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This study is aimed to delineate the subsurface structural elements using geophysical techniques in the Haraza area of Pakistan. We investigated the Oghi and Battal thrust faults, sedimentary and metasedimentary wedge, and the absolute crustal thickness based on terrestrial gravity data. Unlike seismic survey relying on wave propagation, magnetic survey is based on both attraction and repulsion, and electrical and electromagnetics on induction. The attractive gravity field produces relatively simpler patterns of anomalies, like a series of highs and lows over regions with undulating basements and buried structures. A qualitative interpretation of gravity data reveals a good deal of information. During the collision of Indian and Eurasian Plates, compressional structures were developed in the Lesser Himalayas or northwest of the Hazara Kashmir Syntaxis. The study mainly focuses on the western limb of the Hazara Kashmir Syntaxis. The regional and local Bouguer anomalies were incorporated to delineate the regional structural units. The gravity model is computed through geophysical technique along with profile A-A' from Mansehra to the Battal area that demarcates the blind Oghi Thrust and emergent Battal Thrust. Tanol Formation of Precambrian age demarcates the Oghi Thrust near Kotli Pine while the Battal Thrust is demarcated within the Mansehra Granite of Cambrian to Ordovician age near Battal. Along with the Battal Thrust, fault gouge and breccias have been observed during the field studies. The total thickness of the sedimentary/metasedimentary wedge in the Mansehra and Battal areas was estimated to be 13.6 km and 14.2 km. In comparison, the total thickness of crust in the Mansehra and Battal areas was 51.6 km and 52.2 km, respectively.

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.

K-Ar dating of fault gouge from the intersection of the WNW-striking Ohika Detachment of the Paparoa Metamorphic Core Complex and a NNE-striking high-angle normal fault (Ohikaiti Fault) yielded an ...

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

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.

International audience; 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 (

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.

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.

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.

Specularly light reflective fault plane interfaces known as Mirror-Slip Surfaces (MSS’s) are common in seismically active fault zones around the world and thus their role in controlling fault strength and stability is of great interest. MSS’s have been experimentally produced in simulated carbonate faults at relatively high (10-1-100 m/s) and low (10-7-10-5 m/s) sliding velocities (resp. HV and LV). However, their role in controlling fault mechanical properties at sub-seismic vs seismic fault-slip velocities remains enigmatic. With the aim to unravel the structural development of MSS’s with increasing shear displacement (rate) and effective normal stress, we conducted HV and LV shear deformation experiments on simulated faults composed of granular calcite. We employed a ring shear set-up in a HV rotary shear apparatus as well as a saw-cut assembly mounted in a triaxial cell, which enabled fault-slip tests under a wide range of slip velocities (v = 10-7 - 10-1 m/s) and effective normal stresses (σn ≈ 10 – 170 MPa). All experiments were carried out under room-dry conditions, at room temperature. Post-mortem microstructure analysis of recovered fragments was carried out through visual inspection, incident light and scanning electron microscopy, as well as using Raman spectroscopy.MSSs develop at sub-seismic slip velocities (v = 10-7 m/s) initially as visibly striated patches after 0.0062 m (σn ≈ 10 MPa), 0.004 m (σn ≈ 50 MPa) and 0.0026 m (σn ≈ 170 MPa) of shear displacement. The area covered by MSSs systematically increases with displacement to form continuous coatings after 0.042 (σn ≈ 10 MPa), 0.0062 m (σn ≈ 50 MPa) and 0.0036 m (σn ≈ 10 MPa). As displacement rate is increased (10-5 – 10-4 m/s) MSSs are no longer observed however continuous MSSs are visible again at seismic slip velocities (>10-1 m/s). Our microstructural analysis revealed that MSSs are layers of (nano)crystalline calcite some of which contain elongated nanofibrous structures. In addition, discrete, 3 - 20 micron-sized patches of amorphous carbon are produced at seismic slip velocities, and at sub-seismic velocities under high normal stresses (σn > 160 MPa). We could not however identify any microstructural characteristics that are diagnostics of MSSs produced at certain slip rates or normal stress.Our interpretation is that MSSs form by sintering of nm-sized particles within ultrafine-grained shear bands. With increasing shear displacement, MSS patches connect into continuous veneers. The formation of (continuous) MSSs at low as well as high sliding velocities in our experiments implies that natural MSSs are unreliable indicators for palaeoseismicity.

© 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).

The Main Frontal thrust (MFT) shapes the similar to 2500 km-long frontal Himalayan arc between two syntaxes. Surprisingly, the structural geometry and kinematics of the MFT sheet remains poorly constrained along portions of this arc even though it is a major seismogenic plate boundary hazard that affects 53 million people living in the Himalaya and roughly 600 million living in the combined Himalayan drainage basin (Apollo, 2017). Here, we integrate methods to constrain and quantify MFT deformation and its associated topographic growth in the Mohand Range at the northwestern Himalayan front. We use (a) new structural data and trishear modeling to detail the MFT-related structural geometry across a portion of the range and confirm it to be a fault propagation fold with an exposed monocline in the hanging wall as previously hypothesized (Srivastava et al., 2016), and (b) microstructural, finite strain, and grain size analysis from 11 transport-parallel thin section samples of Middle Siwalik sandstones to propose that the fold formed by near-surface (similar to 1-5 km), frictional-sliding and block-supported cataclastic flow. New luminescence dating of fault gouge and uplifted sand bodies indicate that MFT activity is older (similar to 105 ka) in the central compared to the western (similar to 15 ka) Mohand Range, implying that recent MFT motion has been variable and segmented. We next present Boundary Element Method-based dislocation modeling results along 3 transects to confirm that the along-strike topographic growth is variable, but consistent with the existing data and our interpretations. Balanced cross sections indicate that a maximum of 23-26 km(2) area was eroded from the Mohand Range since folding began. Modern era estimates of minimum erosion from our dislocation modeling compared to existing topographic profiles suggest that similar to 1 km(2) erosion occurred in the range. Our integrated approach helps us to better constrain, resolve reported inconsistencies, and advance our understanding of MFT-related deformation, topographic growth and erosion within the Dehradun recess and across its outboard range front. Himalayan arc-segmentation into salients and recesses likely impacts along-strike earthquake rupture propagation, thus it is important to understand how each segment behaves, evolves, and perhaps interrelates as part of a larger perspective towards evaluating active collisional wedge fronts and their potential seismic hazards.

New Zealand's Alpine Fault (AF) ruptures quasi-periodically in large-magnitude earthquakes. Paleoseismological evidence suggests that about half of all recognized AF earthquakes terminated at the boundary between the Central and South Westland sections of the fault. There, fault geometry and the polarity of uplift change. The South Westland AF exhibits oblique-normal fault motion on a structure oriented 052°/82°SE that, for at least 35 km along strike, contains saponite-rich principal slip zone gouges. New hydrothermal friction experiments reveal that the saponite fault gouge is frictionally weak, exhibiting friction coefficients between μ = 0.12 and μ = 0.16 for a range of temperatures (T = 25–210 °C) and effective normal stresses (σn' = 31.2–93.6 MPa). The saponite gouge is rate-strengthening in all velocity steps performed at velocities between 0.01 and 3.0 μm/s, behavior conducive to aseismic creep. A three-dimensional stress analysis shows that the South Westland AF is favorably oriented with respect to the regional stress field for slip within the frictionally weak saponite fault gouge. Geometrically, the fault is severely misoriented for slip in any fault-forming materials with friction coefficients exceeding μ∼0.5. The combination of weak gouges prone to aseismic creep, strong asperities, and low resolved shear stress may impede earthquake rupture propagation along the South Westland Alpine Fault.

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).

We investigate the evolution of the frequency-magnitude b-value during stable and unstable frictional sliding experiments. Using a biaxial shear configuration, we record broadband acoustic emissions (AE) while shearing layers of simulated granular fault gouge under normal stresses of 2–8 MPa and shearing velocity of 11 μm/s. AE event amplitude ranges over 3–4 orders of magnitude and we find an inverse correlation between b and shear stress. The reduction of b occurs systematically as shear stress rises prior to stick–slip failure and indicates a greater proportion of large events when faults are more highly stressed. For quasi-periodic stick–slip events, the temporal evolution of b has a characteristic saw-tooth pattern: it slowly drops as shear stress increases and quickly jumps back up at the time of failure. The rate of decrease during the inter-seismic period is independent of normal stress but the average value of b decreases systematically with normal stress. For stable sliding, b is roughly constant during shear, however it exhibits large variability. During irregular stick–slip, we see a mix of both behaviors: b decreases during the interseismic period between events and then remains constant when shear stress stabilizes, until the next event where a co-seismic increase is observed. Our results will help improve seismic hazard assessment and, ultimately, could aid earthquake prediction efforts by providing a process-based understanding of temporal changes in b-value during the seismic cycle.

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.

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.

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.

To understand the seismogenic potential of shallow plate-boundary thrust faults (decollements) in relatively warm subduction zones, water-saturated Na-montmorillonite gouges were sheared at a pore fluid pressure of 10 MPa, effective normal stresses (σneff) of 10–70 MPa, temperatures (T) of 25–150 °C, and axial displacement rates of 0.03–3 µm/s. The Na-montmorillonite gouges were frictionally very weak at all conditions tested (steady-state friction coefficient μss = 0.05–0.09). At T ≤60 °C, Na-montmorillonite showed a transition from velocity-strengthening to velocity-weakening behavior with increasing σneff, whereas at T ≥90 °C it was largely velocity-neutral or velocity-strengthening, irrespective of σneff. The rates of frictional healing (β) showed extremely low values (mostly

Slow slip events exhibit significant complexity in slip evolution and variations in recurrence intervals. Behavior that varies systematically with recurrence interval is likely to reflect different extents of fault healing between these events. Here we use high-resolution tremor catalogs beneath Guerrero, Mexico, to investigate the mechanics of slow slip. We observe complex tremor propagation styles, including rapid tremor migrations propagating either along the main tremor front or backward, reminiscent of those in northern Cascadia. We also find many migrations that originate well behind the front and repeatedly occupy the same source region during a tremor episode, similar to those previously reported from Shikoku, Japan. These migrations could be driven by slow slip in the surrounding regions, with recurrence intervals possibly modulated by tides. The propagation speed of these migrations decreases systematically with time since the previous migration over the same source area. Tremor amplitudes seem consistent with changes in the propagation speeds being controlled primarily by changes in the slip speeds. One interpretation is that the high propagation speeds and inferred high slip speeds during the migrations with short recurrence intervals are caused by incomplete healing within the host rock adjacent to the shear zone, which could lead to high permeability and reduced dilatant strengthening of the fault gouge. Similar processes may operate in other slow slip source regions such as Cascadia.

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.

To investigate the role of bedding in the evolution of meso- and microstructural fabrics in fault zones, detailed microscopic, mineralogical, and geochemical analyses were conducted on bedding-oblique and bedding-parallel faults that cut a folded Neogene siliceous mudstone that contains opal-CT, smectite, and illite. An analysis of asymmetric structures in the fault gouges indicates that the secondary fractures associated with each fault exhibit contrasting characteristics: those of the bedding-oblique fault are R1 shears, whereas those of the bedding-parallel fault are reactivated S foliation. The bedding-oblique fault shows the pervasive development of S foliation, lacks opal-CT, and has low SiO2/TiO2 ratios only in gouge, whereas the bedding-parallel fault exhibits these characteristics in both gouge and wall rocks. The development of S foliation and the lack of silica can result from local ductile deformation involving the sliding of phyllosilicates, coupled with pressure solution of opal-CT. Although such deformation can occur in gouge, the above results indicate that it may occur preferentially along bedding planes, preceding the formation of a gouge/slip surface. Thus, in sedimentary rocks that contain phyllosilicates and soluble minerals, bedding can influence the rheological evolution of meso- and microstructural fabrics in fault zones.

Thrusting at shallow depths often precludes analysis by means of structural indicators effective in other geological contexts (e.g., mylonites, sheath folds, shear bands). In this paper, a combination of techniques (including structural analysis, magnetic methods, as anisotropy of magnetic susceptibility and paleomagnetism, and paleothermometry) is used to define thrusting conditions, deformation, and transport directions in the Cameros–Demanda thrust (North Spain). Three outcrops were analyzed along this intraplate, large-scale major structure having 150 km of outcropping length, 30 km of maximum horizontal displacement, and 5 km of vertical throw. Results obtained by means of the different techniques are compared with data derived from cross sections and stratigraphic analysis. Mixed-layer illite–smectite and vitrinite reflectance indicating deep diagenetic conditions and mature stage of hydrocarbon generation suggests shallow depths during deformation, thus confirming that the protolith for most of the fault rocks is the footwall of the main thrust. Kinematic indicators (foliation, S/C structures, and slickenside striations) indicate altogether a dominant NNW movement of the hanging wall in the western zone and NE in the eastern zone of the thrust, thus implying strain partitioning between different branches of the main thrust. The study of AMS in fault rocks (nearly 400 samples of fault gouge, breccia, and microbreccia) indicates that the strike of magnetic foliation is oblique to the transport direction and that the magnetic lineation parallelizes the projection of the transport direction onto the k max/k int plane in sites with strong shear deformation. Paleomagnetism applied to fault rocks indicates the existence of remagnetizations linked to thrusting, in spite of the shallow depth for deformation, and a strong deformation or scattering of the magnetic remanence vectors in the fault zone. The application of the described techniques and consistency of results indicate that the proposed multidisciplinary approach is useful when dealing with thrusts at shallow crustal levels.

Friction and deformation in granular fault gouge are among various dynamic interactions associated with seismic phenomena that have important implications for slip mechanisms on earthquake faults. To this end, we propose a mechanistic model of granular fault gouge subject to acoustic vibrations and shear deformation. The grain-scale dynamics is described by the Shear-Transformation-Zone theory of granular flow, which accounts for irreversible plastic deformation in terms of flow defects whose density is governed by an effective temperature. Our model accounts for stick-slip instabilities observed at seismic slip rates. In addition, as the vibration intensity increases, we observe an increase in the temporal advancement of large slip events, followed by a plateau and gradual decrease. Furthermore, slip becomes progressively slower upon increasing the vibration intensity. The results shed important light on the physical mechanisms of earthquake triggering and slow slip and provide essential elements for the multiscale modeling of earthquake ruptures. In particular, the results suggest that slow slip may be triggered by tremors.

Fluid infiltration within fault zones is an important process in earthquake rupture. Magnetic properties of fault rocks convey essential clues pertaining to physicochemical processes in fault zones. In 2011, two shallow holes (134 and 54 m depth, respectively) were drilled into the Yingxiu-Beichuan fault (Longmen Shan thrust belt, China), which accommodated most of the displacement of the 2008 Mw 7.9 Wenchuan earthquake. Fifty-eight drill core samples, including granitic host rock and various fault rocks, were analyzed rock-magnetically, mineralogically, and geochemically. The magnetic behavior of fault rocks appears to be dominated by paramagnetic clay minerals. Magnetite in trace amounts is identified as the predominant ferrimagnetic fraction in all samples, decreasing from the host rock, via fault breccia to (proto-)cataclasite. Significant mass-losses (10.7–45.6%) are determined for the latter two with the “isocon” method. Volatile contents and alteration products (i.e., chlorite) are enriched toward the fault core relative to the host rocks. These observations suggest that magnetite depletion occurred in these fault rocks—exhumed from the shallow crust—plumbed by fluid-assisted processes. Chlorite, interpreted to result from hydrothermal activity, occurs throughout almost the entire fault core and shows high coefficients of determination (R2 > 0.6) with both low and high-field magnetic susceptibility. Close relationships, with R2 > 0.70, are also observed between both low and high-field magnetic susceptibility and the immobile elements (e.g., TiO2, P2O5, MnO), H2O+, and the calculated mass-losses of fault rocks. Hence, magnetic properties of fault rocks can serve as proxy indicators of fluid infiltration within shallow fault zones.

The 2008 Mw 7.9 Wenchuan Earthquake produced two major rupture zones: one in the Yingxiu-Beichuan fault zone (YBF) and another in the Anxian-Guanxian fault zone (AGF). A shallow trench was dug in Bajiaomiao village, Dujiangyan, Sichuan Province, which experienced a ∼4.3 m vertical offset during this large earthquake. The hanging wall of the YBF in this trench includes fault gouge and breccia. Optical microscope observations and X-ray diffraction (XRD) measurements show obvious differences between the fault gouge and breccia. Moreover, rock magnetism measurements were collected and include mass magnetic susceptibility (MS), Isothermal Remnant Magnetization (IRM), Saturation Isothermal Remnant Magnetization (SIRM), high-temperature thermo-magnetism (K–T) and magnetic hysteresis loops. Several cm-thick magnetic mineral anomalies are observed close to the Wenchuan Earthquake surface rupture zone of the YBF. Magnetite and Fe-sulfide are the main magnetic carrier materials for the fault rocks close to the surface rupture zone, including 3 cm-thick fault gouge and 3 cm-thick fault breccia, while the other fault breccia, further from the surface rupture zone, contains the paramagnetic minerals. The possible magnetic change is attributed to newly-formed magnetite from paramagnetic minerals at high temperatures (>500 °C) during the large earthquake, implying that the YBF has ever experienced high-temperature thermal pressurization earthquake slip dynamics. Moreover, the YBF has also experienced high-temperature frictional melting earthquake slip dynamics, constrained by the multiple vein pseudotachylite. These high-temperature earthquake slip processes may be responsible for the high dip angle thrust characteristic of the YBF.

Drilling in an active fault quickly after a large earthquake is an effective way to study earthquake mechanisms. In order to better understand the mechanical, physical, and chemical characteristics of the faults that ruptured during the 2008 Wenchuan earthquake (Mw 7.9), six boreholes were drilled on the two main strands (Yingxiu–Beichuan and Guanxian–Anxian faults) by the Wenchuan earthquake Fault Scientific Drilling project (WFSD). This paper focuses on the cores from the WFSD-3 borehole which drilled across the Guanxian–Anxian fault. A detailed petrological study shows that fault gouge and fault breccia are developed in the WFSD-3 cores in the Late Triassic Xujiahe Formation. The thicknesses of fault gouge range from ~1 mm to ~2.3 m. According to the characteristics of the fault rock combinations and their distribution, at least 22 subsidiary fault zones were recognized in the WFSD-3 cores. The Guanxian–Anxian fault zone is composed of fault rocks from 1192 to 1250.09 m depth, with a real thickness of ~50 m (~60 m thick in the WFSD-3 cores), and an actual damage zone of ~160 m (~980–1192 m depth in the WFSD-3 cores), and shows characteristics of multiple high-strain fault cores. The damage zone is only present in the hanging wall. The actual total thickness of the Guanxian–Anxian fault zone is ~210 m. Based on the analyses of comprehensive logging data, characteristics of the fault gouge, and seismic fault structures, the principal slip zone for the Wenchuan earthquake is identified in the black fault gouge at 1249.95 m depth in the cores, which lies almost at the bottom of the Guanxian–Anxian fault zone, and is also confirmed by surface rupture zone observations. The slip plane of the Wenchuan earthquake is a low-angle thrust fault with a dip angle of ~38° as estimated from the results of the WFSD-3 core analyses. The results from WFSD-1 showed that the Yingxiu–Beichuan segment is a high-angle thrust fault striking NW with a dip angle of ~65°. These two fault segments have different thicknesses and fault structures, which may suggest different faulting mechanisms and evolution history.

Nuclear energy is an important part of a sustainable, economic and secure energy balance for the United Kingdom (UK). Horizon Nuclear Power commissioned Arup to provide seismic hazard assessment (SHA) consultancy services for the proposed Wylfa Newydd Nuclear Power Station in Anglesey, Wales, UK. Advanced Boiling Water Reactors are proposed to provide at least 5400 MW, enough to power around 10 million homes. The SHA comprises a probabilistic seismic hazard assessment of ground motion, a tsunami hazard assessment and a capable faulting assessment. The capable faulting assessment included thoroughly identifying and investigating geological faults at the site, with due consideration of the geological, seismological and tectonic setting. The objective of the capable faulting assessment was to demonstrate that faults are not capable of surface rupture within the current tectonic setting. Should evidence suggest that faults with the potential to affect the safety of the nuclear installation are capable, IAEA guidance dictates that an alternative site shall be considered. A project 3D Geological Fault Model was developed to characterise geological faulting at the site for the capable faulting assessment. A systematic methodology that incorporates expert judgment was developed and applied to the investigation and correlation of geological faults between site investigation data points. A total of 36 geological faults were mapped across the site and three fault sets were identified. Absolute age dating of the fault gouge on a representative selection of samples was subsequently undertaken to demonstrate that faults at the site are not capable in accordance with IAEA guidelines.

Faults weaken during earthquakes. Analysis of the amount of energy released during earthquakes globally suggests that heat-induced pressurization of pore fluids can weaken faults during earthquakes of all sizes. Laboratory simulations of earthquakes show that at high slip rates, faults can weaken significantly, aiding rupture1,2,3. Various mechanisms, such as thermal pressurization and flash heating, have been proposed to cause this weakening during laboratory experiments1,4,5,6, yet the processes that aid fault slip in nature remain unknown. Measurements of seismic radiation during an earthquake can be used to estimate the frictional work associated with fault weakening, known as an event’s fracture energy7,8,9. Here we compile new and existing8,9 measurements of fracture energy for earthquakes globally that vary in size from borehole microseismicity to great earthquakes. We observe a distinct transition in how fracture energy scales with event size, which implies that faults weaken differently during small and large earthquakes, and earthquakes are not self-similar. We use an elastodynamic numerical model of earthquake rupture to explore possible mechanisms. We find that thermal pressurization of pore fluid by the rapid shear heating of fault gouge can account for the observed scaling of fracture energy in small and large earthquakes, over seven orders of fault slip magnitude. We conclude that thermal pressurization is a widespread and prominent process for fault weakening.

The San Andreas Fault (SAF), like other mature brittle faults, exhibits a zone of low seismic velocity hypothesized to result from fluid pressure effects and/or development of a damage zone. To address the relative contributions of these mechanisms in developing low-velocity zones, we measured P and S wave velocities ultrasonically at elevated confining and pore pressures on core samples from the San Andreas Fault Observatory at Depth (SAFOD). We compared those data to wireline and seismic-scale velocities to examine the scale dependence of acoustic properties of the fault core and damage zone. Average laboratory P and S wave velocities of the fault gouge at estimated in situ conditions are 3.1 and 1.5 km/s, respectively, consistent with the sonic log from the same intervals. These data show that fault core has intrinsically low velocity, even if no anomalous pore pressure is assumed, due to alteration and mechanical damage. In contrast, laboratory average P and S wave velocities for the damage zone are 4.7 and 2.5 km/s, up to 41% greater than the sonic log in the damage zone. This scale dependence indicates that stress conditions or macroscale features dominate the damage zone's acoustic properties, although velocity dispersion could play a role. Because no pressure anomaly was detected while drilling the SAFOD borehole, we infer that damage at a scale larger than core samples controls the elastic properties of the broader damage zone. This result bolsters other independent lines of evidence that the SAF does not contain major pore fluid overpressure at SAFOD.

Faults weaken during earthquakes. Laboratory simulations of earthquake rupture show that the nanometric-scale fault gouge created during slip is inherently weak and flows by grain-boundary sliding, providing a mechanism to weaken faults.