fault zones. [ABSTRACT FROM AUTHOR]

A long-term opening problem in geophysics and engineering is the generation mechanism of rock pulverization (e.g., fault gouge) within the shear zone when subjected to triaxial stress environment and dynamic loads. A novel triaxial Hopkinson bar is developed to apply quasi-static triaxial confinement and dynamic loads onto the testing rocks and to measure the energy release from the shearing zone. The results elucidate shear strain-rate threshold (i.e., over 125 s−1) for granite at the triaxial prestress of (10, 10, 10) MPa is required for fine fragments/gouge generation. The effects of shear and normal pre-stresses on dynamic shear strength and surface energy for rock pulverization are investigated, which will help explain fine fragments formed during the high-rate loading events. It is suggested that, except for the quasi-static cumulative slip attrition and multiple dynamic events, rock pulverization or fault gouge could be generated at relatively shallow depths (low geo-stress) in a single dynamic earthquake processes. Highlights: Triaxial static-dynamic coupled shear tests of rock were conducted using a novel triaxial Hopkinson bar (Tri-HB). The effects of strain rate and confinement on dynamic shear behaviors of rock. Abundant fault gouge can be formed at stress deformation conditions associated with a fast shear crack propagating (i.e., shear strain rate over 125 s−1). [ABSTRACT FROM AUTHOR]

The Periadriatic Fault system (PAF) ranks among the largest post-collisional structures of the European Alps. Recent geodetic data suggest that a fraction of the Adria–Europe convergence is still being accommodated in the Eastern Alps. However, the historical seismicity records along the easternmost segment of the PAF are ambiguous, and instrumental records indicate that seismotectonic deformation is mostly concentrated in the adjacent Southern Alps and Dinarides. Both electron spin resonance (ESR) and optically stimulated luminescence (OSL) dating methods can be used to date coseismic slip (with a combined range covering a few decades to a couple of million years) in slowly deforming fault zones, such as the PAF. Since the saturation doses of the quartz ESR signals are larger than quartz and feldspar OSL, ESR enables establishing a maximum age of the last resetting event of the system, while OSL allows constraining their minimum age when the signal is in saturation. We collected fault gouge samples from three localities along the easternmost segment of the PAF. For ESR, we measured the signals from the Al center in quartz comparing the results from the single aliquot additive dose (SAAD) and single aliquot regenerative (SAR) dose protocols. For OSL, we recorded the infrared stimulated luminescence signal at 50°C (IR50) and post-infrared infrared-stimulated luminescence signal at 225 °C (pIRIR225). Our dating results indicate that the studied segment of the PAF system accommodated seismotectonic deformation during the Quaternary, with a maximum age for the last resetting event of the system ranging from 1075 ± 48 to 552 ± 26 ka (ESR SAR) and minimum ages in the range from 196 ± 12 to 281 ± 16 ka (saturation of pIRIR225). The obtained ages suggest that the studied segment of the PAF could be considered at least as a potentially active fault. [ABSTRACT FROM AUTHOR]

Thermal pressurization of the trapped pore fluid is considered to be a widespread fault weakening during fluid injection in confined geological formations. Tremendous amounts of heats will be generated within the narrow shear zone during fault slip. Considering the melting of the fault gouge, the fault seal zone is adopted to construct the fluid pressurization model, thermal pressurization implemented in Comsol Multiphysics is established to illustrate the fault weaken coefficient and effective normal stress during fluid injection. The friction weakening coefficients mT and mF are proposed to research the performance of fault weakening during fluid injection. The results indicate that, the friction coefficients mT and mF both exhibit the initially decreasing and then increasing tendency, and thermal-induced fault weakening of CO2 injection occurs earlier than that of water injection. It was found that initial pore pressure and fault sealing porosity have a negligible influence on the evolutions of friction weakening and effective normal stress. Initial normal stress and fault sealing permeability have certain obvious influences on fault weakening during CO2 injection. Fault thickness is the primary factor influencing the friction weakening coefficient. When the fault thickness is over 1 mm, the variation of fault weakening is totally different from that when the thickness is less than 1 mm. This investigation of friction weakening during fault slip provides an effective reference for fluid injection. [ABSTRACT FROM AUTHOR]

Atera Fault clay gouges were collected for age dating near Kawaue, Nakatsugawa City, Central Japan, and the results integrated within its complex geological history. The results form an internally consistent data set constrained by extensive geochronological data (AFTA, ZFTA, CHIME) and support the application of gouge dating in constraining timing of brittle deformation in Central Japan. The Atera illite age data complete recently obtained limited illite fault gouge age data from underground exposure in the Toki Granite; the new illite age data are identical within error. The age of the heterogenous welded tuff breccia zone (Atera 1) ranges from 40.6 ± 1.0 Ma to 60.0 ± 1.4 Ma, whereas ages of the fault core gouge sample (Atera 2) range from 41.8 ± 1.0 Ma to 52.7 ± 1.2 Ma. The finest < 0.1 µm fraction of the fault breccia and fault core gouge yield ages around 41 Ma, identical within error. The new illite age data indicate brittle faulting and a following geothermal event occurring in the Paleogene–Eocene, similar to the nearby Toki Granite area and confirm they were both synchronous with a post-intrusive pluton exhumation. The Atera Fault illite age data provide additional insights into an integrated, regional-scale record of the tectonic displacement of Central Japan and might be influenced by large-scale tectonic processes such as the Emperor sea mount kink around 55 to 46 Ma with fault initiation around 50 Ma and brittle fault cessation or reactivation around 40 Ma in the Eocene. [ABSTRACT FROM AUTHOR]

The neotectonic processes are crucial for identification of active faults in a seismic unstable area. The Aїn Smara active fault runs along the Middle Rhumel Valley located in Constantine area of Algerian Eastern Tell Atlas mountain chain and it emerges in the south‒western part of the epicentral zone of the earthquake occurred on October 27, 1985 (Mw = 5.8) within a corridor displaying morphological features and deformed Pliocene‒Quaternary layers, which are the consequence of transpressive tectonic regime. In our study the morpho-tectonic and structural research supported by the fieldwork, was performed. The thick fault gouge indicating simple shear deformation, fault breccias and minor faults affecting the Pliocene limestone and Quaternary alluvial terraces, were found. The active fault splits into northern and southern segments. The northern segment corresponds to the El Aria Fault that ruptured during the earthquake in 1985 and the southern segment with the studied Aĭn Smara Fault. Our research showed the significant extent of the active fault and including its parameters for improved seismic hazard assessment of the Constantine area. [ABSTRACT FROM AUTHOR]

CO2 uptake by smectites can cause swelling and self-stressing in shallow clay-rich caprocks under CO2 storage P–T and constrained conditions. However, little data exist to constrain the magnitude of the effects of CO2-H2O-smectite interactions on the sealing properties of clay-rich caprocks and faults. We performed permeability experiments on intact and fractured Opalinus Claystone (OPA) cores (~ 5% smectite), as well as on a simulated gouge-filled faults consisting of Na-SWy-1 montmorillonite, under radially constrained conditions simulating "open" transport pathways (dry and variably wet He or CO2; 10 MPa fluid pressure; 40 °C). Overall, the flow of dry CO2 through intact OPA samples and simulated smectite fault gouge caused a decrease in permeability by a factor of 4–9 or even by > 1 order, compared to dry He permeability. Subsequent to flow of dry and partially wet fluid, both fractured OPA and simulated gouge showed a permeability reduction of up to 3 orders of magnitude once flow-through with wet CO2 was performed. This permeability change appeared reversible upon re-establishing dry CO2 flow, suggesting fracture permeability was dominated by water uptake or loss from the smectite clay, with CO2-water-smectite interactions play a minor effect. Our results show that whether an increases or decreases in permeability of clayey caprock is expected with continuous flow of CO2-rich fluid depends on the initial water activity in the clay material versus the water activity in the CO2 bearing fluid. This has important implications for assessing the self-sealing potential of fractured and faulted clay-rich caprocks. Highlights: Permeability of potential clay-rich caprock (Opalinus Claystone) was systematically measured using variably wet CO2 versus He. Through-flow of dry CO2 caused a decrease in permeability of intact Opalinus Claystone and simulated smectite fault gouge by up to > 1 order, as composed to dry He. Fractured Opalinus Claystone and simulated gouge showed a permeability reduction of up to 3 orders of magnitude with through-flow of wet CO2. Permeability decrease by through-flow of variably wet CO2 appeared reversible upon re-establishing dry CO2 flow. The initial water activity in the clay material versus the water activity in the CO2 bearing fluid determines change in permeability upon CO2 flushing. [ABSTRACT FROM AUTHOR]

Rock joints in faulted zones are usually filled with large amounts of fault gouge, in which clay-rich fillings are very common. Fault water can significantly influence the mechanical parameters of clay-rich fillings and affect the shear mechanical behaviour of rock joints. Based on a tunnelling project passing through a faulted zone in Yunnan Province of China, this paper investigates the mechanical behaviour of rock joints with clay-rich fillings. A series of direct shear tests are conducted on rock joint samples made from rock blocks and fillings collected from a fault zone. The shear stress‒displacement curves and shear failure characteristics of rock joints are obtained, and several key factors of the fillings are considered, such as particle size, mineral composition, clay content, and water content. Furthermore, the peak and residual shear strengths are analysed by considering the influence of the water content and normal loading pressure. The test results show that water content of fillings has a great influence on shear strength of clay-filling rock joints; there is a trend to a negative correlation between the shear strength of rock joints when water content (w) exceeds the plastic limit (wp). Finally, a shear strength model for rock joints with clay-rich fillings is proposed based on the JRC-JCS strength criterion. Influence of water content on the shear strength of rock joints with clay-rich fillings is studied. A series of direct shear tests are conducted on rock joint samples based on a tunnelling project. The shear stress–displacement curves of rock joints with clay-rich filling are obtained. A shear strength weakening model of clay-filled joints is proposed. Highlights: Influence of water content on the shear strength of rock joints with clay-rich fillings is studied. A series of direct shear tests are conducted on rock joint samples based on a tunnelling project. The shear stress-displacement curves of rock joints with clay-rich filling are obtained. A shear strength weakening model of clay-filled joints is proposed. [ABSTRACT FROM AUTHOR]

The injection of fluids into fault gouges in rock formations disturbs the in situ stress conditions, leading to fault slip and increasing the risk of inducing earthquakes. The weakening effect and the permeation of the injected fluid can be influenced significantly by the presence of fault gouges. To investigate this issue, the hydro-mechanical characteristics of fault gouges were evaluated using physical tests to study the combined effects of coupling injecting water and shear deformation. We propose a new experimental apparatus that allows us to measure the spatial distribution of the thickness of a gouge layer sample under combined conditions of shearing and water injection, using 3D scanning technology to evaluate the primary flow path. The test results showed that injecting water had a significant effect in reducing the maximum shear strength, but the degree to which the strength was affected varied with the gouge fill material. The effect of shear deformation is that it will increase the inhomogeneity of the thickness distribution and therefore the distribution of injected water along the fault is not uniformly radial. Additionally, the properties of gouge fill material have an important influence on flow characteristics during fault slipping. [ABSTRACT FROM AUTHOR]

Fault gouge, located in highly deformed fault cores, shows substantial particle size reduction and loss of crystallinity, which could affect the physicochemical properties and thus control the slip behavior and earthquake stability of a fault. To systematically investigate the effect of deformed fault gouge on fault slip behavior in laboratory-scale experiments, a fault gouge analog with variable particle size and crystallinity is required. However, the systematic study for formation of a fault gouge analog (i.e., controlling the particle size and crystallinity) was not performed well. In this study, we investigated the effect of the rotation speed of a high-energy ball mill on particle size reduction rate and degree of amorphization. As the rotation speed increased from 500 to 2000 rpm, the comminution rate linearly increased, and reached a reduced particle size limit. The degree of amorphization and its rate also increased with increasing rotation speed. Upon grinding, the X-ray diffraction (XRD) peaks markedly decreased and reached critical amorphization points within 120 min, except for the case of grinding with a rotation speed of 500 rpm. Talc ground with a rotation speed of 500 rpm did not reach steady-state in mechanical amorphization after 360 min of grinding, thus necessitating prolonged grinding for further amorphization. A comparison of talc powders having similar specific surface areas, but ground at different rotation speeds, shows that grinding with a higher rotation speed for a shorter duration preserves the crystalline structure relatively well compared with grinding at a lower rotation speed for a longer duration. These results indicate that optimizing the grinding rotation speed facilitates the formation of talc fault gouge analogs with systematically varying particle size and crystallinity. The specific surface area of talc increased from 6.1 to 365 m2/g, and the degree of crystallinity decreased from approximately 75% to 11%. Our results indicate that an artificial talc fault gouge can be prepared by varying the particle size and crystallinity using a high-energy ball mill. [ABSTRACT FROM AUTHOR]

Abstract—The paper introduces a new, unique for Russia, meter-scale laboratory setup created in the Institute of Geosphere Dynamics of the Russian Academy of Sciences (IDG RAS) to study the development of different sliding regimes on rock discontinuities. The experimental procedure is described and the results of the first series of tests aimed at studying the formation of different sliding regimes on rock faults are presented. The laboratory fault was a loaded contact of two 75-cm long blocks made of diabase. The fault was filled with granular material (a fault gouge). Normal stresses on the fault can reach 10 MPa. By varying fault gouge composition and loading rate, we reproduced a wide range of sliding regimes: sliding with constant velocity, regular stick-slip, and aperiodic slow slip episodes. It is shown that a variation in the loading rate can cause a significant change in the sliding regime. Intense frictional crushing of gouge grains is detected in the experiments at relatively low normal pressure of 2 MPa. In the case of high-amplitude stick-slip, besides crushing of the gouge material, also structural phase transformations of quartz grains corresponding to a local temperature increase up to 700°C are revealed. A possible set of the problems related to deformation processes in seismogenic fault zones that can be addressed by modeling on such setups—the pre-seismic stage of inelastic behavior of the main fault zone at critical stress—is outlined. [ABSTRACT FROM AUTHOR]

Basalt is a major component of the lunar soil and crust. The frictional properties, and thus the potential for seismicity and associated hazards on basaltic faults are significantly influenced by particle size distribution and normal stress conditions. We conducted velocity-stepping and slide-hold-slide shear experiments on simulated basalt gouges at gouge particle sizes of 50–250 μm, and normal stresses of 5–30 MPa under wet/dry conditions with different gouge structures to investigate the frictional stability and healing of basaltic faults. We observe a transition from velocity-strengthening (a – b = 0.52) to velocity-weakening behavior (a – b = -0.0005) and an enhanced healing rate (from 0.0075 to 0.0044), as particle size decreases. Elevated normal stress is typically associated with reduced grain size, resulting in unstable slip behavior (a – b = -0.0006), though accompanied by a diminished healing rate (from 0.0062 to 0.0050). The presence of fluid contributes to stable slip behaviors and fault compaction. These observations contribute to a better understanding of the potential and nucleation mechanism of lunar seismicity within basaltic faults.HighlightsSmall grain size gouges facilitate fault instability and strength recovery in basaltic faults.High normal stress on basalt gouges assists grain crushing and weakens frictional strength recovery.Fluid enhances fault stability and fault compaction of basaltic faults at room temperature.Deeper crust with higher confining stress and smaller grains suggests higher seismic potential.Small grain size gouges facilitate fault instability and strength recovery in basaltic faults.High normal stress on basalt gouges assists grain crushing and weakens frictional strength recovery.Fluid enhances fault stability and fault compaction of basaltic faults at room temperature.Deeper crust with higher confining stress and smaller grains suggests higher seismic potential. [ABSTRACT FROM AUTHOR]

We study the deformation features preserved in basement granitoids recovered by scientific core drilling from an active seismic zone in the Koyna region, Deccan Traps, India. The region is characterized by recurrent reservoir triggered seismicity for the past 55 years, including the 1967 M6.3 Koyna earthquake, which produced the Donichawadi fissure zone. We investigate the basement granitoids down to a depth of 1.5 km in a set of four boreholes. Salient findings are as follows: (1) Basement rocks are composed of granite, granite-gneiss and migmatitic gneiss, characterized by greenschist to amphibolite facies metamorphism, (2) Distinctive brittle deformation features, such as fault breccias, fault gouge, prominent slickenlines and slickensides on fractures and pseudotachylites indicate proximity to a fault zone, (3) Fault zones are enriched with secondary precipitations of ferruginous and calcium-rich material which are the evidences of fluid-filled fractures at depth, (4) Fault breccia and gouge are preserved along with intense shattering of cores in zones of near-vertical fractures. Combined with information from downhole geophysical logs in a 3 km deep scientific borehole and the distribution of hypocentres, the study provides strong constraints for the subsurface extension of the Donichawadi fissure zone that is active to this day. The new datasets provide evidence for multiple episodes of fracturing and fault rock formation in an intraplate, near-vertical, fault zone and chemical changes of the fault rocks in the presence of water at seismogenic depth. [ABSTRACT FROM AUTHOR]

Elemental contents and Sr isotopic data of calcite and celestite infillings from various tectonic microstructures of the so-called "Main Fault" that occurs in the Opalinus Clay Formation (Switzerland) were used to elaborate on the geochemical impact of this structural system on the fine-grained host sediments. The chemical data of the leachates from vein, slickenside and gouge infillings are significantly different from those of leachates from diffuse calcite of the undeformed and of the highly deformed Opalinus Clay rock matrix called scaly clays. For instance, the fluids flowing within the microstructural veins and slickensides could not derive from pore waters of the host sediments, as the data show that the flowing waters diffused from veins into the matrix. The fluid/rock interactions in the gouges were also different from those in the veins and the slickensides, probably due to an additional pressure-solution impact in the former, complemented by a further local supply of organics from very close sediments. The infillings of the vein and slickenside micro-features and of some scaly clays that precipitated from bi-carbonated and/or sulfated flowing fluids yield an almost constant 87Sr/86Sr ratio of 0.70774 ± 0.00001 (2 σ ). This constrained Sr isotopic value points to a unique homogeneous external source for the tectonic-related fluids that were of probable marine origin, signing those of the early, but also of the late faulting event. Notably different from those seeping presently throughout the host rocks, the initial flowing fluids were apparently related to a late-Eocene marine invasion into the Delémont Basin that occurred during the contemporaneous rifting of the Rhine-Graben tectonic system. The flowing fluids induced by the late Main-Fault system obviously used, at least, some if not most of these earlier tectonic microstructures, as they yield the Sr isotopic signature of their infillings. The regional evolution includes then an initial Rhine-Graben tectonic episode with fluids resulting from a flooding of the deposited sediments and into the faults of the regional active Rhine-Graben rifting by contemporaneous seawater. These fluids precipitated precisely calcite and celestite infillings in the microstructures induced by the rifting that remained then unaffected until the renewed tectonic activation during the late Eocene in the Mont Terri and its fault system. This new tectonic–thermal episode created a new generation of microstructures during the thin-skinned deformation of the Jura Belt within and around the Main Fault, in addition to those from earlier Rhine-Graben event that already existed in the host rocks. Connections between the two generations of veins could be observed at the wall of a new gallery in the rock laboratory. The flowing fluids involved in this late belt event moved apparently along these newly created tectonic microstructures, as well as along older ones when accessible. This duality allowed the preservation of the same initial Sr isotopic signature by partial dissolution of the initial infillings that precipitated about 30 million years earlier in the initial drains of the Rhine-Graben fault system. [ABSTRACT FROM AUTHOR]

Slope stability assessment of vulnerable slopes was conducted along National Highway 58 (NH-58) from Dhari Devi to Rudraprayag (~17 km), Uttarakhand. Kedarnath devastation in June, 2013 due to flash floods has made road cut slopes along the Alaknanda River vulnerable, which has a history of frequent landslides particularly at Kaliasaur (sites K1, K2, K3). At Kaliasaur landslide (site K1), the instability is mainly controlled by the combination of pre-existing tectonic fracturing, plunging fold, structural damage zones formed due to the passage of regional Kaliasaur Fault and at site K3, fault gouge is found due to shearing of quartzite. A total of 35 road cut slopes were investigated along NH-58 and kinematic analysis of joint data shows that 18 slopes are prone to planar failure, 7 for wedge failure and 8 for toppling failure. Slope mass rating (SMR) and continuous SMR show that 18 slopes are vulnerable with 6 slopes falling under the class III category (partially unstable), 9 slopes under class IV (unstable) and the rest of 3 slopes are completely unstable fall under class V causing landslides on NH-58. Maps with location of stable, partially stable, unstable and completely unstable sites are formed to take infrastructure projects at stable zones. Research highlights: Kaliasuar fault causes active landslides along NH-58, Uttarakhand. Maps with location of stable sites are provided for infrastructure projects. Instability at Kaliasaur Landslide is caused by plunging fold, pre-existing tectonic fracturing. Conducted kinematic analysis and geo-mechanical classification based on CSMR of 35 slopes in the study area. [ABSTRACT FROM AUTHOR]

Understanding the slope hydrology and failure processes of rainfall-induced landslides is key to landslide early warning; the heterogeneity of soil (e.g., grain-size distribution in different layers) can markedly affect rainfall infiltration and slope failure patterns. However, the hydrological and failure processes of heterogeneous slopes layered by different soil groups have received little attention. In this study, we use a typical landslide soil composition of rainfall-induced landslide in fault zones as a prototype and via flume experiments to simulate the hydrological evolution, failure processes, and patterns under rainfall conditions on material heterogeneity slopes with a combination of colluvial deposit and fault gouge. Our results showed that rainfall-induced slope settlement and rapid saturation of shallow layers of colluvial deposits led to the occurrence of layer-by-layer shallow flow-slides. The spatial variability of infiltration led to the generation of a relatively dry‒wet interface in deeper layers, causing differential changes in the mechanical properties of the fault gouge; this was conducive to the formation of a steep landslide back wall, perched water table in the shallow layer of the fault gouge, and a rapid increase in porewater pressure, which triggered deep sliding, with a change in the failure pattern to a retrogressive mode. There was a strong linear correlation between the displacement rate before slope instability and the Arias intensity (IA) of the seismic signal; an abrupt change and rapid increase in IA may indicate that the slope entered an accelerating creep stage before failure. The results of this study provide a physical basis for related numerical simulation research and a reference for landslide early warning based on seismic signals. [ABSTRACT FROM AUTHOR]

The M 4.2 2020 December Stanton, TX seismicity sequence marks the first of several M 4 + seismicity sequences in the Midland Basin. In this study, we investigate its causal link to the surrounding field-scale, multi-decadal, multi-zone saltwater disposal (SWD) by 180 + SWD wells through integrated data analysis and high-end hydro-geomechanical numerical modeling. We test six scenarios by permutating three fault upper extents and two fault-zone structures and interpret their plausibility based on their performance in reproducing seismicity onset and distribution as characterized by an aggregated earthquake catalog. We do so via Coulomb faulting analysis at inferred seismogenic depths complemented by rate-and-state friction-based seismological modeling on the event-hosting fault. Our analysis suggests that the basement-rooted faults, despite being poorly seen in the reflection seismic data, likely extend into the deep disposal interval but not the two shallower disposal intervals and contain impermeable fault gouge. The work also strongly suggests that the seismicity sequence was induced mainly by intermediate and deep SWD and negligibly by shallow SWD through an over three-decadal stressing process involving 10 + wells up to 60 km laterally away from the event epicenters. The joint stressing by a cluster of wells to the Northwest and a near-fault well produced two Coulomb stress lobes that explain the bifurcating seismicity near the top of the basement. Meanwhile, the mainshock was likely preceded by a half-year-long nucleation period, over which the joint stressing also drove the development of a seismicity funnel zone that explains the remaining seismicity in the basement. [ABSTRACT FROM AUTHOR]

Seismic records show that seismic activity is strikingly different in the southern and northern Red River Fault Zone (RRFZ). The magnitude and frequency of seismic activity in the southern RRFZ are weak (Less than 3 M) and low, respectively, while the magnitude and frequency of seismic activity in the northern RRFZ are strong (more than 5 M) and high, respectively. However, considerable controversy regarding the reasons for these differences remains. In this paper, to investigate the causes of different seismic activities, we analyze the composition and structure of fault gouges by conducting field investigations, polarized light microscopy, scanning electron microscopy observations, and X-ray diffraction (XRD) experiments. The results indicate that fault gouges in the southern RRFZ with a high illite content have high friction coefficients, and in the nanoscale, illite-rich fault gouges are easily deformed into S–C fabrics or foliated (layers) structure; under shear stress, it can release of stress and strain slowly (creep), so the fault activity shows aseismic creep. While the fault gouges in the northern RRFZ contain mainly montmorillonite, which has a low friction coefficient, and in the nanoscale, montmorillonite-rich fault gouges easily form linear nanogrooves and nanoridges under shear stress, so the fault shows seismic stick–slip, and it can induce earthquakes. Therefore, we propose that the composition and structure of fault gouges are the key factors affecting the different magnitudes and frequencies of seismic activity in the southern and northern RRFZ. [ABSTRACT FROM AUTHOR]

Near-surface disposal of radioactive waste in shales is a promising option to safeguard the population and environment. However, natural faults intersecting these geological formations can potentially affect the long-term isolation of the repositories. This paper characterizes the physical properties and mineralogy of the internal fault core structure intersecting the Opalinus Clay formation, a host rock under investigation for nuclear waste storage at the Mont Terri Laboratory (Switzerland). We have performed porosity, density, microstructural and mineralogical measurements in different sections of the fault, including intact clays, scaly clays and fault gouge. Mercury intrusion porosimetry analysis reveal a gouge that has a pore network dominated by nanopores of less than 10 nm, yet a high-porosity (21%) and low grain density (2.62 g/cm3) when compared to the intact rock (14.2%, and 2.69 g/cm3). Thus, a more permeable internal fault core structure with respect to the surrounding rock is deduced. Further, we describe the OPA fault gouge as a discrete fault structure having the potential to act as a preferential, yet narrow, and localized channel for fluid-flow if compared to the surrounding rock. Since the fault gouge is limited to a millimetres-thick structure, we expect the barrier property of the geological formation is almost not affected. [ABSTRACT FROM AUTHOR]

We present evidence for frictional–viscous flow (or steady creep) behaviour of the blind detachment below the Indo-Burmese wedge (IBW), as characterised by the fault gouge exposed along the Churachandpur–Mao Fault, which is one of the splay faults to the blind detachment. The petrography of the exhumed phyllonitic gouge material (considered the wedge material from the detachment and representative of the detachment) indicates evidence of pressure solution and viscous flow. We use a microphysical model involving frictional–viscous flow, and together with microstructural observations from the exhumed gouge we interpret low shear strength (<20 MPa) within the frictional–viscous transition zone. The results of the model also suggest that the geodetically estimated strain rate in this region lies in the domain of steady creep or velocity strengthening frictional behaviour. We suggest that the presence of phyllosilicate-rich rocks may significantly weaken mature fault cores along the detachment beneath the IBW and facilitate frictional–viscous flow at shallow depth. This new evidence suggests a significantly reduced seismic hazard across the megathrust. Research Highlights: Frictional–viscous flow along the blind detachment beneath the Indo-Burmese Arc. Exhumed fault gouge indicates evidence of pressure solution and viscous flow. Phyllosilicate rich rocks significantly weaken mature fault cores along this detachment. Microphysical model imply low shear strength of the blind detachment. [ABSTRACT FROM AUTHOR]

Fault zones usually present a granular gouge, coming from the wear material of previous slips. This layer contributes to friction stability and plays a key role in the way elastic energy is released during sliding. Considering a mature fault gouge with a varying amount of mineral cementation between particles, we aim to understand the influence of the strength of interparticle bonds on slip mechanisms by employing the discrete element method. We consider a direct shear model without fluid in 2D, based on a granular sample with angular and faceted grain shapes. Focusing on the physics of shear accommodation inside the granular gouge, we explore the effect of an increase of cementation on effective friction (i.e. stress ratio) within the fault. We find that brittleness and the overall shear strength are enhanced with cementation, especially for dense materials. For the investigated data range, three types of cemented material are highlighted: a poorly cemented material (Couette flow profile, no cohesion), a cemented material with aggregates of cemented particles changing the granular flow and acting on slip weakening mechanisms (Riedel shear bands R), and a highly-cemented material behaving as a brittle material (with several Riedel bands followed by fault-parallel shear-localization Y). Effective friction curves present double weakening shapes for dense samples with enough cementation. We find that the effective friction of a cemented fault cannot be directly predicted from Mohr–Coulomb criteria because of the heterogeneity of the stress state and kinematic constraints of the fault zone. [ABSTRACT FROM AUTHOR]

Geological heterogeneity is abundant in crustal fault zones; however, its role in controlling the mechanical behaviour of faults is poorly constrained. Here, we present laboratory friction experiments on laterally heterogeneous faults, with patches of strong, rate-weakening quartz gouge and weak, rate-strengthening clay gouge. The experiments show that the heterogeneity leads to a significant reduction in strength and frictional stability in comparison to compositionally identical faults with homogeneously mixed gouges. We identify a combination of weakening effects, including smearing of the weak clay; differential compaction of the two gouges redistributing normal stress; and shear localization producing stress concentrations in the strong quartz patches. The results demonstrate that geological heterogeneity and its evolution can have pronounced effects on fault strength and stability and, by extension, on the occurrence of slow-slip transients versus earthquake ruptures and the characteristics of the resulting events, and should be further studied in lab experiments and earthquake source modelling. This paper shows that faults comprised of heterogeneously distributed materials, as is typical for tectonic faults in nature, are weaker and more unstable than equivalent faults where the materials are homogeneously mixed together. [ABSTRACT FROM AUTHOR]

Middle-lower crust and mantle rocks are generally widely exposed in metamorphic core complex or gneiss dome, which is an ideal place to study the exhumation process related to regional extension and rheology. The Laojunshan metamorphic complex in southeastern Yunnan is located in a special tectonic position surrounded by the Cathaysia, Yangtze and Indochina blocks. It is composed of different metamorphic-deformation rocks and granitic intrusions. There also are many economic deposits (e.g., tin and tungsten) that are spatially and genetically associated with the formation and exhumation of the Laojunshan gneiss dome. Based on detailed analysis of macro- and microscopic structure, stress field distribution and deformation condition, the tectonic units of the Laojunshan metamorphic complex show obvious characteristics of doming, as well as of typical structural units of metamorphic core complex. It has strongly deformed metamorphic gneiss core (footwall), detachment fault system and sedimentary cover (hanging wall) with lightly metamorphism and deformation. The footwall of gneiss dome presents a strongly ductile deformation domain, accompanied by different ages of granitic intrusions. The distribution of developed foliation and lineation within granitic gneisses are arc-shaped and radial, respectively, with a nearly N-S trending from the footwall to the hanging wall. Mylonitization of deformed rocks gradually weakens and transits to orthogneiss as it moves away from the detachment fault toward the footwall. The low angle detachment fault between the footwall and the hanging wall shows an arc-like shape feature. Mylonite fabrics are preserved in the deformed rocks of the detachment fault, which are mainly composed of chloritized schist, fault breccia, cataclasite and fault gouge. A large number of normal faults are developed in detachment faults and hanging wall, and their stress fields radiate in an arc around the footwall. Zircon U-Pb ages of amphibolite and granitic gneiss from the footwall range from 445 to 420 Ma, indicating the timing of Caledonian magmatic emplacement and the main formation period of the Laojunshan gneiss dome. U-Pb ages of the zircon metamorphic rims are 241–230 Ma, representing the timing of high temperature metamorphism and shortened deformation of the Indosinian collision. In this period, the Laojunshan gneiss dome experienced the tectonic compression in association with high temperature metamorphism-deformation, which was superimposed by detachment and extensional exhumation in association with intense hydrothermal interaction and mineralization in the late stage. [ABSTRACT FROM AUTHOR]

The preparation process of natural earthquakes is still difficult to quantify and remains a subject of debate even with modern observational techniques. Here, we show that foreshock activity can shed light on understanding the earthquake preparation process based on results of meter-scale rock friction experiments. Experiments were conducted under two different fault surface conditions before each run: less heterogeneous fault without pre-existing gouge and more heterogeneous fault with pre-existing gouge. The results show that fewer foreshocks occurred along the less heterogeneous fault and were driven by preslip; in contrast, more foreshocks with a lower b value occurred along the more heterogeneous fault and showed features of cascade-up. We suggest that the fault surface condition and the stress redistribution caused by the ongoing fault slip mode control the earthquake preparation process, including the behavior of foreshock activity. Our findings imply that foreshock activity can be a key indicator for probing the fault conditions at present and in the future, and therefore useful for assessing earthquake hazard. By tuning fault surface heterogeneity, the authors report earthquake preparation processes respectively driven by preslip and cascade-up on a meter-scale laboratory fault. The findings suggest that foreshock activity can be useful for predicting when and how the impending mainshock will occur. [ABSTRACT FROM AUTHOR]

The installed capacity of geothermal systems for direct use of heat is increasing worldwide. As their number and density is increasing, the their interaction with subsurface faults becomes more important as they could lead to safety risks from induced seismicity. Assessment and management of such risks is essential for the further development and extension of geothermal energy for heating. At the same time, the economic output of geothermal systems can be marginal and is hence often supported by subsidy schemes. A combined assessment of fault stability and economic output could help operators to balance economic and safety aspects, but this is currently not common practice. In this study we present a methodology to assess field development plans based on fault stability and Net Present Value (NPV) using reservoir simulations of a fluvial, heterogeneous sandstone representative of the majority of direct-use Dutch geothermal systems. We find that the highest friction coefficient leading to exceedance of the Mohr–Coulomb failure criteria in this sandstone is 0.17; such values could be encountered in clay-rich fault gouges. Similar or lower fault permeability compared to the reservoir results in no changes and an increase respectively of both NPV and fault stability with larger Fault-to-Well Distance (FWD). Fault permeability higher than the reservoir permeability results in a minor increase in NPV with smaller FWD. Our results demonstrate that a combined analysis of thermal, hydraulic, mechanical and economic assessment supports a responsible and viable development of geothermal resources at a large scale. The importance of a high spatial density of supporting stress data will be essential for a better understanding and quantification of economic and fault stability effects of geothermal operations. [ABSTRACT FROM AUTHOR]

The Provence region, located in the south-east of France, has experienced a few destructive earthquakes during the last centuries, such as the 1909 Lambesc earthquake or the 1509 and 1708 Manosque earthquakes. However, faults in the area experience slow slip rates (< 0.1 mm/yr) that preclude quantification of deformation using geodetic measurements. Active faults in Provence have long recurrence intervals, and the region experiences erosion under a Mediterranean climate where surface markers of such deformation are rapidly erased. As a consequence, several faults in the region may go unnoticed despite having the potential to generate earthquakes. This work focuses on the Vinon-sur-Verdon area in Provence, where a relatively narrow fault structure, the Maragrate fault, brings into contact Miocene clays and Pliocene conglomerates. Because of its proximity to a major nuclear research facility, this fault poses a potential earthquake hazard that justifies studies aimed at gaining insight into its mechanics and Quaternary seismic activity. In this context, a multi-disciplinary approach has been implemented. A LiDAR campaign was undertaken to generate a 30-cm high-resolution Digital Elevation Model that depicts in detail the current fault morphology. In parallel, an electrical resistivity campaign was conducted and three trenches were opened. These investigations revealed a fault contact expressed by a fault gouge zone surmounted and sealed by numerous recent Quaternary periglacial and high-energy channel units. Radiocarbon and optically stimulated luminescence dating allowed the chronological reconstruction of sediment deposition and erosion cycles and of the most recent deformation event, older than 20 ka. These investigations did not allow us to demonstrate with certainty that these deformations are of a co-seismic nature. This work highlights the difficulty of identifying faults and studying the seismic hazard in intra-continental contexts. [ABSTRACT FROM AUTHOR]

Accurate determination of the Principal Slip Zone (PSZ) of earthquake fault zones is a key task of earthquake Fault Scientific Drilling for future earthquake control. The fault zone structure of Wenchuan earthquake is complex, and there are many strong earthquakes recorded on the fault zone, which make determining the PSZ in the Wenchuan earthquake Fault Scientific Drilling project-hole 1 (WFSD-1) difficult. At present, core analysis of whole coring is the decisive method for determining PSZ depth, and the fresh fault gouge at 589.2 m is the PSZ in WFSD-1. Abundant and comprehensive logging data can only be used as evidence to judge the PSZ. Based on the discrimination function and hyperplane equation in Bayesian discriminant classification, we derive a new algorithm for computing the PSZ possibility using a Bayesian Discrimination function (PSZP-BDF) based on the simplified model, and set up a mode to determine the PSZ directly using machine learning of well logging. For the verification of WFSD-1, the fault gouges are successfully identified and the PSZ depth is accurately located. The algorithm objectively learns the sample data, which is naturally adaptive to the region. The calculation procedure is simple and does not require expensive coring data or heavy core tests in the well. The calculation speed is fast, using multiple physical data types. The PSZP-BDF algorithm is suitable for processing and interpreting earthquake fault scientific drilling data. [ABSTRACT FROM AUTHOR]

The dynamic permeability evolution of a fracture is a key scientific problem for fluid flows in rock masses within engineering systems. Understanding the dynamic permeability evolution and its mechanism is conducive to design and operation engineering. The dynamic permeability evolution of a rough granite fracture was revealed by laboratory experiments and numerical models. The permeability evolution of six fractured samples with rough fractures were monitored under 1.9–20 MPa effective normal stresses. The results show that the shearing process significantly affects the permeability and that the variation trend of the permeability depends on the magnitude of the effective normal stress. Under effective normal stresses of 1.9–5 MPa, the permeability is first significantly enhanced and then decreased by shearing. When effective normal stresses of more than 5 MPa are applied, the permeability only shows a decreasing trend. A high effective normal stress not only limits the dilatancy of a fracture but also enhances the formation of fault gouges. The mechanism of the dynamic permeability evolution was revealed by numerical simulations based on the discrete element method. The shearing mechanism includes the sliding mechanism and shearing mechanism. Under a low normal stress, first, the sliding mechanism is dominant and decreases the contact area, which is conducive to establishing a flow channel and increases the permeability. Then, the shearing mechanism becomes increasingly impactful, causing the contact area to increase and the permeability to decrease. Under a high normal stress, the sliding and shearing mechanisms are always engaged, which generates many wear products and reduces the permeability. [ABSTRACT FROM AUTHOR]

To explore the grouting reinforcement mechanism of heterogeneous rock and soil in a fault fracture zone, based on the F2 fault grouting project of Yonglian Tunnel, the filling area composed of loose and soft media in the fault is defined as the advantageous grouting diffusion region, which is divided into loose and weak diffusion regions according to the slurry diffusion form. A self-developed grouting reinforcement test system is used to carry out grouting reinforcement tests on the heterogeneous rock and soil. The existence of the advantageous diffusion region can effectively improve the reinforced body strength. The slurry in the reinforced body containing the loose medium mainly expands outwards in the form of first infiltration and then compaction, and the slurry vein mainly remains in the advantageous diffusion region. The slurry in the reinforced body containing the soft medium mainly diffuses in the form of compaction splitting, which can form thicker splitting of the grouting veins. When the reinforced body is destroyed, the body containing the loose medium is mostly conically fractured, indicating complete lithology and homogeneous material. Moreover, the reinforced body containing the soft medium always forms a penetrating crack along the slurry vein or near the grouting vein. Focusing on the heterogeneous stratum consisting of fault breccia and soft plastic fault gouge, controllable slurry dynamic adjustment, same-hole multi-sequence sub-gradient grouting, and localized grouting control are proposed. The grouting control method is applied to the case of Yonglian Tunnel, and a superior grouting reinforcement effect is achieved. [ABSTRACT FROM AUTHOR]

The Lorca earthquake (May 11th, 2011, Mw 5.2) stands as the most destructive one in Spain over the last 50 years, interpreted as having occurred in an intersegment zone of the strike–slip Alhama de Murcia Fault (AMF) (Eastern Betics, Spain). Magnetotelluric data were acquired along a profile to the SW of Lorca (La Torrecilla profile), to characterize its signature at depth, as part of the multidisciplinary project "INTERGEOSIMA". Given the short distance between stations, some station pairs were recorded simultaneously, with magnetic sensors in only one of them. In order to properly understand the resulting impedances (called interstation impedances), and the effects of inverting them, we used synthetic models to compare the impedances and the interstation impedances and to analyze the corresponding inversion results, together with the inversion of the quasi-impedance (inversion of the interstation impedances, considering them as impedances). The results are sensitive to the location of the magnetic sensors and the resistivity underneath, but in general the use of the quasi-impedances in the inversion can be considered a valid procedure. Both the 2D and the 3D resistivity models obtained through the inversion allowed us to complement the previous ERT models and represent the continuation of the main fault gouge in depth showing its extension towards the SE. [ABSTRACT FROM AUTHOR]

Reservoir induced seismicity provides a suitable method for studying the roles of fluid in inducing earthquakes. The fault structure plays a predominant role in the occurrences of earthquakes, and the influences of fluid also cannot be disregarded. In this study, we investigate the active Fairy Mount fault in the Three Gorges Reservoir. Since water impoundment in 2003, more than 4000 detectable earthquakes have occurred along the fault. The vast majority of these earthquakes are associated with the fault and water impoundment. To explore the effects of water-fault interactions on induced earthquakes, a permeability structure of the fault zone is established by a series of geological experiments. Fault rocks, including unconsolidated breccias and fault gouges, collected from a presentative outcrop are employed for detailed microstructural and mineralogical analyses. The results reveal a complex internal fault structure and widespread fluid-rock interactions. The hydrogeological property of the fault exhibits a typical conduit/barrier permeability structure. Highly permeable damage zones act as fluid conduits for the infiltration of reservoir water to the subsurface, while the low permeable fault core renders the fault core as a potential fluids storage area to weaken the fault in the shallow crust. In sum, both the pore pressure changes due to water infiltration and the long-term chemical effect of water on the fault plane promote instability of the fault and induce earthquakes. [ABSTRACT FROM AUTHOR]

We study the stick-slip behavior in a sheared granular fault gouge using a coupled discrete element method and computational fluid dynamics. We compare characteristics of slip events in dry and fluid-saturated granular fault gouge in drained conditions. The granular layer is confined under constant normal load and sheared with a velocity-controlled mechanism. Potential energy is stored through overlaps between particles. We show that the potential energy builds up during the stick phase and drops during slip instability. Our observations show that on average 8% of the drop in potential energy is converted into particle kinetic energy, while the rest dissipates. Our simulations show that drop in potential energy is a good measure of slip size showing a strong correlation with the drop in macroscopic friction coefficient. Our simulations show that in fluid-saturated granular fault gouge, the potential energy drop is higher leading to a higher drop in friction coefficient and in a higher kinetic energy of particles during slip event. [ABSTRACT FROM AUTHOR]

There are two co-seismic bedrock surface ruptures from the Mw 7.9 Wenchuan earthquake in the northern and central parts of the Beichuan-Yingxiu fault, Sichuan Province, southwest China. In this study, we report on the macrostructure of the fault rocks and results from X-ray powder diffraction analysis of minerals from rocks in the fault zone. The most recent fault gouge (the gouge produced by the most recent co-seismic fault movement) in all the studied outcrops is dark or grayish-black, totally unconsolidated and ultrafine-grained. Older fault gouges in the same outcrops are grayish or yellowish and weakly consolidated. X-ray powder diffraction analysis results show that mineral assemblages in both the old fault gouge and the new fault gouge are more complicated than the mineral assemblages in the bedrock as the fault gouge is rich in clay minerals. The fault gouge inherited its major rock-forming minerals from the parent rocks, but the clay minerals in the fault gouge were generated in the fault zone and are therefore authigenic and synkinematic. In profiles across the fault, clay mineral abundances increase as one traverses from the bedrock to the breccia to the old gouge and from the old gouge to the new gouge. Quartz and illite are found in all collected gouge samples. The dominant clay minerals in the new fault gouge are illite and smectite along the northern part of the surface rupture and illite/smectite mixed-layer clay in the middle part of the rupture. Illite/smectite mixed-layer clay found in the middle part of the rupture indicates that fault slip was accompanied by K-rich fluid circulation. The existence of siderite, anhydrite, and barite in the northern part of the rupture suggests that fault slip at this locality was accompanied by acidic fluids containing ions of Fe, Ca, and Ba. [ABSTRACT FROM AUTHOR]

The results of an instrumental and analytical investigation of the products of mineral and textural transformations in the tectonic slickensides and fault gouge in the near-surface terrigenous sedimentary rocks (clays, arkose sandstones, shungites) which have undergone localized deformations in fault zones of presumably seismogenic origin are presented. Based on this, several peculiarities in the behavior of a dynamic slip in the upper transition horizon from aseismic to seismogenic mode of faulting in the Earth's crust are elucidated. The changes in the mineral phase compositions of the fault facies against the protolith composition are estimated; the parameters of the temperature regime and thermal energy balance of deformational metamorphic reactions are determined. The probable causes of instability in the faults, the mechanisms of the loss of strength, the weakening and strengthening during seismogenic dynamic slip are considered. The role of tribochemical phenomena in the course of a rock's transformation into a fault gouge and the related energy effects are discussed. An inventory of the possible energy costs on the processes of transforming material in dynamic slip zones is compiled. [ABSTRACT FROM AUTHOR]

Earthquake precursors are vitally important for warning and risk evaluation, but shedding light on them is a complex task for seismologists and geologists. We attempted to illuminate the precursory rules of fault failure by traditional biaxial-direction shear tests for a zigzag-type gouge with different size and arrangement of simulated faults manufactured from polymethyl methacrylate materials. Shear stress, acoustic emissions, and vibration signals were recorded and comparatively analyzed. Two important findings were obtained: (1) the linearly increasing trend of shear stress peaks generated by foreshocks may indicate the foreshock effect before a larger earthquake triggered by natural fault gouge failure; and (2) four kinds of earthquake precursors were discovered, and earthquake intensity and precursors may be dependent on the time of macro-fracture formation and the quantity of micro-fractures initiated before mainshock. The study contributes to interpreting earthquake shear-slip characteristics and may even help provide warnings of failure and instability. [ABSTRACT FROM AUTHOR]

Microscopic billow-like wavy folds have been observed along slip planes of the Nojima active fault, southwest Japan. The folds are similar in form to Kelvin-Helmholtz (KH) instabilities occurring in fluids, which implies that the slip zone underwent 'lubrication' such as frictional melting or fluidization of fault gouge materials. If the temperature range for generation of the billow-like wavy folds can be determined, we can constrain the physical properties of fault gouge materials during seismic slip. Here, we report on rock magnetic studies that identify seismic slip zones associated with the folds, and their temperature rises during ancient seismic slips of the Nojima active fault. Using a scanning magneto-impedance magnetic microscope and a scanning superconducting quantum interference device microscope, we observed surface stray magnetic field distributions over the folds, indicating that the folds and slip zones are strongly magnetized. This is due to the production of magnetite through thermal decomposition of antiferromagnetic or paramagnetic minerals in the gouge at temperatures over 350 °C. The presence of micrometer-sized finely comminuted materials in the billow-like wavy folds, along with our rock magnetic results, suggests that frictional heating-induced fluidization was the driving mechanism of faulting. We found that the existence of the magnetized KH-type billow-like wavy folds supports that the low-viscosity fluid induced by fluidization after frictional heating decreased the frictional strength of the fault slip zone. [ABSTRACT FROM AUTHOR]

Microstructures developed in the fault gouge zone, Yangbuk, Gyeongju city, Korea, were characterized with emphasis on distribution of remaining grains. Based on XRD analysis, the fault gouges consist of quartz, feldspar and clay minerals composed of abundant illite and chlorite, with vein minerals including laumontite and pyrite. As the fractal dimension (D-value) of grains gradually increases, the pore space in the fault gouge becomes smaller, causing the change in microstructures. Quartz and feldspar exerted a different influence on the development of microstructures in the fault gouge zone, derived from essential reactivity inherent between the two minerals during multiple faulting. Quartz grains intensively suffered grain size reduction while the content and size of coarse grains decreased. Feldspar grains tend to influence the formation of clay minerals made up for the matrix. Although large quartz grains highly fractured with abrasions caused the grain size reduction in the gouge zone during early fault activities, fine-grained quartz in the clay-rich matrix easily remained due to the reduction in friction resulted from clay minerals. On the other hand, feldspar grains underwent more abrasion and dissolution, leading to an increase in the amount of clay minerals newly formed during fault activities. Changes in microstructures were accompanied by precipitation of the secondary minerals during multiple fault activities in connection with fracturing of main rock-forming minerals such as quartz and feldspar derived from the host rock. [ABSTRACT FROM AUTHOR]

Deep-seated rockslides, which are characterised by slow to extremely slow rates of movement, frequently occur in foliated metamorphic rock masses (schists, phyllites, paragneiss series). Many case studies indicate that slope displacement is predominantly localised at basal and internal shear zones. Thus, the deformation and stability behaviour of rockslides is influenced primarily by the properties of these soil-like shear zones. In this study, new findings concerning the structure, mineralogical composition and geomechanical characteristics (residual friction angle, grain size distribution) of the shear zones of deep-seated rockslides are presented. The characteristics of these shear zones are shown by case studies in paragneissic rock masses of the polymetamorphic Austroalpine Ötztal-Stubai crystalline complex in Tyrol, Austria. Differences between the laboratory scale and the in situ scale are discussed, as well as the evolution of the shear zones. Within the framework of this study, structural investigations of the shear zones were performed from surface and subsurface surveys and core logs, as well as mineralogical laboratory analyses, grain size analyses and ring shear tests. The shear zones are characterised by a complex fabric of lensoid-shaped layers of clayey-silty fault gouges embedded in sandy-gravelly fault breccias and block-in-matrix structures. The mineralogical analyses indicated high amounts of phyllosilicates, such as mica and chlorite. Swelling clay minerals were observed in small amounts in very few instances. The ring shear tests of the rockslide fault gouge samples, performed under various normal stress conditions, resulted in residual friction angles in a wide range between 19° and 28°, reached after rather short displacements. [ABSTRACT FROM AUTHOR]

The internal structure and materials of a fault are crucial to understanding its mechanical and fluid flow behaviors because they are interrelated. The Yangsan fault is a major strikeslip fault in SE Korea, and its possible reactivation has been an issue of great interest and debate. However, the internal structure of the fault has not been extensively studied; one reason for this is the scarcity of the fault outcrop. Based on field observations and materials analysis, here we report the characteristics of the internal structure and the materials of the fault in the Bogyeongsa area, where a new outcrop showing the entire section of the fault is found. The fault is composed of a core zone (9−10 m in thickness) and surrounding damage zones (>200 m in thickness). The core zone has two subzones: one is an ~8-9-m thick alternating blue and purple gouge zone (BPZ) and the other is an ~0.5-m thick brown gouge zone (BZ). The former exhibits large fractured lenses and anastomosing gouges, and the latter has a strong network of foliation. Given the smaller content and size of clasts in the BZ than in the BPZ, it follows that larger fault displacement was accommodated in the BZ than in the BPZ. The damage zones around the core zone are characterized by dense fractures and veins and subsidiary faults of multiple generations. According to an analysis of the materials, the core zone gouges are clay-rich (>50 wt%). The BPZ is enriched in illite and chlorite, whereas the BZ is enriched in smectite, which has a notably low frictional strength when wet. Given the occurrence of the Quaternary slip faults in the damage zone of the Yangsan fault in other areas, further studies of the materials and mechanical properties on both the main core zone and the subsidiary faults in the damage zones are necessary to determine which is likely to be reactivated seismogenically under the current stress field. [ABSTRACT FROM AUTHOR]

Increasing fracture aperture by lowering effective normal stress and by inducing dilatant shearing and thermo-elastic effects is essential for transmissivity increase in enhanced geothermal systems. This study investigates transmissivity evolution for fluid flow through natural fractures in granodiorite at the laboratory scale. Processes that influence transmissivity are changing normal loads, surface deformation, the formation of gouge and fracture offset. Normal loads were varied in cycles between 1 and 68 MPa and cause transmissivity changes of up to three orders of magnitude. Similarly, small offsets of fracture surfaces of the order of millimeters induced changes in transmissivity of up to three orders of magnitude. During normal load cycling, the fractures experienced significant surface deformation, which did not lead to increased matedness for most experiments, especially for offset fractures. The resulting gouge material production may have caused clogging of the main fluid flow channels with progressing loading cycles, resulting in reductions of transmissivity by up to one order of magnitude. During one load cycle, from low to high normal loads, the majority of tests show hysteretic behavior of the transmissivity. This effect is stronger for early load cycles, most likely when surface deformation occurs, and becomes less pronounced in later cycles when asperities with low asperity strength failed. The influence of repeated load cycling on surface deformation is investigated by scanning the specimen surfaces before and after testing. This allows one to study asperity height distribution and surface deformation by evaluating the changes of the standard deviation of the height, distribution of asperities and matedness of the fractures. Surface roughness, as expressed by the standard deviation of the asperity height distribution, increased during testing. Specimen surfaces that were tested in a mated configuration were better mated after testing, than specimens tested in shear offset configuration. The fracture surface deformation of specimen surfaces that were tested in an offset configuration was dominated by the breaking of individual asperities and grains, which did not result in better mated surfaces. [ABSTRACT FROM AUTHOR]

A hybrid finite-discrete element method was implemented to study the fracture process of rough rock joints under direct shearing. The hybrid method reproduced the joint shear resistance evolution process from asperity sliding to degradation and from gouge formation to grinding. It is found that, in the direct shear test of rough rock joints under constant normal displacement loading conditions, higher shearing rate promotes the asperity degradation but constraints the volume dilation, which then results in higher peak shear resistance, more gouge formation and grinding, and smoother new joint surfaces. Moreover, it is found that the joint roughness affects the joint shear resistance evolution through influencing the joint fracture micro mechanism. The asperity degradation and gouge grinding are the main failure micro-mechanism in shearing rougher rock joints with deeper asperities while the asperity sliding is the main failure micro-mechanism in shearing smoother rock joints with shallower asperities. It is concluded that the hybrid finite-discrete element method is a valuable numerical tool better than traditional finite element method and discrete element method for modelling the joint sliding, asperity degradation, gouge formation, and gouge grinding occurred in the direct shear tests of rough rock joints. [ABSTRACT FROM AUTHOR]

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 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. [ABSTRACT FROM AUTHOR]

The shear strength of fault gouge plays an important role in the dynamic behavior of faults, ranging from small-scale displacements to severe earthquakes. The characteristics and interactions of constituent materials in fault gouge are the main determinants of shear strength. Assessing the shear strength of fault gouge by means of experiments, however, requires time-consuming procedures, including sampling, shear testing, and reliability checking; consequently, simple and indirect methods to assess shear strength in terms of the characteristics of fault gouge fragments have been investigated. This study focuses on the influence of the shape of fault gouge particles on the shear strength of gouge. We introduce a novel technique to obtain shape parameters of particles using Xray computed tomography (CT), and then show the effects of particle shape on the friction angle of the fault gouge. Samples collected from fault zones developed in various parent rock materials were tested in laboratory experiments to characterize their shear strengths. After shear testing, the particles in the fault gouge were collected, scanned by X-ray CT, and then analyzed for shape characterization. We successfully determined the shape parameters (sphericity, elongation, flatness, and slenderness) of the fault gouge fragments, and found that the parameters are well correlated with the friction angle of the gouge. [ABSTRACT FROM AUTHOR]

How to find more effective way to stabilize the borehole wall in the fault gouge section is the key technical challenge to control the stability of the borehole wall in the Wenchuan fault gouge section during the process of core drilling. Here we try to describe the characters of deep fault gouge in fracture zones from the undisturbed fault gouge samples which are obtained during the core drilling. The X-Ray Diffraction (XRD), X-Ray Fluorescence (XRF) and Scanning Electron Microscope (SEM) provided the detailed information of the fault gouge's microscopic characteristics on the density, moisture content, expansibility, dispersity, permeability, tensile strength and other main physical-mechanical properties. Based on these systematic experimental studies above and analysis of the fault gouge instability mechanism, a new technical procedure to stabilize the borehole wall is proposed - a low water and a low loss low permeability drilling fluid system that consists of 4% clay + 0.5% CMC-HV + 2% S-1 + 3% sulfonated asphalt + 1% SMC + 0.5% X-1 + 0.5% T type lubricant + barite for core drilling in fault gouge sections. [ABSTRACT FROM AUTHOR]

The 2008 M 7.9 Wenchuan earthquake formed two coseismic surface rupture zones with the trend of N35°E, known as the Beichuan-Yingxiu rupture and the Pengguan rupture. The Beichuan-Yingxiu rupture is the principle one with abundant fault gouge development along its length. In the exploratory trench at the Saba village along the Beichuan-Yingxiu rupture, the new fault gouge zone is only ~3 mm wide, which suggests that fault slip was constrained in a very narrow zone. In this study, we thus carried out detailed microstructural and mineral component analysis on the oriented fault gouge samples from the Saba exploratory trench to understand their features and geological implication. The results show that different microstructures of localized brittle deformation can be observed in the fault gouges, including Y-shear, R1-shear, R2-shear, P-shear as well as tension fracture, bookshelf glided structure and so on. These microstructures are commonly recognized as the product of seismic fault slipping. Furthermore, within the area between two parallel Y-shears of the fault gouge, a few of microstructures of distributed ductile deformations were developed, such as P-foliation, elongation and asymmetrical trailing structure of detrital particles. The microstructure features of fault gouges implicate the thrust movement of the fault during the Wenchuan earthquake. In addition, the fault gouge has less quartz and feldspar and more clay than the surrounding rocks, which indicates that some quartz and feldspar in the surrounding rocks were transformed into clay, whereas the fault gouge has more illite and less illite/montmorillonite mixed layers than the surrounding rocks, which shows that the illite/montmorillonite mixed layer was partly converted into illite due to temperature increasing induced by coseismic fault slipping friction (also being affected partly by the chemical action of solutions). Such microstructures features and mineral component changes recorded the information of fault slip and provide criterions for discussing the genesis of fault gouge and recognition of the direction of fault movement. [ABSTRACT FROM AUTHOR]

We report an experimental and microstructural study of the frictional properties of simulated fault gouges prepared from natural limestone (96 % CaCO) and pure calcite. Our experiments consisted of direct shear tests performed, under dry and wet conditions, at an effective normal stress of 50 MPa, at 18-150 °C and sliding velocities of 0.1-10 μm/s. Wet experiments used a pore water pressure of 10 MPa. Wet gouges typically showed a lower steady-state frictional strength ( μ = 0.6) than dry gouges ( μ = 0.7-0.8), particularly in the case of the pure calcite samples. All runs showed a transition from stable velocity strengthening to (potentially) unstable velocity weakening slip above 80-100 °C. All recovered samples showed patchy, mirror-like surfaces marking boundary shear planes. Optical study of sections cut normal to the shear plane and parallel to the shear direction showed both boundary and inclined shear bands, characterized by extreme grain comminution and a crystallographic preferred orientation. Cross-sections of boundary shears, cut normal to the shear direction using focused ion beam-SEM, from pure calcite gouges sheared at 18 and 150 °C, revealed dense arrays of rounded, ~0.3 μm-sized particles in the shear band core. Transmission electron microscopy showed that these particles consist of 5-20 nm sized calcite nanocrystals. All samples showed evidence for cataclasis and crystal plasticity. Comparing our results with previous models for gouge friction, we suggest that frictional behaviour was controlled by competition between crystal plastic and granular flow processes active in the shear bands, with water facilitating pressure solution, subcritical cracking and intergranular lubrication. Our data have important implications for the depth of the seismogenic zone in tectonically active limestone terrains. Contrary to recent claims, our data also demonstrate that nanocrystalline mirror-like slip surfaces in calcite(-rich) faults are not necessarily indicative of seismic slip rates. [ABSTRACT FROM AUTHOR]

An introduction to the Topical Volume "Earthquake Source Physics on Various Scales" of the journal "Pure and Applied Geophysics" is presented in which discusses various reports within the issue on topics including earthquake recurrence and faults, rupture models, and shear tests on fault gouges.