Orientation Of Stress Axes Research Articles

Focal mechanisms of earthquakes in southern Quebec, southeastern Ontario, and northeastern New York with implications for regional seismotectonics and stress field characteristics

Focal mechanisms have been derived for 30 earthquakes in the Adirondack Mountains-western Quebec seismic zone from a modified version of the computer program FOCMEC using first motions of Pn and Pg waves, and SV/P amplitude ratios measured from Sg and Pg phases. Reverse faulting on medium to steeply dipping planes is prevalent. Focal depth and velocity structure can vary within realistic limits without much influence on the focal mechanism solution. Deviatoric compressive-stress axes have a strong horizontal component, but the trend varies. Most align approximately northeast-southwest, in accordance with both the orientation predicted by models based on plate tectonics and that derived from most other studied earthquakes in the region. However, for several earthquakes, mechanisms have been obtained that deviate from this pattern, notably for events near 45.5°N to 46°N, 75°W, and the North Gower main shock-aftershock series of October 1983. Possible explanations for these anomalies in terms of local structural disturbances, or reactivation of preexisting zones of weakness with different structural orientation than those created during the current tectonic regime, are presented . Different hypotheses for the seismotectonic evolution and processes of the area are presented and discussed. In addition to orientations of nodal planes and deviatoric stress axes, there are other indicators, such as the seismicity distribution and patterns of earthquake sequences, of inhomogeneity in the proposed Adirondack Mountains-western Quebec seismic zone .

A possible change in stress field orientation due to the 1976 Tangshan earthquake

Two techniques are used to study the stress field in the Tangshan region. First, the mean directions of principal stress axes in three subregions of the Tangshan aftershock region are determined from 29 focal mechanism solutions. In doing so a slip-fitting technique is applied which determines the mean stress tensor by minimizing the angle between the observed slip direction on every fault plane and the calculated shear stress resolved onto the fault planes. Each of the two nodal planes of the focal mechanism solutions are alternately considered as a possible fault plane. Second, by making a grid search over all possible orientations of stress axes, the meanP,B andT axes in the Tangshan region, both before and after the Tangshan mainshock, are found on the basis of firstP-wave polarity readings from a number of small earthquakes. The results indicate that there is a possible azimuthal change of the principal stress axes in the focal region due to the occurrence of the mainshock.

Source mechanisms and focal depths of East African earthquakes using Rayleigh-wave inversion and body-wave modelling

Summary. The source mechanisms and focal depths of 23 African earthquakes, most of them associated with the East African rift system, have been accurately determined using Rayleigh-wave inversion and body-wave modelling techniques. In so doing we have demonstrated the viability of applying these techniques to intermediate-sized earthquakes in regions of relatively sparse seismicity but high tectonic interest. Earthquake source mechanisms in East Africa are mostly of the normal faulting type. Least compressive stress axes for the events considered exhibit a consistent orientation normal to the rifting trends over a wide region of both the eastern – and western branches of the rift system as well as in seismically active but currently unrifted regions to the south. The mechanisms indicate that the stress system responsible for the current rift faulting is still operative and that the East African rift system may be actively continuing southwards. The length scale of relative homogeneity over which we can use phase velocities determined using a well-constrained reference event or events to retrieve the complex spectra of a neighbouring event is at least 500–600 km for several regions of the East African rift system. The implication is that the Rayleigh-wave travel paths through Africa are largely stable and that lateral heterogeneity in East Africa is not as great as might have been expected in a complex zone of continental rifting. The mechanisms of three events in southernmost Africa show this region to be under relative E–W compression, in contrast to the rifted regions to the north which are subject to general E–W tension. Many of the East African events studied have well-constrained focal depths of 15–30 km, depths that are unusually deep for a region of continental extension if lithospheric thinning by the relative upward movement of asthenosphere is the process responsible for rifting. The consistency of focal mechanisms and tensional stress axis orientations for East African earthquakes having widely ranging epicentres and focal depths suggests that a large-scale process is involved in the continental rifting of East Africa. The results of this study are consistent with the idea that rifting in East Africa is connected with passage of the East African lithosphere over a linear thermally anomalous region in the underlying mantle. It is shown that this idea can also account for many of the temporal and spatial aspects of the East African rift system.

Tectonic analysis of fault slip data sets

Using data that include the direction and the sense of motion on individual fault surfaces determined by slickenside lineations, it is possible to reconstruct reduced stress tensors that correspond to the orientation of stress axes and to the ratio ϕ = (σ2−σ3)/(σ1−σ3) between principal stress values (σ1≥σ2≥σ3, compression being positive). No assumption is made concerning the orientation of fault planes relative to stress axes, so that reactived faults are taken into account as well as newly created ones. Qualitative and quantitative methods for analysis of fault slip data were developed during the last 10 years. The practical limitation of the methods and the necessity for critical field observations are emphasized. These methods can be applied to focal mechanisms of earthquakes. A more complex analysis of heterogeneous data sets, involving an iterative separation of different stress systems, is also discussed. This analysis enables one to distinguish successive faulting events. Careful qualitative study in the field is in all cases essential.

The distribution of earthquakes with depth and stress in subducting slabs

The global variation of Benioff zone seismicity with depth and the orientation of stress axes of deep and intermediate earthquakes is explained using numerical models of subducting slabs. Models that match the seismicity and stress require a barrier to flow at the 670 km seismic discontinuity. The barrier may be a viscosity increase of at least an order of magnitude or a chemical discontinuity. Instantaneous flow is subparallel to the slabs for models with a viscosity increase but contorted for models with a chemical barrier. Log N (number of earthquakes) decreases linearly to 250-300 km depth and increases thereafter. Stress magnitude in the models shows the same pattern, in accord with experiments showing N proportional to e(k-sigma), with k a constant and sigma the stress magnitude. The models predict downdip compression in the slabs at depths below 300-400 km, as observed for earthquake stress axes.

Geometry of the subducted lithosphere beneath the Banda Sea in eastern Indonesia from seismicity and fault plane solutions

The spatial distribution of hypocenters in eastern Indonesia, together with 41 new and previously published fault plane solutions, can be explained by a simple model of two lithospheric plates descending into the upper mantle beneath the Banda Sea. The major one, defined by the shallow to deep hypocenters located along the Banda arc, is a laterally continuous slab that has subducted at the plate boundary defined by the Java trench‐Timor trough‐Aru trough system. The other slab descends toward the southwest to depths of about 100 km in the region of the Seram trough and may be joined to the Banda arc subduction system by the westward extension of the New Guinea Tarera‐Aidoena fault zone, which acts as an arc‐to‐arc transform. The Banda arc slab is contorted at the eastern end of the arc where the trench and the line of active volcanoes curve to the northeast. The contortion appears to be a lateral bend in the subducted slab that is continuous from the surface to depths of 600 km. Fault plane solutions in the contorted portion of the slab have P axes aligned approximately parallel to the local strike of the slab and may be related to lateral bending stresses. Similar orientations of stress axes also occur near the curved ends of other island arcs. Experiments with a lead sheet model suggest that the observed configuration of the Banda slab may be simply related to the curved end of the Banda arc and further suggest that the direction of relative motion between the Banda arc and Australia during the Neogene has been more north‐northwesterly than the direction predicted from major plate motions. The discrepancy can be resolved if Southeast Asia is not considered part of the Eurasian plate. The lack of shallow underthrusting mechanisms and the paucity of shallow earthquakes in the Timor trough‐Aru trough region may have resulted from the youthful collision of the Australian shelf with the Banda arc.

The temperature and orientation dependence of yielding in Mar-M200 single crystals

Abstract The temperature dependence of the yielding behavior of Mar-M200 crystals has been studied during loading at a constant stress rate. Evidence is presented for a shift in the temperature of the yield stress maximum in the range of 1400–1550°F as a function of stress axis orientation, and for slip on cube planes at temperatures greater than 1200° F for a stress axis parallel to 〈111〉. Despite the suppression of the yield stress in the 〈111〉 orientation due to the onset of cube slip, the yield stress in this orientation is higher than in any other orientation, at least up to 1600° F. This results is consistent with the previously determined high resistance to creep at intermediate temperatures in the 〈111〉 orientation.

The effect of orientation and sense of applied uniaxial stress on the morphology of coherent gamma prime precipitates in stress annealed nickel-base superalloy crystals

The coarsening of coherent γ′[Ni3(Al, Ti)] precipitates in single crystals of a representative nickel-base superalloy, Udimet-700, is shown to be affected by a uniaxial stress applied during annealing. Depending on the sense of the applied stress and its crystallographic orientation, stress annealing results in oriented cuboidal, plate, or parallelepiped shaped γ′ precipitates. A general thermodynamic analysis of the effect of stress annealing on precipitate morphology is presented that takes into account free energy changes due to changes in bulk precipitation strain, effective modulus, coherency strain energy, and the total interphase boundary area. The analysis correctly predicts the observed γ′ precipitate morphologies as a function of stress axis orientation, stress sense, the lattice misfit of the precipitate phase, and the elastic constants of the matrix and precipitate phases. The analysis also shows that stress induced morphological changes can be completely precluded, as may be desired to optimize mechanical behavior, only if the elastic constants of the matrix and precipitate phases are equal. Changes in morphology due to changes in bulk precipitation strain, which in the case of Udimet-700 is shown to be the dominant effect, can be eliminated by alloying for zero lattice misfit or, in single crystals, by stressing parallel to . Applications to long-term creep behavior and to the fabrication of composite structures are discussed.

Creep of aluminum strengthened by alumina particles

Abstract Creep of aluminum strengthened by various amounts of alumina particles (4, 7, 10 and 14 wt.%) was investigated in the temperature interval 295–870°K by isothermal tests technique and by microscopy techniques. The major part of alumina particles, having a form of flakes, was situated on grain boundaries, the dislocation density in the grains was very low. The whole temperature interval investigated can be divided into two regions, low temperature Region 1 and high temperature Region 2, separated from each other by a transition region. In both Region 1 and Region 2 the temperature and stress dependence of steady state creep rate ϵg3s, can be described by the equation ϵ g3 s = A(T) exp [B(T,f)σ)] , where A = A 0 exp [ −H sd RT ] , A0 being a function of structure only, Hsd meaning the activation enthalpy of lattice self-diffusion in aluminum, f the volume fraction of alumina and σ the applied tensile stress. The dependence of the parameter B on f in the range of f investigated can be approximately described by the equation B(T,f) = B0.(T)f−m where B0 depends on temperature only and m is equal to about 1 2 In Region 1 (∼295 to ∼500°K) the parameter B decreases with increasing temperature, the apparent activation energy Qc = ∂ In ϵ g3 s /∂( −1 RT ) is temperature independent but it decreases l increasing stress. Lattice self-diffusion probably controls creep in this region, however, the rate controlling dislocation mechanism has not been identified. In Region 2 (above ∼620°K) B increases with temperature. The apparent activation energy Qc increases with increasing temperature and increasing stress and achieves values up to about 250 kcal mol−1. Creep is probably controlled by lattice self-diffusion in aluminum in Region 2, and, as it is not connected with any observable change in dislocation substructure, grain boundary slidings were suggested to be the main deformation mechanism of creep in this region. A model was proposed explaining the strong creep rate dependence on the tensile stress axis orientation with respect to the rolling direction.

Petrofabrics: A critical review

Petrofabric contributions are here classified into seven categories, namely: (1) descriptive techniques; (2) kinematic analyses; (3) geometric analyses of folds; (4) absolute strain analyses; (5) studies of experimentally deformed single crystals and rocks; (6) dynamic analyses of deformation features in naturally deformed rocks; and (7) studies for engineering rock mechanics. The nature of each subfield is given, and items (2) and (6) are discussed in detail.The principles of kinematic analysis through symmetry are reviewed with the aid of simple models of brittle and ductile deformations in which componental displacements are specified and the symmetry of the initial and final fabric is known. The models illustrate the degree of similarity among the symmetries of the fabric, componental displacements, deformation (strain and rigid-body rotation), and states of stress that exist in rocks at the time a fabric element forms. Both isotropic and anisotropic models are considered. In the more realistic cases the symmetry of the pattern of componental displacements is identical to the symmetry of the resulting fabric. If the body behaves isotropically during the deformation, symmetry elements in the final fabric will be common to corresponding elements in the symmetries of the states of stress and deformation. If, however, the body behaves anisotropically one cannot relate symmetry elements in the final fabric to the symmetry of the states of stress and deformation without making assumptions about the symmetry of the anisotropy. The study of fabric symmetry may contribute some detail to the geometric pattern of the strain or displacement fields, but is otherwise of little use in furthering understanding of the mechanics of geologic deformation.Results of dynamic analysis, which involve determination of the orientations and relative magnitudes of principal stresses in naturally deformed rocks, are critically reviewed. The principal axes inferred from fractures, twin lamellae, deformation lamellae, etc., are those within the material at the instant the fabric element forms. As such they correspond to the incremental stresses which are coincident with the incremental strains in linear materials. Certain generalizations are drawn from previous work on slightly and moderately deformed rocks in which the orientation of the incremental stresses and strains are probably coincident with the total stresses and strains. The critique leads to a working philosophy to guide future studies, namely: dynamic petrofabric analyses provide the means for testing hypotheses generated through theoretical and experimental studies of rock deformation. A study should be designed to test specific hypotheses. Samples should be collected from critical domains where the results may provide a basis of choice between alternative hypotheses. The techniques need to be refined to distinguish small differences in the orientations of principal stress axes deduced from fabric data. The chronological sequence in the development of the fabric elements requires attention. Several suggestions along these lines are offered.

Plasticity and Creep in Single Crystals of Zirconium Carbide

High‐temperature deformation in ZrC single crystals was studied. Seeded crystals were grown by a direct rf‐coupling floating‐zone process. Yield stresses were measured from 1080° to 2000°C as a function of stress axis orientation. The Burgers vector was shown to be parallel to the 〈110〉 axes by transmission electron microscopy. Slip was observed on {100}, {110}, or {111} planes, depending on the orientation of the stress axis; it always occurred on the most favorably oriented slip system. The dependence of steady‐state creep rate on the applied stress indicated that recovery occurred by a dislocation climb mechanism. Examination of the dislocation structure in deformed crystals by transmission electron microscopy supported this conclusion.

Principal Stress Directions from Plastic Flow in Crystals

Methods for determining orientations of principal stress axes in deformed rocks involve dynamic analysis of twin-gliding and of extinction bands produced by inhomogeneous translation gliding in crystals. The methods, beginning with Turner9s (1953) technique for dynamic analysis of calcite twins, have been developed using as guides the results from experiments under controlled laboratory conditions. Structures induced by intragranular flow in calcite, dolomite, quartz, micas, orthopyroxenes, clinopyroxenes, olivine, and other common rock-forming materials, may now be used to derive orientations of principal stresses causing the deformation. The various methods, some new, are discussed in detail and examples of their application to tectonites are given. The usefulness of such studies is illustrated by evaluating the observed orientations of principal stresses around folds in light of new data from a theoretical analysis of large amplitude folding of viscous layers in a less viscous matrix. Other areas of research in structural geology in which these methods should prove useful have also been outlined.

On the temperature dependence of flow stress in quenched aluminum crystals

An investigation was made of the quenching effects on the temperature dependence of flow stresses in aluminum crystals. The orientations of stress axes of single crystals of aluminum are shown. The samples were strained in an Instron machine, utilizing a specially designed tensile fixture and cryostat at combinations of the following temperatures: 78, 200, 273, and 293 deg K. In addition to the cycling tests, uninterrupted tensile tests on crystals with widely different orientations were performed at 78 and 293 deg K. Cycling-test results indicate that quenched crystals not only behave in a manner similar to fully annealed ones, but the numerical values for this relation are quite nearly identical. A plot of the results indicated that the flow stress ratio is independent of dislocation density increase produced by plastic deformation. Additional identical behavior was seen in the presence of work softening in the load-extension curves. The evidence presented seems to indicate that, despite the apparent extreme changes in the quenching deformation process, certain areas remain unaffected. This leads to the suggestion that the forest effect is important in explaining the mechanical behavior of aluminum crystals independent of prior thermal history. (B.O.G)

Plastic deformation of silver chloride. II. Photoelastic study of the internal stresses in glide packets

The pattern of birefringence caused by glide in silver chloride is most clearly resolved when the glide planes are observed edge on. It is then possible to sketch the two orthogonal sets of curves giving the directions of polarization at each point of the pattern and, by means of a compensator, to find how the magnitude of the birefringence varies from point to point. The stress distribution can be deduced from the distribution of birefringence, provided that the important influence of the relative orientation of stress axes and crystal axes is taken into account. The state of stress in each plastically deformed grain can be resolved into two parts. (1) Each glide packet is subject to pure bending about two perpendicular axes lying in the glide plane (but with a neutral plane that is not always the middle plane of the packet). This gives stresses that vary discontinuously with a period equal to the spacing of the glide zones. (2) There is also a continuously varying stress distribution which changes only slightly in distances equal to the glide zone spacing. The curvature of the glide packets means either that all the dislocations in one glide zone have the same sign or that positive and negative dislocations are present there in unequal numbers. It is found that, throughout any one grain, the ‘ dislocation charge ’ on each zone (to use an obvious electrical analogy) has the same sign and is never more than about twice the average for the grain.