摘要:

To date, more than 7300 in situ stress orientations have been compiled as part of the World Stress Map project. Of these, over 4400 are considered reliable tectonic stress indicators, recording horizontal stress orientations to within<±25°. Remarkably good correlation is observed between stress orientations deduced from in situ stress measurements and geologic observations made in the upper 1–2 km, well bore breakouts extending to 4–5 km depth and earthquake focal mechanisms to depths of ∼20 km. Regionally uniform stress orientations and relative magnitudes permit definition of broad‐scale regional stress patterns often extending 20–200 times the approximately 20–25 km thickness of the upper brittle lithosphere. The “first‐order” midplate stress fields are believed to be largely the result of compressional forces applied at plate boundaries, primarily ridge push and continental collision. The orientation of the intraplate stress field is thus largely controlled by the geometry of the plate boundaries. There is no evidence of large lateral stress gradients (as evidenced by lateral variations in stress regime) which would be expected across large plates if simple resistive or driving basal drag tractions (parallel or antiparallel to absolute motion) controlled the intraplate stress field. Intraplate areas of active extension are generally associated with regions of high topography: western U.S. Cordillera, high Andes, Tibetan plateau, western Indian Ocean plateau. Buoyancy stresses related to crustal thickening and/or lithospheric thinning in these regions dominate the intraplate compressional stress field due to plate‐driving forces. These buoyancy forces are just one of several categories of “second‐order” stresses, or local perturbations, that can be identified once the first‐order stress patterns are recognized. These second‐order stress fields can often be associated with specific geologic or tectonic features, for example, lithospheric flexure, lateral strength contrasts, as well as the lateral density contrasts which give rise to buoyancy forces. These second‐order stress patterns typically have wavelengths ranging from 5 to 10+ times the thickness of the brittle upper lithosphere. A two‐dimensional analysis of the amount of rotation of regional horizontal stress orientations due to a superimposed local stress constrains the ratio of the magnitude of the horizontal regional stress differences to the local uniaxial stress. For a detectable rotation of 15°, the local horizontal uniaxial stress must be at least twice the magnitude of the regional horizontal stress differences. Examples of local rotations ofSHmaxorientations include a 75°–85° rotation on the northeastern Canadian continental shelf possibly related to margin‐normal extension derived from sediment‐loading flexural stresses, a 50°–60° rotation within the East African rift relative to western Africa due to extensional buoyancy forces caused by lithospheric thinning, and an approximately 90° rotation along the northern margin of the Paleozoic Amazonas rift in central Brazil. In this final example, this rotation is hypothesized as being due to deviatoric compression oriented normal to the rift axis resulting from local lithospheric support of a dense mass in the lower crust beneath the rift (“rift pillow”). Estimates of the magnitudes of first‐order (plate boundary force‐derived) regional stress differences computed from modeling the source of observed local stress rotations magnitudes can be compared with regional stress differences based on the frictional strength of the crust (i.e., “Byerlee's law”) assuming hydrostatic pore pressure. The examples given here are too few to provide a definitive evaluation of the direct applicability of Byerlee's law to the upper brittle part of the lithosphere, particularly in view of uncertainties such as pore pressure and relative magnitude of the intermediate principal stresses. Nonetheless, the observed rotations all indicate that the magnitude of the local horizontal uniaxial stresses must be 1–2.5+ times the magnitude of the regional first‐order horizontal stress differences and suggest that careful evaluation of such local rotations may be a powerful technique for constraining the in situ magnitude

ISSN:0148-0227    

DOI:10.1029/92JB00132

年代:1992

数据来源: WILEY

摘要:

Mantle circulation models capable of predicting satisfactory plate velocities and geoid are tested for their ability to generate adequate lithospheric stresses. The model Earth mantle is driven by mass heterogeneities defined by seismic tomography and slab distribution. Its surface is divided into 11 freely moving plates. The mantle circulation produces shear stresses at the base of each plate. These are used to compute a dynamical component of the lithospheric stresses which is added to another component induced by variations of the Moho depth inside the lithosphère itself. The extensional state of the continents caused by their thick crust is more than compensated by a compressional tendency related to their high seismic velocity roots, interpreted as dense roots. Strong upwellings beneath equatorial regions with geoid highs could also generate anomalous topography and extensional stresses. The long‐wavelength stress pattern proposed by our models cannot easily be compared with available data: the internal loads are still poorly defined, the tested Earth model may be too simple, and the observations are inadequately distribut

ISSN:0148-0227    

DOI:10.1029/91JB00292

年代:1992

数据来源: WILEY

摘要:

Torque poles are calculated for a variety of possible forces acting on the plates, including ridge push, slab pull, and collisional resistance. These torque poles are then compared to the directions of absolute plate motions. There is a strong correlation between ridge torque poles and the azimuth of absolute plate motions for the North American, South American, Pacific, Cocos, and Eurasian plates. Simple slab pull torques correlate well with absolute motion azimuths for the Pacific, Nazca, and Cocos plates and moderately well with the absolute motion azimuth of the IndoAustralian plate. Collisional resistance torque poles correlate with the absolute motion azimuth of the Eurasian plate only. The correlations are presented as further evidence that the absolute reference frame for plate motion is determined by the surface plates themselves. Torque poles for various forces are also compared with several long‐wavelength features of the global intraplate stress field that also tend to be aligned with absolute motion directions. In general, ridge torque directions agree well with the orientations of maximum horizontal stresses for stable North America, western Europe, and South America and provide an alternative explanation for the alignment in terms of ridge push forces rather than basal drag. Collisional resistance forces can also explain the alignment of stresses in western Europe. For the IndoAustralian plate, the torque pole for collisional resistance forces is consistent with the general pattern of stresses in at least the western half of the plate but is not a good predictor of the entire data set for the plate. Other processes, in addition to collisional resistance, must be important for the Indo‐Australian plate. Ridge push forces may account for a significant portion of the long‐wavelength features of the intraplate stress field, especially away from continental collisions. Such a conclusion is consistent with negative buoyancy of the slab being an important component of the driving mechanism. As previously suggested, slab forces may be largely balanced within the subducted slab itself and thus have limited effect on deformation of the surface p

ISSN:0148-0227    

DOI:10.1029/91JB00475

年代:1992

数据来源: WILEY

摘要:

We propose a method to estimate stress magnitudes in oceanic plate interiors from focal depths and focal mechanisms. Using a depth‐dependent rheology, we show it is possible to estimate the differential stress (σ1–σ3), averaged over some reference lithospheric thickness. The resolving power of the method is investigated by evaluating the effect of uncertainties in parameters that are involved in the analysis. We apply the method to the Central Indian Ocean, where intraplate seismicity is high. From well‐studied earthquakes we estimate differential stresses of the order of hundreds of rnegapascals. This result is consistent with the high level of stress that was found from numerical model calculations by Cloetingh and Wortel (1985, 1986). From the few intraplate events in the Pacific plate, we also estimate differential stresses in th

ISSN:0148-0227    

DOI:10.1029/91JB01797

年代:1992

数据来源: WILEY

摘要:

Focal mechanisms of 32 North American midplate earthquakes (mb= 3.8–6.5) were evaluated to determine if slip is compatible with a broad‐scale regional stress field derived from plate‐driving forces and, if so, under what conditions (stress regime, pore pressure, and frictional coefficient). Using independent information on in situ stress orientations from well bore breakout and hydraulic fracturing data and assuming that the regional principal stresses are in approximately horizontal and vertical planes (±10°), the constraint that the slip vector represents the direction of maximum resolved shear stress on the fault plane was used to calculate relative stress magnitudes defined by the parameter ϕ = (S2‐S3)/(S1‐S3) from the fault/stress geometry. As long as the focal mechanism has a component of oblique slip (i.e., theBaxis does not coincide with the intermediate principal stress direction), this calculation identifies which of the two nodal planes is a geometrically possible slip plane (Gephart, 1985). Slip in a majority of the earthquakes (25 of 32) was found to be geometrically compatible with reactivation of favorably oriented preexisting fault planes in response to the broad‐scale uniform regional stress field. Slip in five events was clearly inconsistent with the regional stress field and appears to require a localized stress anomaly to explain the seismicity. Significantly, all five of these events occurred prior to 1970 (when many regional networks were installed), and their focal mechanisms are inconsistent with more recent solutions of nearby smaller events. The frictional likelihood of the geometrically possible slip on the selected fault planes was evaluated in the context of conventional frictional faulting theory. The ratio of shear to normal stress on the fault planes at hypocentral depth was calculated relative to an assumed regional stress field. Regional stress magnitudes were determined from (1)S1/S3ratios based on the frictional strength of optimally oriented faults (the basis for the linear brittle portion of lithospheric strength profiles), (2) the computed relative stress magnitude (ϕ) values, and (3) a vertical principal stress assumed equal to the lithostat. Two end‐member possibilities were examined to explain the observed slip in these less than optimally oriented fault planes. First, the frictional coefficient was held constant on all faults, hydrostatic pore pressure was assumed regionally, and the fault zone pore pressure was determined. Since pore pressure is a measurable quantity with real limits in the crust (P0

ISSN:0148-0227    

DOI:10.1029/92JB00221

年代:1992

数据来源: WILEY

摘要:

Nearly 1500 stress orientation determinations are now available for Europe. The data come from earthquake focal mechanisms, overcoring measurements, well bore breakouts, hydraulic fracturing measurements, and young fault slip studies and sample the stress field from the surface to seismogenic depths. Three distinct regional patterns of maximum compressive horizontal stress (SHmax) orientation can be defined from these data: a consistent NW to NNWSHmaxstress orientation in western Europe; a WNW‐ESE SHmaxorientation in Scandinavia, similar to western Europe but with a larger variability ofSHmaxorientations; and a consistent E‐WSHmaxorientation and N‐S extension in the Aegean Sea and western Anatolia. The different stress fields can be attributed to plate‐driving forces acting on the boundaries of the Eurasian plate, locally modified by lithospheric properties in different regions. On average, the orientation of maximum stress in western Europe is subparallel to the direction of relative plate motion between Africa and Europe and is rotated 17° clockwise from the direction of absolute plate motion. The uniformly oriented stress field in western Europe coincides with thin to medium lithospheric thickness (approximately 50–90 km) and high heat flow values (>80 m W/m2). In western Europe a predominance of strike‐slip focal mechanisms implies that the intermediate principal stress is vertical. The more irregular horizontal stress orientations in Scandinavia coincide with thick continental lithosphere (110–170 km) and low heat flow (<50 m W/m2). The cold thick lithosphere in this region may result in lower mean stresses associated with far‐field tectonic forces and allow the stress field to be more easily perturbed by local effects such as déglaciation flexure and topography. The stress field of the Aegean Sea and western Anatolia is consistent with N‐S extension in a back arc setting behind the Hellenic trench subduction zone. The stress field is influenced in places by regional geologic structures, e.g., in the Western Alps, whereSHmaxdirections show a slight tendency toward a radial stress pattern. Not all major geologic structures, however, appear to affect theSHmaxorientation, e.g., in the vicinity of the Rhine rift system horizontal stress orientat

ISSN:0148-0227    

DOI:10.1029/91JB01096

年代:1992

数据来源: WILEY

摘要:

The recent crustal stress field of Central Europe and especially of the adjoining areas to the east is presented in terms of the directions of maximum horizontal stress (SHmax). The analysis is based on fault plane solutions, in situ stress measurements, geologic fault slip determinations, and repeated precise geodetic triangulations. A bending of the direction ofSHmaxfrom the well‐known NW‐SE direction in the western part of the study area to directions of NE‐SW to E‐W in the eastern part is shown. First results on the recent crustal stress field of the study area were presented by Grünthal and Stromeyer (1986), who substantiated this tendency of bending in the central and eastern parts of the study area by few observations only. Therefore one aim of this paper was to compile observations on the areas with few data points. Generally, these additional data confirm the previously established pattern; in some areas, especially in the Pannonian basin, the stress features became more complicated compared with those solely based on a few data points. The second part of the paper presents steady state elastic finite element model calculations to provide some possible explanations of the observed stress orientations as a result of plate driving forces. Simulation of the North Atlantic seafloor spreading and the northward directed motion of the African plate by appropriate boundary loads produces a pattern ofSHmaxdirections for the western part of the Eurasian plate which is compatible with the broad‐scale observed stress directions. Subregional anomalies such as the fanlike stress pattern perpendicular to the arc of the Western Alps or the radial directions around the Pannonian basin can be explained only when additional stress producing features are introduced overmodulating the regional field. Rigorous introduction of physically constrained model parameters for all these features was not feasible to date. Therefore the preliminary empirical model calculations presented in this paper are attempts to discuss which features have the most influence on the stress field. They are, in a regional scale, the North Atlantic seafloor spreading, the northward motion of the African and the Arabian plate, and obviously, subregionally, increased stiffness of the Apulian promontary as well as of the Bohem

ISSN:0148-0227    

DOI:10.1029/91JB01963

年代:1992

数据来源: WILEY

摘要:

An extensive data set of earthquake focal mechanisms is now available for all of northern Europe and especially for Fennoscandia. These mechanisms are considered to provide representative coverage of the stress field. The maximum horizontal compressional stress orientations are internally very consistent over the area of northern Europe. The dominating NW‐SE compressional stresses appear tied to relative plate motion, with Mid‐Atlantic Ridge spreading and European‐African collision in southern Europe. For Fennoscandia some exceptions to the regional stress pattern exist. The causes of these local anomalies have been investigated. No correlations with geological provinces or province boundaries were found. In addition, there does not appear to be any clear correlation between anomalous stress directions and postglacial uplift in the present earthquake activity. This lack of correlation is in sharp contrast to geological evidence in the form of large faults indicative of large postglacial earthquakes occurring right after the end of the latest ice age, 9000 years ago. Taken together, this evidence suggests a tremendous change of stress field in Holocene time, from one dominated by the postglacial unloading right after the ice age to one dominated by the present plate motion

ISSN:0148-0227    

DOI:10.1029/91JB02011

年代:1992

数据来源: WILEY

摘要:

The stress field has been determined at eight different sites in France (four in crystalline or metamorphic rocks, four in sedimentary formations) by hydraulic tests in boreholes. For seven of these sites the complete stress field has been determined using the hydraulic tests on pre‐existing fractures (HTPF) inversion method. Validity of the results is demonstrated by the good fit between a priori and a posteriori values for the data and by the low values of the a posteriori standard deviation on the unknowns. For four sites, results have been compared with those derived according to the hydraulic fracturing theory. The two methods yield comparable results for the orientation of the maximum horizontal principal stress σH, a generally satisfactory fit for the magnitude of the minimum horizontal principal stress σh, but a very poor agreement for the magnitude of σH. This latter misfit has been attributed to the effect of fluid percolation prior to the actual opening of the fractures. Below a depth of 500 m, a homogeneous σHdirection (N150°E) has been determined at the crystalline sites (Auriat, Echassières, Le Mayet de Montagne, Chassoles), all located in the northern Massif Central. For three of these sites the vertical stress is significantly lower than the weight of overburden as computed from the rock density and the depth of the corresponding measurement. At Auriat and Echassières the stress field is consistent with a mostly strike‐slip faulting regime. At Le Mayet de Montagne and Chassoles the maximum stress is vertical but nearly equal to σH. The stress field has been found to be much more heterogeneous at two of the four sites in sedimentary rocks because of the large variability in mechanical rock properties. In such heterogeneous formations, inversion with the HTPF method must be limited to those data which pertain to the same rock horizon. However, because of its structure this heterogeneity does not restrict the possibility of defining large‐scale uniform principal stress directions. All the results are consistent with the local se

ISSN:0148-0227    

DOI:10.1029/90JB02638

年代:1992

数据来源: WILEY

摘要:

Borehole breakout studies, aligned Quaternary volcanic vents, and kinematic analysis of Quaternary faults indicate that the present‐day least horizontal stress direction (Shmin) in Kenya is aligned approximately NW‐SE and in central Sudan is nearly N–S. Limited data in eastern Ethiopia suggest a NE‐SWShminorientation. The regional pattern forShminis therefore roughly radially disposed about the Afar plate junction. Geologic structures in the Gregory Rift, western Kenya, suggest that up to 0.6–0.4 Ma, Shmin was oriented E‐W, normal to the main rift trend. Similar arguments for the Ethiopian Rift indicate that during its main phases of extension, Shmin was oriented NW‐SE. These data indicate that the central east African stress field underwent a significant realignment during the Quaternary, rotating about 45° in a clockwise sense in the Kenyan Rift. Similar rotations may have occurred in eastern Sudan and eastern Ethiopia on the periphery of the Ethiopian Rift. In Kenya, this stress reorientation resulted in dextral oblique reactivation of older normal faults and formation of new normal faults with dextrally oblique slip components. The large‐scale structural framework of the Kenyan rift is no longer suitably oriented to accommodate large extensional strains. East African extension may now be transferred to the western branch of the rift system (Tanganyika‐Malawi rifts) and rejuvenated basins south of the Gregory Rift in Tanzania (Eyasi‐Manyara rifts). Rapid rotations of near‐surface stress tensors (of the order of 7.5°/105years in the case of East Africa) are now documented in several continental extensional systems. It is unlikely that large‐scale mantle circulation patterns can fluctuate at such rapid rates. This suggests that drag at the base of the lithosphere, at least in continental extensional areas, is not the dominant force controlling the orientation of the near‐surface stress field. Rather, intraplate forces, produced and propagated from distant plate boundaries (ridge‐push, subducting‐slab‐pull), may play a greater role in configuring the

ISSN:0148-0227    

DOI:10.1029/90JB02568

年代:1992

数据来源: WILEY