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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.

The Northern Apennines (NA) are a characteristic example of foreland fold-and-thrust belt progressively migrating towards its foreland. Their tectonic evolution has been quite tightly constrained in time by microfossil biostratigraphy applied to syn-orogenic deposits within foreland basins. This makes the NA well-suited to test the reliability of K-Ar illite dating of Neogene deformation affecting siliciclastic sequences. We sampled two top-to-the ENE thrusts, whose well-defined cores are defined by scaly gouge formed at the expense of the pelitic component of the host rock. X-ray diffraction (XRD) and K-Ar isotopic analysis of multiple grain-size fractions of the gouge, allowed us to discriminate between syn-kinematic and inherited illite crystals in the fault rocks. Illite age analysis (IAA) constrains fault slip along the thrusts to 15.2 ± 7.6 Ma and 15.4 ± 16.6 Ma. The results, in spite of their analytical uncertainty, are fully consistent with the local evolution of the NA as constrained by the independent biostratigraphic studies and confirm the general suitability of this geochronological approach to Neogene deformation.

The fault gouge of the Alhama de Murcia fault (southeast Spain) shows a texture that resembles a mylonite, including a prominent foliation, S-C fabric, and isoclinal folds. It also embeds a large number of isolated pulverized quartz clasts (PQCs). Structural analysis indicates that the gouge fabric was mainly developed by slow frictional sliding along phyllosili- cate-lined Riedel shear bands during continued shearing. In contrast, the PQCs show tensile fracture network features that are typically reported in seismically pulverized rocks found along seismogenic faults. This suggests that quartz-clast pulverization was due to a transient dilatational mechanism rather than shearing. We propose that the PQCs are the result of a rapid confined stress drop related to transient tensile stresses during coseismic ruptures that interrupt creep faulting along the gouge zone. The present study suggests that there is probably a large amount of evidence for paleoseismicity in fault rocks that is currently overlooked, This research was supported by the Spanish Ministry of Economy and Competitiveness under the INTERGEO (CGL2013–47412-C2–1-P) and QUAKESTEP (CGL2017–83931-C3–1-P) projects

China’s Paleozoic deep carbonate effective reservoirs, mainly non-porous reservoirs, are generally formed under the interaction of late diagenesis, hydrothermal fluids, and structural fractures. Faults and their deformation mechanism and internal structure of fault zones play an important role in the formation of carbonate reservoirs and hydrocarbon accumulation. Based on the detailed analysis of outcrop data in Xike’er area, Tarim Basin, this paper systematically studies the deformation mechanism and internal structure of reverse fault in the carbonate rock, and discusses the reservoir characteristics, control factors and development rules. The study shows that the deformation mechanism of the fault in carbonate rocks is faulting and fracturing, and the dual structure of fault core and damage zone is developed. The fault core is mainly composed of fault breccia, fault gouge and calcite zone, and a large number of fractures are formed in the damage zone, which are cemented by calcite locally. The mineral composition and rare earth element tests show that the fault core has the dual effect of hydrothermal fluids and atmospheric fresh water, which is easy to be cemented by calcite; while the damage zone is dominated by atmospheric fresh water, which is a favorable zone for the development of fracture-vuggy reservoirs. Therefore, the damage zone is the “sweet spot” area of carbonate oil and gas enrichment, and generally shows strip distribution along the fault.

Rocks under stress deform by creep mechanisms that include formation and slip on small-scale internal cracks. Intragranular cracks and slip along grain contacts release energy as elastic waves termed acoustic emissions (AE). AEs are thought to contain predictive information that can be used for fault failure forecasting. Here we present a method using unsupervised classification and an attention network to forecast labquakes using AE waveform features. Our data were generated in a laboratory setting using a biaxial shearing device with granular fault gouge intended to mimic the conditions of tectonic faults. Here we analyzed the temporal evolution of AEs generated throughout several hundred laboratory earthquake cycles. We used a Conscience Self-Organizing Map (CSOM) to perform topologically ordered vector quantization based on waveform properties. The resulting map was used to interactively cluster AEs. We examined the clusters over time to identify those with predictive ability. Finally, we used a variety of LSTM and attention-based networks to test the predictive power of the AE clusters. By tracking cumulative waveform features over the seismic cycle, the network is able to forecast the time-to-failure (TTF) of lab earthquakes. Our results show that analyzing the data to isolate predictive signals and using a more sophisticated network architecture are key to robustly forecasting labquakes. In the future, this method could be applied on tectonic faults monitor earthquakes and augment current early warning systems.

Fault age determination using the illite-age-analysis (IAA) method for fault gouges has played a key role in providing absolute age information in tectonic evolution studies for the last 20 years. The accuracy and precision of the IAA method depend on (1) how to reasonably quantify the relative content of 1M/1Md illite generated from fault activity compared to detrital 2M1 illite in the size fractions of the fault gouge, and (2) how to minimize the error factors in K-Ar or Ar-Ar dating analysis. XRD-based quantitative analysis of illite polytype has made great progress in accuracy by generating a simulated XRD pattern of 1M/1Md polytype using WILDFIRE© and full-pattern-fitting it with the XRD pattern measured from size fractions of the fault gauge. Nevertheless, the results of quantitative analysis of illite polytypes may vary depending on the sample state of the size fractions for XRD analysis, especially the preferred orientation due to the layered crystal structure of illite. In addition, the radiometric dating results may be distorted depending on the error factor of the dating method itself and on the mineral composition of the size fractions, that is, the presence of K-containing minerals such as biotite and K-feldspar other than illite. In this study, we reviewed previous studies that determined fault activity ages by applying IAA to fault gouges. From this, the prerequisites and recommendations for each of the five steps (particle size separation process, XRD analysis process, polytype quantification, radiometric dating, IAA plot) for improving the IAA method are summarized and presented. The continuous application of the improved IAA is expected to greatly contribute to the study of tectonic evolution through geological time.

Tamusu area in Inner Mongolia is one of the favorable site candidates for high level radioactive waste (HLW) repository, which requires a stable regional tectonic environment. Field investigation, mossbauer spectroscopy, major elements, carbon and oxygen isotope and quartz micro morphology of fault gouge and host rock of three faults in the site candidate were conducted. The results show that the F2 and F7 faults on the north and south sides of the site candidate are in a relatively stable state with good sealing and weak activity. Part of the F4 fault is located in the site candidate, and the deportment of a small amount of pyrite in the fault zone and the difference of carbon and oxygen isotopes indicate that it may have experienced material exchange with the deep zone. Attention should be paid to induced earthquake risk.

The Nam O - Nam Dong fault is rated as one of the most seismic source zones in Central Vietnam. Field investigation in the mountainous areas in the Cam Le and Hoa Vang districts (Da Nang city), which is a part of the Nam O - Nam Dong fault zone, the authors have discovered fault gouge zone of 5-40 cm width within the Dai Loc granitic rock. Two fault gouge samples were collected for Electron spin resonance (ESR) dating. The results from the ESR dating on the quartz grains from the fault gouge samples showed the youngest age from the smallest fraction, probably indicating that ESR signals in the fractions were completely zeroed at the time of faulting due to frictional heat. The preliminary results from the ESR dating on the quartz grains from the fault gouge indicate that the last major faulting in this site was later the ages of 15.05±3.55 ka to 18.21±4.06 ka ago. Multiple actives during the late Pleistocene - Holocene of this fault had uplifted the fault gouge from a depth-seated to the present-day locality. These data suggest that this fault zone can be classified as a potentially active fault zone and presents some seismic hazards.

Seismological observations highlighted that earthquakes are often followed by changes in elastic properties around the fault zone. Here, we studied the origin of these variations using stick‐slip experiments on saw‐cut granite samples presenting different degrees of bulk damage (i.e., microcracks). Stick‐slip events were induced under triaxial compression configuration with continuous active ultrasonic measurements at confining pressures representative of upper crustal conditions (15–120 MPa). Both the P‐wave velocity (VP) and amplitude (AP) showed drops, concurrently with stress drops, and had a non‐monotonic dependence toward the fault's stress state. Our experimental results suggest that co‐seismic changes in VP were mostly controlled by the elastic re‐opening of microcracks in the bulk, rather than by co‐seismic damage or the formation of fault gouge. Co‐seismic changes in AP were controlled by a combination of elastic re‐opening of microcracks in the bulk and inelastic processes (i.e., co‐seismic damage and gouge formation and dilation)., Key Points Laboratory earthquakes are associated with seismic velocity and amplitude drops in crustal rocksVelocity drops are controlled by the stress acting on the fault and degree of damage in the fault wallAmplitude drops are partly controlled by the state of stress and partly affected by dissipative (inelastic) phenomena occurring on‐fault

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

Temperature is widely believed to act as a primary control on fault rheology, and therefore on the distribution of seismicity along plate boundary faults. However, there are few detailed measurements of the frictional strength and stability of natural fault gouges at elevated temperatures. Here, we report on a suite of shearing experiments designed to investigate the frictional behavior of fault rocks sampled from depths of 111.5–142.9 m along the Alpine Fault in New Zealand obtained by the International Continental Scientific Drilling Program (ICDP) Deep Fault Drilling Project (DFDP). We tested five samples from the DFDP-1B pilot hole: two hanging wall chloritic cataclasites, two footwall granitic cataclasites, and a fault gouge from the principal slip zone (PSZ-1). Each sample was sheared at a range of temperatures from 23 to 500 °C and at an effective normal stress of 80 MPa. The wall rock cataclasites exhibit an increase in the friction coefficient (μ) with temperature, from μ = 0.45–0.64 at 23 °C to μ = 0.87 at 500 °C. The PSZ-1 gouge exhibits lower friction coefficient values than the wall rock at temperatures ≤180 °C (μ = 0.35–0.46 vs 0.45–0.65), but comparable values (μ = 0.87–0.90) at 500 °C. The variation in frictional strength is accompanied by a transition from velocity-strengthening to velocity-weakening behavior at temperatures ≥180 °C for all materials. Extrapolation of the experimentally defined rheological critical stiffness of the fault material and the estimated in situ stiffness of the surrounding crust suggests upper and lower stability boundaries at ~1.8–2.5 km and ~8.5–8.8 km depth, respectively. The upper stability boundary is also consistent with the observed depth-frequency distribution of earthquakes.

Mature faults with rough surface at their core contain granular gouge, however, the presence of fault gouge is mostly disregarded in the analysis of effect of fault’s surface roughness on the mechanics and stability of faults. In this work, we consider a rough fault system with constant roughness at the wall-gouge interaction and study the effect of grain friction on the characteristics of seismic cycles. Our discrete element simulations show that the stick-slip frictional strength and dilation of the rough fault system, as well as their variations, nonlinearly increase with the particle friction, but at high particle friction saturate. By statistical analyses on a large number of slip events, we find that the average recurrence time and its variations decrease with particle friction. A rough fault with higher grain friction shows more small slip events, but also contains a limited number of extreme events and demonstrates a more complex nucleation phase with higher stored energy. We analyze the pseudo acoustic emission, which is based on monitoring of the velocity signal of particles, and find higher temporal and more spatially distributed acoustic emissions for rough fault with higher grain friction. Our findings in this study show that, in rough faults with granular gouge, where the fault zone walls are totally engaged to the granular gouge, the friction at grain scale controls the characteristics of stick-slip cycles showing similar influence on the mechanics of faults as the roughness of fault’s surface in absence of fault gouge.

Temporary excavations during the construction of the Glendoe Hydro Scheme above Loch Ness in the Highlands of Scotland exposed a clay-rich fault gouge in Dalradian Supergroup psammite. The gouge coincides with the mapped trace of the subvertical Sronlairig Fault, a feature related in part to the Great Glen and Ericht–Laidon faults, which had been interpreted to result from brittle deformation during the Caledonian orogeny (c. 420–390 Ma). Exposure of this mica-rich gouge represented an exceptional opportunity to constrain the timing of the gouge-producing movement on the Sronlairig Fault using isotopic analysis to date the growth of authigenic (essentially synkinematic) clay mineralization. A series of fine-size separates was isolated prior to K–Ar analysis. Novel, capillary-encapsulated X-ray diffraction analysis was employed to ensure nearly perfect, random orientation and to facilitate the identification and quantification of mica polytypes. Coarser size fractions are composed of greater proportions of the 2M1 illite polytype. Finer size fractions show increasing proportions of the 1M illite polytype, with no evidence of 2M1 illite in the finest fractions. A series of Illite Age Analysis plots produced excellent R2 values with calculated mean ages of 296 ± 7 Ma (Late Carboniferous–Early Permian) for the oldest (2M1) illite and 145 ± 7 Ma (Late Jurassic–Early Cretaceous) for the youngest (1M) illite. The Late Carboniferous–Early Permian (Faulting event 1) age may represent resetting of earlier-formed micas or authigenesis during dextral displacement of the Great Glen Fault Zone (GGFZ). Contemporaneous WNW(NW)–ESE(SE) extension was important for basin development and hydrocarbon migration in the Pentland Firth and Moray Firth regions. The Late Jurassic–Early Cretaceous (Faulting event 2) age corresponds with Moray Firth Basin development and indicates that the GGFZ and related structures may have acted to partition the active extension in the Moray Firth region from relative inactivity in the Pentland Firth area at this time. These new age dates demonstrate the long-lived geological activity on the GGFZ, particularly so in post-Caledonian times where other isotopic evidence for younger tectonic overprints is lacking.

Drilling investigations and structural analysis of drill cores reveal that a fault gouge zone of 10–30 cm in width was observed at depths of ~260 to 900 m in nine drill holes that intersected the Nojima Fault (NF), on which the 1995 Mw 6.9 Kobe (Japan) earthquake occurred. Logging data and an analysis of mesostructures and microstructures in drill cores show that (i) a ~60‐m wide fault damage zone containing a 10‐ to 30‐cm‐thick fault gouge zone developed in the NF, (ii) the fault gouge zone can be divided into 11–20 thin layers of different color, and (iii) the individually colored layers contain different color breccias of fault gouge that are offset and/or cut by cracks and crack‐filled calcite and quartz veinlets. Our results reveal that the fault gouge zone probably records more than 11–20 paleoseismic faulting events along the NF during the late Pleistocene‐Holocene.

We describe the structural characteristics and tectonic evolution of the NNE–SSW-striking Yangsan Fault, Korea. The surface trace of the fault extends for over 170 km on land, displaying ∼20–30 km of dextral offset. It transects mainly Mesozoic–Cenozoic sedimentary and igneous rocks. Our field observations suggest that the style of deformation within the fault core is controlled by the ductility of the protoliths. Where sedimentary rocks have been faulted, fault cores tend to be relatively wide, having hosted multiple events. Internal fault core structures are characterized by networks of multiple anastomosing strands of fault gouge and breccia, which enclose lenses of fractured protolith. However, where igneous (crystalline) rocks have been faulted, slip tends to be much more localized within narrow fault cores comprising cataclastic rocks. Combining our new data with previous research results, we infer that four main movement phases have occurred on the fault: (1) Late Cretaceous sinistral slip faulting with a component of extensional deformation, under a regime of NW–SE compression; (2) late Paleogene intense dextral slip faulting, which occurred under a regional NE–SW compressional regime; (3) middle Miocene weak sinistral slip faulting, during NNW–SSE compression; and (4) local Quaternary dextral slip faulting with a reverse component, under ENE–WSW to E-W compression. The most intense phase of deformation appears to have occurred during the late Paleogene due to a major reorganization of plate motion in and around the NW Pacific area. Our findings highlight that the fault has been a preferred zone of weakness for episodic reactivations having significant implications for the changing tectonic environments of East Asia from Late Cretaceous to Quaternary, and that its segments underwent distinguishable deformations at each phase probably due to their geometrical features and fault rock properties.

Recent studies on natural and experimental seismic faults have revealed that frictional heating plays an important role in earthquake dynamics. We report changes in the optically stimulated luminescence (OSL)/thermo-luminescence (TL) and electron spin resonance (ESR) signals in quartz fault gouge (grain size 90–250 μm, given dose 40 ± 5 Gy) after frictional experiments. Our results indicate that high-rate (2.0 m/s) frictional heating during seismic events can reset the 'geologic clocks' of fault rocks. Thus, the OSL/TL/ESR signal in quartz from natural fault zones has the potential to directly constrain the age of seismic events. Whereas low-rate (2.0 mm/s) frictional slip, even over long times (1000 s), does not reset the OSL/TL signals in quartz. However, low-rate slip can reset the Al center ESR signal, but not the E’ center signal.

Fault zone usually presents a granular gouge, coming from the wear material of previous slips. Considering a mature fault gouge with mineral cementation between particles, we aim to understand the ...

[Abstract Although seismic and stratigraphic well information put tight constraints on rift basin evolution, eroded rift shoulders commonly expose polydeformed prerift basement whose deformation history may be difficult to constrain. In this work, we apply K‐Ar dating of fault gouge samples from 18 faults to explore the brittle deformation of the well‐exposed eastern rift margin to the northern North Sea rift. We find evidence of clay gouge formation since the Late Devonian, with distinct Permian and Jurassic fault activity peaks that closely match early stages of the two well‐established North Sea rift phases. A marked decay in fault density away from the rift margin confirms a close relationship between rifting and onshore faulting. The results show that initial rift‐related extension affected a much wider area than the resulting offshore rift. Hence our data support a rift model where strain is initially distributed over a several 100 km wide region, as a prelude to the development of the ~150–200 km wide Permo‐Triassic northern North Sea rift as defined by large marginal faults. Towards the end of the second rift phase, strain localises even more strongly to the 25–50 km wide Viking Graben. Interestingly, a period of early widespread extension is seen for both phases of North Sea rifting and may be a general characteristic of continental rifting. The documented prerift faulting and fracturing of the basement since the Devonian weakened the basement and probably facilitated the widespread initial extension that subsequently localised to form the northern North Sea rift, with further localisation to its relatively narrow central part (Viking Graben)., Onshore fault illite dates indicate that the North Sea rifting evolved from an initial phase of widely distributed extension that reached far into its margins, followed by the formation of large‐displacement rift faults. This pattern was repeated during the second phase of rifting, and we suspect that a similar pattern may exist for other rifts. ]

Currently, it is unknown how seismic and aseismic slip influences the recurrence and magnitude of earthquakes. Modern seismic hazard assessment is therefore based on statistics combined with numerical simulations of fault slip and stress transfer. To improve the underlying statistical models we conduct low velocity shear experiments with glass micro‐beads as fault gouge analogue at confining stresses of 5–20 kPa. As a result, we show that characteristic slip events emerge, ranging from fast and large slip to small scale oscillating creep and stable sliding. In particular, we observe small scale slip events that occur immediately before large scale slip events for a specific set of experiments. Similar to natural faults we find a separation of scales by several orders of magnitude for slow events and fast events. Enhanced creep and transient dilatational events pinpoint that the granular analogue is close to failure. From slide‐hold‐slide tests, we find that the rate‐and‐state properties are in the same range as estimates for natural faults and fault rocks. The fault shows velocity weakening characteristics with a reduction of frictional strength between 0.8% and 1.3% per e‐fold increase in sliding velocity. Furthermore, the slip modes that are observed in the normal shear experiments are in good agreement with analytical solutions. Our findings highlight the influence of micromechanical processes on macroscopic fault behavior. The comprehensive data set associated with this study can act as a benchmark for numerical simulations and improve the understanding of observations of natural faults., Plain Language Summary: Earthquakes occur when two continental plates slide past each other. The motion is concentrated at the interface of the two plates which is called a fault. In many cases the fault is filled with granular material, called gouge, that supports the pressure between the plates. Therefore, the properties of this gouge determine how fast and how large an earthquake can be. It also has an influence on the time between earthquakes. In our study, we examine a simplified version of a fault gouge in a simple small‐scale model. Instead of rock material we use glass beads and measure how different conditions affect the motion of the model. We find that our model reproduces features of fault gouge because it shows similar behavior. When there is no motion our model fault becomes stronger with a rate equal to fault gouge. Also, the type of strengthening is analogous to fault gouge. During slip, the glass beads become weaker as the slip velocity increases in a similar manner as in natural faults. These results improve the understanding of computer simulations and natural observations., Key Points: Slip modes in granular gouge are akin to natural fault slip. Glass beads are a suitable granular analogue for fault gouge and show rate‐and‐state dependent friction. Enhanced creep and small scale events are signals for imminent failure and indicate fault criticality., Deutsche Forschungsgemeinschaft (DFG) http://dx.doi.org/10.13039/501100001659, 亥姆霍兹联合会致力, Helmholtz‐Zentrum Potsdam ‐ Deutsches GeoForschungsZentrum GFZ (GFZ) http://dx.doi.org/10.13039/501100010956

International audience; Earthquakes happen with frictional sliding, by releasing all the stresses accumulated in the prestressed surrounding medium. The geological fault gouge, coming from the wear of previous slips, acts on friction stability and plays a key role in this sudden energy release. A large part of slip mechanisms are influenced, if not controlled, by the characteristics and environment of this tribological “third body”. A 2D granular fault (mm scale) is implemented with Discrete Element Modelling (DEM). A displacement-driven model with dry contact is studied to observe kinematics and properties of the slipping zone. Increasing the length of the granular media increases the slip needed to weaken the friction from friction peak to steadystate. Low-angle Riedel shear bands are mostly observed. Their number increases with the inter-particle friction coefficient, which also influences shear bands formation in their orientation angle (higher friction leads to higher angle with the main slip direction).

Reservoir confinement by faults is important for safe storage of liquid waste or hydrocarbons. Having access to 3D seismic and borehole data, we have interpreted the tectonic setting of the Wysoka Kamienska Graben (WKG) in the NW part of the Polish Basin and subsequently made an interpretation of the sealing potential of the graben-bounding faults. The formation and development of the graben in the Late Triassic and Early Jurassic was controlled by mechanical decoupling in the salts of the Zechstein Group. The primary tectonic factor triggering the graben origin was dextral strike-slip movement along the regional fault zone in the Paleozoic basement, transtensional accommodation of which in the Zechstein-Mesozoic cover led to development of a horse-tail pattern of grabens. During the Late Cretaceous, the graben underwent minor tectonic inversion. Sealing potential analysis of the graben-bounding faults was performed for the Triassic–Jurassic sequence including juxtaposition seal and fault gouge seal components. Finally, we have focussed our interpretation on the Jurassic sequence where the best reservoirs have been recognized. Our results indicate good to moderate sealing potential of the Hettangian reservoir, poor to moderate sealing of the Pliensbachian reservoir and lack of sealing of the Bajocian reservoir. Hence, the Hettangian reservoir, characterized by large thickness, low clay content and a large regional extent, acts as a potential storage formation, being confined by the graben-bounding faults of the WKG

Cataclastic rocks, as clay-rich fault gouges, are commonly present in brittle rock masses when fault zones appear during geological engineering projects. Highly deformed rocks that are of poor mechanical quality can lead to technical, safety, and economic problems in rock engineering. The aim of this study is to characterise the resistant behaviour of a highly deformed clay-rich gouge >40 m wide with a marked tectonic fabric that indicates strength anisotropy. We present the results of consolidated-undrained (CU) triaxial tests that were performed at low confining pressures (50, 150, and 300 kPa) on several sets of foliated gouge specimens with four different orientations in the tectonic fabric. Specimens were collected from the encapsulated rock cores of two research boreholes drilled through the Alhama de Murcia Fault (AMF), a main regional fault located in SE Spain. The strain–stress relationships and failure modes were established, indicating that the gouge behaves as hard soil or very soft rock. The test results were adjusted at each orientation using the non-linear Hoek and Brown criteria by considering the fault gouge as an intact material or as a tectonised rockmass. Here, we use the Geological Strength Index (GSI) as an indicator of the rockmass strength that depends on the direction of the tectonic fabric. However, the results from specimens with tectonic fabric that is oriented most favourably for failure were not the weakest in terms of rock strength. Such an anomalous result could be the result of asymmetry in the roughness of the weakness planes that is related to the original gouge microstructure characterised by the strong reorientation of clays in an S-C′ like tectonic fabric. Our results will be useful for practical applications that are related to the stability of slopes and/or shallow underground excavations in brittle fault zones, and provide an inexpensive and easy way to preliminarily evaluate the anisotropic behaviour of this type of brittle fault zones for future engineering projects.

This study provides information about fault motion by statistically presenting shape and orientation information for tens of thousands of grains. The recently developed shape preferred orientation (SPO) measurement method using synchrotron micro-computed tomography was used. In addition, various factors that were not considered in previous SPO analysis were analyzed in-depth. The study area included the Yangsan and Ulsan fault zones, which are the largest fault zones in the southeastern part of the Korean Peninsula. Samples were collected from five outcrops in two regions. According to the field observation results, the samples in the area were largely divided into fault gouge and cataclasite, and as a result of SPO analysis, we succeeded in restoring the three-dimensional fault motion direction for each outcrop and identified the fault type. In addition, the analysis results of the fault gouge and cataclasite samples collected from the thin fault zone were interpreted using the focal mechanism solution. As a result, the statistical SPO analysis approach supplements the shortcomings of previous research methods on two-dimensional planes and can quantitatively infer the three-dimensional fault motion for various fault rock samples in the same sequence, thus, presenting useful evidence for structural analysis.

International audience; The ascent and extrusion of crystal-rich magma is commonly facilitated by deformation partitioned within annular, conduit-parallel shear zones. The physical properties and textures of the shear zone materials, where exposed at surface, provide a record of ascent and eruption dynamics. We describe the shear zone developed in Dome C, part of Chaos Crags in the Lassen Volcanic Center (California, USA). The extruded shear zone comprises volcanic fault gouge and variably densified cataclasites. The competent cataclasites evidence deep-seated gouge production followed by gouge densification within the conduit on the timescale of lava dome ascent. Textural, geochemical and mineralogical data identify solid-state sintering as the densification mechanism. At the temperatures and pressures in the volcanic conduit, solid-state sintering causes rapid porosity and permeability loss within the gouge and concomitant material strengthening. Longer dwell times (i.e., slower ascent) allow for more sintering, producing stronger, denser and less permeable cataclasites. At Chaos Crags, we use the extent of sintering, quantified by residual porosity, to recover minimum in-conduit dwell times necessary to produce the observed cataclasites. Our analysis of the Dome C cataclasites suggests a maximum linear ascent rate of 10 m/day and a minimum ascent time of 100 days. We evaluate the consequences of shear zone lithification by solid-state sintering for the eruption of other crystal-rich, glass-poor magmas. Chaos Crags cataclasites preserve evidence of multiple cycles of fracturing, cataclasis and (re-)sintering suggesting a mechanism for transitions between effusive and explosive phases of dome-building eruptions.

Fault gouges play an important role in the shear deformation of fault zones, by causing weakness and frictional instability in structures. Previous studies have investigated the evolution of shear deformation of fault zones by observing experiments using remolded and synthetic gouge specimens at a micro-scale. However, how the spatial configuration of the rock constituents accounts for the 3D anisotropy of intact structures of fault gouges, particularly at the core-scale, is not well understood. We obtained 3D μ-CT images of directionally cored gouge specimens and performed statistical analysis to quantify the major orientation of the internal structures. Direct shear tests were conducted to investigate the relationship between the distribution of the internal structures and geomechanical behavior. The results show that the undisturbed fault gouge has a clear anisotropy parallel to the fault plane even at the core-scale. Moreover, the direct shear test results show that the frictional resistance of a fault gouge has anisotropy related to the fault plane. The simple, yet robust method proposed in this study confirms that the core-scale structural anisotropy is correlated to the anisotropic shear resistance.

Many high-quality strong motion stations were built prior to the occurrence of the destructive 1999 Chi-Chi earthquake, providing the most comprehensive study of the mechanism of such a damaging event. The general consistent feature in spatial slip distribution of the fault as a large slip of ~12 m at the northern portion of the fault from fault models and geological observation suggest the importance in understanding the physics of faulting of this large slip. The Taiwan Chelungpu-fault Drilling Project (TCDP) aided in the understanding of earthquake energy partitions by revealing the very fine grain (~nm) of the fault gouge with a millimeter-scale slip thickness for a single event. The dynamic parameters obtained from the kinematic slip inversion suggest a heterogeneous shear stress distribution and a complex stress-time history. The study incorporated the examination of the surface energy from identifying the fault gouge from the major slip zone, and the determination of the fracture energy from the dynamic parameters derived from the strong motion data. This allowed for the direct estimation of the energy partition of a single earthquake from the geological and seismological observations. The low-frictional coefficient obtained from temperature measurements made after drilling encouraged similar drilling projects to be undertaken after large earthquakes (e.g. 2008 Wenchuan and 2011 Tohoku earthquakes) to obtain frictional heating measurements. With the success of the TCDP drilling in the slip zone associated with the 1999 Chi-Chi earthquake, in situ borehole seismometers, referred to as the TCDP borehole seismic array (TCDPBHS), were installed to monitor the behavior of the fault zone after a large slip. This paper shows the dynamic parameters obtained from the kinematic slip inversion and, thus, gives a brief review of the current understanding of earthquake kinematics and dynamics. Seismic waveform modeling, drilling of the Chelungpu-fault, and the in situ monitoring of the fault zone after a large slip of ~12 m are all discussed. As a large number of related scientific papers have been published on this subject, some of these papers may not be properly addressed or referred to in this review article. These papers may be identified in the references of related articles.

Mechanism of fluids in modifying mineralogy and geochemistry of the fault zone and the role of rock-fluid interaction in the faulting weakening is still debatable. Through analyzing mineralogical compositions, major elements as well as micro-structural characteristics of outcrop samples including wall rocks, low damage zone, high damage zone and oriented fault gouge samples from principal slip zone gouges, mineralogical and geochemical variations of the fault-rocks is observed from Shaba outcrop of Beichuan-Yingxiu surface rupture zone of the Mw 7.9 Wenchuan earthquake, China. The element enrichment/depletion pattern of fault rock shows excellent consistency with the variation pattern of minerals in terms of the notable feldspar alteration and decomposition, decarbonization, coseismic illitization, and chloritization that occurs in the fault zone. The Isocon analysis indicates that the overall mass loss amount of the Shaba fault zone is ranked as low damage zone&thinsp

In the complex tectonic setting of the Eastern Mediterranean, the westward motion of the Anatolian Block is accommodated mainly along its boundary faults, the North Anatolian Shear Zone (NASZ) and the East Anatolian Shear Zone (EASZ). Although there are relatively limited studies on the active tectonics of the EASZ, horizontal slip rate is suggested to be of about 10 mm/yr, using geodetic data. In terms of instrumental and historical seismicity, this sinistral strike-slip fault generated surface rupturing earthquakes along almost its entire length except two segments, Palu in the northeast and Turkoglu in the southwest, creating two seismic gaps on the East Anatolian Fault (EAF), the most prominent member of the EASZ. In spite of the fact that there are some off-fault seismic activities such as the 2010 Kovancılar Earthquake (M 6.1) in the vicinity of Palu Seismic Gap, recent geodetic measurements show significant aseismic creep, almost retaining the full far plate velocity (~10 mm/yr) for about 100 km-long section of the fault. Hence, the region is continuously monitored by various types of techniques, such as GNSS, InSAR, creepmeter, seismology, and high-resolution photogrammetry.In addition to monitoring, we investigated the mechanical signature of the creep in the fault zone using fault rocks along the Palu Segment. We collected several samples directly from the deformation zone of the EAF, which makes the boundary between limestones of the Kirkgecit Formation and the chaotic alternation of volcanics, mudstones, and limestones of the Maden Complex, at two locations. The Underground Railway Tunnel Section (39.9504°N, 38.6976°E) is cut by the fault zone where the creep signals are recorded by a creepmeter. The X-Ray Diffraction (XRD) analyses of collected samples of this locality suggest the presence of montmorillonite (smectite group) as the main clay mineral in addition to chlorite-kaolinite with a negligible amount of illite-mica minerals within the fault rocks. This preliminary result suggests a linkage between the creeping and petrophysical properties of fault rocks, which are made of the weak smectite mineral and show no-frictional healing as the expected characteristics of the creep. However, the preliminary analyses of fault gouge samples from the Murat River Section (39.9696°N, 38.7043°E) yield a small amount of smectite group clays. We are going to extend our study at different locations in order to increase the spatial resolution on the relation between the fault rocks and creep motion. This study is supported by the TUBITAK Project no. 118Y435.

Coupled fluid characterization and absolute dating of fracture networks may provide insights into the understanding of critical stages of evolution of a growing orogen. As a result of the collision between the European and Apulian plates, the Alps have experienced several evolutionary stages comprising continental subduction, nappe stacking, thick- to thin-skin tectonics in relation with the frontal propagation of a fold and thrust belt, and extensional reactivation of the major Penninic Frontal Thrust (PFT). Current evolution of the orogen (Tricart et al., 2001, 2007 and Sue et al., 2007) shows an ongoing extensional seismic activity along PFT while borders of the orogenic system remain in compression. The transition from compression to extension along the PFT remains unconstrained.This study aims to constrain the time of the PFT inversion and provide a characterization of the tectonic structures through time during the formation of upper Durance normal fault system. For this, we applied several novel dating techniques (in-situ U-Pb calcite and (U-Th)/He hematite dating techniques). In addition, we determined the geochemical signature of the fluids trapped (calcite crystallization) deformation by δ13C and δ18O stable isotope analysis of calcites to constrain the fluid reservoirs, and thus the size of the involved tectonic structures. Stable isotopes show that the fluids associated with the early extensive structures bear isotopic signatures close to those of their host rocks, indicating a fluid at equilibrium and thus a close system in agreement with the small (mm-cm) size of mostly ductile structures. In a second stage, connection of veins and fractures lead to major fault formation (metric to kilometric scale structures) show isotopic signatures in agreement with ascending metamorphic fluids, featuring an open system along the PFT.U-Pb dating on calcite was successful on several samples despite high common lead concentrations. Two fault gouge samples associated with kilometric scale faults gave ages between 3.5 Ma and 2.5 Ma. These structures are a signature of the paleoseismic activity wich occured some 2.5-3.5 Ma ago when the wall domain of PFT was few km depth. Moreover, (U-Th)/He hematite dating was used on slickensides of the same fault system. Preliminary ages of 2.5, 1.5 and 15 Ma were obtained. The 15 Ma age is interpreted as a minimum age inversion of the PFT, while other ages overlap with the U-Pb calcite ages. This multidisciplinary inverstigation in the Western Alps helps to constrain the exhumation history of the paleo-seismogenic zone related to the inversion of the PFT.

© The Author(s) 2020., 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., This research was funded by the INGERGEOSIMA project: CGL2013-47412 from the Ministerio de Economía y Competitividad of the Spanish Govern‑ ment and by project CGL2017-82169-C2-2-R from Agencia Estatal de Investigación (AEI) of the Spanish Government and Fondo Europeo de DesarrolloRegional (FEDER).

Naturally and experimentally deformed gouges from sliding surfaces within the Galera Fault Zone were analyzed using scanning and transmission electron microscopy (SEM, TEM) to identify changes in the fault rocks as a consequence of ongoing deformation. The two gouges studied have a particular mineral association that includes planar (mainly smectite and illite) and fibrous clay minerals (sepiolite and palygorskite). Microstructural findings include a radical difference in grain alignment between the two gouges, a phenomenon that strongly influences gouge permeability. Smectite crystals are aligned on the same orientation and show a great number of layer terminations and delamination on the basal planes that contribute to a distributed mode of deformation in the gouge. In contrast, the sepiolite-rich gouge exhibits a grid-like microfabric that results in localized deformation limited to small areas where the needle-like crystals are bent and broken producing “feather-like” structures, without the presence of lattice distortions. Meanwhile, significant chemical results include: (1) Al content identified in sepiolite fibers through analytical electron microscopy (AEM), together with variability in the (110) d-spacing of sepiolite across single fibers, suggest the existence of a progressive transformation from sepiolite to palygorskite. (2) Mg content in smectite suggests that a portion of the smectites within the fault plane could have an authigenic origin and may be the result of a transformation reaction from palygorskite, however, the similarity of the 2:1 layer compositions between the smectites in the two contexts do not allow to either confirm nor deny such possibility. (3) Chemical continuity of Mg-decrease and Al+Fe-increase in the octahedral cation content of the sepiolites, palygorskites, and smectites within the gouges indicate a sequence of mineral transformations that is favored by a depleted Mg content and an increase of Al content in the fluid. In this setting, deformation promotes grain size reduction and fluid-rock interaction with the wall rocks resulting in a local supply of Al to the fault gouge that drives phase transformations. Structural differences between smectites and fibrous clay minerals affect important chemical and physical properties of the gouge including their mechanical properties. We propose that the permeability of the gouges in the Galera Fault is strongly affected by their mineralogy. Furthermore, the extent of the mineral authigenesis and mineral transformations could be a controlling factor that progressively changes both the permeability and the strength of the fault.

Liaodong Peninsula on the northeastern North China Block that witnessed the spectacular Late Mesozoic tectonics is a key area to probe the transition from compression to extension. Borehole data clearly reveals that the Cambrian limestone overlay the Jurassic limestone, which identified the Yuantai thrust system in Liaodong Peninsula. Zircon grains from a granodiorite dike that intruded the thrust system yielded U-Pb age of 128.3 ± 1.3 Ma, further constraining the timing of the thrusting between Late Jurassic to early Early Cretaceous. A detailed field study of three-dimensional fault-zone exposures in the Zhayao tectonic window are conducted to decipher the thrust tectonics before the extension in Liaodong Peninsula. The key observations are: (1) meter-scale asymmetric folds in deformed mid-Cambrian rocks; (2) decameter-scale recumbent folds and secondary imbricate thrusts in deformed Middle Jurassic rocks; and (3) non-foliated breccia, fault gouge, and brittle porphyroclasts adjacent to fault planes. The combined brittle–ductile style of deformation indicates that the fault zone operated under near-surface conditions. In addition, we infer that the folds were formed first at the propagating fault tip and subsequently dismembered by normal faults. We interpret the Zhayao thrust, which is located around the margin of the tectonic window, as the frontal thrust of the Yuantai thrust system. Furthermore, by applying the theory of fault-related folds, we deduce that the Zhayao frontal thrust is a secondary duplex. Based on our field observations and borehole data, we propose an imbricate thrust model for the fold-and-thrust belt of the southern Liaodong Peninsula. The structural data indicate that the transport direction of the Yuantai thrust system was top-to-the-southeast, suggesting that the thrust system was resulted due to northwest-ward subduction of the paleo-Pacific plate during the Mesozoic.

Author(s): Jung, Y; Doughty, C; Borgia, A; Lee, KJ; Oldenburg, CM; Pan, L; Daley, TM; Zhang, R; Altundas, B; Chugunov, N; Ramakrishnan, TS | Abstract: With the goal of detecting and characterizing faults and fractures in enhanced geothermal systems (EGS), a new technology involving CO2 push-pull testing, active-source geophysical imaging, and well logging has recently been proposed. This technique takes advantage of (1) the contrasting properties of supercritical CO2 and water which cause CO2 to appear distinct from surrounding brine in seismic and other geophysical logging approaches, (2) the non-wetting nature of CO2 which keeps it localized to the faults and fractures to create contrast potentially sufficient for active seismic and well-logging approaches to image faults and fracture zones at EGS sites. In this study, we evaluate the feasibility of using pressure transient monitoring during CO2 push-pull tests to complement active seismic and wireline well logging for EGS characterization. For this purpose, we developed a 2D model of a prototypical geothermal site (Desert Peak, NV) that includes a single fault. The fault zone consists of a slip plane, fault gouge, and damage zone, and is bounded by the surrounding matrix of the country rock. Through numerical simulation using iTOUGH2, we found that the pressure transient at the monitoring wells in the fault gouge shows unique traits due to the multiphase flow conditions developed by CO2 injection, and varies sensitively on the arrival of the CO2 plume and the degree of CO2 saturation. A sensitivity analysis shows the pressure transient is most sensitive to the fault gouge permeability, but also depends on multiphase flow parameters and the boundary conditions of the fault. An inversion study reveals that the fault gouge permeability can be best estimated with the pressure transient data, whereas additional CO2 saturation data do not improve the accuracy of the inversion significantly.

Radiometric dating of fault gouges has become a useful tool for regional tectonics studies and for exploring and understanding fault and earthquake processes. Methods to define the absolute age of faults achieved a solid scientific foundation almost 25 years ago when the development and application of illite age analysis for investigating sedimentary burial and thermal histories found a new potential application – defining the age of fold-and-thrust development. Since then, the methods have benefitted from further development and incorporation of the 40Ar/39Ar micro-encapsulation method and quantitative clay mineral evaluation to distinguish polytypes (Wildfire). These refinements to the methods have improved their application in fold-and-thrust terrains and have opened up applications in normal and strike-slip fault environments. Another important development is the use of absolute dating methods in retrograde clay gouges in which clays in a fault develop from igneous or metamorphic wall rocks that contain no clays. In addition, the method has also been shown to be useful at dating folds in fold-and-thrust belts. We think the method is now an established part of the geological toolkit, look forward to future fault structural and tectonic studies that incorporate fault ages and hope that researchers continue to probe and discover ways that the method can assist fault process studies, including earthquake fault studies.

To investigate the influence of foliation and structure within fault zones, synthetic fault gouges of quartz containing various thin layers of biotite were sheared in a triaxial testing system at hydrothermal conditions to acquire the faulting behaviour, along with quartz-biotite mixtures tested for comparison. First, in Series-1 experiments, gouges were sheared at a temperature of 100 °C, with effective confining pressure of 200 MPa, pore pressure of 30 MPa and loading rate of 1.22 μm/s. The results revealed that strength of quartz-biotite mixture weakened linearly with increasing biotite proportion, whereas the gouges with a biotite interlayer weakened much more significantly even for biotite content as little as ~5 wt%. Series-2 experiments were performed on simulated gouge with 15 wt% biotite interlayer at temperatures of 25–600 °C, effective normal stress of 200–230 MPa and pore pressure of 30 MPa, with stepped shear rates of 0.0488 to 1.22 μm/s. At effective normal stress of 200 MPa, the simulated foliate gouge showed a transition from velocity strengthening to velocity weakening around 200 °C, which persisted up to 600 °C. Stick-slips above 400 °C were associated with minor a–b averagely of ~−0.0006 and submicron dc values. At effective normal stress of 230 MPa and temperature of 600 °C, stick-slip motions evolved with slip distance to viscous-like behaviour at a displacement of ~3 mm. Microstructural observation on deformed samples revealed that shear deformation mainly occurred within the biotite interlayer in most cases, with surrounding quartz gouge only slightly deformed but increasingly compacted as temperature increased. Viscous-like behaviour corresponded to migration of dominant shear deformation to the quartz gouge, as indicated by the development of Riedel shears therein. Our results may help constrain fault strength and depth range of seismogenesis within fault zones associated with biotite in form of foliations and/or interconnected networks.

Well-preserved fault gouge along brittle faults in Paleoproterozoic, volcano-sedimentary rocks of the Raipas Supergroup exposed in the Alta–Kvænangen tectonic window in northern Norway yielded latest Mesoproterozoic (approximately 1050 ± 15 Ma) to mid-Neoproterozoic (approximately 825–810 ± 18 Ma) K–Ar ages. Pressure–temperature estimates from microtextural and mineralogy analyses of fault rocks indicate that brittle faulting may have initiated at a depth of 5–10 km during the opening of the Asgard Sea in the latest Mesoproterozoic–early Neoproterozoic (approximately 1050–945 Ma) and continued with a phase of shallow faulting to the opening of the Iapetus Ocean–Ægir Sea and the initial breakup of Rodinia in the mid-Neoproterozoic (approximately 825–810 Ma). The predominance and preservation of synkinematic smectite and subsidiary illite in cohesive and non-cohesive fault rocks indicate that Paleoproterozoic basement rocks of the Alta–Kvænangen tectonic window remained at shallow crustal levels (

Author(s): Lee, KJ; Oldenburg, CM; Doughty, C; Jung, Y; Borgia, A; Pan, L; Zhang, R; Daley, TM; Altundas, B; Chugunov, N | Abstract: Characterizing the faults and fractures that provide flow pathways for efficient geothermal energy production is critical for design of sustainable geothermal energy production. Both natural faults and stimulated fractures in enhanced geothermal systems (EGS) are difficult to image and map by seismic methods because hot brine filling the fractures and faults does not create a strong seismic property contrast relative to surrounding rock. We investigate here the technical feasibility of using supercritical CO2 (scCO2) injection into faults in a single-well push-pull scenario to characterize the hydraulic properties of the fault zone by emplacing scCO2 that can serve as a contrast fluid for seismic monitoring. We develop a conceptual and numerical reservoir model of two intersecting faults based on the Dixie Valley geothermal system in Nevada, USA. The 2D conceptual model consists of a system with a main fault and an intersecting conjugate fault. The corresponding numerical model is discretized using irregular grid blocks with fine discretization around the slip plane, gouge, and damage zones. We perform forward modeling along with sensitivity and data-worth analyses of scCO2 push-pull to investigate the CO2 distribution in the fault gouge during 30 days of push (injection) and 30 days of pull (production). Formal sensitivity analysis is conducted to determine the most controlling unknown parameters in the fault zones. Using the selected set of unknown parameters and output responses, we perform data-worth analysis to reveal the most valuable output response to be measured for the best prediction of CO2 distribution in the fault zones and its uncertainty. From the results of data-worth analysis, we determine the optimal properties to target in monitoring, their locations, and the minimum observation time. Our results provide information on the optimal design of scCO2 push-pull testing in a conjugate fault system modeled after Dixie Valley that can be used to enhance monitoring by active seismic and well-logging methods to better characterize the transmissive fault(s).

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.

Claystones are considered as a geological barrier. However, the properties of claystone may be modified if these are cut by brittle faults and fractures. Investigations of fault rocks are therefore crucial to evaluate the barrier properties of clay rich formations. The present study is dealing with the characterization of naturally and artificially deformed Opalinus Clay of the Mont Terri rock laboratory in NW Switzerland. Complete core sections, covering the artificially excavation damaged zone (EDZ) and several tectonic fault zones, have been studied using a multidisciplinary approach consisting of geophysical, geotechnical, mineralogical/geochemical and lithological/structural data. The fault zones encountered are characterized by a high density of planar discontinuities, which often show slickenside striations. Under the microscope, the fault zones turned out to be pervasively deformed resulting in open veins and pore space, now filled with calcite and celestine. Fault zone reactivation led to very fine-grained, cohesionless fault gouge and fragmentation of the previously formed calcite veins. Packer tests in boreholes reveal excavation induced, enhanced permeabilities up to 3 m borehole depth (1.9 m ┴ to gallery floor). Seismic borehole measurements indicate that seismic attributes, which are typical for undisturbed Opalinus Clay, are not reached until 7.5 m borehole depth (4.8 m ┴ to gallery floor), which is larger than in the usual gallery configuration. We interpret the anomalies in geophysical measurements as well as the elevated permeabilities, measured in the Main Fault (3.6 m to 4.2 m ┴ to gallery floor), to result from an enlarged EDZ, influenced by the presence of the brittle fault structures. The up to two orders of magnitude higher permeability (compared to the intact claystone) most probably results from excavation induced stress and a local reactivation of fault planes. The results suggest that the presence of a fault zone can alter the extent of the EDZ significantly, and thereby affect the rock integrity, at least in the near field of a repository tunnel.

Microstructures from a fault gouge formed during the Ms. 8.0 Wenchuan earthquake in China were investigated in detail. In samples collected near the highest vertical surface offset, two types of amorphous materials were found in the high-strain shear zone: 1) material formed by creeping during the inter-seismic period, which is characterized by ground sandstone clasts; and 2) coseismic material, characterized by the comminution of the sandstone clasts of the former type. Interaction of amorphous material with fluids during creeping resulted in Fe-rich nano-sized grains in the pores, Mg- and Al-rich stratified zones near the pores, amorphous silica next to the stratified zones, as well as new clay minerals in the stratified zones. Feldspar and phyllosilicates were transformed into clay minerals and amorphous silica. We propose that amorphous materials and clay minerals contributed to the dynamic weakening of the Yingxiu–Beichuan fault.

Pore-fluid pressure is an important parameter in controlling fault mechanics as it lowers the effective normal stress, allowing fault slip at lower shear stress. It is also thought to influence the nature of fault slip, particularly in subduction zones where areas of slow slip have been linked to regions of elevated pore-fluid pressure. Despite the importance of pore-fluid pressure on fault mechanics, its role on controlling fault stability, which is determined by the friction rate parameter ( a − b ), is poorly constrained, particularly for fault materials from subduction zones. In the winter of 2018-19 the accretionary complex overlying the Nankai Trough subduction zone (SW Japan) was drilled as part of Integrated Ocean Drilling Program (IODP) Expedition 358. Here we test the frictional stability of the accretionary sediments recovered during the expedition by performing a series of velocity-stepping experiments on powdered samples (to simulate fault gouge) while systematically varying the pore-fluid pressure and effective normal stress conditions. The Nankai gouges, despite only containing 25% phyllosilicates, are strongly rate-strengthening and exhibit negative values for the rate-and-state parameter b. We find that for experiments where the effective normal stress is held constant and the pore-fluid pressure is increased the Nankai gouges become more rate-strengthening, and thus more stable (an increase in ( a − b ) of ∼6 × 10−5 MPa−1 with increasing pore-pressure). In contrast, when the pore-fluid pressure is held constant and the effective normal stress is varied, there is minimal effect on the frictional stability of the gouge. The increase in frictional stability of the gouge at elevated pore-fluid pressure is caused by an evolution in the rate-and-state parameter b, which becomes more negative at high pore-fluid pressure. These results have important implications for understanding the nature of slip in subduction zones and suggest the stabilizing effect of pore-fluid pressure could promote slow slip or aseismic creep on areas of the subduction interface that might otherwise experience earthquake rupture.

Faults are key components in defining fluid migration pathways and seals in sedimentary basins. The sealing capacity of faults is closely related to the petrophysical and geomechanical properties of fault gouge. Clay smear, cataclasis, and diagenesis favor a high capillary breakthrough pressure and low permeability in clastic sediments, and therefore fault gouge seal. However, significant uncertainty remains in accurately predicting the sealing capacity of faults for CO2 storage. We conducted a series of experiments, including absolute permeability, breakthrough pressure, and post-breakthrough CO2 permeability measurements on synthetic fault gouge samples, made from homogeneous mixtures of Frio sand and Anahuac shale, lithofacies of tertiary sediments in the Gulf of Mexico basin. The results show that the permeability of synthetic fault gouge decreases by about one order of magnitude with increments of 10 wt% of clay (mostly smectite) from the Anahuac shale. Independent of clay content and bulk porosity, all permeability measurements scale with the void ratio of the clay fraction. The breakthrough pressure of synthetic fault gouge increases by approximately half order of magnitude with increments of 10 wt% clay. The samples with clay content above 40 wt% reach a breakthrough pressure equivalent to a (supercritical) CO2 column height of more than 100 m. The measurements on fault gouge properties are meaningful to quantitatively evaluate fault sealing capability and migration of buoyant fluids through faults in sand-shale sequences.

The 2000 Western Tottori Earthquake (Mw 6.6, Mjma 7.3) in Japan occurred in an area where no active faults are known to exist. We examined the distribution, occurrence, paleostress field, and K–Ar ages of minor faults in the aftershock region of this earthquake to understand the tectonics and deformation process in a blind active fault area. Five stress stages were identified: NW–SE compressional strike-slip and NE–SW extensional normal faulting stress field in Stage 1; clockwise rotation of ~ 40° between 18 and 16 Ma in Stage 2; N–S compressional strike-slip and N–S extensional normal faulting stress field in Stage 3; stress state with σHmin (σ3) oriented NE–SW in Stage 4; and E–W compressional strike-slip faulting stress field in Stage 5. The results from the relationship between stress fields and K–Ar ages of fault indicate that most minor faults in the epicentral area of the 2000 Western Tottori Earthquake were formed along pre-existing joints during rifting and opening of the Japan Sea during the late Oligocene. Some minor faults show signs of reactivation under the present stress field. However, a large active fault with a clear topographic expression has not yet been developed due to the small displacement of minor faults. These findings indicate that detailed structural analysis of minor faults and reconstruction of stress history are important for seismic hazard assessment in areas where fault activities are initial stages.

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.

This paper presents the pre-injection fault characterisation that has been conducted at the CO2CRC Otway Project in Victoria, Australia, aimed at understanding the response of faults to pressure increases in the formation, fault transmissibility horizontally and vertically, and also presents a methodology to extract key rheological fault properties using novel rock mechanical techniques. The initial modelling results show that the likelihood of injection induced fault reactivation is exceedingly small, due to the very small pressure increases associated with injection (∼ 0.05 MPa after about 100 days of injection during the Stage 2C experiment). The potential for CO 2 flow across the main reservoir-cutting faults was quantified using the shale gouge ratio algorithm. Results indicate that the faults should be sealing to some degree and should therefore restrict the lateral movement of CO 2 . Despite these positive results with regard to fault behaviour during current operations, there remains significant uncertainty with regard to certain fault properties such as friction and cohesion that have a large effect on fault stability. Here we develop a workflow which involves several rock mechanical testing techniques to gain information regarding a key fault at the Otway site. One relatively novel technique that is used is the scratch test, which measures strength heterogeneity of the host rock at the centimetre scale. The scratch data can be correlated to certain well logs via a multivariate analysis in order to develop proxies for rock strength that can be applied to other wells and also to wells that traverse fault zones, thereby providing some estimate of fault strength. This scratch analysis is complemented by a series of triaxial experiments designed to measure poroelastic properties and also assess the rate and state frictional parameters of potential fault gouge materials. While the work presented here is aimed specifically at understanding the properties of key faults at the Otway Project, it is hoped that the knowledge gained and the transforms developed here can be used as a guide to constrain fault properties at other CCS sites.

The EW-trending Kunlun Fault System (KFS) is one of the major left-lateral strike-slip faults on the Tibetan Plateau. It forms the northern boundary of the Bayan Har block. Heretofore, no evidence has been provided for the most recent event (MRE) of the ~70-km-long eastern section of the KFS. The studied area is located in the north of the Zoige Basin (northwest Sichuan province) and was recognized by field mapping. Several trenches were excavated and revealed evidence of repeated events in late Holocene. The fault zone is characterized by a distinct 30–60-cm-thick clay fault gouge layer juxtaposing the hanging wall bedrock over unconsolidated late Holocene footwall colluvium and alluvium. The fault zone, hanging wall, and footwall were conformably overlain by undeformed post-MRE deposits. Samples of charred organic material were obtained from the top of the faulted sediments and the base of the unfaulted sediments. Modeling of the age of samples, earthquake yielded a calibrated 2σ radiocarbon age of A.D. 1489 ± 82. Combined with the historical earthquake record, the MRE is dated at A.D. 1488. Based on the over ~50 km-long surface rupture, the magnitude of this event is nearly M w ~7.0. Our data suggests that a ~200-km-long seismic gap could be further divided into the Luocha and Maqu sections. For the last ~1000 years, the Maqu section has been inactive, and hence, it is likely that the end of its seismic cycle is approaching, and that there is a potentially significant seismic hazard in eastern Tibet.

The clay mineralogy of fault gouges has important implications for the frictional properties of faults, often identified as a major factor contributing to profound fault weakness. This work compares the frictional strength of a group of Mg‐rich minerals common in the Mg‐Al‐Si‐O compositional space (talc, saponite, sepiolite, and palygorskite) by conducting triaxial frictional tests with water or argon as pore fluid. The studied minerals are chemically similar but differ in their crystallographic structure. Results show that fibrous Mg‐rich phyllosilicates are stronger than their planar equivalents. Frictional strength in this group of minerals is highly influenced by strength of the atomic bonds, continuity of water layers within the crystals, and interactions of mineral surfaces with water molecules, all of which are dictated by crystal structure. The formation and stability of the minerals studied are mainly controlled by small changes in pore fluid chemistry, which can lead to significant differences in fault strength.