Showing papers on "Stress field published in 2010"

Stress Field of the Earth's Crust

Abstract: Foreword Preface Dedication Acknowledgement List of Permissions 1. Introduction 1.1 Stresses in a Body 1.2 Importance of Rock Stress 1.3 History of Interest in Rock Stress Part I - Definition and Terminology 2. Stress Definition 2.1 Stress Tensor 2.2 Principal Stresses 2.3 Mohr Circle of Stress 2.4 Visualizing Stress 3. Rock Fracture Criteria 3.1 Phenomenological Theories 3.2 Mechanistic Failure Theories 3.3 Fracture Mechanics 3.4 Nonlinear Fracture Mechanics 4. Rock Stress Terminology 4.1 Gravity Stress 4.2 Tectonic Stress 4.3 Residual Stress 4.4 Structural Stress 5. Crustal Stress Models 5.1 Lithostatic Stress 5.2 Biaxial State of Stress 5.3 Tectonic Stress Field 5.4 Effective Stress 5.5 Laboratory Stress Profiles Part II - Measuring Stress 6. Physics of Stress Measurements 6.1 Mechanical Methods 6.2 Strain Gages 6.3 Diffraction Methods 6.4 Optical Methods 6.5 Ultrasonic Wave Speed 6.6 Micromagnetic Method 7. Measuring Crustal Stress - Borehole Methods 7.1 Classification of Measurement Techniques 7.2 Hydraulic Fracturing 7.3 Borehole Breakouts 8. Measuring Crustal Stress - Core-Based Methods 8.1 Anelastic Strain Recovery 8.1.1 Rheological Basis 8.1.2 Relaxation Apparatus 8.2 Kaiser Effect Part III - Interpreting Stress Data 9. Local Stress Data 9.1 Continental Deep Drilling Site KTB, Germany 9.2 Nuclear Waste Site Olkiluoto, Finland 9.3 San Andreas Fault Observatory at Depth, USA 10. Generic Stress Data 10.1 Magnitude-Depth Profiles 10.2 Orientation Maps and Smoothing 10.3 Stress State-Scale Relations 10.4 Best-Estimate Stress Model 11. Global Stress 11.1 European Stress 11.2 World Stress Map 11.3 Plate Tectonic Interpretation Epilogue Stress References Index Stress Movies Content on DVD-ROM DVD-ROM included inside back cover

The application of failure mode diagrams for exploring the roles of fluid pressure and stress states in controlling styles of fracture‐controlled permeability enhancement in faults and shear zones

Abstract: Geofluids (2010) 10, 217–233 Abstract Permeability enhancement associated with deformation processes in faults and shear zones plays a key role in facilitating fluid redistribution between fluid reservoirs in the crust. Especially in high fluid flux hydrothermal systems, fracture-controlled permeability can be relatively short-lived, unless it is repeatedly regenerated by ongoing deformation. Failure mode diagrams in pore fluid factor and differential stress space, here termed λ–σ failure mode diagrams, provide a powerful tool for analysing how fluid pressure and stress states drive failure, associated permeability enhancement and vein styles during deformation in faults and shear zones. During fault-valve behaviour in the seismogenic regime, relative rates of recovery of pore fluid factor, differential stress and fault cohesive strength between rupture events impact on styles of veining and associated, fracture-controlled permeability enhancement in faults and shear zones. Examples of vein-rich fault zones are used to illustrate how constraints can be placed, not just on fluid pressure and stress states at failure, but also on the fluid pressurization and loading paths associated with failure and transitory permeability enhancement in faults and shear zones. This provides insights about when, during the fault-valve cycle, various types of veins can form. The use of failure mode diagrams also provides insights about the relative roles of optimally oriented faults and misoriented faults as hydraulically conductive structures. The analysis highlights the dynamics of competition between fluid pressures and loading rates in driving failure and repeated permeability regeneration in fracture-controlled, hydrothermal systems.

Stress, stress asymmetry and couple stress: from discrete particles to continuous fields

Abstract: Explicit and closed expressions for the stress and couple-stress fields for discrete (classical) mechanical systems in terms of the constituents’ degrees of freedom and interactions are derived and compared to previous results. This is done by using an exact and general coarse graining formulation, which allows one to predetermine the resolution of the continuum fields. Since the full dynamics of the pertinent fields is considered, the results are not restricted to static states or quasi-static deformations; the latter comprise mere limiting cases, which are discussed as well. The fields automatically satisfy the equations of continuum mechanics. An explicit expression for the antisymmetric part of the stress field is presented; the question whether the latter vanishes, much like its nature when it does not, have been debated in the literature. Physical explanations of some of the obtained results are offered; in particular, an interpretation of the expression for the stress field provides an argument in favor of its uniqueness, yet another topic of debate in the literature. The formulation and results are valid for single realizations, and can of course be used in conjunction with ensemble averaging. Part of the paper is devoted to a biased discussion of the notion of coarse graining in general, in order to set the presented results in a certain perspective. Although the results can be applied to molecular (nanoscale included) and granular systems alike, the presentation and some simplifying assumptions (which can be easily relaxed) target granular systems. The results should be useful for the analysis of experimental and numerical findings as well as the development of constitutive relations.

Elastic moduli evolution and accompanying stress changes with increasing crack damage: implications for stress changes around fault zones and volcanoes during deformation

Abstract: SUMMARY The elastic moduli of rock in areas susceptible to crack damage, such as within fault zones or volcanic edifices, can be subject to large modifications. Knowledge of how elastic moduli may vary in such situations is important for both the reliable modelling of volcano deformation and stability and for linear and non-linear elastic crack models for earthquake rupture. Furthermore, it has previously been shown that changes in elastic moduli can induce changes in the stress field surrounding faults. Here we report both uniaxial experimental measurements of changes in elastic moduli during increasing-amplitude cyclic stressing experiments on a range of different rock types (basalts, sandstones and granite) and the results of modelled stress modifications. The trend in elastic moduli evolution with increasing damage was remarkably similar for each rock type, with the exception of essentially crack-free intrusive basalt that exhibited very minor changes. In general, Young’s modulus decreased by between 11 and 32 per cent and Poisson’s ratio increased by between 72 and 600 per cent over the total sequence of loading cycles. These changes are attributed to an increasing level of anisotropic crack damage within the samples. Our results also show that acoustic emission (AE) output during any loading cycle only commenced when new crack damage was generated. This corresponded to the level of stress where AE ceased during the unloading portion of the previous cycle. Using the multilayer elastic model of Faulkner et al. we demonstrate that the damage-induced changes in elastic moduli also result in significant decreases in differential stress, increases in mean stress and rotation of the applied greatest principal stress relative to the orientation of the mechanical layering. The similar trend in the evolution of the elastic moduli of all the rocks tested suggests that stress modification in the damage zone of faults might take the same form, regardless of the lithology through which the fault runs. These observations are discussed in terms of their applicability to both fault zones and deformation at volcanoes.

In situ stress state in the Nankai accretionary wedge estimated from borehole wall failures

Abstract: We constrain the orientations and magnitudes of in situ stress tensors using borehole wall failures (borehole breakouts and drilling-induced tensile fractures) detected in four vertical boreholes (C0002, C0001, C0004, and C0006 from NW to SE) drilled in the Nankai accretionary wedge. The directions of the maximum horizontal principal stress (SHmax), indicated by the azimuths of borehole wall failures, are consistent in individual holes, but those in C0002 (margin-parallel SHmax) are nearly perpendicular to those in all other holes (margin-normal SHmax). Constrained stress magnitudes in C0001 and C0002, using logged breakout widths combined with empirical rock strength derived from sonic velocity, as well as the presence of the drilling-induced tensile fractures, suggest that the stress state in the shallow portion of the wedge (fore-arc basin and slope sediment formations) is predominantly in favor of normal faulting and that the stress state in the deeper accretionary prism is in favor of probable strike-slip faulting or possible reverse faulting. Thus, the stress regime appears to be divided with depth by the major geological boundaries such as unconformities or thrust faults. The margin-perpendicular tectonic stress components in the two adjacent sites, C0001 and C0002, are different, suggesting that tectonic force driven by the plate pushing of the Philippine Sea plate does not uniformly propagate. Rather, the stress field is inferred to be influenced by additional factors such as local deformation caused by gravitation-driven extension in the fore arc and thrusting and bending within individual geologic domains.

The dynamics of the eastern Mediterranean and eastern Turkey

Abstract: SUMMARY In this study we investigate the dynamics of the region that includes Greece, the Aegean Sea and Asia Minor. In a least-squares inversion we solve for a continuous strain rate field, and corresponding velocity field, that satisfies 872 GPS data. The estimate of the geodetic strain rate field provides constraints for our dynamic analysis. Next, we separately solve the depth integrated 3-D force balance equations for depth-integrated deviatoric stresses within the lithosphere, in which body force input comes from differences in vertically integrated vertical stress, or differences in gravitational potential energy per unit area (GPE). These GPE estimates calibrate the absolute magnitudes of deviatoric stresses that are acting within the lithosphere. Further, we investigate the sensitivity of our stress field solutions by using two different crustal structure models: one from compiled crustal structure estimates obtained primarily from relatively recent seismic observations and the other from the Crust 2.0 model. In an iterative least-squares inversion we then solve for stress field boundary conditions that, when added to the contribution of deviatoric stresses associated with GPE differences, provides a best fit to the directions of principal axes and relative magnitudes of the principal axes of the rates of strain obtained in the kinematic analysis. Robust features that arise from the boundary condition solution are NNE forcing along the southern boundary east of about 33° E (0.5–1.2 × 1012 N m−1), with a rapid anticlockwise rotation of forces to the west of this, along with an outward pulling force (∼0.4 × 1012 N m−1) directed SSW along the entire Hellenic Arc segment. This force system along the Hellenic Arc can be interpreted as a result of slab rollback. The total depth integrated 3-D deviatoric stresses in the final dynamic solution provides an excellent match to the deformation indicators throughout the region, with vertically integrated stress magnitudes of order 0.5–2.5 × 1012 N m−1. We use constraints from derived stress magnitudes, together with GPS-defined scalar values of strain rate magnitude, to define bulk effective viscosities of the lithosphere. Depth-averaged effective viscosities for the entire lithosphere are high within the Black Sea, of order 0.7–3 ×1023 Pa-s, relative to surrounding continental lithosphere. North Anatolian shear zone, northern Aegean Sea and Gulf of Corinth are characterized by low depth averaged viscosities of order 1–5 ×1021 Pa-s. Deviatoric stresses from GPE differences and boundary condition effects combine in surprising ways in some regions, resulting in near total stress cancellation in areas such as the southern Aegean Sea and portions of the central Anatolian block. GPE differences combine with boundary condition effects along the eastern segment of the North Anatolian Fault (NAF) in a way that is compatible with the hypothesis that motion on the NAF was facilitated by slab detachment beneath East Anatolia and dynamic uplift of East Anatolian Plateau. In general, GPE differences play a nearly equal role as boundary condition influences in their contribution to the total deviatoric stress field. The low depth integrated deviatoric stress magnitudes throughout the region suggest that zones of active deformation are facilitated by dramatic weakening mechanisms throughout the lithosphere.

State of stress in central and eastern North American seismic zones

Abstract: We use a Bayesian analysis to determine the state of stress from focal mechanisms in ten seismic zones in central and eastern North America and compare it with regional stress inferred from borehole measurements. Comparisons of the seismologically determined azimuth of the maximum horizontal compressive stress (SHS) with that determined from boreholes (SHB) exhibit a bimodal pattern: In four zones, the SHS and regional SHB orientations are closely parallel, whereas in the Charlevoix, Lower St. Lawrence, and Central Virginia zones, the SHS azimuth shows a statistically significant 30°–50° clockwise rotation relative to the regional SHB azimuth. This pattern is exemplified by the northwest and southeast seismicity clusters in Charlevoix, which yield SHS orientations strictly parallel and strongly oblique, respectively, to the regional SHB trend. Similar ∼30° clockwise rotations are found for the North Appalachian zone and for the 2003 Bardwell earthquake sequence north of the New Madrid zone. The SHB/SHS rotations occur over 20–100 km in each seismic zone, but they are observed in zones separated by distances of up to 1500 km. A possible mechanism for the stress rotations may be the interaction between a long-wavelength stress perturbation source, such as postglacial rebound, and local stress concentrators, such as low-friction faults. The latter would allow low-magnitude (<10 MPa) postglacial rebound stresses to locally perturb the preexisting stress field in some seismic zones, whereas postglacial rebound stresses have little effect on the intraplate state of stress in general.

On Higher Order Terms in the Crack Tip Stress Field

Abstract: The influence of higher order terms on the stress field of a cracked plate under in plane loading may not be negligible in some engineering applications. The aim of this note is to present a more accurate analysis of these terms.

Imaging hydraulic fractures in a geothermal reservoir

Abstract: [1] An injection experiment at the Coso geothermal field in eastern California in March 2005 caused a swarm of microearthquakes that was recorded by a local network of three-component digital seismometers. High-resolution relative hypocenter locations propagated upward and northward on a 700 × 600 m plane striking N 20°E and dipping 75° to the WNW. This plane is a pre-existing fault, whose surface projection coincides with an active scarp. The earthquakes have similar non-double-couple mechanisms that involve volume increases, and the fault plane bisects their dilatational fields, implying a process dominated by tensile failure. The source types require the additional involvement of subsidiary shear faulting, however. Events before and after the swarm have variable orientations and volume changes of both signs. Similar tensile-shear failure is observed in some natural microearthquake swarms, for example at Long Valley caldera, California. Its occurrence under low fluid pressure may imply a heterogeneous stress field or the induction of thermal stresses by introduction of cold fluid.

Compaction-induced stress variations with depth in an active anticline: Northern Apennines, Italy

Abstract: [1] It is shown that the stress field can vary with depth, even within a homogeneous tectonic setting, as documented for the active Mirandola fault-related fold along the buried front of the northern Apennines. Analyses of borehole breakouts and other well data, integrated with seismological information and field evidence, show that extension perpendicular to the fold axis, above approximately 1200 m, changes to a strike-slip stress field and finally to compression near the main detachment at depth. Similar along-depth strain variations recognized in non-active anticlines are usually explained by invoking tangential longitudinal strain folding (i.e., stretching above and shortening below a neutral surface) or gravitational instabilities. However, in this study, we propose that differential compaction may play a significant and generally overlooked role. With the aid of numerical modeling, it is shown that where fold limbs are onlapped by highly compactable deposits (as in the Mirandola area), differential compaction induces stretching at shallow (<1–2 km) depths. The amount of stretching is a function of the shape of the fold and of the thickness of the syn-tectonic sediments. We conclude that in the Mirandola case study, the stress variations observed with depth are the result of a combination of a regional compression at depth and local tension driven by differential compaction of growth strata on the limbs of the anticline with respect to its crest.

Effect of path planning on the laser powder deposition process: thermal and structural evaluation

Abstract: In this study, ANSYS finite element software is used to simulate the temperature and stress field in the laser powder deposition process. The model is used to determine the effect of the deposition pattern on the final stress distribution. Four deposition patterns are defined to cover the same area: long bead, short bead, spiral in, and spiral out. The results show that the deposition pattern significantly affects the temperature history of the process, and consequently, the stress distribution. Among the four deposition patterns, the spiral-in pattern shows the highest and the short-bead pattern shows the lowest maximum residual stress. The modeling results are verified with experiments.

Role of initial stress rotations in rupture dynamics and ground motion: A case study with implications for the Wenchuan earthquake

Abstract: [1] Motivated by observations in the 2008 Mw 7.9 Wenchuan earthquake, we study effects of systematic changes in the principal stress orientation along the fault strike on rupture dynamics and ground motion using a 3-D finite-element method. Based on Anderson's theory of faulting, we set up the initial stress field with rotations in stress orientations along strike for a dynamic rupture model of a shallow dipping fault. We find that initial stress rotations along strike can cause dramatic changes in rupture speed and produce distinct patterns in slip distribution and peak ground motion. When a mismatch (unfavorable for faulting) between fault geometry and initial stress orientations is encountered, rupture can spontaneously stop. Some first-order features in the Wenchuan event may be partially caused by rotations in the principal stress orientation along strike, such as the rupture arrest at the northeast end and two severe destruction zones in the observed seismic intensity distribution. These results may have important implications for assessing seismic hazards imposed by faults with changes in the initial stress field along strike worldwide.