Regional Stress State Research Articles

Thermal stress state in the end regions of multilayered shells

Thermal stress state in the end regions of multilayered shells

Mechanical energy budget of a fold-and-thrust belt

Recent studies1,2 have shown that an active fold-and-thrust belt is mechanically analogous to the wedge of soil or snow that forms in front of a moving bulldozer; such a wedge attains a critical taper in which the regional state of stress is everywhere on the verge of brittle failure. Here this critical-taper model of the regional stress field is combined with a simple kinematic description of the steady-state deformation of an active, eroding fold-and-thrust belt, to examine the mechanical energy balance of brittle, frictional mountain building. Part of the work performed by the compressive stress at the rear of a fold-and-thrust belt is dissipated by frictional heating on the decollement fault, part is dissipated by internal frictional processes that accompany the brittle deformation within the critically tapered wedge, and part is expended against gravitational body forces. The total power expenditure in the steady-state fold-and-thrust belt of western Taiwan is about five gigawatts, of which 45% is being dissipated against basal friction, 39% is being expended against gravity and only 16% is being dissipated internally.

Regional state of stress in Western Europe : Klein, R J; Barr, M V Proc International Symposium on Rock Stress and Rock Stress Measurement, Stockholm, 1–3 Sept 1986P33–44. Publ Lulea: Centek, 1986

Regional state of stress in Western Europe : Klein, R J; Barr, M V Proc International Symposium on Rock Stress and Rock Stress Measurement, Stockholm, 1–3 Sept 1986P33–44. Publ Lulea: Centek, 1986

A numerical method for determining the state of stress using focal mechanisms of earthquake populations: application to Tibetan teleseisms and microseismicity of Southern Peru

The numerical method described in this paper enables the study in terms of stress of the kinematics of seismic faults provided by focal mechanisms. This method assumes a mean state of stress in the source region and is based on the simple mechanical model used for fault population analysis which supposes slip in the direction of the resolved shear stress acting on the fault plane. The proposed algorithm first defines compressional and tensional zones resulting from superimposition of the compressional and tensional quadrants limited by the nodal planes. This enables one to test the data homogeneity. Furthermore, this restricts the space where the principal stress axes have to be searched. Then, for each principal stress reference whose location is constrained by above confined zones, the R value (chosen equal to (σ 2′ − σ 1′)/(σ 3′ − σ 1′) ) is computed which fits the slip vector on each nodal plane. This permits one to select a set of preferred seismic fault planes from a set of auxiliary planes. Finally, a state of stress is computed from the preferred seismic fault plane set using the non-linear simplex method already applied to fault populations. This algorithm is constructed so as to avoid two major difficulties: misleading estimation of the deviatoric stress tensor which may result from excessive emphasis by minimizing the residuals by a least squares method if some of the data are wrong and a lengthy prospection of the stress references over all the space of directions. This calculation does not take into account possible changes of the strike and dip of the nodal planes. However, we use rotations of nodal planes in agreement with the first arrival data to test the compatibility of these data with the computed state of stress. Tibetan teleseisms and southern Peruvian microseisms are analyzed and results are compared with kinematics of superficial recent and active faults measured in field in the same regions. This suggests that major seismic events may give a good evaluation of the regional (mean) state of stress. Focal mechanisms of the small events are also modelled by a mean stress deviator, but this differs significantly from the regional state of stress in southern Peru deduced from Quaternary faulting. This may be due to imprecise location of the small events. However, some movements are strongly different from those predicted by the computed mean stress deviator. This suggests that small fault kinematics surely depend on the regional state of stress but that local effects may control the small deformations.

Structural analysis of recent and active faults and regional state of stress in the epicentral area of the 1978 Thessaloniki earthquakes (northern Greece)

During spring and summer 1978, Macedonia (Greece) suffered damaging seismic activity. During the two main shocks which occurred on May 23 (Mb = 5.7) and June 20 (Mb = 6.1), seismic open cracks and faults were formed in the epicentral area, 25 km NNE from Thessaloniki. They have been mapped and their kinematics analyzed. These studies have shown that they result from the reactivation of existing faults of Pleistocene, Recent, and historical age and that the crust is stretching along a N‐S to NNE‐SSW direction in that part of the North Aegean. Principal stress directions have been computed from the slip vectors measured on recent neotectonic faults and seismic cracks and faults, this computation utilizing a simple mechanical model (homogeneous deformation through the addition of small and independent displacements of rigid blocks in a highly fractured body). A striking feature of this analysis is that the tensional directions σ3 deduced from seismic faulting and from recent neotectonic faulting are nearly the same. They are compatible with the regional tension axis deduced from focal mechanisms of North Aegean and with the focal mechanisms of the 1978 Thessaloniki main shocks. They are also compatible with in situ stress measurements in this region. Studies of superficial seismic faulting strongly support previous assumptions proposed from aftershock distribution: the 1978 Thessaloniki earthquakes resulted from reactivation of several different faults situated within the same NE dipping fault zone of Upper Eocene‐Lower Oligocene age (and older) separating the Serbo‐Macedonian Massif from the Peonias (East Vardar) zone.

Implications of an elastic analysis of in situ stress measurements near the San Andreas Fault

Twenty‐nine measurements of in situ stress obtained with the hydraulic fracturing technique near Palmdale, California, are the basis of an elastic analysis of the state of stress in the Mojave Desert adjacent to the San Andreas fault. The measurements were made at depths extending from 80 to 849 m and at distances from the fault between 2 and 34 km. The elastic solution indicates a state of deviatoric stress typical for continents in that the inferred depth gradient of the maximum shear stress is about 7.9 MPa/km. Extrapolation yields an average shear stress in the upper 14 km of the crust of about 56 MPa, a result that is higher than estimates of the average shear stress on the San Andreas fault based on the analysis of heat flow data. This finding is consistent, however, with estimates of fault strength based on laboratory determinations of the coefficient of friction for samples of San Andreas fault gouge if the regional state of deviatoric stress is limited by the strength of the fault zone. If so, then the coefficient of friction of the San Andreas fault zone inferred from the stress field results is about 0.45. The state of stress does not appear to vary systematically with distance from the San Andreas fault although considerable localized variation is observed. The observations suggest an upper bound of about 0.1 MPa/km for the horizontal gradient of the maximum shear stress in the direction perpendicular to the San Andreas fault, a result that implies a corresponding limit of about 1.4 MPa on the shear traction applied to the base of the seismogenic layer. Finally, we demonstrate the potential application of in situ stress data to the direct assessment of accumulated slip, which could be released in a large earthquake. We show that on the basis of a model involving a locked fault, extending to about 22 km, the total fault slip below the locked portion is less than 13 m. A more comprehensive set of stress data could permit the estimation of an even lower bound.

Finite element study of the Nevis Bluff (New Zealand) rock slope failure

Finite Element Study of the Nevis Bluff (New Zealand) Rock Slope Failure In June 1975, part of the Nevis Bluff rock slope failed along State Highway 6 in the South Island of New Zealand. The slope is near the bottom of the Kawarau River valley, which has been deeply entrenched in chlorite schist rocks. The detailed geology of the slope, and the mechanical properties of the rocks comprising the slope are described. For the purposes of finite element modelling, the schist was assumed to be a transversely isotropic material with four independent deformability parameters. A two dimensional finite element program was used to establish the regional state of stress, the results from which were then used in a larger scale model of the rock slope. The results of finite element analyses suggested that slope failure occurred by flexural toppling, where continuous columns broke in flexure as they slid along schistosity surfaces. Flexural cracking was then responsible for propagating the plane along which sliding occurred. When the shear strength available along this plane was exceeded by shear stresses, the slope failed.

An approximate analysis of an internally loaded elastic plate containing an infinite row of closely spaced parallel cracks

The stress transfer which occurs in an internally loaded infinite elastic plate containing an array of closely spaced parallel cracks of finite width is examined. The internal loading corresponds to a doublet of concentrated forces which act at finite distances from the cracked region. The solution presented is approximate to the extent that the state of stress in the strip regions contained between adjacent cracks is considered to be one-dimensional. Such a simplification enables the derivation of certain general results for the stress distribution in a strip region contained within internally loaded half-planes of differing elastic characteristics. These solutions are obtained by Fourier transform methods. Attention is particularly focussed on the estimation of the stress magnification which occurs in the strip region.

Seismicity and earthquake focal mechanisms in the Baikal rift zone

The seismicity of the Baikal rift zone is considered on the basis of instrumental and field observations. The spatial pattern of epicentres, the frequency of earthquakes and the relations between seismicity and the elements of fault tectonics are analyzed. The regional and local stress states in the crust of the Pribaikalye region, obtained from studying earthquake focal mechanisms for various energies are summarized.