Fault Nucleation Research Articles

(Invited) Influence of Wafering, CMP and Subsurface Damage on Epitaxial Defects and Surface Quality in SiC

The Silicon Carbide power device industry continues to experience very robust increases and adoption in many long-term growth markets like renewables, cloud and electric vehicles. The continuous year over year increases in demand has led to widespread raw material shortages of substrates and epi wafers. This has resulted in incumbent manufacturers putting in more capacity and the emergence of many new SiC substrate manufacturers. With many supply sources, there is also a wide variety of quality in the material. In some of our previous works, we have looked at some of the issues arising from immature crystal quality. However, often overlooked as a source of defects and surface quality, is the damage left over from wafering, grinding and CMP. In this work, we observe the effects of sub-surface damage and how it affects epitaxial layer quality. We also look at ways to minimize this effect both at the CMP stage and at the epi growth stage. Defect metrology tools for Silicon Carbide wafers have seen great improvements in capability and throughput. As wafers are measured both pre-epi and post-epi, a wide category of defects are identified and classified. Full wafer scans for substrates yield several defect categories like micropipes, bumps/particles as well as shallow and deep scratches. Recent tools with photoluminescence (PL) are also able to detect crystal defects like grain boundaries and bar stacking faults among others. All of these defects have an effect on the epitaxial layers either nucleating or transforming into other defects. However, none of the detection methods are able to detect residual damage left over from the wafering/grind process. This damage can be present just below the surface even if the surface is extremely smooth and scratch free. We detect the presence of these damage layers after epitaxial growth. Depending on the amount of damage present a very high density of stacking faults and triangular defects (and partials) are seen in the epitaxial layers. To remove the damage layers, a higher amount of CMP removal was used. As the damaged layer is reduced, the nucleation of the stacking faults and triangular defects also reduce. In some wafers, we observe some residual damage layers present near the wafer edges and flats where the CMP process is not uniform. Moreover, even when the nucleation of the defects are suppressed, the resulting epi is much rougher than epi grown on a substrate without any sub-surface damage. Wafers that displayed sub-surface damage were re-polished back to the substrate using a known good process. These wafers did not show the nucleation of defects. The suppression of the nucleation of defects was also attempted by enhanced in-situ etching before epi growth. Though this resulted in suppression of nucleated defects, by removal of the damage layer, it also resulted in much rougher epitaxial layers. The characterization results and processes from these experiments will be presented. Additional methods to detect these damage layers will be discussed.

Seismic evidence for repeated vertical fluid flow through polygonally faulted strata in the Canterbury Basin, New Zealand

Abstract Seismic imaging techniques are widely used to determine fluid migration pathways in the subsurface. Understanding the timing and distribution of focused fluid migration pathways is important for assessing reservoir seal quality and hydrocarbon exploration risk, as well as seafloor biogeochemical processes in the case of fluid venting from the seafloor. The frontier Canterbury Basin southeast of New Zealand is well studied, but questions remain regarding the dynamic movement of fluids through the basin. Using 3D seismic data combined with high-resolution boomer seismic data, we characterise focused fluid migration and sediment remobilisation events in the Canterbury Basin. We show how polygonal fault formation and nucleation is affected in the vicinity of vertically ascending fluids and sediments. A cylindrical region devoid of polygonal faulting around the feeder system of a buried sediment volcano points to the possible existence of an axisymmetrical stress field induced by upward migrating fluids and sediments. Sediment remobilisation and intrusions of permeable stingers (i.e. sand injectites) could enable dewatering of their host strata and thereby prevent polygonal faults from nucleating. Amplitude variations with angle (AVA) and a detailed analysis of two tiers of polygonal faults reveal three phases of fluid migration and sediment remobilisation and intrusion through, and into, low permeability sediments. Our integrated conceptual model provides insights into vertical fluid migration mechanisms and the implications for polygonal fault formation.

Normal fault growth influenced by basement fabrics: The importance of preferential nucleation from pre‐existing structures

AbstractReactivation of pre‐existing intra‐basement structures can influence the evolution of rift basins, yet the detailed kinematic relationship between these structures and overlying rift‐related faults remains poorly understood. Understanding the kinematic as well as geometric relationship between intra‐basement structures and rift‐related fault networks is important, with the extension direction in many rifted provinces typically thought to lie normal to fault strike. We here investigate this problem using a borehole‐constrained, 3D seismic reflection dataset from the Taranaki Basin, offshore New Zealand. Excellent imaging of intra‐basement structures and a relatively weakly deformed, stratigraphically simple sedimentary cover allow us to: (a) identify a range of interaction styles between intra‐basement structures and overlying, Plio‐Pleistocene rift‐related normal faults; and (b) examine the cover fault kinematics associated with each interaction style. Some of the normal faults parallel and are physically connected to intra‐basement reflections, which are interpreted as mylonitic reverse faults formed during Mesozoic subduction and basement terrane accretion. These geometric relationships indicate pre‐existing intra‐basement structures locally controlled the position and attitude of Plio‐Pleistocene rift‐related normal faults. However, through detailed 3D kinematic analysis of selected normal faults, we show that: (a) normal faults only nucleated above intra‐basement structures that experienced late Miocene compressional reactivation, (b) despite playing an important role during subsequent rifting, intra‐basement structures have not been significantly extensionally reactivated, and (c) preferential nucleation and propagation of normal faults within late Miocene reverse faults and folds appears to be the key genetic relationship between contractionally reactivated intra‐basement structures and rift‐related normal faults. Our analysis shows that km‐scale, intra‐basement structures can control the nucleation and development of newly formed, rift‐related normal faults, most likely due to a local perturbation of the regional stress field. Because of this, simply inverting fault strike for causal extension direction may be incorrect, especially in provinces where pre‐existing, intra‐basement structures occur. We also show that a detailed kinematic analysis is key to deciphering the temporal as well as simply the spatial or geometric relationship between structures developed at multiple structural levels.

Outcrop evidence of polygonal faulting in Ypresian marine clays (Southern North Sea Basin) leads to a new synthesis

Layer-bound, polygonal fault systems (PFS) in marine, fine-grained sediments have been documented with seismic data over large areas in over 100 basins. The questions of how, when and how fast these muds broke without tectonic stress were still open. This study covered the missing observational length scales from 100 m to μms with a continuous range of outcrop observations and related lab results from two quarries in Ypresian marine unlithified swelling clays in the Belgian part of the North Sea Basin.The observed coeval dilatant hydrofractures and folded faults imply highly unstable and transitional tensile fracturing behaviour under shallow overpressure. The random fault plane orientations, radial striation vectors and conal micro-faults exclude far-field stress and slope instability as the driving force, and show that gravity was the major principal stress. A contributing driving force was overpressure in the oxidising fluviatile and lagoonal sands below, which further weakened its clayey caprock and supplied at least part of the water evacuating through it. This fracture water flow eroded and oxidised bits of wall clay and tell-tale microfossil mixtures along the way up. The sharp contrast of black gouge against barely stained wall clays implies that such dilatant fracture and fault growth was almost instantaneous. This resulted in winner-takes-all shear localisation along often razor-thin, slickensided fault mirrors. Faults that locked-up by different degrees of compaction folding and local rotational flow imply: 1) that this clayey mud was still an underconsolidated viscoplastic gel just prior to plastic faulting, capable of syneresis and therefore capable of supplying isotropic contractional stress; 2) not episodic growth but episodic nucleation of additional fractures and faults and punctuated dewatering along them, until the clayey gel matrix and the overpressure in it were fully deflated.

Atomic-scale mechanisms of single crystal plasticity in CoCrFeMnNi high-entropy alloys

The behavior of CoCrFeMnNi high-entropy alloys under mechanical loading was studied within the framework of the molecular dynamics method. The mechanisms of local structural transformations of the crystal lattice responsible for the onset of plasticity at the tension of CoCrFeMnNi single crystals were determined. A crystallographic analysis of the structure and identification of the defects formed was carried out. Irrespectively on stoichiometric composition of samples the plasticity is realized by the nucleation, growth and intersection of intrinsic stacking faults. Rearrangement of fcc lattice to bcc structure in the regions where the local atom fraction of Fe, Mn and Ni is above average was found to be the mechanism of stacking fault formation. The effect of the stoichiometric composition and crystallographic orientation of single-crystal CoCrFeMnNi samples upon the structural rearrangements at the nucleation and development of plastic deformation under tension was studied.

Basement fabric controls rift nucleation and postrift basin inversion in the continental margin of NE Brazil

In passive continental margins, the brittle reactivation of shear zones and their role in the deformation and deposition of sedimentary basins are still a matter of debate. In this research, we investigated the role of the brittle reactivation of Precambrian shear zones in the nucleation of rift and postrift faults in the onshore portion of the Sergipe-Alagoas and Pernambuco basins in northeastern Brazil. We combine and interpret a dataset of aeromagnetic and topographic data, associated with reflection seismic and borehole data, to analyze the evolution of a portion of the Atlantic continental margin of Brazil. Our results indicate that in the crystalline basement, the magnetic lineaments are correlated with ductile structures as shear zones, and the continuity of these lineaments in the Sergipe-Alagoas and Pernambuco basins is interpreted as the shear zones below the sedimentary cover of these basins. We document the following phases of the brittle reactivation of basement shear zones: (1) the opening of the South Atlantic Ocean in the Early Cretaceous under an extensional stress regime and (2) tectonic inversion induced by the Mid-Atlantic Ridge push and the Andean Cordillera rise in the Neogene-Quaternary under a predominantly strike-slip stress regime. During the rift phase, the brittle reactivation of the shear zones controlled the locations and architectures of the rifts. These structures acted as zones of weakness and were reactivated as normal faults. The brittle reactivation of shear zones was still active during the postrift phase and was responsible for the development of compressional structures. The reverse faulting and related folding pattern indicate tectonic inversion in the Late Cretaceous-Cenozoic. The structures formed during the postrift phase under a strike-slip regime are consistent with the present-day stress field, indicating that tectonic inversion is an active phase of the Brazilian margin.

On Shear Testing of Single Crystal Ni-Base Superalloys

Shear testing can contribute to a better understanding of the plastic deformation of Ni-base superalloy single crystals. In the present study, shear testing is discussed with special emphasis placed on its strengths and weaknesses. Key mechanical and microstructural results which were obtained for the high-temperature (T ≈ 1000 °C) and low-stress (τ ≈ 200 MPa) creep regime are briefly reviewed. New 3D stereo STEM images of dislocation substructures which form during shear creep deformation in this regime are presented. It is then shown which new aspects need to be considered when performing double shear creep testing at lower temperatures (T < 800 °C) and higher stresses (τ > 600 MPa). In this creep regime, the macroscopic crystallographic [11−2](111) shear system deforms significantly faster than the [01−1](111) system. This represents direct mechanical evidence for a new planar fault nucleation scenario, which was recently suggested (Wu et al. in Acta Mater 144:642–655, 2018). The double shear creep specimen geometry inspired a micro-mechanical in-situ shear test specimen. Moreover, the in-situ SEM shear specimen can be FIB micro-machined from prior dendritic and interdendritic regions. Dendritic regions, which have a lower γ′ volume fraction, show a lower critical resolved shear stress.

Architecture, deformation style and petrophysical properties of growth fault systems: the Late Triassic deltaic succession of southern Edgeøya (East Svalbard)

AbstractThe Late Triassic outcrops on southern Edgeøya, East Svalbard, allow a multiscale study of syn‐sedimentary listric growth faults located in the prodelta region of a regional prograding system. At least three hierarchical orders of growth faults have been recognized, each showing different deformation mechanisms, styles and stratigraphic locations of the associated detachment interval. The faults, characterized by mutually influencing deformation envelopes over space‐time, generally show SW‐ to SE‐dipping directions, indicating a counter‐regional trend with respect to the inferred W‐NW directed progradation of the associated delta system. The down‐dip movement is accommodated by polyphase deformation, with the different fault architectural elements recording a time‐dependent transition from fluidal‐hydroplastic to ductile‐brittle deformation, which is also conceptually scale‐dependent, from the smaller‐ (3rd order) to the larger‐scale (1st order) end‐member faults respectively. A shift from distributed strain to strain localization towards the fault cores is observed at the meso to microscale (&lt;1 mm), and in the variation in petrophysical parameters of the litho‐structural facies across and along the fault envelope, with bulk porosity, density, pore size and microcrack intensity varying accordingly to deformation and reworking intensity of inherited structural fabrics. The second‐ and third‐order listric fault nucleation points appear to be located above blind fault tip‐related monoclines involving cemented organic shales. Close to planar, through‐going, first‐order faults cut across this boundary, eventually connecting with other favourable lower‐hierarchy fault to create seismic‐scale fault zones similar to those imaged in the nearby offshore areas. The inferred large‐scale driving mechanisms for the first‐order faults are related to the combined effect of tectonic reactivation of deeper Palaeozoic structures in a far field stress regime due to the Uralide orogeny, and differential compaction associated with increased sand sedimentary input in a fine‐grained, water‐saturated, low‐accommodation, prodeltaic depositional environment. In synergy to this large‐scale picture, small‐scale causative factors favouring second‐ and third‐order faulting seem to be related to mechanical‐rheological instabilities related to localized shallow diagenesis and liquidization fronts.

Linking sintering stresses to nano modification in the microstructure of BaLa4Ti4O15 by transmission electron microscopy

High quality factor and a temperature stable resonant frequency make BaLa4Ti4O15 (BLT) ceramics attractive materials for microwave applications. Aiming to exploit the effects of external stresses on the development of textured and anisotropic microstructures to optimise MW properties, the influence of applied external pressure during sintering of BLT ceramics is analysed. HRTEM and geometric phase analysis (GPA) showed that stresses applied during sintering, trigger the nucleation and growth of faults hypothesised to be due to the errors in the AO3 layer (basal plane) stacking sequence of the hexagonal perovskite structure. The results reveal a strong correlation between the high concentration of structural defects and the development of anisotropic microstructures, which tune the properties of BLT. Stresses applied during sintering are therefore a promising tool to design material properties.

Effect of the fcc-hcp martensitic transition on the equation of state of solid krypton up to 140 GPa

Solid krypton (Kr) undergoes a pressure-induced martensitic phase transition from a face-centered cubic (fcc) to a hexagonal close-packed (hcp) structure. These two phases coexist in a very wide pressure domain inducing important modifications of the bulk properties of the resulting mixed phase system. Here, we report a detailed in situ x-ray diffraction and absorption study of the influence of the fcc-hcp phase transition on the compression behavior of solid krypton in an extended pressure domain up to 140 GPa. The onset of the hcp-fcc transformation was observed in this study at around 2.7 GPa and the coexistence of these two phases up to 140 GPa, the maximum investigated pressure. The appearance of the hcp phase is also evidenced by the pressure-induced broadening and splitting of the first peak in the XANES spectra. We demonstrate that the transition is driven by a continuous nucleation and intergrowth of nanometric hcp stacking faults that evolve in the fcc phase. These hcp stacking faults are unaffected by high-temperature annealing, suggesting that plastic deformation is not at their origin. The apparent small Gibbs free-energy differences between the two structures that decrease upon compression may explain the nucleation of hcp stacking faults and the large coexistence domain of fcc and hcp krypton. We observe a clear anomaly in the equation of state of the fcc solid at \ensuremath{\sim}20 GPa when the proportion of the hcp form reaches $\ensuremath{\sim}20%$. We demonstrate that this anomaly is related to the difference in stiffness between the fcc and hcp phases and propose two distinct equation of states for the low and high-pressure regimes.

Hematite (U-Th)/He thermochronometry constrains intraplate strike-slip faulting on the Kuh-e-Faghan Fault, central Iran

The Kuh-e-Faghan strike-slip fault system (KFF), located to the northern edge of the Lut Block in central Iran, developed through a Neogene-Quaternary pulsed history of eastward fault propagation and fault-related exhumation. This system is a consequence of the residual stresses transmitted from the Arabia-Eurasia convergent plate boundary. Here we integrate structural and textural analysis with new and previously published apatite fission-track (AFT) and apatite (U-Th)/He (apatite He) results, chlorite thermomentry, and hematite (U-Th)/He data from hematite-coated brittle fault surfaces to constrain the timing of tectonic activity and refine patterns of late Miocene-Pliocene burial and exhumation associated with the propagation of the KFF. Twenty-nine hematite (U-Th)/He (hematite He) dates from three striated hematite coated slip surfaces from the KFF fault core and damage zone yield individual dates from ~12–2 Ma. Petrographic analysis and chlorite thermometry of a polyphase, fossil fluid system in the KFF fault core document that fluid circulation and mineralization transitioned from a closed system characterized by pressure solution and calcite growth to an open system characterized by hot hydrothermal (T = 239 ± 10 °C) fluids and hematite formation. Hematite microtextures and grain size analysis reveal primary and secondary syntectonic hematite fabrics, no evidence of hematite comminution and similar hematite He closure temperatures (~60–85 °C) in each sample. Integration of these results with thermal history modeling of AFT and apatite He data shows that KFF activity in the late Miocene is characterized by an early stage of fault nucleation, fluid circulation, hematite mineralization, and eastward propagation not associated with vertical movement that lasted from ~12 to 7 Ma. Hematite He, AFT, and apatite He data track a second phase of fault system activity involving fault-related exhumation initiating at ~7 Ma and continuing until present time. Our new data constrain the onset of the recognized Late Miocene-Pliocene tectonic reorganization in north-central Iran.

Generation of amorphous carbon and crystallographic texture during low-temperature subseismic slip in calcite fault gouge

Identification of the nano-scale to micro-scale mechanochemical processes occurring during fault slip is of fundamental importance to understand earthquake nucleation and propagation. Here we explore the micromechanical processes occurring during fault nucleation and slip at subseismic rates (∼3 × 10−6 m s–1) in carbonate rocks. We experimentally sheared calcite-rich travertine blocks at simulated upper crustal conditions, producing a nano-grained fault gouge. Strain in the gouge is accommodated by cataclastic comminution of calcite grains and concurrent crystal-plastic deformation through twinning and dislocation glide, producing a crystallographic preferred orientation (CPO). Continued wear of fine-grained gouge particles results in the mechanical decomposition of calcite and production of amorphous carbon. We show that CPO and the production of amorphous carbon, previously attributed to frictional heating and weakening during seismic slip, can be produced at low temperature during stable slip at subseismic rates without slip weakening.

A km-scale “triaxial experiment” reveals the extreme mechanical weakness and anisotropy of mica-schists (Grandes Rousses Massif, France)

The development of Andersonian faults is predicted, according to theory and experiments, for brittle/frictional deformation occurring in a homogeneous medium. In contrast, in an anisotropic medium it is possible to observe fault nucleation and propagation that is non-Andersonian in geometry and kinematics. Here, we consider post-metamorphic brittle/frictional deformation in the mechanically anisotropic mylonitic mica-schists of the Grandes Rousse Massif (France). The role of the mylonitic foliation (and of any other source of mechanical anisotropy) in brittle/frictional deformation is a function of orientation and friction angle. According to the relative orientation of principal stress axes and foliation, a foliation characterized by a certain coefficient of friction will be utilized or not for the nucleation and propagation of brittle/frictional fractures and faults. If the foliation is not utilized, the rock behaves as if it was isotropic, and Andersonian geometry and kinematics can be observed. If the foliation is utilized, the deviatoric stress magnitude is buffered and Andersonian faults/fractures cannot develop. In a narrow transition regime, both Andersonian and non-Andersonian structures can be observed. We apply stress inversion and slip tendency analysis to determine the critical angle for failure of the metamorphic foliation of the Grandes Rousses schists, defined as the limit angle between the foliation and principal stress axes for which the foliation was brittlely reactivated. This approach allows defining the ratio of the coefficient of internal friction for failure along the mylonitic foliation to the isotropic coefficient of friction. Thus, the study area can be seen as a km-scale triaxial experiment that allows measuring the degree of mechanical anisotropy of the mylonitic mica-schists. In this way, we infer a coefficient of friction μweak=0.14 for brittle-frictional failure of the foliation, or 20% of the isotropic coefficient of internal friction.

Influence of a pre-existing basement weakness on normal fault growth during oblique extension: Insights from discrete element modeling

We use discrete element modeling to investigate three-dimensional fault geometry and the three-dimensional evolution of a fault network that develops above a 60° dipping planar pre-existing weakness striking 60° relative to the extension direction. The evolution of the fault network comprises three stages: (i) reactivation of pre-existing structure and nucleation of new faults (0–10% extension); (ii) radial propagation and interaction between reactivated structure and new faults (15%–20% extension); and (iii) linkage between reactivated structure and adjacent faults (20%–25% extension). During the first stage, the pre-existing structure mostly reactivates, forming a long and under-displaced fault. New faults are mainly extension-perpendicular and dip at 60°. During the second stage, ‘saw-tooth’ fringes grow upwards from the upper tip of the reactivated structure (which becomes the major fault) and influence the density and orientation of surrounding faults. During the third stage, the reactivated structure links laterally and vertically with adjacent faults, creating non-planar fault geometries. Following linkage, the reactivated structure enhances the displacement of linked faults along branch lines. Our study demonstrates that pre-existing weak faults can be reactivated, propagating upwards in an irregular (‘saw-tooth’) pattern, and affecting fault density, orientation, dip and displacement, and providing the nucleation site of new faults.

Three-dimensional fault geometries and interactions within experimental models of multiphase extension

ABSTRACT We use experimental (analog) models to examine the three-dimensional (3-D) fault geometries and interactions that develop during two phases of noncoaxial extension. In the models, a homogeneous layer of wet clay undergoes two phases of extension whose directions differ by 45°. The resulting fault pattern varies significantly with depth. At shallow levels, second-phase normal faults accommodate most second-phase extension. At depth, both second-phase normal faults and reactivated, first-phase faults with oblique slip accommodate most second-phase extension. A variety of interactions occurs between first-phase and second-phase faults. One interaction involves the upward propagation of second-phase faults from tips of reactivated, blind, first-phase faults. These hybrid faults have deep segments that strike subperpendicular to the first-phase extension direction and shallow segments whose strike varies with depth, becoming increasingly subperpendicular to the second-phase extension direction at shallow levels. A second interaction involves the nucleation of second-phase normal faults on the surfaces of reactivated, first-phase faults. These splay faults propagate upward and laterally from their nucleation sites into the hanging walls of the first-phase faults. As they propagate, they commonly encounter and link with different first-phase faults. The resulting composite faults have zigzag geometries in both map and cross-sectional views. A third interaction involves either the termination of second-phase antithetic normal faults against or near first-phase faults or the offset of first-phase faults by second-phase antithetic normal faults. The 3-D fault patterns and interactions within our models closely resemble those within the Taranaki basin of offshore New Zealand and Milne Point on Alaska’s North Slope.

On the nucleation of planar faults during low temperature and high stress creep of single crystal Ni-base superalloys

The present work studies the nucleation of planar faults in the early stages of low temperature (750 °C) and high stress (800 MPa) creep of a Ni-base single crystal superalloy (SX). Two families of 60° dislocations with different Burgers vectors were detected in the transmission electron microscope (TEM). These can react and form a planar fault in the γ′ phase. A 2D discrete dislocation model helps to rationalize a sequence of events which lead to the nucleation of a planar fault. First, one 60° channel dislocation approaches another 60° interface dislocation with a different Burgers vector. At a distance of 5 nm, it splits up into two Shockley partials. The interface dislocation is pushed into the γ′-phase where it creates a small antiphase boundary. It can only move on when the leading Shockley partial joins it and creates an overall 1/3<112> superdislocation. This process is fast and therefore is difficult to observe. The results obtained in the present work contribute to a better understanding of the processes which govern the early stages of low temperature and high stress primary creep of SX.

Discrete Fracture Network modelling of a hydrocarbon-bearing, oblique-slip fault zone: Inferences on fault-controlled fluid storage and migration properties of carbonate fault damage zones

Discrete Fracture Network (DFN) modelling is a useful tool to assess both fluid storage and migration properties of naturally fractured carbonate rocks. This work presents the results of multiple DFN models, which represent the structural network associated with a ca. 100 m–offset, oblique-slip, normal fault zone crosscutting Miocene ramp carbonates of the Majella Mountain, Italy. Specifically, by taking advantage of a previously published conceptual model of fault nucleation and growth, the DFN models are aimed at computing the amount of fracture porosity and equivalent permeability of representative portions of the evolving fault damage zone. In fact, by first considering only background structural elements, those pre-dating the fault nucleation stage, all fault-related structural elements such as joints, small and medium faults are added, one by one, according to their relative time of formation. Small faults are characterized by several cm-offset, whereas the latter ones have several 10's of cm-offset. The following fracture parameters are inputted for each structural element: (i) length; (ii) aspect ratio; (iii) mechanical and hydraulic aperture; (iv) fracture intensity and (v) orientation. A sensitivity analysis is carried out in order to test the seeding procedure of the software, and the input fracture aperture values. Results of DFN modelling are consistent with the small faults forming the main repository for underground geofluids. Differently, the fluid migration paths through the fault damage zone are mainly controlled by opening-mode-related fractures, which mainly form clusters at the extensional quadrants of sheared structural elements. Due to their attitude and distribution, these features mainly enhance fault-parallel fluid migration paths. In fact, the computed values of horizontal equivalent permeability obtained along this direction are about one order of magnitude higher than those along the orthogonal direction. The approach and methodologies applied in this work, together with the results of the multiple DFN model configurations show the relative influence and role of the different structural elements on fluid storage and migration properties of limestone-hosted, fault damage zones.

Microscale characterization of rupture nucleation unravels precursors to faulting in rocks

Precursory signals, manifestations of microscale damage that precedes dynamic faulting, are key to earthquake forecasting and risk mitigation. Detections of precursors have primarily relied on measurements performed using sensors installed at some distance away from the rupture area in both field and laboratory experiments. Direct observations of continuous microscale damage accumulated during fault nucleation and propagation are scarce. Using an X-ray transparent triaxial deformation apparatus, we show the first quantitative high resolution three-dimensional (3D) information about damage evolution of rocks undergoing brittle failure. The dynamic microtomography images documented a spectrum of damage characteristics and different fault growth patterns. The interplay between various deformation mechanisms can result in either a positive, negative, or constant net volume change. Consequently, changes in rock density and acoustic wave velocities before faulting are expected to vary in different tectonics settings, hence making failure forecasting intrinsically dependent on rock type at depth.

Low temperature stacking fault nucleation and expansion from stress concentrators in 4H-SiC

Despite of intensive studies during the last two decades of nucleation and expansion of Shockley-type single or adjacent stacking faults (double stacking faults) in 4H-SiC, the involved mechanisms are far from being deeply understood. Thus, in this paper, the temperature and carrier injection dependence of nucleation and expansion of these defects from local stress concentrators have been investigated by cathodoluminescence and low energy electron beam irradiation. Their corresponding behavior depends on their multiplicity. Upon annealing, the single stacking faults are found to be nucleated at lower temperature than the double stacking faults and their mobility is lower. Under electron injection at room temperature, only single stacking faults are nucleated. The involved partial dislocations dragging the stacking faults are assumed to have different nature cores.

Investigation of the Surface Morphology and Stacking Fault Nucleation on the (000-1)C Facet of Heavily Nitrogen-Doped 4H-SiC Boules

The stacking fault formation during physical vapor transport growth of heavily nitrogen-doped (mid-1019 cm−3) 4H-SiC crystals was investigated. Low-voltage scanning electron microscopy (LVSEM) observations detected the stacking fault formation on the (000-1) facet of heavily nitrogen-doped 4H-SiC crystals. Stacking faults showed characteristic morphologies, and atomic force microscopy (AFM) studies revealed that these morphologies of stacking faults stemmed from the interaction between surface steps and stacking faults. Based on these results, the stacking fault formation mechanism in heavily nitrogen-doped 4H-SiC crystals is discussed.