Shear Faults Research Articles

Fast and Localized Temperature Measurements During Simulated Earthquakes in Carbonate Rocks.

The understanding of earthquake physics is hindered by the poor knowledge of fault strength and temperature evolution during seismic slip. Experiments reproducing seismic velocity (∼1 m/s) allow us to measure both the evolution of fault strength and the associated temperature increase due to frictional heating. However, temperature measurements were performed with techniques having insufficient spatial and temporal resolution. Here we conduct high velocity friction experiments on Carrara marble rock samples sheared at 20 MPa normal stress, velocity of 0.3 and 6 m/s, and 20 m of total displacement. We measured the temperature evolution of the fault surface at the acquisition rate of 1 kHz and over a spatial resolution of ∼40 µm with an optical fiber conveying the infrared radiation to a two‐color pyrometer. Temperatures up to 1,250°C and low coseismic fault shear strength are compatible with the activation of grain size dependent viscous creep.

Fluid pressurisation and earthquake propagation in the Hikurangi subduction zone

In subduction zones, seismic slip at shallow crustal depths can lead to the generation of tsunamis. Large slip displacements during tsunamogenic earthquakes are attributed to the low coseismic shear strength of the fluid-saturated and non-lithified clay-rich fault rocks. However, because of experimental challenges in confining these materials, the physical processes responsible for the coseismic reduction in fault shear strength are poorly understood. Using a novel experimental setup, we measured pore fluid pressure during simulated seismic slip in clay-rich materials sampled from the deep oceanic drilling of the Pāpaku thrust (Hikurangi subduction zone, New Zealand). Here, we show that at seismic velocity, shear-induced dilatancy is followed by pressurisation of fluids. The thermal and mechanical pressurisation of fluids, enhanced by the low permeability of the fault, reduces the energy required to propagate earthquake rupture. We suggest that fluid-saturated clay-rich sediments, occurring at shallow depth in subduction zones, can promote earthquake rupture propagation and slip because of their low permeability and tendency to pressurise when sheared at seismic slip velocities.

Fracture Shape and Orientation Contributions to P-Wave Velocity and Anisotropy of Alpine Fault Mylonites

P-wave anisotropy is significant in the mylonitic Alpine Fault shear zone. Mineral- and texture-induced anisotropy are dominant in these rocks but further complicated by the presence of fractures. Electron back-scattered diffraction and synchrotron X-ray microtomography (micro-CT) data are acquired on exhumed schist, protomylonite, mylonite, and ultramylonite samples to quantify mineral phases, crystal preferred orientations, microfractures, and porosity. The samples are composed of quartz, plagioclase, mica and accessory garnet, and contain 3–5% porosity. Based on the micro-CT data, the representative pore shape has an aspect ratio of 5:2:1. Two numerical models are compared to calculate the velocity of fractured rocks: a 2D wave propagation model, and a differential effective medium model (3D). The results from both models have comparable pore-free fast and slow velocities of 6.5 and 5.5 km/s, respectively. Introducing 5% fractures with 5:2:1 aspect ratio, oriented with the longest axes parallel to foliation decreases these velocities to 6.3 and 5.0 km/s, respectively. Adding both randomly oriented and foliation-parallel fractures hinders the anisotropy increase with fracture volume. The anisotropy becomes independent of porosity when 80% of fractures are randomly oriented. Modeled anisotropy in 2D and 3D are different for similar fracture aspect ratios, being 30 and 15%, respectively. This discrepancy is the result of the underlying assumptions and limitations. Our numerical results explain the effects that fracture orientations and shapes have on previously published field- and laboratory-based studies. Through this numerical study, we show how mica-dominated, pore-free P-wave anisotropy compares to that of fracture volume, shape and orientation for protolith and shear zone rocks of the Alpine Fault.

3D Modeling of Subsurface Lawanopo Fault In Southeast Sulawesi, Indonesia Using Grablox and its Consequence to Geohazard

Lawanopo Fault is a horizontal shear fault (sinistral strike-slip) found in Southeast Sulawesi province and is thought to be active during Plio-Pleistocene or mid-late Miocene to the present. This study has been carried out which aims to find out the geometric shapes below the surface of the Lawanopo fault using complete Bouguer anomaly (ABL) data. The ABL data is projected onto a flat plane using the Dampney method at an altitude of 8 km, and the separation of local and regional anomalies is carried out using the upward continuation method at an altitude of 60 km. Three-dimensional (3D) modeling under the surface of the Lawanopo fault is done using the computer program Grablox. Data processing techniques using Singular Value Decomposition (SVD) and Occam inversion. The results showed that a high gravity anomaly of 190-225 mGal was caused by an igneous rock below the surface with a density of 2.7-3.33 gr/cm3 and a thickness of about 13 km, a moderate anomaly of 175-187 mGal caused by Paleozoic igneous rocks aged Carbon with a density of 2.6-2.9 gr/cm3 and a thickness of about 25 km. Low anomaly 115-160 mGal is caused by rocks with a density of 2.0-2.5 gr/cm3 and a thickness of about 22-23 km. The Lawanopo fault constituent rocks consist of alkaline rocks in the basement covered by sediment and metamorphic with a depth of Lawanopo fault more than 15 km and begin to be seen at a depth of 4.3 km of the surface. it is known that the area around the Lawanopo fault is an area prone to earthquakes. But, based on the soil and rock structure around the Lawanopo fault, the compactness and attenuation levels in reducing earthquake waves are quite good, so that land use around the Lawanopo fault tends to be safe.

Hydrothermal Alteration and Ore Metal Mineralisation at Temon, Pacitan, East Jawa, Indonesia.

Pacitan area is known as Tertiary volcanic arc in Java, as the result of subduction zone of the Indian-Australian Plate beneath the Eurasian Plate since Oligocene. It was superimposed volcanism which formed a wide area of hydrothermal alteration zone, resulting potential ore metals mineralization, such at Temon and its vicinities, Pacitan Regency, East Java Province, Indonesia. The aim of study was to analyze hydrothermal alteration and ore metal mineralization zones. Method was surface mapping, thin section analyses, mineragraphic analyses and X-Ray Diffraction (XRD) analyses. Field study observed denuded and deformed volcanic crater geomorphology. There are ore placer deposits within the sand dunes of Grindulu River, which it consists of andesitic lava and breccia of Early Oligocene Mandalika Formation; Early Miocene lithic and vitric tuffs; and dacitic intrusion. The dikes of dacite as the last of volcanism was the host rock controlling the zonation of alteration and mineralization stages. Oblique normal faults and shear faults were cross over dilating formed fractures, which were as bodies to depositing the ore metals. There are (zone 1st) the argillic clay consists of quartz+alunite+dickite+kaolinite±illite with vuggy structures, (zone 2nd) the argillic clay consists of quartz+montmorillonite±illite zone with quartz vents, brecciated and sulfide massive, and (zone 3rd) as the chloritized zone with low grade and supergene on the edge of hydrothermal alteration. It was fluid overprinted that very acid to the core of zone 1st (pH2-4) into more netral pH 4-6 (zone 2nd) and (pH5-6) in the edge zone 3rd. The potentials ore metal mineralization are Fe and Cu by pyrite, chalcopyrite, hematite, and covellite. Other potential ore metal mineralization was also from enargite by the supergene alteration.

Microstructural responses of Zirconia materials to in-situ SEM nanoindentation

Development of optimal shaping processes for pre-sintered and sintered zirconia materials requires a fundamental understanding of damage and deformation mechanisms at small-scale contacts with diamond tools. This paper reports on responses of zirconia materials with distinct microstructures to nanoindentation associated with diamond machining using a Berkovich diamond indenter. In-situ nanoindentation was performed in a scanning electron microscope (SEM) and in-process filmed to record small contact events. Indentation morphology was SEM-mapped at high-magnifications. Although both pre-sintered porous and sintered dense zirconia materials mechanically revealed the quasi-plastic behavior in indentation, there were distinct responses of the two materials to quasi-plasticity at the microstructural level. For pre-sintered porous zirconia, the quasi-plasticity was attributed to shear faults resulting from breaking pore networks as microstructurally discrete interfaces, to lead to compression, fragmentation, pulverization and microcracking of zirconia crystals in indentation imprints. In contrast, sintered dense zirconia had shear band-induced quasi-plastic deformation, accompanied with localized tensile microfracture. A material index associated with the mechanical properties ranked the lower quasi-plasticity for pre-sintered porous zirconia than its sintered dense state, predicting more machining-induced damage in the former than the latter. Significantly higher indentation imprint volumes induced in indented pre-sintered porous zirconia than sintered dense state previses higher machining efficiency for the former than the latter. The microstructure-dependent indentation mechanisms provide the fundamental knowledge into micromechanics of abrasive machining of zirconia materials and may lead to a new microstructural design for zirconia materials to achieve a balanced machining efficiency and damage control.

Andaman–Nicobar–Sumatra Margin Revisited: Analysis of the Lithospheric Structure and Deformation Based on Gravity Modeling and Distribution of Seismicity

The Andaman–Nicobar–Sumatra subduction margin, with a well-developed Benioff zone down to ~ 200 km depth, extends over 1300 km along strike and has a lateral extent of ~ 200 km. Two-dimensional (2D) profiles based on generalized inversion of free-air gravity anomaly data across different segments of the Andaman–Nicobar sector of the margin were analyzed by reconstructing the geometry of the converging India–Eurasia lithospheric plates. Detailed 2D structures of the Ninetyeast Ridge (NER), fore-arc basin, volcanic arc, back-arc basin, spreading ridge, Sewell Rise, Mergui Ridge and Mergui Basin, and depths of Moho and the lithosphere–asthenosphere boundary were delineated. The volcanic arc is located at a distance of ~ 150–200 km from the trench and is marked by a zone of positive gravity anomaly separated from the trench by the fore-arc basin with a zone of gravity low. The topographic and gravity anomaly patterns show complex geometrical patterns over the volcanic arc due to the presence of a number of regional faults striking parallel to the trench axis developing a series of pull-apart (transtensional) basins as well as the interaction of the spreading ridges on the overriding plate. A narrow zone of negative gravity anomaly (~ − 185 to − 110 mGal) observed in the gravity model of the margin is interpreted to be associated with fore-arc shear fault above a zone of fractured oceanic crust with a thick sedimentary layer, located above a zone of sharp bending of the eastward converging Indian lithosphere in the fore-arc basin. A high concentration of seismicity and a trench-parallel band of moderate moment energy release along this fault might have been caused by concentrated deformation within the zone of flexing of the descending plate. A wide fore-arc (> 200 km) and the enhanced deformation of the subducting Indian oceanic plate in central sectors possibly resulted from increased interaction between the NER and the Andaman trench and extension in the Andaman back-arc. Appearance of a second moderate energy band and shifting of seismicity toward the trench axis following the 2004 MW 9.2 mega-event are apparently caused by the migration of the stress field from deeper to shallower part of the Indian lithosphere. A similar type of seismicity migration toward shallower part of the descending plate has been recorded along other subduction margins around the globe.

High resolution tomography of the Tanlu fault zone near Hefei with passive seismic and magnetotelluric linear array data

As the largest fault trending NNE-SSW to NE-SW in the eastern Eurasia Continent, the Tanlu fault zone (TLFZ) extends over 2,400 km within China, roughly from Wuxue, Hubei Province, to Russia. Since the Quaternary period, the TLFZ has been an earthquake-prone area in eastern China where several major earthquakes resulted by tectonic compression occurred, causing tremendous casualties and significant economic losses. Many studies on different segments of the TLFZ have been carried out in the past few decades. However, numerous key questions regarding the fault zone remain unanswered due to a lack of clear subsurface characterization and fault delineation. In this study, we present high-resolution tomographic results across the TLFZ to the east of Hefei, where one 22-km-long passive seismic array with densely spaced short-period nodes, and a 24-km-long magnetotelluric array were deployed adjacent to each other. We find the velocity and resistivity variations are highly consistent with known surface geology. Sharp property contrasts in both the seismic shear wave velocity and electrical resistivity profiles clearly delineate the Tanlu F1 fault (TLF-1) near Hefei. More interestingly, an upwelling with distinct high velocity is imaged within the Hefei Basin to the west of the TLF-1, whereas a slanted block with low-velocity and low-resistivity seems to cut into or thrust upon the high-grade to low-grade middle-pressure rocks in the Zhangbaling uplift right below the Tanlu F2 ductile shear fault (TLF-2). The presented results show a new approach to characterize deep subsurface structure of the TLFZ beyond 2-km depths using passive data, which it is often difficult for active seismic surveys with refracted and reflected waves to image.

Microtwinning in Ni-base supalloy Mar-M247 at intermediate temperature and low stress

After creep deformation at 760 °C and 470 MPa, approximately half of the yield strength, for 7075 h, the deformation microstructures of Mar-M247 are investigated using transmission electron microscope. Experimental results reveal that pairs of a/2 〈110〉 dislocations, superlattice stacking faults and microtwins are produced in the deformed specimens, suggesting anti-phase boundary shearing, stacking fault shearing and microtwinning operate simultaneously during creep deformation. This study provides new insights into understanding the creep deformation mechanisms in Ni-base superalloy.

Dependence on temperature of compression behavior and deformation mechanisms of nickel-based single crystal CMSX-4

The compression behavior under a temperature range from room temperature to 1100 °C were conducted on the single crystal superalloys CMSX-4. The objective of this study is to identify the effect of the temperature on the compression behavior and understand the deformation mechanism by transmission electron microscopy after yielding. Experimental results show that with increasing temperature, the yield strength increases until 850 °C. After that, there is a opposite trend in yield strength.At room temperature, plastic deformation is dominated by anti-phase boundary shearing. From 700 °C to 850 °C, the major anti-phase boundary and minor stacking fault shearing is observed. Besides, Kear–Wilsdorf locks are frequently observed especially at 800 °C and 850 °C, resulting in the best yield strength at these temperatures. At 950 °C, the deformation mechanism is mainly controlled by stacking fault shearing. With temperature increasing to 1100 °C, the dislocation network and rafted structure are formed, where the deformation mechanism is Orowan by-passing and dislocation climb. Finally, the formation of stacking faults with temperatures and the relationship between yield strength and deformation mechanism under compression tests are deduced.

Structural and kinematic evolution of strike-slip shear zones around and in the Central Tianshan: Insights for eastward tectonic wedging in the southwest Central Asian Orogenic Belt

In order to better understand the late Paleozoic tectonic evolution of the southwestern Central Asian Orogenic Belt (CAOB), we carried out structural and geochronological studies on the poorly investigated Xiaergou and Wulasitai shear zones around and in the Chinese Central Tianshan. The Xiaergou shear zone is the connecting segment between the North Tianshan Fault and Main Tianshan Shear Zone along the northern margin of the Yili - Central Tianshan blocks, it strikes NW-SE with a width of ~3–5 km and shows predominant dextral kinematics. Zircon U–Pb ages of pre- and syn-kinematic granitic dykes within the Xiaergou shear zone indicate that the dextral shearing was active at ~312-295 Ma. The Wulasitai shear zone is a high-strain belt occurring in the interior of the Central Tianshan block, it extends NW-SE for more than 40 km with variable widths of ~1–5 km, steep mylonitic foliations and sub-horizontal stretching lineation are well developed and various kinematic indicators suggest prevailing sinistral shearing. New biotite 40Ar/39Ar ages of two meta-sedimentary rocks, together with the published metamorphic zircon ages constrain the timing of the sinistral shearing at ~312-301 Ma. Our new results combined with the previous studies reveal that the dextral strike-slip shear zones framing the Central Tianshan formed almost simultaneously in the latest Carboniferous (~310 Ma) and lasted until the middle to late Permian. They resulted from the eastward tectonic wedging and relative rotations between continental blocks in the SW CAOB. The sinistral shearing of the Wulasitai shear zone within the Central Tianshan was likely generated due to differential eastward motions of the northern and southern parts of the Central Tianshan.

Factors that affect the stability of roads around rocks

The control method of deformation of large sections of soft rock formations in coal mines is a major problem in mining practice. Taking Wangzhuang Coal Mine in Shanxi Province, China, for example, the support method of large sections of soft rock formations was studied to control the deformation of the top and bottom plates and two gangs, and to obtain the damage of the surrounding rocks at different depths through drilling imaging. In addition, the displacement, plastic area and stress field distribution patterns of surrounding rocks are obtained by simulating in FLAC3D. Coupling support for broken, plastic and elastic zones is achieved using bolts, grout anchors, and long anchor cables. The bottom of the road is effectively controlled by floor grout and concrete reverse arches. The results show that under the combination of anchorage and grout, the fault depth and degree of the surrounding rocks are less than the original support method, which can effectively reduce the subsidon of the roof and the deformation of the two gangs. It is important to note that anchorage cable set support can effectively reduce shear fault areas in floor and two gang plastic areas, and reduce the weight of the pavement floor. This study provides a technical reference for the support of large sections of soft rock roadways in coal mines.

Ufa mantle ridge fault – main tectonic, geological and minerogenic consequences

Relevance of the work. The data on the structure and properties of the subsoil, obtained on the basis of geophysical studies, make it possible to significantly revise the ideas about the structure and patterns of formation of mineral deposits within the Ufimsky ridge fault and its zone of influence (55º–56º north latitude in the Urals). Purpose of the work: constructing a model of the formation of the Ufa mantle ridge fault and identifying its mineragenic features using the example of deposits of ferruginous quartzite, kyanites and rare earth mineralization. The methodology of the research. The published databases and their summarizing materials on deep seismic sounding of the Urals with the allocation of a mineragenic load characteristic of the Ufa mantle ridge fault and its influence zone were used as a factual basis for the research. Research results. The Ufa ridge fault is a structure formed by the uplift of the upper mantle, presumably at the intersection of intramantle shear faults. The transverse asymmetry of the mantle ridge fault and the later rift-collisional processes are reflected in the overlying complexes and lead to a distortion of the lines of the outcropping of deep faults at the level of the modern erosional section. Horst formation was complicated by rifting-collisional tectonic events with accompanying magmatic, metamorphic, metasomatic, and dislocation processes, accompanied by various types of numerous manifestations and mineral deposits exposed by erosional processes. Schemes of the location of deposits and manifestations of ferruginous quartzites, kyanites and rare earth mineralization in its more eroded part are given as examples confirming the confinement of various types of minerals to the structure of the Ufa mantle ridge fault. Conclusions. The cropping of deposits and occurrences of ferruginous quartzites, kyanite, rare earth mineralization and other minerals is closely related to the development of the Ufa mantle ridge fault.

Complex co- and postseismic faulting of the 2017–2018 seismic sequence in western Iran revealed by InSAR and seismic data

The largest earthquake in the Zagros Mountains struck the city of Azgeleh on the Iran–Iraq border on 12 November 2017. This Mw 7.3 earthquake was followed by an intense seismic sequence. Implementing the double-difference earthquake location technique, we relocate 1069 events recorded by our local seismic network, deployed after the mainshock. The spatial distribution of the epicenters indicates linear alignments of the events nucleated along at least four notable clusters. The clusters are characterized by at least one significant earthquake, such as the Tazehabad earthquake of 25 August 2018 (Mw 5.9) along a dense, east–west trending cluster and the Sarpol-e Zahab earthquake of 25 November 2018 (Mw 6.3) along the cluster with a northeast–southwest trend. We use two-pass differential SAR interferometry (DInSAR) and Small BAseline Subset (SBAS) methods to study the coseismic permanent displacements of the Azgeleh, Tazehabad and Sarpol-e Zahab events as well as the one-year postseismic deformation field of the 2017–2018 seismic sequence, respectively. We use non-linear and linear optimization algorithms to derive the source geometry and the slip distribution along the fault planes. The inversion is conducted by introducing also seismological constraints, leading to the definition of a listric geometry for the Azgeleh mainshock rupture that accommodates the slip area at depth of 10–16 km along a sub-horizontal plane (dipping ~3°) and a low-angle (~16°) ramp. The thrust and dextral movements along this NNW-striking (~345°) fault have triggered a tear fault responsible for the Tazehabad event ruptured an east–west trending (~267°), north-dipping (~78°) sinistral shear fault. We present the dextral slip distribution of the Sarpol-e Zahab event along a NE-striking (~34°) fault, as a synthetic Riedel structure for the southern segment of the Khanaqin fault, dipping 63° to the southeast. We find the postseismic deformation field associated with the seismic sequence is not confined only to the mainshock source (the Azgeleh fault), but also develops along the Tazehabad and Sarpol-e Zahab faults. We additionally propose afterslip along a duplex, flat-ramp-flat structure down-dip and up-dip of the Azgeleh coseismic slip area. The up-dip afterslip develops onto the shallow detachment (~3°) at depth of ~8 km and the down-dip afterslip propagate onto the mid-crustal décollement level within the Pan-African basement. The Azgeleh, Tazehabad, Sarpol-e Zahab and Khanaqin faults mark the Lurestan Arc–Kirkuk Embayment sharp margin in the Northwest Zagros and play a key role in the lateral escape of the Lurestan Salient and vertical strain partitioning in the Zagros front.

The moderate size 2019 September Mw 5.8 Silivri earthquake unveils the complexity of the Main Marmara Fault shear zone

SUMMARYThe unbroken section of the North Anatolian Fault beneath the Sea of Marmara is a major source of seismic hazard for the city of İstanbul. The northern and currently the most active branch, the Main Marmara Fault (MMF), is segmented within a shear zone and exhibits both partially creeping and locked behaviour along its 150 km length. In 2019 September, a seismic activity initiated near MMF, off-coast the town of Silivri, generating 14 earthquakes ≥ Mw 3.5 in a week. The Mw 5.8 Silivri earthquake, is the largest in the Marmara Sea since the 1963 Mw 6.3 Çınarcık earthquake. Our analyses reveal that the activity started in a narrow zone (∼100 m) and spread to ∼7 km following an Mw 4.7 foreshock within ∼2 d. The distribution of relocated aftershocks and the focal mechanisms computed from regional waveforms reveal that the Mw 5.8 earthquake did not occur on the MMF, but it ruptured ∼60° north-dipping oblique strike-slip fault with significant thrust component located on the north of the MMF. Finite-fault slip model of the main shock shows 8 km long rupture with directivity toward east, where the ruptured fault merges to the MMF. The narrow depth range of the slip distribution (10–13 km) and the aftershock zone imply that the causative fault is below the deep sedimentary cover of the Marmara Basin. The distribution of aftershocks of the Mw 5.8 event is consistent with Coulomb stress increase. The stress changes along MMF include zones of both stress decrease due to clamping and right-lateral slip, and stress increase due to loading.

3D outcrop modelling of large discordant breccia bodies in basinal carbonates of the Apulian margin, Italy

Large discordant breccia bodies (LDBBs) are important record keepers of the tectonic and gravitational evolution in platform-to-basinal settings, and have important implications for fluid-flow migration and compartmentalization of tight carbonate reservoirs. In the Gargano Promontory of southern Italy, LDBBs occur within a Cretaceous slope and basinal carbonate succession. We use field mapping and Unmanned Aerial Vehicle (UAV) -based Structure from Motion (SfM) Photogrammetry to document otherwise inaccessible cliff-side exposures of seismic-to subseismic-scale vertical discordant breccias. LDBBs are up to 50 m in width, more than 80 m in height and display internal chaotic or aligned clast fabrics. The formation which generally contains the LDBBs is characterized by beds of cherty pelagic limestone intercalated with calcarenites, calciturbidites and horizons of mass transport deposits. The mass-transport deposits can be correlated across the chaotic breccia bodies, indicating only slight or no vertical displacement across the adjacent walls. The bases of the breccia bodies are always hidden below current sea level, while the rarely exposed tops are capped by bedded intervals of the host rock formation. Timing and origin of the studied breccias were determined using several lines of evidence, such as stratigraphic provenance of clasts in breccias, mutual relationships of structural and sedimentologic features, and previous studies which establish that the age of dolomitizing cements in the LDBBs formed at different times and by different processes (fault shearing and solution collapse). This work investigates the size, shape and geometry of these breccia bodies whilst also providing cm-scale detail of the textural features in otherwise inaccessible outcrops. We suggest that breccias formed as a result of solution exploiting a pre-existing fracture network characterized by large-scale vertical strike-slip or oblique-slip faults. Initial displacement along these faults created a wide fault damage zone, where fluid migration was later focused to create a zone highly susceptible to solution and subsequent periodic sidewall collapse.

Stick-slip failure in heterogeneous sheared fault with a variety of fault roughness

Stick-slip failure, as a possible mechanism for destructive shallow focus earthquakes, often occurs in heterogeneous faults. To investigate the physical processes of stick-slip failure in heterogeneous faults, direct shear tests are performed in the laboratory, combined with the use of multi-channel high-speed dynamic strain acquisition system, on the bimaterial samples. The bimaterial samples consist of PMMA (poly methyl methacrylate) and PC (poly carbonate). This experimental study is concerned with the effects of fault roughness on the stick-slip behaviors along the heterogeneous faults. The metallographic microscope is used to quantitatively measure the surface morphology of the heterogeneous faults. The stress distribution along the simulated fault is highly non-uniform, which dictates the initiation locations of stick-slip failure. The nucleation starts at a firstly-yielded material point, and as the energy accumulates in the heterogeneous fault, the nucleation zone is progressively formed. The crack tip propagates at an increasing velocity until the entire fault is penetrated. It is found that a rougher fault is corresponding to a slower rupture velocity, a smaller local stress drop, a larger critical slip-weakening displacement, and a larger critical crack length.

The role of {111} slip in the initial plastic deformation of Ni-base superalloys at room temperature

After just yielding at room temperature, the deformation mechanisms in the Ni-base superalloys René N5, CMSX-4, PWA 1483, and CM247LC are investigated by transmission electron microscope. It is found that anti-phase boundary shearing controls the initial plastic deformation of René N5, CMSX-4 and PWA 1483, although stacking fault shearing also operates in the latter two alloys. Whereas, besides many pairs of a/2 〈110〉 dislocations, a high density of isolated superlattice stacking faults and extended stacking faults is also created in CM247LC after around 0.2% plastic deformation, indicating that stacking fault shearing prevails in the initial stage of plastic deformation. These observations demonstrate that plastic deformation is not accomplished solely by anti-phase boundary shearing in Ni-base single crystal superalloys at room temperature, and the formation of superlattice stacking faults does not necessarily require reordering of atoms, hence providing new insights into understanding the interaction mechanisms between matrix dislocations and γ' precipitates.

Evolution of Faulting Induced by Deep Fluid Injection, Paradox Valley, Colorado

ABSTRACT High-pressure fluid injection into a subhorizontal confined aquifer at 4.3–4.6 km depth induced >7000 earthquakes between 1991 and 2012 within once seismically quiescent Paradox Valley in Colorado, with magnitudes up to Mw 3.9. Earthquake hypocenters expanded laterally away from the well with time, defining the margins of the aquifer pressurized by injection at the well. Within 5 km of the well, alignment of earthquake hypocenters defines strikes of nine vertical fault zones. Previous studies show that these fault zones predate injection, producing left-stepping offsets in the normal faults of the Wray-Mesa fault system that cradles Paradox Valley. Hypocenters, rakes, and strikes of 2041 well-constrained focal mechanisms show that most injection-related earthquakes occur where these vertical faults intersect the pressurized aquifer. Well-defined focal mechanisms show that this induced seismicity consists of Riedel shear faults at acute angles to the strikes of these fault zones. These small faults develop an anastomosing fault structure of focal planes along each planar fault zone, as fluid injection continues, even as their hypocenters define a single planar fault zone. Failure conditions at each hypocenter are found using a fully coupled poroelastic analysis of stress induced by fluid injection, and this analysis indicates a minimum Coulomb failure condition of 0.1 MPa. This failure condition is primarily a result of aquifer pore-fluid pressurization, as almost all well-located seismicity is within the pressurized aquifer. Reducing the rate of injection and frequent well shutdowns in the second decade nearly eliminated induced seismicity, except very near the well where gradients in pressurization are the largest. Despite these decreases in failure conditions and seismicity, some fault zones continued to produce earthquakes larger than M 3 as injection continued.

Mineral precipitation as a mechanism of fault core growth

Faults vary in structural style, from simple planes to complex systems composed of fault cores and damage zones. Increased fault complexity results from the interaction of mechanical and chemical processes, including fracture growth, shear, and linkage, and mineral dissolution and precipitation. Although water-rock interaction is traditionally associated with fault rock weakening and shear localization, we investigate processes of fault core widening by water-rock interactions that resulted in quartz precipitation. We combine field and petrographic observations with prior mechanical characterization to assess the impact of alteration and cementation on fault architecture at the Dixie Comstock epithermal gold deposit, Nevada, USA. Mineralized portions of the fault contain strong, thick, silicified fault cores and wide, weak damage zones, with evidence for widening of the core through entrainment of damage zone material and repeated cycles of embrittlement, dilation, and cementation. We present a model of fault zone evolution in which the hydrothermal regimes favoring either alteration-weakening or precipitation-strengthening result in distinct fault zone architecture and mechanical and flow properties of fault systems. Alteration-weakening favors localization of the fault into thinner, clay-rich, low permeability fault cores. Precipitation-strengthening promotes thick, strong, and low permeability fault cores, with mineralization-embrittlement enhancing transient permeability following coseismic failure.