Showing papers on "Stress field published in 1996"

Slip-tendency analysis and fault reactivation

Abstract: Slip-tendency analysis is a new technique that permits rapid assessment of stress states and related potential fault activity. The tendency of a surface to undergo slip in a given stress field depends on its frictional characteristics (primarily controlled by rock type) and the ratio of shear to normal stress acting on the surface, here defined as slip tendency (determined by orientation of the surface within the stress field). An interactive computer tool displays the stress tensor in terms of its associated slip-tendency distribution and the relative likelihood and direction of slip on surfaces of all orientations. The technique provides easy visualization and rapid evaluation of stress in terms of its potential for causing slip on individual faults or fault populations for use in seismic-risk and fault-rupture–risk assessment, exploration for high-risk and earthquake-prone blind faults, selection of likely earthquake focal mechanism solutions, and for use in analysis of compatibility of geologic structures.

Growth model of the inner core coupled with the outer core dynamics and the resulting elastic anisotropy

Abstract: We present a growth tectonic model of Earth's inner core and the resulting model of the seismic anisotropy. The inner core grows anisotropically if the convection in the outer core is of Taylor column type. The anisotropic growth produces a flow field of the poloidal zonal order 2 type as a result of the isostatic adjustment of the viscous inner core. Crystals in the inner core align themselves under the stress field produced by the flow. We model the anisotropic structure of the inner core, using the theory of Kamb [1959] and elastic constants of Stixrude and Cohen [1995b]. We consider models for both hcp iron and fcc iron, which are the probable crystal structures for the inner core iron according to Stixrude and Cohen [1995a]. We have found that the c axis for hcp iron and [111] direction for fcc iron align in the polar direction. The alignment is consistent with seismic observations, which have revealed that the P wave velocity is faster in the polar direction. Our model predicts that the degree of the alignment decreases near the inner core boundary in accord with recent body wave observations. The radial dependence of the alignment would result from the following three effects: (1) crystals near the surface have not undergone stressed state long enough to acquire anisotropy after precipitation, (2) stress near the surface is different from that in the interior of the inner core due to shear stress free boundary condition, and (3) partially molten structure results in transversely isotropic stress condition near the inner core surface due to compaction. Thus the application of Kamb's theory successfully explains the seismic anisotropy in the inner core provided that the crystals have been subjected under the same stress condition for the timescale of the order of 109 years.

Stress pattern in Portugal mainland and the adjacent Atlantic region, West Iberia

Abstract: The Portuguese mainland territory is located close to the Azores-Gibraltar plate boundary, in a tectonic setting responsible for significant neotectonic and seismic activities. However, few data concerning the present regional lithospheric stress field were available, as testified by recently published maps of stress indicators for the Europe and Mediterranean regions. One of the authors already presented a synthesis on this subject [Cabral, 1993], where geological and geophysical stress indicators were considered. In this paper we introduce new information, mainly a considerable amount of borehole breakout data. The updated data set comprises 32 reliable stress indicators showing a mean azimuth of 145° (standard deviation 21°) for the maximum horizontal stress direction (SHmax). On the average, the geological data are rotated clockwise and the focal mechanism data deviated anticlockwise to that azimuth, while the borehole elongation results are consistent with the mean SHmax trend. These differences in stress trend suggest a regional progressive rotation of the SHmax direction from NNW-SSE to WNW-ESE since the upper Pliocene. To estimate stress trajectories, new and published stress indicators in the adjacent Atlantic area and northern Africa were also investigated, showing a very uniform NW-SE SHmax trend in west Iberia. A high level of horizontal compressive stress acting oblique to the western Portuguese continental margin is inferred and interpreted in view of a proposed regional geodynamical model, of activation of this passive margin, with the nucleation of a subduction zone in the Atlantic SW of Iberia, at the Gorringe submarine bank, which is propagating northward along the base of the continental slope, at the transition between thinned and normal continental crust.

Induced stresses and fault potential in eastern Canada due to a realistic load: a preliminary analysis

Abstract: SUMMARY In order to understand the causal relation between postglacial rebound and earthquakes, a simple disc load model is used to: (1) calculate stresses induced in the lithosphere and mantle by glacial loading, melting and postglacial rebound; and (2) evaluate the effect of glacial loading/rebound on the failure potential for earthquakes in the upper crust. The dependence of the failure potential and the actual mode of failure on the coefficient of friction, the ambient tectonic stress magnitude/direction, the stress due to the overlying rocks, and lithospheric thickness are investigated. Prominent features of this paper are the inclusion of: (1) a viscoelastic mantle and thus the migration of stress; and (2) the ambient tectonic stress and overburden stress contributions in the calculation of the total stress field. It is assumed that, throughout the Earth, there are optimally oriented pre-existing virtual faults that are initially close to but not at failure; thus, a time-dependent quantity called dFSM (related to the Coulomb-Mohr failure criterion) can be defined such that a negative value of dFSM would advocate faulting or earthquake activities whereas a positive value of dFSM would promote stability. The results indicate that, under all combinations of tectonic stress magnitude and overburden stress, crustal loading promotes fault stability directly underneath the load. Upon the removal of the load, thrust faulting is predicted within the ice margin if the horizontal stress (S,) induced by the overburden is greater than or equal to the vertical component (S,) of the overburden stress (121, where (=SJS,). Under this condition, theory predicts that faulting or earthquake activity should have reached a maximum immediately after deglaciation. If the horizontal stress induced by the overburden is less than the vertical component of the overburden stress (1 < l), then theory predicts fault stability within the ice margin. The theory predicts fault instability both north and south of the ice margin. The mode of failure, however, is completely determined by the value of i. The trade-off between the tectonic stress magnitude and the overburden stress parameter (1) is also investigated. It is shown that a larger tectonic stress magnitude can be used to compensate a smaller value of 1. The results of this analysis show that variations in the coefficient of friction, lithospheric thickness and a ductile zone below the upper crust do not significantly affect the above conclusions.

On the Elliptic Inclusion in Anti-Plane Shear

Abstract: In this paper, we consider the anti-plane shear problem of an elliptic inclusion embedded in an infinite, isotropic, elastic medium, subjected at infinity to a uniform stress field. Using complex v...

Analysis of the South American intraplate stress field

Abstract: The first-order South American intraplate stress field was modeled through a finite element analysis to evaluate the relative contribution of plate boundary forces and intraplate stress sources. The finite element mesh consisted of 3100 nodes in a network of 5993 equal-area triangular elements which provided a spatial resolution of about 1° at the equator. An important aspect of our modeling is the inclusion of topographic forces due to the cooling oceanic lithosphere along the Mid-Atlantic Ridge (e.g., ridge push), the continental margins along the east coast of Brazil and Argentina, and the elevated continental crust (e.g., the Andean Cordillera). Predicted intraplate stresses for two representations of the western collisional boundary forces are evaluated: pinned collisional boundaries and applied collisional boundary forces. Constraint for the modeling was provided by information about the orientation of the maximum horizontal compressive stress, SHmax, provided by 217 stress indicators from the World Stress Map Project as well as by SHmax magnitude estimates and torque information from previous investigations. Our modeling results demonstrate that the first-order features of the observed stress field can be explained with simple tectonic models which balance the torque acting on the plate either with a fixed western margin or drag forces applied along the base of the plate. The predicted intraplate stress field is characterized by a nearly uniform E-W SHmax orientation throughout most regions of the plate, with stress magnitudes generally less than 20 MPa averaged over a 100-km-thick lithosphere. Significant perturbation of this regional stress field occurs in the western part of the plate in response to forces associated with the high topography of the Andes. Although the magnitude of the collisional boundary forces acting along the western margin remains poorly constrained, we estimate a plausible upper bound on the force per unit length acting along the Peru-Chile Trench to be about 2.5 × 1012 N m−1. While some of our models are consistent with a driving basal drag to balance the torques acting on the plate, the magnitude of the drag torque is small compared to the contribution from other sources of stress such as the ridge push force.

Stress field fluctuations along the Dead Sea rift since the middle Miocene

Abstract: The Dead Sea transform (DST) is the suture between the African and the Arabian plates, both of which subduct northward under the Eurasian plate. The collision of these plates with Eurasia and their relative motion are the source for their internal deformation, especially in the vicinity of the Dead Sea transform. Geological observations indicate that two distinct paleostrain regimes operated adjacent to the Dead Sea rift and in the Sinai-Israel subplate: (1) WNW shortening (less than 10%) and NNE extension, beginning in the Turonian, is associated with the development of the Syrian Arc fold belt and attributed to the Syrian Arc stress field (SAS); and (2) middle Miocene to Recent NNW shortening and ENE extension, associated with the 105-km sinistral displacement along the Dead Sea transform and the opening of the Red Sea, is attributed to the Dead Sea stress field (DSS). It was originally suggested that the SAS terminated during the middle Miocene with initiation of the DSS. However, trend and age analysis of many structures shows that formation of SAS-compatible structures continued after the middle Miocene up to the Recent. In some middle Miocene and younger rocks structures, such as faults, dikes, volcanic lineaments, and tectonic stylolites compatible with both stress fields were reported. In a few locations, structures compatible with both stress fields crosscut each other. It is suggested that the movements which resulted in the development of the Syrian Arc and other SAS-compatible structures are continuing to the Recent and that DSS movements are superimposed upon them. The overlapping of SAS and DSS structures adjacent to the DST results from spatial and temporal fluctuations in the overall stress state. We propose that such variations could be caused by the superposition of large earthquake stress drops, associated with movements along the DST, on a steady background, plate scale, stress regime associated with the SAS. Thus DSS-compatible structures should form in preseismic activity periods, when DST-related elastic strain is high. On the other hand, the SAS-compatible structures should form during interseismic activity times, subsequent to large local stress release along the DST.

Present‐day stress field changes along the Baikal rift and tectonic implications

Abstract: Intraplate extension, in a frame of a global compressional stress field, seems linked to local lithospheric perturbations (lithospheric thinning or thickening) able to modify the resulting state of stress [Zoback, 1992]. The Baikal Rift Zone (BRZ), Siberia, is located north of the India-Asia collision zone and exhibits no direct communication with any oceanic domain. It can thus be fully considered as an area of continental extension, dominated by the “global compressional intraplate stress field” resulting from plate driving forces. In order to address the problem of its dynamics and kinematics and their links with the India-Asia collision, a comprehensive stress tensor analysis is presented, based on 319 focal mechanisms of earthquakes located along the whole Baikal rift. The stress field is varying at different scales of observation: when looking at central Asia (several thousands kilometers), the maximum horizontal stress SHmax directions remain rather constant (with a fan-shape geometry) when the tectonic regime goes from compressional (Himalayas) to extensional (Baikal). When observing the Baikal rift (about 1000 km long), clear variations of the stress regime are observed, from an extensional regime in the central part of the rift to wrench ones in its northern and southern ends. Finally, at the scale of 100 km, systematic SHmax reorientations occur close to major rift faults. We thus infer that the interaction between collisional processes and inherited structures may have a strong influence on rift dynamics. We then use computed stress tensors to predict slip vectors on major rift faults. Deformation patterns show two distinct parts of the rift: the South Baikal Rift (SBR) is characterized by a constant trending (around N100°E) slip vector, meanwhile the North Baikal Rift (NBR) exhibits a complex block rotation behavior involving at least three crustal blocks. We propose to interpret these surficial structures and motions as the result of an interaction between the regional compression coming from the India-Asia collision and the geometry of the hardly deformable Siberian platform. This particular setting can explain most of the surficial deformation patterns, which suggest a large-scale cracking of the lithosphere in the Baikal region. Other possible sources of stress could also be considered, like deep mantellic upwelling, or trench suction linked to the Pacific subduction.

Stress perturbation associated with the Amazonas and other ancient continental rifts

Abstract: The state of stress in the vicinity of old continental rifts is examined to investigate the possibility that crustal structure associated with ancient rifts (specifically a dense rift pillow in the lower crust) may modify substantially the regional stress field. Both shallow (2.0–2.6 km depth) breakout data and deep (20–45 km depth) crustal earthquake focal mechanisms indicate a N to NNE maximum horizontal compression in the vicinity of the Paleozoic Amazonas rift in central Brazil. This compressive stress direction is nearly perpendicular to the rift structure and represents a ∼75° rotation relative to a regional E-W compressive stress direction in the South American plate. Elastic two-dimensional finite element models of the density structure associated with the Amazonas rift (as inferred from independent gravity modeling) indicate that elastic support of this dense feature would generate horizontal rift-normal compressional stresses between 60 and 120 MPa, with values of 80–100 MPa probably most representative of the overall structure. The observed ∼75° stress rotation constrains the ratio of the regional horizontal stress difference to the rift-normal compressive stress to be between 0.25 and 1.0, suggesting that this rift-normal stress may be from 1 to 4 times larger than the regional horizontal stress difference. A general expression for the modification of the normalized local horizontal shear stress (relative to the regional horizontal shear stress) shows that the same ratio of the rift-normal compression relative to the regional horizontal stress difference, which controls the amount of stress rotation, also determines whether the superposed stress increases or decreases the local maximum horizontal shear stress. The potential for fault reactivation of ancient continental rifts in general is analyzed considering both the local stress rotation and modification of horizontal shear stress for both thrust and strike-slip stress regimes. In the Amazonas rift case, because the observed stress rotation only weakly constrains the ratio of the regional horizontal stress difference to the rift-normal compression to be between 0.25 and 1.0, our analysis is inconclusive because the resultant normalized horizontal shear stress may be reduced (for ratios >0.5) or enhanced (for ratios <0.5). Additional information is needed on all three stress magnitudes to predict how a change in horizontal shear stress directly influences the likelihood of faulting in the thrust-faulting stress regime in the vicinity of the Amazonas rift. A rift-normal stress associated with the seismically active New Madrid ancient rift may be sufficient to rotate the horizontal stress field consistent with strike-slip faults parallel to the axis of the rift, although this results in a 20–40% reduction in the local horizontal shear stress within the seismic zone. Sparse stress data in the vicinity of the seismically quiescent Midcontinent rift of the central United States suggest a stress state similar to that of New Madrid, with the local horizontal shear stress potentially reduced by as much as 60%. Thus the markedly different levels of seismic activity associated with these two subparallel ancient rifts is probably due to other factors than stress perturbations due to dense rift pillows. The modeling and analysis here demonstrate that rift-normal compressive stresses are a significant source of stress acting on the lithosphere and that in some cases may be a contributing factor to the association of intraplate seismicity with old zones of continental extension.

Implementing image stresses in a 3D dislocation simulation

Abstract: A three-dimensional (3D) dislocation simulation has been developed in the last few years in order to fill the gap between atomistic simulations and the more macroscopic approaches. The specific role of such a simulation is to combine all mechanisms responsible for the hardening (e.g. the forest mechanism) and the multiplication of dislocations which typically occur in three dimensions. Many problems involve the presence of interfaces, which can come from cracks, oxide layers, particles etc. The present work deals with a method to tackle such problems in a 3D simulation. It is firstly restricted to the image stress case. A method of calculating the image stress field of dislocation segments due to the presence of a semi-infinite free surface has been proposed by Gosling and Willis. An alternative method is proposed here based on the Boussinesq problem of point loading in a half-space. It is firstly shown on simple cases that the methods are equivalent. The latter one is used in the present simulation, and the image stress field due to populations of dislocations is investigated. From a practical point of view, we calculate the depths within which all dislocations have to be taken into account, in order to get reasonable accuracy on the image stresses, and we also investigate how this stress field decays in space.

Two-parameter characterization of the near-tip stress fields for a bi-material elastic-plastic interface crack

Abstract: A particular case of interface cracks is considered. The materials at each side of the interface are assumed to have different yield strength and plastic strain hardening exponent, while elastic properties are identical. The problem is considered to be a relevant idealization of a crack at the fusion line in a weldment. A systematic investigation of the mismatch effect in this bi-material plane strain mode I dominating interface crack has been performed by finite strain finite element analyses. Results for loading causing small scale yielding at the crack tip are described. It is concluded that the near-tip stress field in the forward sector can be separated, at least approximately, into two parts. The first part is characterized by the homogeneous small scale yielding field controlled by J for one of the interface materials, the reference material. The second part which influences the absolute value of stresses at the crack tip and measures the deviation of the fields from the first part can be characterized by a mismatch constraint parameter M. Results have indicated that the second part is a very weak function of distance from the crack tip in the forward sector, and the angular distribution of the second part is only a function of the plastic hardening property of the reference material.

Tectonic stress within the New Madrid seismic zone

Abstract: Refraction data indicate a significant high-density rift pillow beneath the New Madrid seismic zone. We present results of linear and nonlinear viscoelastic finite element modeling to determine whether support of the rift pillow may contribute significantly to the total present-day stress field, and we consider the implications for intraplate seismicity. These models were run for a loading time of 100 m.y. to account for relaxation and transfer of stress since the last reactivation of the rift in the mid-Mesozoic. Results indicate that the nonlinear viscoelastic model with rheological stratification based on composition and temperature agrees well with the observed deformation within the seismic zone and with estimates of regional stress magnitudes. The model predicts a maximum compression of 30–40 MPa above the rift pillow in the center of the rift axis. If the magnitude of local compression predicted by the nonlinear model produces the inferred clockwise rotation of the order of 10°–30° in the direction of SHmax (maximum horizontal compression) near the rift axis, the magnitude of regional compression is a factor of 1 to 2 times the magnitude of local compression and consistent with an origin due to ridge push forces. The addition of the local stress associated with the rift pillow, however, results in an approximately 30% reduction in the resolved maximum horizontal shear stress. Thus, while the stress associated with the rift pillow can rotate the stress field into an orientation favorable for failure, reduction in the resolved shear stress requires a separate mechanism for strength reduction. Results of the modeling indicate that stresses from the load of the rift pillow may still be present in the upper crust even after 100 m.y. and may still play a role in present-day deformation and seismicity of the New Madrid seismic zone. Local stress fields of significant tectonic magnitudes may also occur around other ancient rift pillows and help explain the observed correlation between intraplate seismicity and failed rift zones.

The orientation dependence of elastic strain energy in cubic crystals and its application to the preferred orientation in titanium nitride thin films

Abstract: Elastic strain energy under some conditions provides the major contribution to the total energy of a film growing on a substrate from condensing vapour. Polycrystalline films grown with intrinsic stress induced by energetic bombardment are expected to show orientations which minimize total energy. Even for a cubic crystal in a non-hydrostatic stress field the energy is a function of the relative orientation of the stress field and the crystallographic axes. The Gibbs free energy is minimized under constant stress and temperature conditions at thermal equilibrium. In this paper we derive expressions for the Gibbs free energy of a cubic crystal in uniaxial and biaxial stress fields and find the conditions under which it is a minimum. The sign of the expression is the quantity which determines the behaviour of a cubic crystal and if negative, predicts that the [111] direction of the crystal will align with the principal stress of a uniaxial stress field and will lie normal to the plane of principal stresses in a biaxial stress field. Experimental evidence is presented which shows that titanium nitride, TiN, which has a negative value of , obeys these predictions. If is positive, then the [100] direction of the crystal obeys the above rules rather than the [111] direction.

The Effect of Matrix Plasticity on the Stress Fields in a Single Filament Composite and the Value of Interfacial Shear Strength Obtained from the Fragmentation Test

Abstract: An axisymmetrical finite-element model has been used to study the effect of matrix properties (elastic modulus, yield and/or cold draw strengths and yield strain) on the interfacial shear stress in a short embedded fibre and, consequently, the value of interfacial shear strength obtained from the fragmentation test. It is observed that the maximum shear stress at the fibre-matrix interface is related to matrix yield strength. The maximum shear stress at the interface is limited only to a very small portion of the fibre which is not the fibre end. However, at higher applied strains, a major portion of the fibre is subjected to a slightly lower value of interfacial shear stress, defined as `plateau shear stress', which corresponds to the cold draw strength of the matrix. Matrix yield strain is observed to be the major parameter controlling the fibre fragmentation process and the number of fibre fragments at saturation. It has been shown that the use of the elastic theories, such as the shear lag and finite difference models, for the normalization of the value of interfacial shear strength obtained from the fragmentation test is not appropriate since the data reduction technique for the fragmentation test assumes a perfectly plastic matrix. The value of the plateau shear stress is compared with the fragmentation test results and it is observed that the interfacial shear strength calculated from the fragmentation test can exceed the plateau value of the interfacial shear stress in certain cases. This discrepancy can be explained on the basis of limitations of the constant shear model. Further, the stress field developed around a short fibre embedded in a matrix is compared with existing one-dimensional and bi-dimensional models. It has been observed that one of the serious limitations of the various micromechanical models is to predict the area of influence caused by the presence of the fibre. Finite-element analysis is used to study the area of influence.

Subsonic and intersonic crack growth along a bimaterial interface

Abstract: An experimental investigation has been conducted to study the dynamic failure of bimaterial interfaces. Interfacial crack growth is observed using dynamic photoelasticity and characterized in terms of crack-tip velocity, complex stress intensity factor, and energy release rate. On the basis of crack-tip velocity two growth regimes are established, viz. the subsonic and transonic regimes. In the latter regime crack-tip velocities up to 1.3 times the shear wave velocity of the more compliant material are observed. This results in the formation of a line of discontinuity in the stress field surrounding the crack tip and also the presence of a pseudo crack tip that travels with the Rayleigh wave velocity (of the more compliant material).

Phase transition buoyancy contributions to stresses in subducting lithosphere

Abstract: The sequence of phase transitions undergone by minerals with increasing depth in Earth's mantle is perturbed within subducting lithospheric slabs by their thermal structure. Such perturbation of equilibrium phase relations gives rise to relative buoyancy contrasts between slab and mantle that contribute to the state of stress within the slab. While other factors contribute to overall slab stresses, thermal and phase transition effects largely control the structure of the stress field within the slab. The resulting maximum in down-dip compressive stress within the slab corresponds to the observed peak in depth distribution of deep seismicity. Furthermore, metastable persistence of lower pressure phases within the cold slab should give rise to localized shear stresses whose distribution corresponds to observed features of subduction zone seismicity. These observations are independent of the variety of failure mechanisms proposed for deep seismogenesis.

Estimates of stress directions by inversion of earthquake fault‐plane solutions in sicily

Abstract: SUMMARY The tectonic stress orientation is estimated in the lithosphere of northern Sicily, the southern Tyrrhenian sea and southern Calabria, and in the Wadati-Benioff zone below the Tyrrhenian, by inversion of fault-plane solutions of earthquakes covering a magnitude range from 2.5 to 7.1. Focal mechanisms of 97 earthquakes are taken from the literature, after a critical evaluation of their data quality. An average misfit of F= 13" indicates that the set of all shallow (<50 km) earthquakes is generated by a heterogeneous stress field. For three subsets, based on regional and magnitude separation, F was small enough (2.8"lFs5.9") to support the assumption of a homogeneous stress direction; for an additional subset, with F= 7.4, such a condition is close to being fulfilled even though some heterogeneity appears to be present. The number of earthquakes in these subsets ranged from nine to 22, and the uncertainties of the principal stress directions were generally of the order of 20" at the 90 per cent confidence level. The earthquakes with A425 define a regional stress field with the greatest principal stress, ul, dipping at a shallow angle to the south. In north-eastern Sicily and south-western Calabria the stress field estimated by earthquakes is extensional, with o3 in a direction of WNW, and a near-vertical ol, in agreement with the graben tectonics mapped geologically in this area. In western Sicily the c1 direction is oriented WNW, but this result is judged less reliable than the others, based on the broader confidence limits of the solution and the average misfit of 7.4". The earthquakes in the Wadati-Benioff zone define ol dipping at about 70" to the NW, subparallel to the zone, with o2 horizontal and striking parallel to the zone.

Large-deflection and stress analysis of multilayered plates with induced-strain actuators

Abstract: Due to the growing interest in adaptive structures, recently a number of specific plate theories with various degrees of accuracy and complexity, depending on the object under investigation, have been developed. In fact, the modelling of the thicknesswise stress, strain and displacement distributions induced by the actuator activation is of increasing interest, together with the prediction of the effect of control on static and dynamic deflections. Hence, plate models including the layerwise kinematics are well suited to the purpose. In spite of the need for layerwise plate models for stress analyses of adaptive laminated beams, plates and shells, work to date has left layerwise kinematics out of consideration. Another topic that has received rather scant attention is the active control of large deflections of these structural elements. To fill the gaps, the present paper deals with a detailed investigation of the 3-D stress field of multilayered intelligent plates based on the von Karman strain - displacement relations. A third-order zigzag layerwise plate theory is developed for multilayered, intelligent, anisotropic plates with a surface-bonded piezoelectric actuator layer. The theory accounts for a piecewise cubic through-the-thickness variation of the in-plane displacements with discontinuous derivatives at the interfaces, in order to satisfy the transverse stress continuity conditions, and for a piecewise linear variation of the static electric scalar potential. Numerical results are presented for simply supported cross-ply plates with top and bottom actuators in cylindrical bending under distributed transverse loading. Comparisons with available elasticity solutions and other approximate analyses are made. For all of the cases examined it is found that the active control of deflections is effective, the effectiveness increasing for decreasing length-to-thickness ratios. The pre-existing stress field is considerably modified by the activation: cases are found where a tensile exists that can lead to delamination. The large deflections reduce and increase at the midplane, whereas they reduce both and close to the upper surface.

Seismicity and deformation at convergent margins due to heterogeneous coupling

Abstract: Studies of the mechanics of subduction as inferred from earthquake cycle observations suggest that the distribution and style of seismicity in the seafloor, between the trench and the outer rise, and in the slab at intermediate depth, can in some cases serve to identify asperity locations along the thrust interface [Dmowska and Lovison, 1992]. Such asperities, identified from seismic wavefield modeling, are the zones of highest seismic moment release in large underthrusting events. To the extent that asperity locations are relatively stationary from one event to the next, their locations provide the zones of highest expected moment release in future large earthquakes, and rupture often nucleates at the border of an asperity. The region of the thrust interface outside such asperities is, apparently, less well coupled and releases moment throughout the great earthquake cycle in some combination of aseismic creep and moderate seismicity. Thus it is reasonable that stress and deformation rates associated with the earthquake cycle should be most pronounced near asperities, and that this should have seismic and geodetic consequences. Three-dimensional finite element modeling is used here to understand such stress and deformation patterns and their variation in time, in relation to heterogeneity of coupling along thrust interfaces. The stress field helps to explain the observed clustering of seafloor seismicity along the strike of the convergent margin. In cases of convergence at approximately normal incidence, like for the region of the Valparaiso, Chile, 1985 thrust event, the modeling is consistent with the observation that areas of large earthquakes in the seafloor toward the outer rise and in the slab tend to lie within corridors through thrust zone asperities, running perpendicular to the line of the trench. We seek to learn if such model stress fields are consistent with observations, for the strongly oblique subduction margin of the Rat Islands, western Aleutians, 1965 event, that active areas of the outer rise and slab at intermediate depth are offset along strike from asperity locations. Modeling results here for the stress in the seafloor raise the possibility that to explain this offset, the asperity zones along the thrust interface may have to be strung out along the direction of oblique slip, perhaps reflecting the contact path of subducting seamounts or geometric irregularities along the interface. Shear stress patterns created in the upper plate, when there is oblique subduction, suggest that favorable areas for back-arc strike slip activity following underthrusting, as in the Adak Island, central Aleutians, region of the 1986 Andreanof Island earthquake [Ekstrom and Engdahl, 1989], will also be shifted along strike from asperity locations. Our analyses show how deformation patterns on the earth's surface above asperities differ from patterns above nonasperities, and hence provide tools to identify inhomogeneous coupling from geodetic observations. We discuss possible bathymetric, topographic, and structural signals of strength of coupling, and of asperities, particularly noting that the density and extent from the trench of seafloor normal faults correlates with seismically inferred zones of strongest coupling in the central Aleutians.

Temporal variation in the stress field of the Aegean Region

Abstract: We use a forward numerical model to analyse the intra-plate stress field in an area of active extension of continental lithosphere : the Aegean region. Recent tomographic images of the Hellenic subduction zone offer the unique opportunity to address the relation between the evolving Aegean stress field and temporal variations in the subduction process. To this extent we apply our model to both the present-day and the Late Pliocene situation. Regarding the current situation it is found that subduction-related forces largely explain the observed pattern of tension. Results obtained for the Pliocene indicate that detachment of the slab subducted below western Greece may have controlled the observed temporal variation in the pattern of tension.