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While the structural deformation at the Arabia-Anatolia-Africa junction is critical for understanding eastern Mediterranean tectonics, it remains debatable (extensional or compressional). Field survey of the 2023 Mw7.7 Pazarcik earthquake surface rupture along the Amanos Range, and microstructure and composition analyses of related fault rock were performed to investigate this issue. The surface rupture shows transtensional branches, with the main and secondary strands displaying sinistral (normal) offsets of 2.0–4.0 (0.4–0.9) and 1.0 (0.2) m. Slip plane of the secondary fault is marked by a ∼20 cm wide extensional fracture dividing fault breccia from gouge. Meanwhile, the gouge (26 % calcite, 38% serpentine and 36% smectite) is also dominated by tension cracks microscopically. In contrast to the widespread extensional deformation, geological evidence imply that this fault had experienced reverse slip in the Quaternary, as shown by thrusting of serpentine onto alluvium and the consistency of R foliations with reverse shear. We interpret it as the SW continuation of the East Anatolia fault (EAF) that was formerly characterized by transpressional deformation, and reactivated transtensionally due to the change of fault strike during this earthquake. In the broader Mediterranean tectonics framework, the Amanos segment might be interpreted as a recently formed component linking the Karasu fault to the EAF. • The coseismic rupture along the Amanos Range is overall transtensional. • The largest branch fault reactivated was previously transpressional. • This transformation happened in the Quaternary. • It highlights the tectonic evolution in the Arabia-Anatolia-Africa junction zone. [ABSTRACT FROM AUTHOR]

The Sertengshan Range-front Fault (SRF) and Langshan Range-front Fault (LRF) constitute a normal fault system on the northern boundary of the Linhe Depression in its northwest, and an investigation of the LRF–SRF is significant for understanding the seismic activities of normal faults. We set four trenches at three study sites to reveal paleo-earthquakes in the western segment of the SRF (W-SRF) in this system, and established 5 surface rupture events along W-SRF since the Late Pleistocene. By integrating paleoseismic data from 27 trenches on multiple fault segments, we reconstructed the paleoseismic sequence for the LRF–SRF region since 15 ka, and identified 10 paleoseismic events with corresponding rupture segments and magnitudes. The slip rates at the three study sites decreased gradually from the west to east on the W-SRF, by comparing with previous studies, further evidencing that the tectonic activities of the two faults have gradually synchronized. Besides, based on the timing of the latest paleoseismic event (1.88−2.03 ka BP), earthquakes of > M 8 surface ruptures of >200 km can occur in the northwestern Ordos Block and its seismic behavior revealed the seismogenic possibility of normal faults triggering an M > 8 earthquake. • Identified at least 5 surface rupture events since the late Pleistocene on the W-SRF. • Reconstructed paleoseismic sequence of the SRF-LRF by integrating data of trenches. • The vertical slip rates decrease gradually from west to east on the W-SRF. • Confirmed the normal fault system can occur > M8 earthquake and cause surface ruptures >250 km. [ABSTRACT FROM AUTHOR]

The spectrum of slip behavior in crustal faults generates various rock types that can inform the mechanics of earthquake genesis. However, a single fault exposure may contain evidence of slip at various depths and temperatures due to progressive fault rock formation and overprinting during exhumation. Here, we unravel the spatiotemporal evolution of mechanical transitions along the Boundary Canyon detachment, a low-angle normal fault northeast of Death Valley, USA. Field, microstructural, and geochemical characterizations of fault rocks are compared to existing laboratory experiments and combined with a thermo-kinematic model of fault evolution. Together, these constrain the depths of mechanical transitions along the fault and reveal the evolution of earthquake nucleation zone thickness. Fault exposures from different initial paleodepths passed through the mechanical transitions during footwall exhumation, resulting in overprinting of mylonite by cataclasite and ubiquitous late formation of foliated, illite-rich gouge within the uppermost crust. We present evidence of coseismic low-angle normal fault slip (e.g., injection veins of cataclasite, laminar and grain-inertial fluidization). Coseismic slip likely nucleated at strength contrasts within the fault zone (i.e., contacts between quartzite breccia and calc-mylonite; quartz ribbons and mylonite matrix; breccia and clay gouge) at ≈ 5–9.5 km depth. Observations including mutually overprinting cataclasite/ultramylonite and exposures of pulverized gouge support that dynamic rupture propagated down-dip through the brittle-ductile transition zone (≈10–11 km depth) and up-dip through velocity-strengthening fault patches (≈0–5 km depth). Rapid fault exhumation increased the geotherm, leading to upward advection of the brittle-ductile transition and shallowing/thinning of the earthquake nucleation zone. This process may explain the rarity of large magnitude earthquakes on low-angle normal faults. [Display omitted] • Field, micro-structural, and geochemical analysis of fault rocks. • Fault rocks reconstructed to paleo-depth and temperature. • Reconstructed mechanical transitions and 2D static stress models. • Coseismic low-angle, normal-sense slip and dynamic rupture. • Thinning and shallowing of earthquake nucleation zone with exhumation. [ABSTRACT FROM AUTHOR]

The Nanbei River–Boli Fault is one of the largest NW-trending faults in Northeast China. However, limited data on the style and timing of recent earthquake activity on this fault greatly restrict the accuracy of seismic risk assessment. Here, we provide evidence of late Pleistocene to Holocene activity of the eastern segment of this fault for the first time. Specifically, the eastern segment of the Nanbei River–Boli Fault has experienced multiple earthquakes since the late Quaternary. Trench excavations indicate a recent strong earthquake at 7.28–2.11 ka, with approximately 1 m of co-seismic vertical displacement and preliminary estimates of the minimum vertical slip rates of 0.07–0.12 mm/a. According to a comprehensive analysis, we estimate that the eastern segment of the Nanbei River–Boli Fault can generate strong earthquakes of magnitude 7.0. This research provides important information for seismic risk assessments and seismic zoning, which can help seismic hazard assessment and mitigate their effects in Northeast China. • We analyze the Nanbei River–Boli Fault, southern margin of Boli Basin, Northeast China. • Radiocarbon and OSL dating provide the timing of recent strong earthquakes. • We estimate the maximum earthquake magnitude and minimum vertical slip rate. • The fault has experienced multiple surface rupture events in the late Quaternary. • This is the first evidence of late Pleistocene to Holocene activity along the fault. [ABSTRACT FROM AUTHOR]

Uncertainty concerning the processes responsible for slip-rate fluctuations associated with temporal clustering of surface faulting earthquakes is a fundamental, unresolved issue in tectonics, because strain-rates accommodated by fault/shear-zone structures are the key to understanding the viscosity structure of the crust and seismic hazard. We constrain the timing and amplitude of slip-rate fluctuations that occurred on three active normal faults in central Italy over a time period of 20–30 kyrs, using in situ 36Cl cosmogenic dating of fault planes. We identify five periods of rapid slip on individual faults lasting a few millennia, separated time periods of up to 10 millennia with low or zero slip-rate. The rapid slip pulses migrated across the strike between the faults in two waves from SW to NE. We replicate this migration with a model where rapid slip induces changes in differential stress that drive changes in strain-rate on viscous shear zones that drive slip-rate variability on overlying brittle faults. Earthquakes increase the differential stress and strain-rate on underlying shear zones, which in turn accumulate strain, re-loading stress onto the overlying brittle fault. This positive feedback produces high strain-rate episodes containing several large magnitude surface faulting earthquakes (earthquake clusters), but also reduce the differential stress on the viscous portions of neighbouring fault/shear-zones slowing the occurrence of large-magnitude surface faulting earthquakes (earthquake anticlusters). Shear-zones on faults experiencing anticlusters continue to accumulate viscous strain at a lowered rate, and eventually this loads the overlying brittle fault to failure, initiating a period of rapid slip through the positive feedback process described above, and inducing lowered strain-rates onto neighbouring fault/shear-zones. We show that these patterns of differential stress change can replicate the measured earthquake clustering implied by the 36Cl data. The stress changes are related to the fault geometry in terms of distance and azimuth from the slipping structure, implying that (a) strain-rate and viscosity fluctuations for studies of continental rheology, and (b) slip-rates for seismic hazard purposes are to an extent predictable given knowledge of the fault system geometry. [ABSTRACT FROM AUTHOR]

Shallow creep has been observed in the Laohushan segment which lies within a seismic gap between the co-seismic ruptures of the 1920 Haiyuan M8.5 earthquake and the 1927 Gulang M8.0 earthquake. However, there has been a lack of geological and mechanical evidence to explain the mechanism behind this shallow creep. In this study, we carried out detailed fieldwork along the Laohushan fault, analyzed the mineral compositions and microstructures of the fault rocks, and performed frictional experiments on fault gouges under various temperature and pressure conditions in the laboratory. Our findings revealed that clay minerals are prevalent in the fault rocks along the Laohushan creep segment, especially in the fault gouge at the fault core. Microstructural observations showed that extensive development of clay foliation in the fault gouges, along with carbonate veins healing cracks and minerals dissolution in the fault rocks. The frictional experiments indicated that the clay-rich gouges showed low frictional coefficients of 0.46–0.58, with positive a - b values of 0.0035–0.0052, reducing to 0–0.0015 at 200 °C. The experimental samples exhibited microstructural evolution of pervasive well-developed clay foliation and fewer clast fractures, which controls the velocity-strengthening sliding behavior of fault gouges. Our results suggest that the enrichment of weak clay minerals in the fault zone generates a well-foliated microfabric, which, as demonstrated by frictional experiments, controls the Laohushan shallow creep along the Haiyuan fault at depths of less than 8 km. This finding is consistent with the shallow creep inferred from geodetic data and repeating earthquakes. • Clay minerals are major components in fault rocks along the Laohushan creep segment. • Weak clay-rich fault gouges display velocity-strengthening stable sliding. • Well-developed clay foliation in fault rocks and test gouges control shallow creep. [ABSTRACT FROM AUTHOR]

We made field and petrographic observations of antitaxial crystal fibres in the gypsum veins of the Mercia Mudstone (SW England) to examine the kinematic and dynamic implications of these fibres. The gypsum fibres are predominantly oriented oblique to vein walls, indicating that the veins were formed in a shear-extension mode rather than pure tension mode. Fibres in single veins are parallel aligned, regardless of the vein-wall curvatures, except those in the tapering tip zones. Field data demonstrate that there is no clear correlation between fibre plunges and vein dips. Fibres in many vein steps exhibit the same orientations as those in the main vein segments, suggesting that these steps were formed as reactivated pre-existing faults/fractures instead of dilatational jogs. Vein tips that propagated progressively, commonly curved towards the planes of neighbouring larger veins, or interacted with the tips of adjacent sub-parallel veins, suggesting that vein propagation may have been dominated by the local stress field rather than remote stresses. Fibres in vein branches may not share the same orientations as those in the parent veins. The host marls are commonly segmented into multiple parts by interconnected vein arrays that contain differently oriented fibres in each vein. Our study suggests that the gypsum veins lack evidence for crack-seal increments, and the crystal fibres in the veins had a continuous accretion towards vein walls. The fibres are parallel to vein opening directions as indicated by the correlative vein-wall irregularities, and are of great potential for kinematic analysis and deformation restoration of the host rocks. Vein dilation could be driven by force of crystallization of veins exerted on the enclosing rocks, which could have caused a growth-related compaction and fracturing rather than uplifting the overlying strata that was accompanied with a bulk extension. • We made field and petrographic observations of antitaxial gypsum veins. • Fibrous gypsum veins were formed in a shear-extension mode. • Gypsum fibres are parallel to vein opening directions. • Vein dilation led to growth-related compaction of the enclosing hosts. [ABSTRACT FROM AUTHOR]

Normal faulting earthquakes rarely rupture the entire extent of active normal faults, and can also jump between neighbouring faults. This confounds attempts to use segmentation models to define the likelihood of future rupture scenarios. We attempt to study this problem comparing the offsets produced in single earthquakes with those produced by multiple earthquakes over longer timescales, together with detailed studies of the structural geology. We study the active normal fault system that caused the Mw 6.3 2009 L'Aquila earthquake in central Italy, comparing the spatial distribution of coseismic offsets, cumulative offsets that have developed since 15 ± 3 ka, and the total offsets that have accumulated since the faults initiated at 2–3 Ma. Our findings suggest that: 1) faults within a segmented fault system behave as a single interacting fault segment over time periods including multiple earthquake cycles (e.g. 2–3 Ma or 15±3ka), with single earthquakes causing either partial or total ruptures of the entire system; 2) an along-strike bend causes throw and throw-rates enhancements within the bend throughout the seismic history of the fault system. We discuss the synchronised and geometrically controlled activity rates on these faults in terms of the propensity for floating earthquakes, multi-fault earthquakes, and seismic hazard. • Faults within a segmented fault system can behave as a single interacting fault segment over multiple seismic cycles. • Single earthquakes can float within the fault system, causing either partial and total ruptures of the system. • Along-strike fault bends in segmented fault systems cause throw enhancement within the bend. • Structural geology provides important information to assess the propensity of multi-fault earthquakes. [ABSTRACT FROM AUTHOR]

Fault duplexes were recognized as early as the late nineteenth century and have been identified in most fold - thrust belts. Duplexes appear to provide a mechanical advantage to the emplacement of large thrust sheets by transferring displacement upward from one detachment horizon to another. However, faulting is not the only mechanism for displacement transfer; pressure-solution cleavage and folding may play the same role. Folds or cleavage within a duplex absorb the shortening of the underlying detachment, so that the displacement on the floor thrust decreases toward the foreland. As displacement on the floor decreases, displacement on the upper detachment (roof thrust) increases. In fold duplexes, the geometries of the detachment folds vary from open, concentric and kink-band folds to vertical and recumbent isoclines. A range of duplex styles, from fault duplexes to cleavage and fold duplexes, can be found within a single a single thrust belt, and this variation raises several important questions to guide future study. They include what controls the geometry of the folds within fold duplexes; and, perhaps more importantly, what conditions (lithology, pressure, temperature, strain rate) determine the type of duplex (fault, cleavage or fold) that will develop. • The term "fold duplex" is introduced. • Fold duplexes play the same role as fault duplexes – displacement transfer. • Examples are described from the thrust belts of Utah, Montana and Idaho. • Hybrid duplexes, including cleavage-fold duplexes, are introduced. [ABSTRACT FROM AUTHOR]

The mechanical behavior of materials is not affected by our choice of a coordinate system. Thus, the description of the behavior should not be affected by this choice. This is known as the principle of coordinate invariance, and is important not only for plasticity theory but for theoretical investigations in structural geology. The deviatoric stress-strain space, which fulfills the principle, is shown to be useful for the formulation of stress inversion. Problems in the inversion schemes of fault data are transformed into geometrical problems so that we can solve the problems using geometrical interpretations. In addition, the formulation gives the good basis for defining the classes of dissimilarities between reduced stress tensors that are the solutions of the inversion. We redefine the classes, here, from the standpoint of probability. It is demonstrated finally that the violation of the principle spoils the accuracy and resolution of the inversion. • Deviatoric stress-strain space is useful for paleostress analysis. • The space fulfills the principle of coordinate invariance. • Inversion schemes violating the principle have anisotropic accuracy and resolution. • Mathematical apparatus using the space are provided for the analysis. • Dissimilarity classes of reduced stress tensors are redefined based on the principle. [ABSTRACT FROM AUTHOR]

Vein networks of pseudotachylytes and ultracataclastic rocks are composed mainly of ultrafine-to fine-grained materials, including fault gouge and microbreccia, and are widely considered indicators of past seismic faulting events. I show that such vein networks of pseudotachylyte with both melt and crush origins, as well as ultracataclastic rocks, form by the rapid injection of ultrafine-to fine-grained material sourced from pulverized ultracataclastic rocks in seismogenic fault zones under thermal pressurization and fluidization during seismic events. The thermal expansion of seismic slip zones caused by frictional heating results in the rapid fluidization of ultrafine-to fine-grained materials along with expanded fluids and gases (e.g., water vapor and melt) that are injected under pressure into fracture void spaces within fault zones in a gas–solid–liquid system during large earthquakes. I propose that the thermal expansion of water vapor and ultrafine-to fine-grained materials caused by frictional heating in the fault slip zone is the main mechanism responsible for the dramatic increase in pore pressure that results in the dynamic coseismic weakening of faults. • Vein networks of pseudotachylyte (Pt) are indicators of seismic faulting. • Pt veins are sourced from by pulverized ultracataclastic rocks in seismic fault zones. • Thermal pressurization and fluidization occurred in fault zone during seismic faulting. • Thermal expansion of seismic slip zones caused by frictional heating in fault zones. • Thermal expansion is a main mechanism of dynamic coseismic weakening of faults. [ABSTRACT FROM AUTHOR]

Some fault gouges (e.g., kaolinite) exhibit velocity-strengthening behavior and slide stably at low, subseismic slip rates (e.g., 1–10 μm/s), accompanied by the preferential development of foliation rather than a principal slip zone or a discrete narrow band of large shear strain. However, a remarkable observation from experimental gouges sheared at seismic slip rates (>0.1 m/s) is that all the gouges show a well-developed principal slip zone, irrespective of their frictional behaviors (velocity weakening or velocity strengthening) at subseismic slip rates. These observations imply that in some gouges, a principal slip zone develops only at seismic slip rates and can be used as a seismic slip indicator. To test this idea, we conducted shear tests on dry kaolinite powder and wet kaolinite paste at low (0.1–100 μm/s), intermediate (∼1 mm/s), and high (1 m/s) slip rates and at a normal stress of 3 MPa. The dry powder specimen recovered after the experiments showed a principal slip zone at all slip rates. In contrast, the paste specimens showed a principal slip zone only at the seismic slip rate. They display foliation and R-shears at low and intermediate slip rates, indicating the occurrence of distributed shear. The microstructure of the wet paste at the seismic slip rate is similar to that reported from a natural kaolinite-rich seismic slip zone. Considering the experimental results, the structures of the experimental and natural seismic slip zones, and the unlikely fault slip under dry conditions, we suggest that kaolinite gouges with principal slip zone(s) can be a seismic slip indicator. Further studies may be extended to other gouge materials showing similar velocity strengthening and stable sliding at subseismic slip rates to determine whether the same conclusion can be drawn. • Dry powder and wet paste of kaolinite sheared at 1–100 μm/s, 0.001 m/s, and 1 m/s. • Wet pastes showed V-strengthening and foliation development at subseismic Vs. • Principal slip zones (PSZs) developed in the dry powder layers at all V. • A PSZ in the wet paste only at seismic V. • A PSZ in wet kaolinite gouge can indicate seismic slip. [ABSTRACT FROM AUTHOR]

Abstract Fracture corridors, elongated zones of narrowly spaced, highly connected, fractures, can generate the main fluid flow pathways in natural reservoirs and can be responsible for an early water break-through in hydrocarbon reservoirs. Understanding this fracturing is necessary for reservoir behaviour prediction. An outcrop analogue of a chalk fractured reservoir from the Isle of Thanet, in SE England is used to understand the development of fracture corridors, and to characterize their relationship with faulting and folding. We show that fracture corridors are associated with strike-slip reactivation on sub-seismic faults in a context of polyphase deformation. They nucleate on slip patch tips or on areas of stress concentration due to fault plane dip/strike irregularities, steps or fault intersections. Fracture corridors generated are oblique to the fault which they initiate on. With increasing deformation, fractures composing the corridor are sheared and a fault can develop within the fracture corridor. The relationship between folding and fracture corridor development in this area is indirect. Both are localized by a pre-existing basement fault that allows stress concentration and reorientation and promotes fault reactivation in the chalk cover. This work emphases the importance of the distribution of sub-seismic faults in fractured reservoirs as they can control the development of fracture corridors. Highlights • Fracture corridors (FC) observed in chalk outcrops are interpreted as fault related. • FC are due to subseismic fault reactivation during a polyphased deformation. • FC nucleate on areas of stress concentration due to slip difficulties (tips, bends ...). • FC are oblique to the pre-existing faults and form the damage zone of new ones. • FC are not fold related but basement structural inheritances give their distribution. [ABSTRACT FROM AUTHOR]

Abstract The formulation for the 3D triangular displacement discontinuity boundary element method with frictional constraints is described in detail. Its accuracy in comparison to analytical solutions is then quantified. We show how this can be used to approximate stress intensity factors at the crack tips. Using this method, we go on to quantify how slip is reduced on fault surfaces with topography, where the asperities are approximated as a sinusoidal waveform, i.e. corrugations. We use stress boundary conditions (compressive) orientated such that frictional contacts shear. We show that slip reductions relative to planar faults for 2D line and 3D penny-shaped crack models are comparable within 10% when slip is perpendicular to the corrugations. Using the 3D model, we then show how slip is reduced more when corrugation wavelengths are doubled compared to the reduction due to corrugation misalignment with the slip direction. When slip is parallel with the corrugation alignment we show that reducing the out-of-plane stress, from the normal traction acting on the fault when planar to that resolved on a perpendicular plane, has the same effect as halving the length of the corrugation waveform in terms of slip reduction for a given amplitude. Highlights • 3D displacement discontinuity method friction and stress intensity formulations. • Quantifies how slip reduction changes due to boundary conditions and fault shape. • Shear fractures can open even when pore pressure doesn't exceed confining stress. [ABSTRACT FROM AUTHOR]

Abstract This study uses 3D seismic mapping, NNCC attribute analysis and 2D SDEM modelling of soft-sediment deformation to analyse hanging wall deformation related to faulting across a listric fault detaching along the top of a major salt roller. The computational model experiments used the overall fault and salt-structure evolution as boundary conditions to predict the distribution of faults within the hanging wall. The modelling successfully reproduced geometries similar to the syn- and antithetic faults that are visible on seismics, but it also showed the presence of well-defined vertical deformation zones, both in the hanging wall and the foot-wall of the listric fault. In the model, salt movement controls the formation of the deformation zones. The similarity of NNCC-inferred vertical chimneys and the modelled deformation zones suggests that the subtle vertical chimneys identified in the 3D seismic data might actually be related to vertical to sub-vertical deformation zones caused by the presence and movement of the salt structure. Highlights • SDEM modelling reproduces hanging wall deformation above rising salt-roller. • Subtle deformation controls chimney formation within the hanging-wall succession. • Improved fault mapping beyond seismic resolution due to deformation modelling. [ABSTRACT FROM AUTHOR]

Abstract In this contribution we present numerical simulations of the development of faults in extensional environments, in rifts with coupled extension and dyking as well as in scenarios with only dyking. The numerical model allows for the study of fault initiation and propagation with dynamic development of faults and couples fluid dynamics for the dyking-induced with tectonically-induced stresses. Results show that faults nucleate at a depth of around 250 m in hybrid extensional-shear mode under pure extension when the material has a Young's Modulus range between 5 and 20 GPa and as mode I cracks at shallow depth of around 100 to 80 m at a Young's Modulus of 40–80 GPa. Following nucleation, faults propagate into vertical fissures towards the Earth's surface and into shear fractures at depth resulting in listric geometries. Continued extension leads to a complex network of conjugate faults and to central collapse structures. Dyking into an extensional system produces at least two nucleation points for faults, one on top of the dyke and a second point between dyke and the Earth's surface. Both of these nuclei develop into curved faults but with opposite curvatures forming fissures or mode I fractures at the Earth's surface and at the top of the dyke. Further extension of these coupled dyke and extension systems leads to relatively localized collapse structures on top of dykes with rhomboid blocks that are subsiding. Thin and shallow dyking leads to more concentrated collapse structures. In cases where dykes are the only structures producing stresses faults on top of the dykes propagate upwards as mode II shear fractures and develop shallow branches when they become close to the Earth's surface. Overall the models indicate that the most common and concentrated collapse structures develop when dyking and extension are both taking place and that the collapse structures develop complex zones with opening fissures at the Earth's surface, rhomboid collapsing blocks and faults with both normal and reverse movement. Highlights • This contribution presents modelling of fissures and faults in rifts induced by extension and dyking. • Faults nucleate as hybrid shear surfaces and migrate upwards as fissures and downwards as shear fractures. • Dyking tends to localize faults on top of dykes and produces narrow vertical collapse structures. • Collapse structures are rhomboid blocks that form along conjugate fault sets and move down with normal and reverse sense. • The most localized collapse structures develop on top of thin and shallowly intruding dykes. [ABSTRACT FROM AUTHOR]

Abstract The 2010 Ms 7.1 Yushu earthquake generated a 30-km-long surface rupture along the Gyêgu segment of the Yushu fault within the eastern margin of the Tibetan Plateau. Comparison of the surface slip distribution from this earthquake with the cumulative slips provides a basis to examine repeated activity through several earthquake cycles along the Yushu fault. To better understand the seismic behavior of the Yushu fault, we conducted geomorphological and geological investigations on the 2010 slip and cumulative offsets along the fault at several typical sites. Field observation and satellite image interpretation shows that the coseismic surface ruptures typical of the 2010 Yushu earthquake are discontinuous and usually do not show distinct slip at places where the accumulated displacements are large. Measurements made both in the field and on the satellite images show that the surface slip of the 2010 event was comparatively small and contributed little to cumulative displacement of the fault. Repeated offsets of 6–8 m characterize slip associated with events prior to 2010 along the Yushu fault, whereas the 2010 earthquake produced offsets only on the order of 1 m. In addition, surface ruptures from the magnitude 7.1 event involved the failure of less than half of the Gyêgu fault segment and did not terminate at major geometrical and geological breaks in the fault zone. Geomorphic evidence implies that the fundamental behavior of the Gyêgu fault segment is typically expressed as full-length rupture activity. Thus, data from this study suggests that the characteristic slip along the Yushu fault is 6–8 m, the Ms 7.1 earthquake was not likely a characteristic event along the Yushu fault and the 2010-type events may make some contribution to the low slip rate and long recurrence interval of large earthquakes on the Yushu fault. Highlights • The smallest cumulative offset (6–8 m) likely represents the characteristic slip along the Yushu fault. • Geomorphological and geological data along the Yushu Fault suggest non-characteristic slip occurred during the 2010 earthquake. • Characteristics of the cumulative slip distribution and geometrical irregularities suggest the fundamental behavior of the Gyêgu fault segment is expressed as full-length rupture. • The 2010-type events may make some contribution to the low slip rate and long recurrence interval of large earthquakes on the Yushu fault. [ABSTRACT FROM AUTHOR]

Abstract A novel numerical approach enables determining contemporary stress states at a single fault near the ground surface using three dimensional fault displacement data, i.e. displacements that include movement component perpendicular to the fault surface. This approach is restricted to specific near-surface conditions and is based on three assumptions: (i) the near-surface faults contain apertures and the blocks can move in all directions at a sub-millimetric scale, so the movement is not restricted only to the fault plane; thus the stress vector (traction) acting on the fault surface has the same direction as the fault displacement vector, (ii) the isotropic component of the stress tensor near the ground surface is negligible, and (iii) one of the principal stress orientations must be vertical as being close to the ground surface. These generalizations enable distinct formula simplification and the reduced stress tensor calculation. Input data for the mathematical solution are the orientation of the fault surface (i.e., the down-directed fault surface normal) and the fault displacement (i.e., the vector of the hanging wall displacement). The orientation of the principal stresses and the shape parameter of the stress ellipsoid are the outputs of the method. We demonstrate the approach on a single fault reactivation event on 7 November 2014 in Obir Cave near the Periadriatic Fault in the Eastern Alps. Highlights • Contemporary stress calculation based on a single fault 3D displacement. • The stress tensor calculation possible thanks to specific near-surface conditions. • Possibility of observation of stress variations in real time. [ABSTRACT FROM AUTHOR]

Abstract Outcrop mapping of a chalk cliff and wavecut platform in Thanet, Southeast England show a complex fracture pattern that seems to be controlled by meso-scale strike-slip faults within the chalk. The response of these faults to changes to in situ stress and fluid pressure is thought to control the nucleation and propagation of fractures in the chalk. In this study the DEM (Discrete Element Method) technique has been employed as a follow up to previous field and numerical (boundary and finite element method) investigations to ascertain the role of the faults in the initiation and nucleation of fractures The role of fluid pressure, in-situ stress, and fault geometry are recognised as focal factors. The generation of localised areas of tensile stresses due to fluid pressure and stress perturbations have been shown to cause the initiation of fractures around the fault bends. For releasing bends, localised tensile stresses tend to occur along the central segment of the fault bend, whereas for restraining bends, tensile stresses are more likely to develop on the outer edges of the fault bend. The dissimilarity in the fracturing process due to differences in the geometry of pre-existing faults demonstrates the significance of both fault geometry and fluid behaviour in controlling fracturing. Highlights • We investigated the behaviour of faults at a chalk cliff and wavecut platform at Thanet, UK. • The discrete element numerical technique was implemented. • Fluid pressure, in-situ stresses and fault geometry greatly influence fracture proliferation. • The orientation of propagating fractures is found to be perpendicular to the fault plane. • Fluid pressure and stress perturbations are shown to initiate fracturing around fault bends. [ABSTRACT FROM AUTHOR]

Previous studies using paleoseismic trenches and numerical models proposed two different fault rupture models for the Pingdingshan-Annanba segments of the Altyn Tagh Fault, which are both poorly validated because of a lack of measurements of surface rupture displacements. In this paper, we use high-resolution satellite imagery to measure offset geomorphic markers. Taking the fault geometry into consideration, the most likely paleoseismic rupture parameters are determined by comparing the results of the cumulative offset probability densities (COPDs) and the empirical relationships of seismic parameter. The results show that the most recent earthquake (1270A.D.-1775A.D.) along the Suoerkuli segment only ruptured the Suoerkuli Valley, with a surface rupture length of 140 km, an average horizontal displacement of 3.12 m and a moment magnitude (Mw) of 7.4–7.6. The penultimate earthquake (676A.D.-1347A.D.) ruptured both the Annanba segment and part of the Suoerkuli segment with an average horizontal displacement of greater than 5 m, a surface rupture length of 120–260 km and Mw 7.4–7.8. The rupture parameters of the above two earthquakes demonstrate that the Suoerkuli and Annanba segments are individual fault segments which can rupture independently with a surface rupture length more than 100 km. The Pingdingshan double bend to the west of the Suoerkuli segment is likely to be an endpoint that inhibits the rupture propagation towards further west. [ABSTRACT FROM AUTHOR]

Fault damage zones affect the preservation and enrichment of shale gas. Most fault damage zone studies are based on directly observed outcrops. In this study, we provide an example of fault damage zone analysis using 3D seismic data of deep shales buried more than 3500 m in the Sichuan Basin, China. The 3D seismic variance attribute and the cumulative frequency method were used to define the background value and then estimate the damage zone widths of different sizes of faults. We found that the damage zone widths range from 80 m to 1570 m, and trend to stabilize with the increase in displacements. The scattering caused by the relay zone can be interpreted as segment linkage mode and damage superposition related to subsidiary faults. The damage intensity decays following a power law with distance from the fault core, in agreement with field observations and the distribution of seismic variance. Damage zone widths display a significant asymmetry across the fault, and the hanging wall damage zone is generally wider than the footwall damage zone. We discuss the impact of the fault damage zone on shale gas preservation, which is related to the sealing of the direct caprock. It appears that the seismic variance attribute is an effective tool for subsurface characterization of fault damage zones and can help identify the location of deep unconventional energy. • Estimation method of fault damage zone widths based on seismic variance attribute. • Determination of D-W relationship for reverse faults in the shale. • We identified two sizes of reverse faults according to seismic profiles in the study area. • Determination of impact of damage zones on shale gas preservation. [ABSTRACT FROM AUTHOR]

Microfabrics of a gabbroic ultramylonitic shear zone from the Atlantis Bank oceanic core complex have been studied to investigate strain localization processes during exhumation of the lower oceanic crust through detachment faults. The ∼0.5 cm wide ultramylonite band is hosted in a coarse-grained deformed gabbro. Microfabric analysis reveal undulatory extinction, kink bands, bent exsolution lamellae or twins and healed microfractures decorated by new grains within porphyroclasts of diopside and plagioclase in the ultramylonite, as well as the host gabbro close to the border of the ultramylonite. These microstructures suggest that strain was initially accommodated by dislocation glide associated with microfracturing, i.e. high-stress crystal plasticity. Heterogenous fluid influx along microfractures is suggested to have led to localized phase transformation, as amphibole is abundant in the fine-grained matrix of the ultramylonite but not in the host gabbro. Fluid-rock interaction resulted in the development of the ultramylonite via grain-size sensitive fluid-assisted granular flow. Strain localization is concomitant to increasing fluid-rock interaction and the replacement of the former deformed anhydrous mineral assemblage into a hydrated ultramylonite composed of a plagioclase-amphibole mixture. Our study highlights the importance of fluid-assisted metamorphic reaction in decreasing the viscosity of the metastable lower oceanic crust and creating zones of extreme rheological weakening that deform by diffusion creep on the long term. Fluid infiltration plays a key role in facilitating ductile deformation and tectonic spreading through large-scale detachment faults in the hot, lower oceanic crust. • Thin high-temperature ultramylonite band is hosted in a coarse-grained gabbro. • Strain is initially accommodated by dislocation glide associated with microfracturing. • Microfractures allow fluid-rock interaction and development of the ultramylonite via fluid-assisted granular flow. • Strain localization is coeval with fluid-rock interaction and growth of amphibole-plagioclase mixtures. • Fluid infiltration facilitates tectonic spreading along lower oceanic shear zones. [ABSTRACT FROM AUTHOR]

The Alpine Fault in New Zealand is a major plate-bounding structure that typically slips in ∼M8 earthquakes every c. 330 years. To investigate the near-surface, high-velocity frictional behavior of surface- and borehole-derived Alpine Fault gouges and cataclasites, twenty-one rotary shear experiments were conducted at 1 MPa normal stress and 1 m/s equivalent slip velocity under both room-dry and water-saturated (wet) conditions. In the room-dry experiments, the peak friction coefficient ( μ p = τ p / σ n ) of Alpine Fault cataclasites and fault gouges was consistently high (mean μ p = 0.67 ± 0.07). In the wet experiments, the fault gouge peak friction coefficients were lower (mean μ p = 0.20 ± 0.12) than the cataclasite peak friction coefficients (mean μ p = 0.64 ± 0.04). All fault rocks exhibited very low steady-state friction coefficients ( μ ss ) (room-dry experiments mean μ ss = 0.16 ± 0.05; wet experiments mean μ ss = 0.09 ± 0.04). Of all the experiments performed, six experiments conducted on wet smectite-bearing principal slip zone (PSZ) fault gouges yielded the lowest peak friction coefficients ( μ p = 0.10–0.20), the lowest steady-state friction coefficients ( μ ss = 0.03–0.09), and, commonly, the lowest specific fracture energy values ( E G = 0.01–0.69 MJ/m 2 ). Microstructures produced during room-dry and wet experiments on a smectite-bearing PSZ fault gouge were compared with microstructures in the same material recovered from the Deep Fault Drilling Project (DFDP-1) drill cores. The near-absence of localized shear bands with a strong crystallographic preferred orientation in the natural samples most resembles microstructures formed during wet experiments. Mechanical data and microstructural observations suggest that Alpine Fault ruptures propagate preferentially through water-saturated smectite-bearing fault gouges that exhibit low peak and steady-state friction coefficients. [ABSTRACT FROM AUTHOR]

The Huangshi structure, as one of the NWW-trending S-shaped structures in the southwestern Qaidam Basin, holds important implications for unraveling the regional structural pattern. There are four dominant sets of surface strike-slip fractures at the core of the Huangshi structure. The fractures with orientations of N28°E, N47°E and N65°E correlate well with conjugate Riedel shears (R′), tension fractures (T) and Riedel shears (R) in the Riedel shear model, respectively. Two conjugate strike-slip fracture sets occur at the surface of the Hongpan structure (secondary to the Huangshi structure) and the southwestern part of the Huangshi structure. In seismic sections, the Huangshi structure is present as a positive flower or Y-shaped structure governed by steeply dipping faults, whereas Hongpan and Xiaoshaping structures, located symmetrically to the Huangshi structure, are thrust-controlled anticlines. The Riedel shear pattern of surface strike-slip fractures, the positive flower or Y-shaped structure in seismic sections and the NW-trending secondary compressional anticlines consistently demonstrate that the Huangshi structure is dominated by left-lateral strike-slip faults which comprise a strike-slip fault network. Considering the similar S-shaped configuration and NWW trend of structures across the southwestern Qaidam Basin, it can be further speculated that these structures are also predominantly of left-lateral strike-slip types. [ABSTRACT FROM AUTHOR]

Flow through fractures in shales is of importance to many geoengineering purposes. Shales are not only caprocks to hydrocarbon reservoirs and nuclear waste or CO 2 storage sites, but also potential source and reservoir rocks for hydrocarbons. The presence of microfractures in shales controls their permeability and transport properties. Using X-ray micro-tomography and white light interferometry we scanned borehole samples obtained from 4 km depth in the Pomeranian shales in Poland. These samples contain open exhumation/drying cracks as well as intact vein-rock interfaces plus one striated slip surface. At micron resolution and above tensile drying cracks exhibit a power-law roughness with a scaling exponent, called the Hurst exponent H , of 0.3. At sub-micron resolution we capture the properties of the clay interface only, with H = 0.6. In contrast, the in-situ formed veins and slip surface exhibit H = 0.4–0.5, which is deemed representative for in-situ fractures. These results are discussed in relation to the shale microstructure and linear elastic fracture mechanics theory. The data imply that the Hurst roughness exponent can be used as a microstructural criterion to distinguish between exhumation and in-situ fractures, providing a step forward towards the characterization of potential flow paths at depth in shales. [ABSTRACT FROM AUTHOR]

The chemo-mechanical feedbacks associated with hydration of periclase immersed in a calcite matrix were investigated experimentally. Dense calcite-periclase aggregates with <5% porosity and with a calcite to periclase ratio of 90/10 and 95/5 by volume were prepared by hot isostatic pressing. Subsequent hydration experiments were performed in a hydrothermal apparatus at temperatures of 580–610 °C and a pressure of 200 MPa for run durations of 5–60 min. The rate of the periclase to brucite transformation was primarily controlled by the access of fluid. Where fluid was present, the reaction was too fast for the associated positive volume increase of the solids of about 100% to be accommodated by creep of the calcite matrix, and fracturing was induced. The newly formed cracks greatly enhanced the access of fluid leading to a positive feedback between hydration and fracturing. Mostly the newly formed cracks follow pre-existing grain boundaries in the calcite matrix. Comparison of experimental results with numerical 2D discrete element modelling (DEM) of crack formation revealed that the geometry of the crack pattern around a reacting particle depends on the shape of the original periclase particle, on the mechanical strength of the particle-matrix interface and on the mechanical strength and arrangement of grain boundaries in the calcite matrix in the immediate vicinity of the swelling particle. [ABSTRACT FROM AUTHOR]

Although it is well documented that during faulting there is an overall fault zone porosity loss in porous sandstone, little is known about the effect of shear-induced compaction on subsequent deformation bands petrophysics. We studied the impact of fault-induced compaction on deformation band's petrophysical properties developed in poorly lithified sandstones of the Rio do Peixe Basin, Brazil. We compared undeformed sandstone and deformation bands associated with syn-rift normal faults and post-rift strike-slip faults by integrating structural, petrophysical, mineralogical and uniaxial compressive strength (UCS) measurements. The sealing behavior was interpreted from capillary pressure obtained from mercury injection porosimetry and in situ air permeability. Host rock consists of poorly lithified sandstone with 25–30% porosity, permeability of 1.3 Darcy, capillary pressure of 5 psi, and UCS of 20 MPa. The normal faults developed from soft-sediment conditions with moderate to intense cataclasis, and resulted in an overall fault zone compaction (deformation bands and zones between deformation bands) with localized porosity and permeability loss in deformation bands (10% and 12 mD) and rock volumes between deformation bands (20% and 0.7 Darcy). Compared to syn-rift deformation bands, the strike-slip deformation bands show similar porosity (12–13%), but lower permeability (1–10 mD) and greater capillary pressures (up to 114 psi). XRD analysis on clay minerals indicates that both syn-rift and post-rift faults formed under similar shallow burial depths (<1–2 km). Therefore, we propose a model that quantifies fault-induced compaction and may predict fault sealing in structurally complex, deformation bands-dominated sandstone reservoirs affected by multiple tectonic stages. • We study the petrophysical and microstructural properties of syn-rift and post-rift DBs of the Rio do Peixe Basin, NE Brazil. • The rock fabric is progressively compacted as new faults and deformation bands are formed. • Fault-induced compaction is directly related to the strain intensity of subsequent deformation bands. • Post-rift strike-slip DBs presents a higher fluid-flow barrier potential in respect to syn-rift DBs. [ABSTRACT FROM AUTHOR]

In order to determine whether slip during an earthquake on the 26th September 1997 propagated to the surface, structural data have been collected along a bedrock fault scarp in Umbria, Italy. These collected data are used to investigate the relationship between the throw associated with a debated surface rupture (observed as a pale unweathered stripe at the base of the bedrock fault scarp) and the strike, dip and slip-vector. Previous studies have suggested that the surface rupture was produced either by primary surface slip or secondary compaction of hangingwall sediments. Some authors favour the latter because sparse surface fault dip measurements do not match nodal plane dips at depth. It is demonstrated herein that the strike, dip and height of the surface rupture, represented by a pale unweathered stripe at the base of the bedrock scarp, shows a systematic relationship with respect to the geometry and kinematics of faulting in the bedrock. The strike and dip co-vary and the throw is greatest where the strike is oblique to the slip-vector azimuth where the highest dip values are recorded. This implies that the throw values vary to accommodate spatial variation in the strike and dip of the fault across fault plane corrugations, a feature that is predicted by theory describing conservation of strain along faults, but not by compaction. Furthermore, published earthquake locations and reported fault dips are consistent with the analysed surface scarps when natural variation for surface dips and uncertainty for nodal plane dips at depth are taken into account. This implies that the fresh stripe is indeed a primary coseismic surface rupture whose slip is connected to the seismogenic fault at depth. We discuss how this knowledge of the locations and geometry of the active faults can be used as an input for seismic hazard assessment. [ABSTRACT FROM AUTHOR]

Triaxial shear tests are performed to assess the effects of displacement velocity and confining pressure on shear strengths and dilations of tension-induced fractures and smooth saw-cut surfaces prepared in granite, sandstone and marl specimens. A polyaxial load frame is used to apply confining pressures between 1 and 18 MPa with displacement velocities ranging from 1.15 × 10 −5 to 1.15 × 10 −2 mm/s. The results indicate that the shearing resistances of smooth saw-cut surfaces tend to be independent of the displacement velocity and confining pressure. Under each confinement the peak and residual shear strengths and dilation rates of rough fractures increase with displacement velocities. The sheared-off areas increase when the confining pressure increases, and the displacement rate decreases. The velocity-dependent shear strengths tend to act more under high confining pressures for the rough fractures in strong rock (granite) than for the smoother fractures in weaker rocks (sandstone and marl). An empirical criterion that explicitly incorporates the effects of shear velocity is proposed to describe the peak and residual shear strengths. The criterion fits well to the test results for the three tested rocks. [ABSTRACT FROM AUTHOR]

Fault zone architecture and its internal structural variability play a pivotal role in earthquake mechanics, by controlling, for instance, the nucleation, propagation and arrest of individual seismic ruptures and the evolution in space and time of foreshock and aftershock seismic sequences. Nevertheless, the along-strike architectural variability of crustal-scale seismogenic sources over regional distances is still poorly investigated. Here, we describe the architectural variability of the >40-km-long exhumed, seismogenic Bolfin Fault Zone (BFZ) of the intra-arc Atacama Fault System (Northern Chile). The BFZ cuts through plutonic rocks of the Mesozoic Coastal Cordillera and was seismically active at 5–7 km depth and ≤ 300 °C in a fluid-rich environment. The BFZ includes multiple altered fault core strands, consisting of chlorite-rich cataclasites-ultracataclasites and pseudotachylytes, surrounded by chlorite-rich protobreccias to protocataclasites over a zone up to 60-m-thick. These fault rocks are embedded within a low-strain damage zone, up to 150-m-thick, which includes strongly altered volumes of dilatational hydrothermal breccias and clusters of epidote-rich fault-vein networks at the linkage of the BFZ with subsidiary faults. The strong hydrothermal alteration of rocks along both the fault core and the damage zone attests to an extensive percolation of fluids across all the elements of the structural network during the activity of the entire fault zone. In particular, we interpret the epidote-rich fault-vein networks and associated breccias as an exhumed example of upper-crustal fluid-driven earthquake swarms, similar to the presently active intra-arc Liquiñe-Ofqui Fault System (Southern Andean Volcanic Zone, Chile). • We describe the structural variability of the Bolfin Fault Zone (BFZ). • The BFZ has a wide fault core and an extensive hydrothermally altered damage zone. • Ancient seismicity is attested by widespread occurrence of pseudotachylytes. • Ancient seismicity was associated with intense fluid-rock interaction. • Clusters of fault-vein networks represent ancient swarm-type sequences. [ABSTRACT FROM AUTHOR]

The mechanisms by which fluids modify the mineralogy and geochemistry of fault zones and the role of rock–fluid interactions in fault weakening are subjects of debate. We assessed the mineralogical and geochemical variations in fault rocks in the Shaba area of the Yingxiu–Beichuan surface rupture zone of the Mw 7.9 Wenchuan earthquake using analyses of their mineralogical compositions, major elements, and the microstructural characteristics of wall rocks, damage zones, and oriented fault gouge samples from the principal slip zones. The elemental and mineralogical compositions show a certain regularity across the fault zone from the wall rocks to the fault gouge so that (1) Al 2 O 3 , Fe 2 O 3 T, and K 2 O contents increase and exhibit significant enrichments in the fault gouge; (2) Na 2 O and P 2 O 5 contents decrease gradually and exhibit significant depletions in the fault gouge; (3) quartz, feldspar, and carbonate mineral contents decline; and (4) clay mineral contents increase dramatically. Isocon analysis indicates that mass losses in the Shaba fault zone occur with losses in the damage zone < the fault gouge. The mechanisms of material loss and transformation in the fault zone are complex, and mechanical fracturing, dehydration reactions, and fluid–rock interactions are important factors in changing and controlling the material compositions and fault zone evolution, with material losses and transformation especially important within the fault core. Moreover, due to the damage zone having a higher permeability than the fault core, it was conducive to hydrothermal upwelling, fluid–rock interactions, and fracture healing. • Mineral assemblages and major element contents exhibit variations across the fault zone. • Enrichment/Depletion of elements and mass balance transfers are recorded in the fault zone. • Fault rocks exhibit numerous features for fluid-rock interactions. • Material loss occurred in the fault zone varies with time and space. [ABSTRACT FROM AUTHOR]

Natural beds at different stratigraphic levels display different degrees of folding. Variable propagation-to-slip ratios in the trishear models are more appropriate for the reproduction of natural folds. An improved trishear model with self-adaptive propagation-to-slip ratio, called TMSP, is proposed in this paper. Transverse isotropy elastic constitutive relationships are utilized to formulate the mechanic behaviors of the overlying sediments. The von Mises failure criterion is introduced to determine whether the material points are destroyed in TMSP. Compared with previous trishear models, TMSP has the advantage that propagation-to-slip ratio can be updated with the proceeding of fault rupture propagation and is not required to be predetermined. A natural fault-propagation fold, Tejerina fold, can be reproduced well by TMSP. Finally, a series of simulations are conducted to investigate the influences of three key parameters of TMSP, including apical angle, yield stress, and elastic modulus, on faulting process and folding growth. • We proposed an improved trishear model with self-adaptive propagation-to-slip ratio, called TMSP. • The material failure criterion is introduced to determine whether overlaying sediments are destroyed. • Propagation-to-slip ratios are variable in the processes of faulting. • Apical angle, yield stress, and elastic modulus have influences on propagation-to-slip ratios in TMSP. [ABSTRACT FROM AUTHOR]

Closely spaced, sub-parallel fracture networks contained within localized tabular zones that are fracture corridors may compromise top seal integrity and form pathways for vertical fluid flow between reservoirs at different stratigraphic levels. This geometry is exemplified by fracture corridors found in outcrops of the Jurassic Entrada Formation in Utah (USA). These fracture corridors exhibit discolored (bleached) zones, interpreted as evidence of ancient fracture-enhanced circulation of reducing fluids within an exhumed siliciclastic reservoir-cap rock succession. Extensive structural and stratigraphic mapping and logging provided fracture data for analysis with respect to their occurrence and relationships to larger faults and folds. Three types of fracture corridors, representing end-members of a continuum of possibly interrelated structures were identified: 1) fault damage zone including segment relays; 2) fault-tip process zone; and 3) fold-related crestal-zone fracture corridors. The three types exhibit intrinsic orientations and patterns, which in sum define a local- to regional network of inferred vertical and lateral, high-permeability conduits. The results from our analysis may provide improved basis for the evaluation of trap integrity and flow paths across the reservoir-cap rock interface, applicable to both CO2 storage operations and the hydrocarbon industry. [ABSTRACT FROM AUTHOR]

Bimaterial faults can be found in nonhomogeneous rock media between dissimilar rock layers. An analytical model was extended for the right lateral strike-slip fault between two dissimilar rock layers to predict the activated tensile, right and left lateral shear zones around the fault tip. The effect of elastic modulus contrast (E 2 /E 1 , the ratio of the Young's moduli of the lower to the upper layer), rupture velocity (c), precompression stress ratio (-ϭ xx o/-ϭ yy o), residual to peak strength ratio (τ r /r p) and fracture energy (G) were considered. Results show that under an intermediate rupture speed of (0.5 of the Rayleigh wave speed): the extension of activated tensile and shear zones depends strongly on the ratio of E 2 /E 1 and it increases with increasing this ratio. The preferred direction of the new branches is little affected by the elastic modulus contrast. Increasing precompression stress ratio encourages right and left lateral shear branching on the compressional side of the fault and decreases the size of activated tensile and shear zones. Under an extreme rupture speed of (0.98 of the Rayleigh wave speed): the mechanism of branching differs, for E 2 /E 1 < 1, the three activated zones are located in the lower layer and there is only a possibility for unilateral branching on the extensional side of the fault. For E 2 /E 1 ≥ 1, right and left lateral shear zones extend to the upper layer (compressional side), which means the possibility of bilateral shear branching on the two sides of the fault. For all values of E 2 /E 1 , c, -ϭ xx o/-ϭ yy o, tensile branches can only be formed in the lower layer on the extensional side of the fault. • 2D dynamic analytical model for bimaterial faults was developed. • Activated tensile and shear zones with preferred directions of new branches. • Dependency of off-fault cracks on rupture speed, E 2 /E 1 and -ϭ xx o/-ϭ yy o. • Tensile branches are only unilateral on the extensional side of the fault. • Shear branches can grow on the compressional side of the fault. [ABSTRACT FROM AUTHOR]

This study focuses on the complexity of fault-fold system developed in the Hasishan section of the Haiyuan fault zone in the northeastern margin of the Tibetan Plateau, along which a 237-km-long coseismic surface rupture zone formed during the 1920 M 8.5 Haiyuan earthquake. Interpretations of satellite imageries combined with field investigations and structural analyses of fault geometry and kinematics reveal that the fault-fold system developed in the Hasishan section are characterized by four main elements (i) the left-lateral strike-slip Haiyuan primary fault F1; (ii) a discrete branch fault F2 with an identical sense of movement; (iii) a tension structure, characterized by a NE-trending normal fault F3 developed in piedmont terrace deposits; and (iv) a compression structure P, characterized by a NW-trending en echelon folds within the Cretaceous–Cenozoic sedimentary sequences including the most recent late Pleistocene–Holocene unconsolidated and/or weakly-consolidated deposits. Previous geologic and geophysical observations show that the slip rate along the whole Haiyuan fault zone is non-uniform, indicating an eastward decrease with a gradient of ∼2.65 mm/100 km. In contrast, the slip rate for the primary fault F1 in the Hasishan section was reported to be 3.2 ± 0.2 mm/yr, ∼3 mm/yr less than the result of 6.2 mm/yr inferred from the slip rate gradient. The finding in this study demonstrates that the fault-fold system consisting of faults F2, F3, and fold P has partitioned ∼3 mm/yr slip within the Hasishan section of the Haiyuan fault zone. This study documents that the slip rates at different sections along the primary strike-slip faults are partially dependent on local geometric structure and fault complexity, and the recognition and analyses of related fault-fold system within large strike-slip fault zone are important for better characterizing the total slip rate budget across primary strike-slip faults, which will contribute to improve seismic hazard assessments. • The fault-fold system consists of three faults F1–F3 and en echelon folds P. • The fault-fold system accommodates ∼3 mm/yr slip of the Haiyuan primary fault. • Slip rates of strike-slip fault are partially dependent on local fault complexity. [ABSTRACT FROM AUTHOR]

Faults are characterized by a complex internal architecture. In carbonates, the geometry, attitude, and distribution of fault-related fractures and subsidiary faults can largely affect the petrophysical properties and hydraulic behavior of the fault zone. This work investigates the footwall damage zone of a seismic-scale normal fault (throw ∼ 300 m) from a structural, petrophysical and seismic point of view. The studied Venere Fault (VF) bounds the intra-mountain Fucino Basin (central Italy) and crosscuts Lower Cretaceous platform carbonates. A significant portion of the footwall VF damage zone (VF-DZ) is well exposed in the 400 × 200 m Santilli Quarry. There, we assess the amount of outcrop-scale fracture porosity and permeability by in-situ fracture analyses and permeability measurements. The results show a composite power-law decay of fracture intensity away from the main slip surfaces, strongly influenced by subsidiary faults. An outcrop-based, digital 2D model of the VF-DZ is constructed and populated with acoustic properties (Vp, Vs and density) derived from both the matrix and fracture porosities. This model is enlarged five times and used for seismic modelling to investigate the seismic signature of the VF-DZ under different but realistic geological and geophysical conditions. Seismic modelling suggests that within the modelled damage zone and for wave frequencies of 20–40 Hz, seismic impedance contrasts associated with subsidiary faults may be imaged, depending on the degree of fracture porosity, fracture aperture, and the illumination angle (a measure of the maximum dip that can be imaged), the last two parameters being controlled by overburden depth. These results have implications for the seismic interpretation and characterization of fault zones in carbonates, and hence for the evaluation of fluid migration through these structures. • Detailed field measurements of fault damage zone in tight carbonates. • Power-law decay of fracture intensity away from main slip surfaces. • Subsidiary faults largely affect the petrophysical properties of the damage zone. • Subsurface imaging of the damage zone is investigated via seismic modelling. • Fault seismic response depends on geological and geophysical parameters. [ABSTRACT FROM AUTHOR]

Abstract: The Moresby Seamount detachment (MSD) in the Woodlark Basin (offshore Papua New Guinea) is a large active low-angle detachment excellently exposed at the seafloor, and cutting through mafic metamorphic rocks. Hydrothermal infiltration of quartz followed by that of calcite occurred during cataclastic deformation. Subsequent deformation of these a priori softer minerals leads to mylonite formation in the MSD. This study aims at a better understanding of the deformation mechanism switch from cataclastic to plastic flow. Deformation fabrics of the fault rocks were analyzed by light-optical microscopy. Rheologically critical phases were mapped to determine distributions and area proportions, and EBSD was used to measure crystallographic preferred orientation (CPO). Strong calcite CPOs indicate dominant dislocation creep. Quartz CPOs, however, are weak and more difficult to interpret, suggesting at least some strain accommodation by diffusion creep mechanisms. When quartz aggregates are intermixed with the polymineralic mylonite matrix diffusion creep grain boundary sliding may be dominant. The syntectonic conversion from mafic cataclasites to more siliceous and carbonaceous mylonites induced by hydrothermal processes is a critical weakening mechanism enabling the MSD to at least intermittently plastic flow at low shear stresses. This is probably a crucial process for the operation of low-angle detachments in hydrated and dominantly mafic crust. [Copyright &y& Elsevier]

Stepovers in strike-slip fault systems play important roles in controlling the propagation of earthquake ruptures, depending partially on the competition between the stepover width and the coseismic slip gradient approaching the step. The 1920 M w 7.9 Haiyuan earthquake is the most recent major earthquake that has occurred along the Haiyuan fault. The earthquake rupture broke through multiple stepovers along the fault and finally ended at the 4 km-wide Jingtai pull-apart basin (releasing stepover) at the western end. To understand the process of this termination of the Haiyuan earthquake rupture, we conducted detailed mapping of the surface rupture geometry, coseismic slip measurement and slip gradient calculation in the vicinity of the endpoint based on the 0.2 m-resolution SfM-derived DEM along the 20 km section east of the Jingtai pull-apart basin. Combining coseismic slip measurements from this study and published slip data from fieldwork in the 1980s, we calculated slip gradients of 84–160 cm/km as the rupture approached the Jingtai releasing stepover. These values are high for the 4 km-wide Jingtai releasing stepover compared to those from a dataset of worldwide historical earthquakes compiled by Elliott et al. (2009). The high values imply that the rupture could have a relatively high likelihood of breaching through the Jingtai releasing stepover. Alternatively, the slip gradient may be overestimated. Detailed mapping and field investigation, however, show that the rupture may extend ~1.86 km farther west of the end location indicated in previous studies. We acquired the new slip gradient with lower value of 50–82 cm/km, which is still considerably high compared to dataset of Elliott et al. (2009). We speculate that the slip gradient could be larger variation along the stepover boundary to stop fault slip in different geologic setting. Another factor control slip termination is related to fault properties. Creep along the southern boundary fault of the Jingtai stepover, a velocity-strengthening region, may also have played a role in stopping the rupture at this location. Our observations further indicate that the increasing LOS velocity of InSAR data within the Jingtai stepover is probably related to fault behavior rather than a nontectonic signal of subsidence. [ABSTRACT FROM AUTHOR]

Abstract: A 3-D method to evaluate the reactivation potential of fault planes is proposed. The method can be applied to cohesive or noncohesive faults whatever their orientation and without any conditions on the regional stress field. It allows computation of the effective stress ratio required to reactivate any fault plane and to determine whether the plane is favorably oriented, unfavorably oriented or severely misoriented with respect to the ambient stress field. The method also includes a graphical sorting tool that involves plotting poles of fault planes on stereoplots for which the boundaries separating the three domains corresponding to favorable orientations, unfavorable orientations and severe misorientations cases are shown. The delineation of these domains is based on the value of the ratio that depends on the orientation of the fault plane with respect to the principal stress axis orientations, the stress shape ratio (Φ = (σ 2 − σ 3)/(σ 1 − σ 3)), the coefficient of static friction μ s of the fault, and the fault cohesion C 0. The method is applied on 145 focal mechanisms of the 2011 March 11th Tohoku-Oki (Japan) earthquake sequence. This application delineates, along or in the vicinity of the Pacific-Okhotsk plate interface, three types of domains characterized by favorable orientations, unfavorable orientations or severe misorientations of mainshock/aftershock fault planes. Aftershock focal mechanisms that plot in the ‘severe misorientation’ domains are interpreted to have occurred because of pore fluid pressures exceeding the regional minimum principal stress at those locations. The distribution of these ‘severe misorientation’ domains partly overlaps the asperities or the low-velocity anomalies mapped on the plate interface off NE Japan. The proposed 3-D fault reactivation analysis appears complementary to geophysical investigations. [Copyright &y& Elsevier]

Abstract: The 23 October 2011 M w 7.1 (USGS, 2011a) Van earthquake was generated by the rupture of a main thrust fault plane striking N60°E and dipping 55°NW with a very small left-lateral offset in the western part of the Bardakçı-Saray thrust fault zone located in the north of the Bitlis-Zagros Suture Zone. The incompatibly between the subsurface seismologic data and surface deformation data indicates that the fault that generated the earthquake could be blind. The main thrust fault plane developed between the northern (Erciş) and southern (Van) continental blocks and indicates that the region is under the influence of a constant NW–SE pure compressional tectonic regime. Along with the primary thrust rupture, a secondary back thrust fault, a left lateral fault with NE strike and a right lateral fault with NW strike occurred in the contraction province. Additionally, the tensional ground cracks that developed on the hanging wall block of the ruptured thrust fault plane due to intra-continental collision are either transtensional that mainly fits to left lateral strike-slip faulting, or extensional deformations on the elevated block. [Copyright &y& Elsevier]

Abstract: As a confined static grain mass is increasingly stressed, elastic strain energy accumulates in the grains; when local stress somewhere exceeds grain strength, grain breakage radiates this stored energy to the surrounding grains as a brief high-intensity pulse of pressure energy. Local grain stresses in a stressed static fault-zone follow a known probability density distribution in which the maximum local grain stress resulting from a given applied stress increases with fault-plane area. Tectonic stress buildup on a fault increases the probability of local stresses sufficient to break grains. Brittle failure of a grain releases strain energy that can trigger cascading failure of a large fault at relatively low applied stress, culminating in fault rupture, if the fault strength distribution is sufficiently homogeneous. Such ruptures can occur at lower driving stresses in larger faults. This process correctly explains the magnitude of the failure stress of the peninsula segment of the San Andreas fault, so may significantly affect fault rupture strength. Grain fragmentation also plays a significant role in dynamic fault friction; in dense comminuting granular flows, grain fragmentation causes continuous recycling of elastic-strain energy. During cataclastic fault slip, widespread grain comminution thus generates and maintains an intense high-frequency elastic energy field that causes dynamic weakness. This process is sufficiently powerful to explain the magnitude of the dynamic slip resistance of the San Andreas fault. [Copyright &y& Elsevier]

Abstract: We use scaled experimental (analog) models to investigate how the properties of a population of pre-existing normal faults influence fault development during a subsequent phase of extension. In the models, a homogeneous layer of wet clay undergoes two phases of extension whose extension directions differ by 45°. To vary the properties of the first-phase fault population, we vary the magnitude of the first-phase extension. As the magnitude of the first-phase extension increases, the number, average and maximum length, and average and maximum displacement of the first-phase normal faults increase. For a poorly developed first-phase fault population, new normal faults (which strike perpendicular to the second-phase extension direction) form during the second phase. For a well developed first-phase fault population, many first-phase normal faults are reactivated as oblique-slip faults during the second phase. New normal faults also form; these second-phase normal faults are shorter and have displacement maxima adjacent to the reactivated first-phase faults. They are less likely to cut the pre-existing first-phase faults than second-phase normal faults that form in models with a less developed first-phase fault population. In all models, pre-existing faults serve as nucleation sites for new faults. In models with a well developed first-phase fault population, the pre-existing faults also act as obstacles to the propagation of the second-phase normal faults. Fault geometries in the models vary considerably; parallel, zig-zag, or intersecting fault geometries develop depending on whether the first-phase fault population is poorly, moderately, or well developed, respectively. [Copyright &y& Elsevier]

Abstract: We present a case study of Riedel shear structures related to the co-seismic surface ruptures produced during the 2001 Mw 7.8 Kunlun earthquake in northern Tibet, which are related to strike-slip movement along the Kunlun Fault. Field investigations and interpretations of high-resolution remote sensing images show that the 2001 co-seismic surface ruptures, striking WNW–ESE, are mainly characterized by Riedel shear structures, including T fractures, and R, Y, and P shears. A left-lateral shear sense is indicated. To assess the Riedel shear fabrics quantitatively, we measured the azimuth of 19,455 co-seismic surface rupture strands, the width of rupture zones at 474 profiles, and 336 fold axes of mole track structures, using 1-m-resolution IKONOS and 0.61-m-resolution QuickBird images acquired soon after the earthquake. The analytical results show that i) the co-seismic surface ruptures are generally concentrated in up to five subparallel sub-rupture zones, with individual sub-rupture zones varying in width from 3 to 350 m (generally <100 m); ii) the total width across all sub-rupture zones is generally <500 m (though locally >1–2 km); iii) T fractures are mainly developed within the alluvial deposits at counterclockwise angles of 15–40° relative to the general trend of the rupture zone; iv) P shears are also mainly found in the alluvial deposits, but at counterclockwise to clockwise angles of 5–10° relative to the general trend; and v) Y and R shears are developed within both the alluvial deposits and basement rocks, mainly representing the reactivation of pre-existing fault traces. The results demonstrate that the co-seismic Riedel shear structures are primarily controlled by the local geology during surface rupture formation, consistent with the idea that Riedel shear structures are common fault patterns within strike-slip shear zones and that their development is related to the early stages of fault evolution. [Copyright &y& Elsevier]

Abstract: We conducted a variety of measurements and analyses at Chihshang Active Fault Observatory in eastern Taiwan, including surface-rupture mapping, three shallow borehole core analysis and kinematic analysis of geodetic measurements. We found that the Chihshang fault exhibits a three-branch fault system in the Chinyuan alluvial fan, which is composed of at least 100 m thick gravel deposits. Outside of the Chinyuan River, the Chihshang fault shows a single fault system with a sharp lithological contact. Combining the leveling results and trench excavation, we interpret that the three fault branches are coupled with a 50−60-m-wide pop-up structure in the hangingwall. Based on the ratio between vertical and horizontal displacements, we obtained dip angles of 38°, 62° and 16° for two west-vergent thrusts and an east-vergent backthrust, respectively. This pop-up was estimated to develop at the uppermost 30–40 m unconsolidated gravels during the last few thousand years above the main fault with a dip angle of 42°. By compiling the available ages data, we obtained an uplift rate of 2.3 cm/yr of the Chihshang fault and an alluvial sedimentation rate of 1.1 cm/yr during the past thousands years. Consequently, the individual uplift rate for each fault branch at the Chihshang Observatory was slightly less than the deposition rate of the Chinyuan River. No geomorphic fault scarp can thus be observed in the three-branch fault system area. [Copyright &y& Elsevier]

Abstract: We mapped fractures at Sant Corneli anticline, an E–W trending non-cylindrical, fault-related anticline in the Spanish Pyrenees. Fracture mapping in syn-tectonic strata directly links fold-related fracturing to fold evolution because the relationship of fractures to syn-tectonic strata can constrain structural timing. Rather than using absolute fracture orientation as a primary means of grouping fracture sets, we used relative fracture timing, mineral fill, fracture size (length and height), and orientation with respect to bed strike to delineate five bed-orthogonal fracture sets that we interpret to be associated with folding and faulting events. We observe several early sets of joints with calcite fill (J1, J2, J3) many of which are interpreted to be related to fold axis perpendicular normal faulting of the anticline. Two late-stage joint sets with associated iron oxide mineralization in the surrounding wall rock (J4, J5) are oblique to bedding, maintain orientation with respect to bed strike, and are interpreted to result from flexure of the anticline. We infer the timing of J4/J5 jointing relative to syn-tectonic events. This research demonstrates how fracture studies that integrate syn-tectonic strata and distinguish fracture sets on the basis of multiple characteristics can better constrain the timing of fracturing relative to regional deformation events. [ABSTRACT FROM AUTHOR]

Abstract: Active tectonics within the northern Basin and Range province provide a natural laboratory for the study of normal fault growth, linkage, and interaction. Here, we present new geologic mapping and morphologic fault-scarp modeling within the Centennial Valley, Montana to characterize Pleistocene – Holocene ruptures of the young and active Lima Reservoir fault. Geologic relationships and rupture ages indicate Middle Pleistocene activity on the Henry Gulch (>50 ka and 23–10 ka), Trail Creek (>43 ka and ∼13 ka), and reservoir (∼23 ka) segments. Offset Quaternary deposits also record Holocene rupture of the reservoir segment (∼8 ka), but unfaulted modern streams show that no segments of the Lima Reservoir fault have experienced a large earthquake in at least several millennia. The clustered pattern of rupture ages on the Lima Reservoir fault segments suggests a seismogenic linkage though segment length and spacing make a physical connection at depth unlikely. Trail Creek and reservoir segment slip rates were non-steady and appear to be increasing. The fault helps accommodate differential horizontal surface velocity measured by GPS geodesy across the boundary between the northern Basin and Range province and the Snake River Plain. [ABSTRACT FROM AUTHOR]

Abstract: Increases in the dimensions of strike-slip faults including fault length, thickness of fault rock and the surrounding damage zone collectively provide quantitative definition of fault growth and are commonly measured in terms of the maximum fault slip. The field observations indicate that a common mechanism for fault growth in the brittle upper crust is fault lengthening by linkage and coalescence of neighboring fault segments or strands, and fault rock-zone widening into highly fractured inner damage zone via cataclastic deformation. The most important underlying mechanical reason in both cases is prior weakening of the rocks surrounding a fault''s core and between neighboring fault segments by faulting-related fractures. In this paper, using field observations together with effective medium models, we analyze the reduction in the effective elastic properties of rock in terms of density of the fault-related brittle fractures and fracture intersection angles controlled primarily by the splay angles. Fracture densities or equivalent fracture spacing values corresponding to the vanishing Young''s, shear, and quasi-pure shear moduli were obtained by extrapolation from the calculated range of these parameters. The fracture densities or the equivalent spacing values obtained using this method compare well with the field data measured along scan lines across the faults in the study area. These findings should be helpful for a better understanding of the fracture density/spacing distribution around faults and the transition from discrete fracturing to cataclastic deformation associated with fault growth and the related instabilities. [Copyright &y& Elsevier]

Abstract: Over the last years, field-based studies have shown that fault surfaces can exhibit a considerable self-affine topography. It is reasonable to assume that similar undulations are also present in fault interpretations from 3D reflection seismic data, however both the interpretation uncertainty and geophysical resolution limits hinder their analysis. This study analyses a set of small-scale, non-reactivated faults in the Upper Cretaceous Chalk Group (Upper Ommelanden Formation) of the NW-part of the Groningen Block, the Netherlands, in a high quality Pre Stack Depth Migrated 3D seismic data set. The studied faults are fully contained inside the Chalk Group, in an area located between the major tectonic-bounding faults of the NW Groningen Block. Over 200 faults, with offsets in the order of 30–50m, were interpreted across an area of ca. 150km2, showing a clear preferential orientation for strike, dip and dip-direction. Detailed interpretations and 3D fault plane analyses show undulations on the fault plane. We show that these undulations are not an interpretation or gridding artefact, and interpret these to indicate direction of fault slip. These results were used to calculate a paleostress tensor, using all faults to calculate a single stress tensor for the entire study area by Numerical Dynamic Analysis. Based on the orientation, position and a thickness analysis, it is interpreted that these faults formed due to the tectonic reactivation of salt structures in the Latest Cretaceous. The calculated paleostress state shows a general NW–SE-extension, with a vertical maximum principle stress, and a stress ratio of about 0.3, indicating that the studied faults are not the result of dewatering. This interpretation agrees both with a nearby salt-tectonic reconstruction, as well as field-based paleostress results from the UK, Belgium and France. A first look at other surveys from the Dutch sector indicates that similar faults are present in other areas, with different orientations. We propose that a dedicated analysis of these faults across on- and offshore Europe would allow extending the stress map of the Late Cretaceous into areas where the Chalk is not outcropping. [Copyright &y& Elsevier]

Abstract: Two-dimensional analysis of folded surfaces oblique to the mechanical layering can shed light on the kinematic mechanisms that operated during the development of folds. A new version of the program ‘FoldModeler’, developed in the Mathematica™ environment, is used to obtain the deformed configuration of an initial pattern of oblique surfaces deformed by any combination of the most common kinematic folding mechanisms: flexural flow, tangential longitudinal strain, with or without area change and heterogeneous simple shear. The layer can also undergo any form of homogeneous strain at any moment of the folding process. The outputs of the program provide complete information about the strain distribution in the folded layer that includes graphs of the angle between the oblique surfaces as a function of the inclination of the layering through the fold. These graphs can be very useful to discriminate between the mechanisms that operate in the development of natural folds, and they have been obtained and discussed for the most common combinations of strain patterns. The program is applied to obtain theoretical folds that give a good fit of some natural examples of folded oblique surfaces. [Copyright &y& Elsevier]