Finite Shear Strain Research Articles

Coseismic surface ruptures of 20 strike-slip earthquakes measured from geodetic imaging data

Observations of coseismic surface ruptures are widely used for examining faulting mechanics and kinematics, and in fault displacement hazard analysis. Here, first, we provide new remote sensing observations of 20 historic surface rupturing continental strike-slip earthquakes (6.1 ≤ Mw≤ 7.9). To measure the near-field surface deformation pattern, we have processed a range of raw radar and optical images from satellite and aerial platforms with pixel tracking techniques, and in two cases using standard interferometric synthetic aperture radar (InSAR). We provide a range of observations, including surface displacement maps that constrain at least the horizontal component of surface motion (2D), and for three events the full 3D components. Second, we provide strain invariants for each event, which includes the finite dilatational strain, strain magnitudes, the vorticity, and maximum shear strain. Third, we provide a total of 134 surface fault rupture traces that are mapped manually from the displacement maps. Finally, we provide 2648 measurements of coseismic horizontal fault-parallel surface fault slip that are measured objectively using swath profiles with a function fitting method that we have developed.

Mechanical stability of homogeneous holographic solids under finite shear strain

We study the linear stability of holographic homogeneous solids (HHS) at finite temperature and in presence of a background shear strain by means of a large scale quasi-normal mode analysis which extends beyond the hydrodynamic limit. We find that mechanical instability can arise either as a result of a complex speed of sound — gradient instability — or of a negative diffusion constant. Surprisingly, the simplest HHS models are linearly stable for arbitrarily large values of the background strain. For more complex HHS, the onset of the diffusive instability always precedes that of the gradient instability, which becomes the dominant destabilizing process only above a critical value of the background shear strain. Finally, we observe that the critical strains for the two instabilities approach each other at low temperatures. We conclude by presenting a phase diagram for HHS as a function of temperature and background shear strain which shows interesting similarities with the physics of superfluids in presence of background superfluid velocity.

Eigenvalue analysis of stress-strain curve of two-dimensional amorphous solids of dispersed frictional grains with finite shear strain.

The stress-strain curve of two-dimensional frictional dispersed grains interacting with a harmonic potential without considering the dynamical slip under a finite strain is determined by using eigenvalue analysis of the Hessian matrix. After the configuration of grains is obtained, the stress-strain curve based on the eigenvalue analysis is in almost perfect agreement with that obtained by the simulation, even if there are plastic deformations caused by stress avalanches. Unlike the naive expectation, the eigenvalues in our model do not indicate any precursors to the stress-drop events.

Thermoelectric transport in holographic quantum matter under shear strain

We study thermoelectric transport under shear strain in two spatial dimensional quantum matter using the holographic duality. General analytic formulae for the DC thermoelectric conductivities subjected to finite shear strain are obtained in terms of black hole horizon data. Off-diagonal terms in the conductivity matrix also appear at zero magnetic field, resembling an emergent electronic nematicity, which cannot nevertheless be identified with the presence of an anomalous Hall effect. For an explicit model study, we numerically construct a family of strained black holes and obtain the corresponding nonlinear stress–strain curves. We then compute all electric, thermoelectric, and thermal conductivities and discuss the effects of strain. While the shear elastic deformation does not affect the temperature dependence of thermoelectric and thermal conductivities quantitatively, it can strongly change the behavior of the electric conductivity. For both shear hardening and softening cases, we find a clear metal-insulator transition driven by the shear deformation. Moreover, the violation of the previously conjectured thermal conductivity bound is observed for large shear deformation.

Strain localisation and transcurrent reactivation in the granulite facies Kalinjala Shear Zone at Port Neill, South Australia

The central portion of the Kalinjala Shear Zone, in the Port Neill region, South Australia, is recognised as a northeast-trending anomalously magnetic corridor coinciding with a sub-vertical transpressional dextral shear zone. This shear zone is a primary lithospheric structure, separating the nucleus of the Archean Gawler Craton from a suite of Paleoproterozoic granites (ca 1850 Ma) and earlier paragneiss. At Port Neill, there is an apparent 17 km dextral offset of a pre-existing Bouguer gravity anomaly by the Kalinjala Shear Zone. This is attributed to Kimban-aged (ca 1740–1690 Ma) deformation that is accompanied by peak granulite facies metamorphic conditions reaching pressures of ∼1.0 GPa and temperatures of ∼800 °C. This is consistent with the presence of leucosomes, within the sheared fabric, in the axial surface of mylonitic folds and in mafic gneisses. Strain has been localised within and adjacent to mafic dykes, predominantly hosted in the granitic rocks, with the highest strained areas having the greatest evidence of leucosome extraction. The local orientation of stretching lineations in this shear zone do not necessarily correlate with the transport direction but may reflect along- and across-strike variations in finite shear strain into alternating zones dominated by either constrictional or a superimposed flattening strain. Overprinting the dominant dextral movement are sinistral shears, with horizontal strike-slip movements, which post-date the peak metamorphic assemblages and are generally confined to sites containing late K-feldspar-rich pegmatites. The major movement recorded in the Kalinjala Shear Zone was both horizontal and vertical, occurring in response to regional exhumation of the Cornian-aged (ca 1850 Ma) sequences in the east, during the Kimban Orogeny, and was basically controlled by the nature of lithospheric plate convergence during accretion of a continental backarc onto the Gawler Craton. KEY POINTS Dextral transpression with late-sinistral along-strike extension and east-up exhumation is associated with 17 km of horizontal offset along the central section of the Kalinjala Shear Zone. Deformation is dominated by three major Kimban-aged (ca 1730–1690 Ma) events, which overprint older ensialic backarc metasedimentary rocks, orthogneiss and granites of the Donington Suite. Granulite facies metamorphic conditions control the nature of deformation and strain distribution within the shear zone. The Kimban events record the collision of the Gawler Craton nucleus with the southern margin of the North Australian Craton.

Twinning mechanism asymmetry in body-centered cubic tantalum under [001] uniaxial compression/tension

Plasticity of body-centered cubic tantalum with preexisting dislocations is investigated under $[001]$ uniaxial compression/tension loading using a molecular dynamic simulation method. At low temperature or high strain rate, twinning is the main deformation mechanism under both $[001]$ compression and tension. However, the twinning mechanism under compression is different from the conventional twinning mechanism under tension, which is formed by $1/6[1\overline{1}1]$ twinning dislocations moving on adjacent $(\overline{1}12)$ planes along the twinning direction. The twin nucleates from six-layer dissociation of a $1/2[111]$ screw dislocation and grows by the glide of disconnections that step the twin boundary by two $(\overline{1}12)$ planes. Further study shows that this type of twinning mode generates a finite antitwinning shear strain, the magnitude of which is one-half of that generated by the conventional deformation twinning. Finally, the two twinning mechanisms are discussed in terms of the energy landscape.

Theory of history-dependent multi-layer generalized stacking fault energy— A modeling of the micro-substructure evolution kinetics in chemically ordered medium-entropy alloys

In this study, a chemical order related concept “history-dependent multi-layer generalized stacking fault energy” (HDML-GSFE) was proposed, and it was then demonstrated by employing the recent, very interesting multi-principal element alloy (CoCrNi medium-entropy alloys; MEA) with different chemical short-range order (CSRO) levels using a density functional theory (DFT)-based neural network interatomic potential. To demonstrate the impacts of the history dependency and interlayer (atomic interlayers of the slip system) coupling effect on the GSFE of CSRO MEAs, HDML-GSFEs were computed for different shear deformation pathways of the MEAs with different CSRO levels, such as interlayer multiple-time slipping, twin growth, and γ−ε (FCC-HCP) phase transformation. It was demonstrated that multiple-time slipping induces CSRO collapse, leading to local shear softening due to the history dependency of GSFE. In addition, it was found that the slipping of neighboring atomic interlayers is affected by the slipping resulting from the induced CSRO collapse of present interlayers because of the interlayer coupling effect of GSFE. Eventually, by employing a novel kinetic Monte Carlo (kMC) simulation method based on dislocation/disconnection loop nucleation events and using the HDML-GSFE with the history dependency and interlayer coupling effect, we proposed a laminated micro-substructure evolution that involves twinning and γ−ε phase transformations subject to a finite shear strain rate and finite temperature.

Coseismic deformation of the ground during large-slip strike-slip ruptures: Finite evolution of “mole tracks”

Abstract To evaluate ground deformation resulting from large (~10 m) coseismic strike-slip displacements, we focus on deformation of the Kekerengu fault during the November 2016 Mw 7.8 Kaikōura earthquake in New Zealand. Combining post-earthquake field observations with analysis of high-resolution aerial photography and topographic models, we describe the structural geology and geomorphology of the rupture zone. During the earthquake, fissured pressure bulges (“mole tracks”) initiated at stepovers between synthetic Riedel (R) faults. As slip accumulated, near-surface “rafts” of cohesive clay-rich sediment, bounded by R faults and capped by grassy turf, rotated about a vertical axis and were internally shortened, thus amplifying the bulges. The bulges are flanked by low-angle contractional faults that emplace the shortened mass of detached sediment outward over less-deformed ground. As slip accrued, turf rafts fragmented into blocks bounded by short secondary fractures striking at a high angle to the main fault trace that we interpret to have originated as antithetic Riedel (R′) faults. Eventually these blocks were dispersed into strongly sheared earth and variably rotated. Along the fault, clockwise rotation of these turf rafts within the rupture zone averaged ~20°–30°, accommodating a finite shear strain of 1.0–1.5 and a distributed strike slip of ~3–4 m. On strike-slip parts of the fault, internal shortening of the rafts averaged 1–2 m parallel to the R faults and ~1 m perpendicular to the main fault trace. Driven by distortional rotation, this contraction of the rafts exceeds the magnitude of fault heave. Turf rafts on slightly transtensional segments of the fault were also bulged and shortened—relationships that can be explained by a kinematic model involving “deformable slats.” In a paleoseismic trench cut perpendicular the fault, one would observe fissures, low-angle thrusts, and steeply dipping strike-slip faults—some cross-cutting one another—yet all may have formed during a single earthquake featuring a large strike-slip displacement.

Исследование механического поведения эластомерных нанокомпозитов при больших циклических сдвиговых деформациях

The paper presents the results of experimental studies of viscoelastic properties of elastomeric nanocomposites based on styrene-butadiene rubber and various nanofillers: carbon black, detonation nanodiamonds, graphene nanoplates and single-walled graphene nanotubes. Corresponding tests were also carried out for pure unfilled rubber (as a basis for comparison and analysis of results). Non-standard samples and equipment specially designed for testing at finite shear strains (deformation by hundreds of percent) were used. In order to reveal the specific features of the viscoelastic behavior of the materials under consideration, test programs with complex loading trajectories were carried out — symmetric cyclic shear deformation with increasing amplitude and stops for relaxation when the direction of the tensile machine grips movement is changed. In addition, to evaluate the strength properties of these nanocomposites, they were all subjected to standard uniaxial tensile to rupture tests at a constant strain rate. The analysis of the obtained experimental data is carried out. It was found that the greatest enhancement effect is provided by the input of carbon nanotubes — several times in comparison with other samples. In this case, the largest increase in the hysteresis loop (i.e. dissipative loss) is also observed. This circumstance is very important when using such materials as damping elements. According to the authors, the effect of mechanical enhancement at large shear strains depends on the shape of the nanofiller particles. Nanotubes, due to their huge (compared to the transverse dimensions) length and flexibility, have a much stronger effect on the rearrangement of the composite microstructure compared to other types of fillers, thereby increasing dissipative losses

Theory of History-Dependent Multi-Layer Generalized Stacking Fault Energy: A Modeling of the Micro-Substructure Evolution Kinetics in Chemically Ordered Medium-Entropy Alloys

In this study, a chemical order related concept “history-dependent multi-layer generalized stacking fault energy” (HDML-GSFE) was proposed, and it was then demonstrated by employing the recent, very interesting multi-principal element alloy (CoCrNi medium-entropy alloys; MEA) with different chemical short-range order (CSRO) levels using a density functional theory (DFT)-based neural network interatomic potential. To demonstrate the impacts of the history dependency and interlayer (atomic interlayers of the slip system) coupling effect on the GSFE of CSRO MEAs, HDML-GSFEs were computed for different shear deformation pathways of the MEAs with different CSRO levels, such as interlayer multiple-time slipping, twin growth, and γ - e (FCC- HCP) phase transformation. It was demonstrated that multiple-time slipping induces CSRO collapse, leading to local shear softening due to the history dependency of GSFE. In addition, it was found that the slipping of neighboring atomic interlayers is affected by the slipping resulting from the induced CSRO collapse of present interlayers because of the interlayer coupling effect of GSFE. Eventually, by employing a novel kinetic Monte Carlo (kMC) simulation method based on dislocation/disconnection loop nucleation events and using the HDML-GSFE with the history dependency and interlayer coupling effect, we proposed a laminated micro-substructure evolution that involves twinning and γ - e phase transformations subject to a finite shear strain rate and finite temperature.

Out-of-equilibrium dynamical equations of infinite-dimensional particle systems. II. The anisotropic case under shear strain

As an extension of the isotropic setting presented in the companion paper Agoritsas et al (2019 J. Phys. A: Math. Theor. 52 144002), we consider the Langevin dynamics of a many-body system of pairwise interacting particles in d dimensions, submitted to an external shear strain. We show that the anisotropy introduced by the shear strain can be simply addressed by moving into the co-shearing frame, leading to simple dynamical mean field equations in the limit . The dynamics is then controlled by a single one-dimensional effective stochastic process which depends on three distinct strain-dependent kernels—self-consistently determined by the process itself—encoding the effective restoring force, friction and noise terms due to the particle interactions. From there one can compute dynamical observables such as particle mean-square displacements and shear stress fluctuations, and eventually aim at providing an exact benchmark for liquid and glass rheology. As an application of our results, we derive dynamically the ‘state-following’ equations that describe the static response of a glass to a finite shear strain until it yields.

Recrystallization processes, microstructure and crystallographic preferred orientation evolution in polycrystalline ice during high-temperature simple shear

Abstract. Torsion experiments were performed in polycrystalline ice at high temperature (0.97 Tm) to reproduce the simple shear kinematics that are believed to dominate in ice streams and at the base of fast-flowing glaciers. As clearly documented more than 30 years ago, under simple shear ice develops a two-maxima c axis crystallographic preferred orientation (CPO), which evolves rapidly into a single cluster CPO with a c axis perpendicular to the shear plane. Dynamic recrystallization mechanisms that occur in both laboratory conditions and naturally deformed ice are likely candidates to explain the observed CPO evolution. In this study, we use electron backscatter diffraction (EBSD) and automatic ice texture analyzer (AITA) to characterize the mechanisms accommodating deformation, the stress and strain heterogeneities that form under torsion of an initially isotropic polycrystalline ice sample at high temperature, and the role of dynamic recrystallization in accommodating these heterogeneities. These analyses highlight an interlocking microstructure, which results from heterogeneity-driven serrated grain boundary migration, and sub-grain boundaries composed of dislocations with a [c]-component Burgers vector, indicating that strong local stress heterogeneity develops, in particular, close to grain boundaries, even at high temperature and high finite shear strain. Based on these observations, we propose that nucleation by bulging, assisted by sub-grain boundary formation and followed by grain growth, is a very likely candidate to explain the progressive disappearance of the c axis CPO cluster at low angle to the shear plane and the stability of the one normal to it. We therefore strongly support the development of new polycrystal plasticity models limiting dislocation slip on non-basal slip systems and allowing for efficient accommodation of strain incompatibilities by an association of bulging and formation of sub-grain boundaries with a significant [c] component.

Development of crystallographic preferred orientation during cataclasis in low-temperature carbonate fault gouge

Grain size reduction due to cataclasis is a key process controlling fault frictional properties during the seismic cycle. We investigated the role of cleavage planes on fracturing and microstructural evolution during cataclasis in wet and dry carbonate fault gouges (50 wt% calcite, 50 wt% dolomite) deformed in a rotary-shear apparatus over a wide range of slip rates (30 μms−1 to 1 ms−1) and displacements (0.05–0.4 m). During shearing, progressive strain localization forms a narrow slip zone that undergoes significant frictional heating (at high slip rates), but the bulk gouge always accommodates low finite shear strains and deforms at low temperatures. Microstructural analysis of the bulk gouges indicates that deformation occurred by brittle fracturing and twinning. Microfractures in calcite are closely spaced, often exploit {101¯4} cleavage r-rhomb planes, and occur mainly subparallel to the expected principal stress orientation (σ1). Instead, twin planes typically occur sub-perpendicular to σ1. Electron backscatter diffraction analysis of the bulk gouges shows that calcite develops a well-defined crystallographic preferred orientation (CPO) at all investigated deformation conditions. The CPO is defined by a clustering of the calcite c-axes around an orientation sub-parallel to σ1. The calcite CPO is interpreted to result from grain rotation during granular flow, followed by brittle fracturing that occurred preferentially along calcite cleavage planes. This interpretation is supported by measurements of calcite grain shape-preferred orientations that show a population of elongate calcite grains oriented with their long axes sub-parallel to σ1. Our experimental results indicate that well-defined CPOs can form at low temperature in cataclastic fault rocks, and that mineral cleavage can strongly influence the evolution of grain sizes and shapes during comminution.

Experimental Techniques for Finite Shear Strain Measurement within Two Advanced High Strength Steels

There is a growing need to experimentally characterize local shear deformation in advanced high strength steels (AHSS) for the calibration of stress-state dependent fracture criteria and to better understand sheared edge cracking during secondary forming operations. Planar shear test specimens with digital image correlation (DIC) strain measurement are now commonly performed tests but may not be able to resolve the local strains during the final stage of fracture when the macroscopic shear band collapses to a micro-shear band with intense local strains. Studies of sheared edge stretching of AHSS have shown that the microstructure at the sheared edge experiences extreme local shear deformation with the shear-affected zone (SAZ) that can be much larger than the macroscopic strains reported using DIC on planar shear tests. In this work, two independent experimental techniques are proposed to characterize the residual strain distribution within the shear-affected zone for two AHSS grades with a similar strength level: a complex-phase (CP) steel, CP800, and a dual-phase (DP) steel, DP780. The first method uses finite strain theory to calculate the work-conjugate equivalent strain from grain rotations within the shear bands of interrupted in-plane shear tests. A comparison between the local DIC strain measurements and the grain rotation measurements were found to be in excellent agreement until just prior to failure. The second technique used micro-hardness measurements taken from the interrupted shear tests to develop correlations with the measured equivalent strain from the DIC system. The hardness and grain rotation techniques were then used to characterize the local strain distribution within the SAZ of hole expansion test specimens for punch clearances of 12% and 28%. Both methods provided similar strain distributions with the grain rotation method having the highest strain resolution. The residual strain field within the SAZ of both AHSS was found to be strongly dependent upon the punch clearance. Finally, a homogenization procedure was applied to the SAZ strain distributions to facilitate the length scale transition from the grain-level to length scales appropriate for finite-element modelling of sheet metal forming operations with sheared edges.

The role of anisotropy on the static and wave propagation characteristics of two-dimensional architectured materials under finite strains

In the current work, we analyze the role of anisotropy on the static and wave propagation characteristics of architectured materials. In particular, we study the mechanical behavior of nearly isotropic, moderately and highly anisotropic metamaterials under finite shear and normal strains. Thereupon, we quantify the effect of finite deformations with respect to their magnitude and loading direction, relating the metamaterials' initial degree of anisotropy to its finite strain static attributes. We distinguish between the effect of finite shear and normal strains, noting that highly anisotropic material designs do not uniquely entail shear strain sensitive structural behaviors. Contrariwise, we observe that finite normal loads in certain loading directions can lead to instabilities at rather low strain magnitudes already for moderately anisotropic material designs. Moreover, we show that strain-induced instabilities can be detected as negative phase velocity increments before the stiffness matrix loses its positive definiteness. What is more, we demonstrate that anisotropy and non-reciprocity can be used as mechanisms for wave propagation tuning. We provide evidence that for enhanced tuning capabilities to be observed, inner material density variations need to be exploited. The latter require the application of normal rather than shear strains, as shear deformations are volume preserving.

Geometry and the onset of rigidity in a disordered network.

Disordered spring networks that are undercoordinated may abruptly rigidify when sufficient strain is applied. Since the deformation in response to applied strain does not change the generic quantifiers of network architecture, the number of nodes and the number of bonds between them, this rigidity transition must have a geometric origin. Naive, degree-of-freedom-based mechanical analyses such as the Maxwell-Calladine count or the pebble game algorithm overlook such geometric rigidity transitions and offer no means of predicting or characterizing them. We apply tools that were developed for the topological analysis of zero modes and states of self-stress on regular lattices to two-dimensional random spring networks and demonstrate that the onset of rigidity, at a finite simple shear strain γ^{★}, coincides with the appearance of a single state of self-stress, accompanied by a single floppy mode. The process conserves the topologically invariant difference between the number of zero modes and the number of states of self-stress but imparts a finite shear modulus to the spring network. Beyond the critical shear, the network acquires a highly anisotropic elastic modulus, resisting further deformation most strongly in the direction of the rigidifying shear. We confirm previously reported critical scaling of the corresponding differential shear modulus. In the subcritical regime, a singular value decomposition of the network's compatibility matrix foreshadows the onset of rigidity by way of a continuously vanishing singular value corresponding to the nascent state of self-stress.

Ultramylonite generation via phase mixing in high‐strain experiments

AbstractDynamic recrystallization and phase mixing are considered to be important processes in ductile shear zone formation, as they collectively enable a permanent transition to the strain‐weakening, grain‐size sensitive deformation regime. While dynamic recrystallization is well understood, the underlying physical processes and timescales required for phase mixing remain enigmatic. Here, we present results from high‐strain phase mixing experiments on calcite‐anhydrite composites. A poorly mixed starting material was synthesized from fine‐grained calcite and anhydrite powders. Samples were deformed in the Large Volume Torsion apparatus at 500°C and shear strain rates of 5 × 10−5 to 5 × 10−4 s−1, to finite shear strains of up to γ = 57. Microstructural evolution is quantified through analysis of backscattered electron images and electron backscatter diffraction data. During deformation, polycrystalline domains of the individual phases are geometrically stretched and thinned, causing an increase in the spatial density of interphase boundaries. At moderate shear strains (γ ≥ 6), domains are so severely thinned that they become “monolayers” of only one or two grain's width and form a thin compositional layering. Monolayer formation is accompanied by a critical increase in the degree of grain boundary pinning and, consequently, grain‐size reduction below the theoretical limit established by the grain‐size piezometer or deformation mechanism field boundary. Ultimately, monolayers neck and disaggregate at high strains (17 <γ <57) to complete the phase mixing process. This “geometric” phase mixing mechanism is consistent with observations of mylonites, where layer (i.e., foliation) formation is associated with strain localization, and layers are ultimately destroyed at the mylonite‐ultramylonite transition.

Shear Yielding and Shear Jamming of Dense Hard Sphere Glasses.

We investigate the response of dense hard sphere glasses to a shear strain in a wide range of pressures ranging from the glass transition to the infinite-pressure jamming point. The phase diagram in the density-shear strain plane is calculated analytically using the mean-field infinite-dimensional solution. We find that just above the glass transition, the glass generically yields at a finite shear strain. The yielding transition in the mean-field picture is a spinodal point in presence of disorder. At higher densities, instead, we find that the glass generically jams at a finite shear strain: the jamming transition prevents yielding. The shear yielding and shear jamming lines merge in a critical point, close to which the system yields at extremely large shear stress. Around this point, highly nontrivial yielding dynamics, characterized by system-spanning disordered fractures, is expected.

Evolution and progressive geomorphic manifestation of surface faulting: A comparison of the Wairau and Awatere faults, South Island, New Zealand: COMMENT

It is well known that continuum deformation of media with strain weakening rheology causes deformation zones to progressively decrease in width and structural complexity as shear strain is localized (Ben-Zion and Sammis, 2003). Strain localization occurs on faults at seismogenic depth (e.g., Chester and Chester, 1998; Boulton et al., 2012) and at the surface, as evidenced from detailed mapping of surface ruptures and analogue experiments in granular media (Tchalenko, 1970). In Geology, Zinke et al. (2015) hypothesize that large differences in accumulated strike-slip displacement on the Wairau (~460 km) and Awatere (~20 km) faults in New Zealand have resulted in narrower and less complex off fault deformation zones (OFD) along the Wairau fault despite similar slip rates, lithologies, and tectonic settings. Their hypothesis is not supported by the data they present because (1) the compared sites are structurally and topographically dissimilar, and these differences may independently account for the observed rupture patterns, (2) large along-strike variations in the width and complexity of surface rupture zones occur along other nearby analogous faults with uniform structural maturity, (3) wide and structurally complex OFD zones are common features along faults with more accumulated slip and greater structural maturity, and (4) strain localization in granular media with simple boundary conditions occurs at low shear strain, implying that a factor other than structural maturity may be responsible for variations in OFD width and complexity. The Saxton River site (SR) is located at a restraining bend along the Awatere Fault, where the fault strike averaged over a distance of 15 km along the surface trace in both directions changes ~12, from 64±1 NE of SR to 76.5±0.5 SW of SR). The trace of the Wairau Fault either side of the Branch River (BR) site is comparably straight (NE=67±0.5, SW = 61±0.5) with the possibility of a slight (~6) releasing divergence in fault strike. On the nearby Hope Fault, Khajavi et al (2014) showed increases in OFD width and complexity correspond with deviations in fault strike of ~10-20. Restraining bends on strike slip faults are commonly associated with an increase in the width and complexity of surface ruptures (e.g., Duffy et al., 2014). The SR and BR sites are also topographically distinct. Topographic relief on the Awatere fault trace exceeds 250 m within 6 km of the SR site whereas the Wairau Fault trace is largely confined to the Wairau River valley with 400 m, where it is manifested as a structurally complex mosaic of faults and folds, along strike lengths of 400 km of lateral displacement (Sutherland, 1999) and is one of Earth’s most structurally mature active faults, are kinematically partitioned and notoriously complex, with surface rupture zones commonly exceeding 300 m (Barth et al., 2012). Clearly, structural maturity is not a primary control of OFD complexity and width variations along these faults. Strain localization onto structurally simple, continuous strike-slip faults at the surface occurs at low finite shear strains in quasihomogeneous granular materials with simple boundary conditions (e.g., steep underlying fault, low topographic relief). Shear localization occurred after lateral surface displacements of ~3m in the Dasht-e Bayaz earthquake (Tchalenko and Ambraseys, 1970) and ~2-3 m in the Darfield earthquake (Van Dissen et al., 2013). Perhaps other controls on surface rupture morphology, including near-surface strain localization (independent of fault maturity) and variations in material properties of faulted media, depth-to-basement, near-fault topography, and fault strike need to be more carefully considered in future analyses.

Semi‐brittle flow of granitoid fault rocks in experiments

AbstractField studies and seismic data show that semi‐brittle flow of fault rocks probably is the dominant deformation mechanism at the base of the seismogenic zone at the so‐called frictional‐viscous transition. To understand the physical and chemical processes accommodating semi‐brittle flow, we have performed an experimental study on synthetic granitoid fault rocks exploring a broad parameter space (temperature, T = 300, 400, 500, and 600°C, confining pressure, Pc ≈ 300, 500, 1000, and 1500 MPa, shear strain rate, ≈ 10−3, 10−4, 10−5, and 10−6 s−1, to finite shear strains, γ = 0–5). The experiments have been carried out using a granular material with grain size smaller than 200 µm with a little H2O added (0.2 wt %). Only two experiments (performed at the fastest strain rates and lowest temperatures) have failed abruptly right after reaching peak strength (τ ~ 1400 MPa). All other samples reach high shear stresses (τ ~ 570–1600 MPa) then weaken slightly (by Δτ ~ 10–190 MPa) and continue to deform at a more or less steady state stress level. Clear temperature dependence and a weak strain rate dependence of the peak as well as steady state stress levels are observed. In order to express this relationship, the strain rate‐stress sensitivity has been fit with a stress exponent, assuming γ̇ ∝ τn and yields high stress exponents (n ≈ 10–140), which decrease with increasing temperature. The microstructures show widespread comminution, strain partitioning, and localization into slip zones. The slip zones contain at first nanocrystalline and partly amorphous material. Later, during continued deformation, fully amorphous material develops in some of the slip zones. Despite the mechanical steady state conditions, the fabrics in the slip zones and outside continue to evolve and do not reach a steady state microstructure below γ = 5. Within the slip zones, the fault rock material progressively transforms from a crystalline solid to an amorphous material. We present and interpret the experimental results both in terms of sliding friction and viscous flow, and we discuss the possible effect that the formation of nanocrystalline and amorphous layers may have on earthquake nucleation.