Showing papers on "Stress field published in 2007"

Hindered Crack Propagation in Materials with Periodically Varying Young's Modulus—Lessons from Biological Materials†

Abstract: Many biological materials, such as bone, nacre or biosilica, are known to be both stiff and tough. Their structure is hierarchical and appears to be optimized at all levels of hierarchy to combine the properties of its primary components, which are a tough protein and stiff mineral. Bone, for example, is a nanocomposite and the deformation pattern is clearly hierarchical. In lamellar cortical bone, fibrillar units aggregate into laminate sheets, in analogy to plywood. This lamellar structure has a dramatic effect on fracture toughness. Nacre and biosilica are also layered structures where thin organic layers separate sheets of aragonite mineral and biosilica, respectively. The high toughness of such layered biological structures is intriguing and may serve as a model for artificial layered composites. A possible origin for the toughness in layered structures is the deflection of racks at weak interfaces. However, we know from theoretical fracture mechanics that a variation of the material properties alone (even without inherently weak interfaces) may result in a shielding or anti-shielding effect to the crack tip, which leads to a change of the crack driving force and the energy consumed by the fracture process. In the current work, we therefore analyse the driving force onto cracks propagating inside a material where the Young’s modulus varies in a periodic way in a given direction (perpendicular to the lamellae). We derive a simple design criterion to obtain laminate structures without driving force for crack propagation perpendicular to the lamellae. We consider a crack in a plane configuration of unit thickness with the crack tip located at the point P, see Figure 1. Globally, the material is assumed to be elastic with a constant Young’s modulus Efar and a Poisson’s ratio m far away from the crack tip P. Inside a circular region with the radius R, however, the Young’s modulus E varies in space. We assume that this variation is periodic in x-direction with an average value E0. The specimen is loaded by a stress ∑ in y-direction on the upper and lower parts of the boundary Cfar (indicated as “o” and “u” in Fig. 1). The crack flanks are assumed to be stressfree. The stress field r near the crack tip T is described by the classical “near-tip field” expressed in polar coordinates r,h and the stress intensity factor KI, for details see the fracture mechanics literature, e.g., Gross et al., Ch. 4.2. The specific elastic strain energy density r e 2 can be calculated analytically for constant E and plane stress or strain conditions as

Plate boundary forces are not enough: Second‐ and third‐order stress patterns highlighted in the World Stress Map database

Abstract: [1] The World Stress Map Project compiles a global database of contemporary tectonic stress information of the Earth's crust. Early releases of the World Stress Map Project demonstrated the existence of first-order (plate-scale) stress fields controlled by plate boundary forces and second-order (regional) stress fields controlled by major intraplate stress sources such as mountain belts and zones of widespread glacial rebound. The 2005 release of the World Stress Map Project database provides, for some areas, high data density that enables us to investigate third-order (local) stress field variations, and the forces controlling them such as active faults, local inclusions, detachment horizons, and density contrasts. These forces act as major controls on the stress field orientations when the magnitudes of the horizontal stresses are close to isotropic. We present and discuss examples for Venezuela, Australia, Romania, Brunei, western Europe, and southern Italy where a substantial increase of data records demonstrates some of the additional factors controlling regional and local stress patterns.

On the Forces Driving Plate Tectonics: Inferences from Absolute Plate Velocities and Intraplate Stress

Abstract: Summary Absolute plate motions and intraplate stress both serve as tests of models for the forces acting on plate boundaries. Plate velocities relative to a presumedly fixed underlying mantle are calculated from the hypothesis that no net torque is exerted on the lithosphere. Intraplate stress is calculated by solving the equilibrium equations for thin elastic shells in the membrane state of stress. The absolute velocity fields predicted from a wide assortment of physical and geological models are all very similar. While the global pattern of absolute velocity is probably close to those predicted by such models, the absolute motions do not therefore provide a strong test of the driving mechanism. Comparison of predicted intraplate stress with the long-wavelength features of the global stress field, however, as determined by in situ measurements, earthquake mechanisms, and stressinduced geological structures, does prove to be a powerful test of possible driving forces. All absolute velocity models have several interesting properties. Lithosphere in the equatorial half of the Earth is moving significantly faster than lithosphere in the polar half. Some connection to the Earth's rotation is implied since this statement is demonstrably untrue for co-ordinate poles much different from the geographic pole. Subducted slabs are characterized by a slow horizontal translation perpendicular to strike that is independent of the plate convergence rate, confounding attempts to explain the dip angles of Benioff zones in terms of a uniform vertical sinking and a variable absolute velocity for the overthrust plate. Ridges must migrate at a wide range of velocities relative to their underlying source of new lithosphere; such rapid migration may be a necessary but is not a sufficient condition for ridge jumps. Force models considered in velocity and stress calculations include driving forces at spreading centres and subduction zones and various parameterizations of drag at the base of the lithosphere. From the rms absolute velocities of individual plates, there is a weak indication that pull by subducted lithosphere at trenches is an important driving force and that drag may be greater beneath continental than oceanic lithosphere. The predicted intraplate deviatoric stress cannot match the well-determined stress fields in North America and Europe unless the driving force exerted at ridges is at least comparable in magnitude to other forces in the system. The mid-plate stresses are very sensitive to the nature of drag at the base of the lithosphere and thus measured stresses may ultimately provide a sensitive test of absolute plate velocities.

FEM calculation of residual stresses induced by laser shock processing in stainless steels

Abstract: Laser shock processing, also known as laser shock peening, generates through a laser-induced plasma, plastic deformation and compressive residual stresses in materials for improved fatigue or stress corrosion cracking resistances. The calculation of mechanical effects is rather complex, due to the severity of the pressure loading imparted in a very short time period (in the ns regime). This produces very high strain rates (106 s−1), which necessitate a precise determination of dynamic properties.Finite element techniques have been applied to predict the residual stress fields induced in two different stainless steels, combining shock wave hydrodynamics and strain rate dependent mechanical behaviour. The predicted residual stress fields for single or multiple laser processes were correlated with those from experimental data, with a specific focus on the influence of process parameters such as pressure pulse amplitude and duration, laser spot size or sacrificial overlay.Among other results, simulations confirmed that the affected depths increased with pulse duration, peak pressure and cyclic deformations, thus reaching much deeper layers (> 0.5 mm) than with any other conventional surface processing. To improve simulations, the use of experimental VISAR determinations to determine pressure loadings and elastic limits under shock conditions (revealing different strain-rate dependences for the two stainless steels considered) was shown to be a key point.Finally, the influence of protective coatings and, more precisely, the simulation of a thermo-mechanical uncoated laser shock processing were addressed and successfully compared with experiments, exhibiting a large tensile surface stress peak affecting a few tenths of micrometres and a compressive sub-surface stress field.

Significance of the elastic peak stress evaluated by FE analyses at the point of singularity of sharp V-notched components

Abstract: The paper presents an expression useful to estimate the notch stress intensity factor (NSIF) from finite element analyses carried out by using a mesh pattern with a constant element size. The evaluation of the NSIF from a numerical analysis of the local stress field usually requires very refined meshes and then large computational effort. The usefulness of the presented expression is that (i) only the elastic peak stress numerically evaluated at the V-notch tip is needed and no longer the whole stress-distance set of data; (ii) the adopted meshes are rather coarse if compared to those necessary for the evaluation of the whole local stress field. The proposed expression needs the evaluation of a virtual V-notch tip radius, i.e. the radius which would produce the same elastic peak stress than that calculated by FEM at the sharp V-notch tip by means of a given mesh pattern. Once such a radius has been theoretically determined for a given geometry, the expression can be applied in a wide range of notch depths and opening angles.

Stress-induced large-area lift-off of crystalline Si films

Abstract: A new implantation-free lift-off process is presented. We deposit a layer with mismatched thermal expansion coefficient with respect to the substrate. Upon cooling, the differential contraction induces a large stress field which is released by the initiation and the propagation of a crack parallel to the surface. The principle is demonstrated on both single and multi-crystalline silicon. Films with an area of 25 cm2 and a thickness of 30–50 μm have been obtained. Some Si layers were further processed into solar cells. An energy conversion efficiency of 9.9% was reached on a 1 cm2 sample.

A Bayesian approach to estimating tectonic stress from seismological data

Abstract: SUMMARY Earthquakes are conspicuous manifestations of tectonic stress, but the non-linear relationships between the stresses acting on a fault plane, its frictional slip, and the ensuing seismic radiation are such that a single earthquake by itself provides little information about the ambient state of stress. Moreover, observational uncertainties and inherent ambiguities in the nodal planes of earthquake focal mechanisms preclude straightforward inferences about stress being drawn on the basis of individual focal mechanism observations. However, by assuming that each earthquake in a small volume of the crust represents a single, uniform state of stress, the combined constraints imposed on that stress by a suite of focal mechanism observations can be estimated. Here, we outline a probabilistic (Bayesian) technique for estimating tectonic stress directions from primary seismological observations. The Bayesian formulation combines a geologically motivated prior model of the state of stress with an observation model that implements the physical relationship between the stresses acting on a fault and the resultant seismological observation. We show our Bayesian formulation to be equivalent to a well-known analytical solution for a single, errorless focal mechanism observation. The new approach has the distinct advantage, however, of including (1) multiple earthquakes, (2) fault plane ambiguities, (3) observational errors and (4) any prior knowledge of the stress field. Our approach, while computationally demanding in some cases, is intended to yield reliable tectonic stress estimates that can be confidently compared with other tectonic parameters, such as seismic anisotropy and geodetic strain rate observations, and used to investigate spatial and temporal variations in stress associated with major faults and coseismic stress perturbations.

Earthquake source characteristics from dynamic rupture with constrained stochastic fault stress

Abstract: One of the challenging tasks in predicting near-source ground motion for future earthquakes is to anticipate the spatiotemporal evolution of the rupture process. The final size of an event but also its temporal properties (propagation velocity, slip velocity) depend on the distribution of shear stress on the fault plane. Though these incipient stresses are not known for future earthquakes, they might be sufficiently well characterized in a stochastic sense. We examine the evolution of dynamic rupture in numerical models of a fault subjected to heterogeneous stress fields with varying statistical properties. By exploring the parameter space of the stochastic stress characterization for a large number of random realizations we relate generalized properties of the resulting events to the stochastic stress parameters. The nucleation zone of the simulated earthquake ruptures in general has a complex shape, but its average size is found to be independent of the stress field parameterization and is determined only by the material parameters and the friction law. Furthermore, we observe a sharp transition in event size from small to system-wide events, governed mainly by the standard deviation of the stress field. A simplified model based on fracture mechanics is able to explain this transition. Finally, we find that the macroscopic rupture parameters ( e. g., moment, moment rate, seismic energy) of our catalog of model quakes are generally consistent with observational data.

Off‐fault damage patterns due to supershear ruptures with application to the 2001 Mw 8.1 Kokoxili (Kunlun) Tibet earthquake

Abstract: We extend a model of a two-dimensional self-healing slip pulse, propagating dynamically in steady state with slip-weakening failure criterion, to the supershear regime in order to study the off-fault stressing induced by such a slip pulse and investigate features unique to the supershear range. Specifically, we show that there exists a nonattenuating stress field behind the Mach front that radiates high stresses arbitrarily far from the fault (practically this would be limited to distances comparable to the depth of the seismogenic zone), thus being capable of creating fresh damage or inducing Coulomb failure in known structures at large distances away from the main fault. We allow for both strike-slip and dip-slip failure induced by such a slip pulse. We show that off-fault damage is controlled by the speed of the slip-pulse, scaled stress drop, and principal stress orientation of the prestress field. We apply this model to study damage features induced during the 2001 Kokoxili (Kunlun) event in Tibet, for which it has been suggested that much of the rupture was supershear. We argue that an interval of simultaneous induced normal faulting is more likely due to a slip partitioning mechanism suggested previously than to the special features of supershear rupture. However, those features do provide an explanation for otherwise anomalous ground cracking at several kilometers from the main fault. We also make some estimates of fracture energy which, for a given net slip and dynamic stress drop, is lower than for a sub-Rayleigh slip pulse because part of the energy fed by the far-field stress is radiated back along the Mach fronts.

Non‐local stress approach for fatigue assessment based on weakest‐link theory and statistics of extremes

Abstract: In the present paper a non-local stress approach for fatigue assessment based on weakest-link theory and statistics of extremes is presented. It is a non-local stress approach in the sense that it takes the complete stress field into account rather than just the highest local stress. The statistical distribution of fatigue strength data from smooth standard specimens serves as a starting point for the computation of the probability of fatigue failure of a mechanical component under cyclic loading. The probability of fatigue failure can be obtained by post-processing results from a standard finite element stress analysis. It is shown that the non-local stress approach can be linked to the probability of finding the fatigue critical defect in the most highly stressed volume of the component. A numerical procedure is presented that is fully compatible with the results from a standard finite element stress analysis. It is further shown how the fatigue strength distribution can be transformed into a fatigue life distribution by using Basquin's equation. Finally, the non-local stress approach is used for predicting the fatigue limit of several specimens and predictions are compared with test results.

Emergence of singular structures in Oldroyd-B fluids

Abstract: Numerical simulations reveal the formation of singular structures in the polymer stress field of a viscoelastic fluid modeled by the Oldroyd-B equations driven by a simple body force. These singularities emerge exponentially in time at hyperbolic stagnation points in the flow and their algebraic structure depends critically on the Weissenberg number. Beyond a first critical Weissenberg number the stress field approaches a cusp singularity, and beyond a second critical Weissenberg number the stress becomes unbounded exponentially in time. A local approximation to the solution at the hyperbolic point is derived from a simple ansatz, and there is excellent agreement between the local solution and the simulations. Although the stress field becomes unbounded for a sufficiently large Weissenberg number, the resultant forces of stress grow subexponentially. Enforcing finite polymer chain lengths via a FENE-P penalization appears to keep the stress bounded, but a cusp singularity is still approached exponentially in time.

Continental Fault Structure and Rheology from the Frictional-to-Viscous Transition Downward

Abstract: Faulting is an expression of the interaction between rock rheology, kinematic boundary conditions, and associated stress fields The structure and rheology of faults vary with depth, such that pressure-dependent frictional behavior predominating in the upper, brittle part of the crust is transitional to strongly temperature- and rate-dependent behavior in the lower part of the crust and mantle This frictional-to-viscous transition (FVT) is characterized by changes in rock structure, rheology, and fluid activity that are closely tied to the earthquake cycle As such, the FVT is a first-order decoupling zone, whose depth and lateral extent vary in time Brittle, sometimes seismic, instabilities perturb the ambient stress field within the lithosphere on timescales ranging from seconds to years These instabilities are measurable as transient motions of the Earth's surface and are manifest both at, and below, the FVT by the development of structural anisotropies (fractures, foliations) Surface motion studies of plate-boundary strike-slip faults indicate that shearing below the FVT is more localized in the lower crust than in the upper mantle Structural investigations of exhumed shear zones reveal that this localization involves the nucleation of fractures at the FVT, as well as the buckling and rotation of existing foliations below the FVT In some cases, rotation of these surfaces can initiate transient deformation, transferring stress upward and potentially triggering earthquakes The networking of shear zones on several length scales allows them to function as decoupling horizons that partition three-dimensional strain within the lithosphere The simplification of fault geometry with progressive strain lends justification to the use of laboratory-derived flow laws to estimate the bulk rock rheology on length scales at which strain is homogeneous In general, the longer the timeand length scales of faulting considered, the greater the potential influence of the kinematic and thermal history on the rheology of the fault system Taken together, studies suggest that future fault modeling must include parameters that quantify the thermal and structural aspects of rock history, as well as the fluid activity in and around faults

Evolution of stress in Southern California for the past 200 years from coseismic, postseismic and interseismic stress changes

Abstract: SUMMARY The seismicity of southern California results from stresses that arise from the relative motion of the Pacific and North American Plates being accommodated along the San Andreas Fault (SAF) system and the Eastern California Shear Zone (ECSZ). Here we calculate how the stress field in southern California has evolved over the past two centuries due to interseismic loading, as inferred from current GPS observations of surface velocities, from redistributions of static stress induced by large (M w ≥ 6.5) earthquakes since the 1812 Wrightwood quake, and postseismic viscoelastic relaxation associated with these events that serves to transfer coseismic stresses from the deep, warm, lower crust and upper mantle to the overlying seismogenic upper crust. We calculate Coulomb stress changes on vertical strike-slip faults striking parallel to the SAF and at the hypocenters on the rupture planes of all M w ≥ 6 events over the past two centuries. Our results suggest that the 1857 M w = 8.2 Fort Tejon earthquake, by far the largest event to have occurred in the region over the past two centuries, had a profound influence on the state of stress in Southern California during the 19th century, inducing significant stress increases to the north (Parkfield region and adjoining creeping SAF) and south (southern SAF and San Jacinto fault), and stress relief across the southern ECSZ. These stress changes were then greatly magnified by postseismic relaxation through the early part of the 20th century. Slow interseismic build-up of stress further loads all major strike-slip faults and works to reload the areas of the ECSZ where stress was relieved by the 1857 quake. Our calculations suggest that only 56% of hypocenters were pushed closer to failure by preceding coseismic stress changes, suggesting that the occurrence of large earthquakes is not strongly determined by coseismic Coulomb stress changes. This percentage rises to 70% when postseismic stress changes are also considered. Our calculations demonstrate the importance of postseismic viscoelastic relaxation in the redistribution of stress following large earthquakes. We find, however, that postseismic processes associated with events more than about a decade old are near completion and thus do not significantly influence the regional velocity field presently observed in southern California.

Stress orientations of Taiwan Chelungpu-Fault Drilling Project (TCDP) hole-A as observed from geophysical logs

Abstract: [1] The Taiwan Chelungpu-fault Drilling Project (TCDP) drilled a 2-km-deep research borehole to investigate the structure and mechanics of the Chelungpu Fault that ruptured in the 1999 Mw 7.6 Chi-Chi earthquake. Geophysical logs of the TCDP were carried out over depths of 500–1900 m, including Dipole Sonic Imager (DSI) logs and Formation Micro Imager (FMI) logs in order to identify bedding planes, fractures and shear zones. From the continuous core obtained from the borehole, a shear zone at a depth of 1110 meters is interpreted to be the Chelungpu fault, located within the Chinshui Shale, which extends from 1013 to 1300 meters depth. Stress-induced borehole breakouts were observed over nearly the entire length of the wellbore. These data show an overall stress direction (N115E) that is essentially parallel to the regional stress field and parallel to the convergence direction of the Philippine Sea plate with respect to the Eurasian plate. Variability in the average stress direction is seen at various depths. In particular there is a major stress orientation anomaly in the vicinity of the Chelungpu fault. Abrupt stress rotations at depths of 1000 m and 1310 m are close to the Chinshui Shale’s upper and lower boundaries, suggesting the possibility that bedding plane slip occurred during the Chi-Chi earthquake. Citation: Wu, H.-Y., K.-F. Ma, M. Zoback, N. Boness, H. Ito, J.-H. Hung, and S. Hickman (2007), Stress orientations of Taiwan Chelungpu-Fault Drilling Project (TCDP) hole-A as observed from geophysical logs, Geophys. Res. Lett., 34, L01303, doi:10.1029/2006GL028050.

The dynamics of western North America: stress magnitudes and the relative role of gravitational potential energy, plate interaction at the boundary and basal tractions

Abstract: SUMMARY We investigate the forces involved in driving long-term large-scale continental deformation in western North America, and quantify the vertically averaged deviatoric stress field arising from internal buoyancy forces and the accommodation of relative plate motions. In addition, we investigate the ability of regional models to resolve the level of tractions acting at the base of the lithosphere. We directly solve force-balance equations for vertically averaged deviatoric stresses associated with differences in values of 1/(lithospheric thickness) times the gravitational potential energy per unit area (GPE). The GPE values are inferred using both the ETOPO5 topographic data set and the CRUST2.0 crustal thickness model. Deviatoric stresses associated with basal tractions are calculated globally, with inputs determined from an isoviscous upper mantle (η = 10 21 Pa s) 3-D large-scale convection model in which mantle density variations were inferred from tomographic data and the history of subduction. In a 211parameter iterative inversion we then solve for a stress field boundary condition by fitting stress field indicators (i.e. the directions and relative magnitudes of the principal axes of kinematic strain rates). Magnitudes of the total vertically averaged deviatoric stress field (sum of GPE solution with the boundary condition solution) range from 5 to 10 MPa within a 100-km thick lithosphere. These magnitudes are calibrated by the GPE differences, along with the spatial variation in deformation style. There is a trade-off between the scaling of the basal traction deviatoric stress field and the boundary condition solution. However, the combined boundary conditions plus basal traction solution is robust (in both magnitude and style), and when added to the contribution from GPE differences provides a global minimum of misfit between the total deviatoric stress solution and the stress field indicators. GPE variations account for ∼50 per cent of the deviatoric stress magnitudes driving deformation, while boundary condition stresses account for the remaining ∼50 per cent of deviatoric stress magnitude. By comparing possible end-member strength profiles with our vertically averaged deviatoric stresses we infer that the bulk of the strength within the lithosphere in western North America lies within the brittle seismogenic layer.

The early evening boundary layer transition

Abstract: Various mechanisms which lead to nocturnal accelerations and formation of the low-level jet are examined by analysing Wangara data. the stress field is inferred from the wind field and equations of motion. the evolution of the stress divergence, during the evening transition from daytime mixed layer flow to nocturnal boundary layer flow, is found to increase the ageostrophic flow and subsequent nocturnal accelerations by roughly a factor of two. During this transition the stress divergence in the lowest few hundred metres increases due to the fact that the influence of decreasing boundary layer depth exceeds the effect of decreasing surface stress. This leads to temporary deceleration and rotation of the low-level wind vector towards low pressure and thus increases the ageostrophic flow. Diurnal variation of the geostrophic wind is also found to significantly strengthen the nocturnal flow. This diurnal variation is apparently due to heating and cooling over terrain which slopes gently upward, east of the location of the Wangara experiment.

Aftershock asymmetry on a bimaterial interface

Abstract: To better understand the asymmetric distribution of microearthquake aftershocks along the central San Andreas fault, we study dynamic models of slip-weakening ruptures on an interface separating differing elastic half-spaces. Subshear ruptures grow as slightly asymmetric bilateral cracks, with larger propagation velocities, slip velocities, and normal stress changes at the rupture front moving in the direction of slip of the medium with the lower shear wave speed (the southeast front, in the context of the San Andreas). When the SE front encounters a stress barrier, the tensile stress perturbation behind the rupture front continues forward and for a wide range of barrier strengths nucleates a dying slip pulse. This slip pulse smooths the stress field and reduces the static stress change beyond the SE front. Furthermore, because the tensile stress that carried the slip pulse into the barrier is a purely dynamic phenomenon, the SE rupture front can be left far below the failure threshold, while the NW front remains quite close to failure. Both mechanisms could contribute to the observed aftershock asymmetry. Formation of a robust slip pulse requires a peak tensile stress perturbation that approaches the nominal strength drop of the slip-weakening law. To achieve this while minimizing off-fault damage requires either substantial velocity contrasts or small reductions in friction. The simulations also show a pronounced asymmetry in the timescales over which barriers to the SE and NW experience increasing stresses, a result that has implications for the asymmetric distribution of subevents in compound earthquakes.

Ensemble averaging stress–strain fields in polycrystalline aggregates with a constrained surface microstructure – Part 1: anisotropic elastic behaviour

Abstract: The effect of three-dimensional (3D) grain morphology on the deformation at a free surface in polycrystalline aggregates is investigated by means of a large-scale finite element and statistical approach. For a given two-dimensional surface at z = 0 containing 39 grains with given crystal orientations, 17 random 3D polycrystalline aggregates are constructed having different 3D grain shapes and orientations except at z = 0, based on an original 3D image analysis procedure. They are subjected to overall tensile loading conditions. The resulting stress–strain fields at the free surface z = 0 are analyzed. Ensemble average and variance maps of the stress field at the observed surface are computed. In the case of an anisotropic elastic behaviour of the grains, fluctuations ranging between 5% and 60% are found in the equivalent stress level at a given material point of the observed surface from one realization of the microstructure to another. These results have important implications in the way of comparing fin...

Present‐day stress field in the Gibraltar Arc (western Mediterranean)

Abstract: [1] The Gibraltar Arc in the western Mediterranean consists of the Betic and Rif Alpine chains and the Alboran Sea Basin. Four types of stress indicators (wellbore breakouts, earthquake focal plane mechanisms, young geologic fault slip data, and hydraulic fracture orientations) indicate a regional NW–SE compressive stress field resulting from Africa-Eurasia plate convergence. In some particular regions, deviations of SHmax are observed with respect to the regional stress field. They are gentle-to-moderate (22°–36°) anticlockwise rotations located along the North Alboran margin and moderate-to-significant (36°–78°) clockwise rotations around the Trans-Alboran Shear Zone (TASZ). This is a broad fault zone composed of different left-lateral strike-slip fault segments running from the eastern Betics to the Alhoceima region in the Rif and resulting in a major bathymetric high in the Alboran Sea (the Alboran Ridge fault zone). Some of these stress rotations appear to be controlled by steep gradients of crustal thickness variation across the North Alboran margin and/or differential loading imposed by thick sedimentary accumulations in basin depocenters parallel to the shoreline. Other stress perturbations may be related to active left-lateral, strike-slip deformation within the TASZ that crosscuts the entire orogenic arc on a NE–SW trend and represents a key element to understand present-day deformation partitioning in the western Mediterranean.

Nonuniform prestress from prior earthquakes and the effect on dynamics of branched fault systems

Abstract: [1] To examine the effects of branched fault geometry on the dynamics of fault systems in the long term, we perform multicycle simulations on generic faulting models. An explicit finite element algorithm is used to simulate spontaneous dynamic rupture of earthquakes. The fault stress during the interseismic period is evaluated by an analytical viscoelastic model. We find that the fault prestress field becomes highly nonuniform near the branch point and on the two branch segments over multiple earthquake cycles, owing to the branched fault geometry and stress interaction between the two segments. The principal prestress on faults rotates over multiple earthquake cycles and departs from the regional stress field significantly near the branch point. After a number of earthquake cycles, the branched fault systems evolve to a steady state in which several patterns of the fault prestress and earthquake rupture repeat. The nonuniform prestress developed from previous earthquakes has large effects on the rupture and slip patterns. Several different rupture scenarios can occur on a given branched fault system. In addition, backward branching can occur in the nonuniform prestress field, either driven by slip on the ‘‘stem’’ of the fault system or through a triggering mechanism. These modeling results may have important implications for understanding fault-branching behavior observed in the 1992 Landers, the 1999 Hector Mine, and the 2002 Denali fault earthquakes and for seismic hazard analysis in the areas where branched fault systems exist.

Anisotropy of seismic and mechanical properties of Opalinus clay during triaxial deformation in a multi-anvil apparatus

Abstract: It is generally accepted that the dilatancy concept offers a reliable basis for the assessment of underground buildings in various host rock formations, e.g., a repository for radioactive waste. Stress conditions are predictable where creep failure and increasing permeability will inevitably develop. However, in argillaceous rocks an experimental detection of microcrack opening by volumetric strain measurements fails because the damage induced volume increase is overlapped by the dominating bedding plane compaction, also during deviatoric loading. To solve remaining uncertainties regarding the stress dependent onset of dilatant deformation in clay rocks it is inalienable to investigate the stress and deformation induced variations of various independently measured physical parameters, e.g., volumetric strain resp. porosity, p- and s-wave velocities, permeability. The aim of our spatial velocity measurements in a multi-anvil apparatus is not only to quantify the seismic anisotropy of the Opalinus clay but also to identify the onset of dilatancy and to monitor its evolution at various states of stresses. We found that the crack sensitive p- and s-velocities is a powerful tool for the determination of the so-called dilatancy boundary. Differences in the sensitivity of V p and V s , resp. anisotropy effects are closely related to the addressed physical process of crack opening depending on stress field orientation and bedding plane orientation. In summary, our results lead to a new and comprehensive synoptic view of the stress and deformation induced changes of rock properties in argillaceous rocks, which impressively confirms the general concept of dilatancy.

Calcite twinning constraints on late Neogene stress patterns and deformation mechanisms in the active Zagros collision belt

Abstract: Mechanically induced calcite twins in veins and host rocks of Late Cretaceous to Miocene age in Iran have been used to determine regional Arabia-Eurasia collisional stresses. A late folding stress regime with a compression oriented 025° (±15°) has been identified across the Zagros belt and the southern Iranian Plateau. This late Neogene stress pattern agrees with the current stress field determined from the focal mechanisms of basement earthquakes and suggests that the Hormuz salt decollement poorly decouples the basement and cover stress fields. Our data show that the collisional state of stress has been relatively constant since ca. 5 Ma. The magnitudes of the stresses obtained from the twinning analysis are unexpectedly low, and, to a first approximation, they are constant across the Zagros simply folded belt. This result supports an overall mechanism of buckling of the detached Zagros cover. Internal viscous-plastic processes help to relieve stress within this cover, thus lowering its seismogenic potential. Beyond these regional implications, this study underlines the potential of paleostress analyses in constraining both the tectonics and the mechanics of ancient and active foreland fold belts.