Showing papers on "Stress field published in 1983"

The 1979 Homestead Valley Earthquake Sequence, California: Control of Aftershocks and Postseismic Deformation

Abstract: The coseismic slip and geometry of the March 15, 1979, Homestead Valley, California, earthquake sequence are well constrained by precise horizontal and vertical geodetic observations and by data from a dense local seismic network. These observations indicate 0.52±0.10 m of right-lateral slip and 0.17±0.04 m of reverse slip on a buried vertical 6-km-long and 5-km-deep fault and yield a mean static stress drop of 7.2±1.3 MPa. The largest shock had MS = 5.6. Observations of the ground rupture revealed up to 0.1 m of right-lateral slip on two mapped faults that are subparallel to the modeled seismic slip plane. In the 1.9 years since the earthquakes, geodetic network displacements indicate that an additional 60±10 mm of postseismic creep took place. The rate of postseismic shear strain (0.53±0.13 μrad/yr) measured within a 30×30-km network centered on the principal events was anomalously high compared to its preearthquake value and the postseismic rate in the adjacent network. This transient cannot be explained by postseismic slip on the seismic fault but rather indicates that broadscale release of strain followed the earthquake sequence. We have calculated the postearthquake stress field caused by the modeled coseismic slip. We assume that failure is promoted when the sum of the shear stress plus 0.75 times the faultopening stress increases. Most aftershocks concentrate at points where the stresses are enhanced by 0.3 MPa (3 bars) or more; aftershocks are nearly absent where postearthquake stresses decrease by 0.3–0.5 MPa. Isolated off-fault clusters of aftershocks that locate at one fault length from the rupture plane are explainable by this hypothesis. We find that ground rupture and postseismic creep take place where near-surface stresses are calculated to increase within the preexisting fault zones. Two patches that extend 4 km from both ends of the seismic fault exhibited neither aftershocks nor measurable postseismic creep. The sensitivity of aftershocks and ground rupture to changes in stress that are less than 5% of the earthquake stress drop demonstrates that the region around the earthquakes was within a few percent of its failure threshold before the main shocks. The preearthquake stress field and the stress required for failure must also have been nearly uniform.

Tectonic analysis of the Dead Sea Rift Region since the Late-Cretaceous based on mesostructures

Abstract: We have measured the orientations of small faults, slickensides, tectonic stylolites, vein with secondary mineralization, small folds and dikes in Israel and Sinai. These structures are indicators of paleo-stress or strain and provide the pattern of tectonic deformation. Data were collected at 130 stations, most with tens of separate measurements; most stations showed consistent deformation. Stations were located on exposures ranging from Precambrian crystalline rocks to Pleistocene sediments. We have defined two tectonic stress fields, each relatively uniform in both time and space. One stress field, with dominating maximum horizontal compression trending W to WNW, in the Late Cretaceous to Eocene rocks in the folds and plateaus west of the Dead Sea rift. The second field, with dominating horizontal extension trending E to ENE, in all rocks inside the rift and proximal thereto. The first stress field is called the Syrian Arc stress, and the second is called the Dead Sea stress. A change in style of tectonic deformation, which corresponds to the two stress fields, is manifested also in the major structures in Israel. The Late Cretaceous to Neogene deformation is characterized by long wavelength folds and monoclines, whereas the Neogene to Recent deformation is characterized by normal and strike slip faults and volcanic activity.

Tectonic stresses in the lithosphere

Abstract: Various types of observables (earthquake focal mechanisms, in situ measurements and geological deformations) give information about the large scale lithospheric stress field. The latter has often been explained by postulating appropriate forces acting at the edges and beneath the plates. This approach ignores the role of mass heterogeneities within the lithosphere. Here we analyze the effect of both boundary and internal forces on the stress pattern and show that both contributions are of comparable magnitude. The presence of internal sources makes the problem three-dimensional. We show however that it can be reduced to a two-dimensional plane stress formulation, whereby the edge forces are expressed by the ‘non hydrostatic stresses’ and the basal shear is increased by the addition of a term proportional to the gradient of the mean vertical stress. For the oceanic lithosphere we derive a compression that increases with age. The comparison with geophysical observables yields an upper bound of a few bars on the magniude of the basal drag. For the continents we infer the existence of an underlying upper mantle somewhat denser than under oceans.

Modeling of large-scale accretionary wedge deformation

Abstract: Instantaneous stress and velocity fields within an active accretionary wedge can be calculated using plastic slip line theory, given the assumptions of a perfectly plastic rheology and plane strain conditions. After selecting a plastic yield stress and an average density, a stress field (slip line field) can be computed for a wedge of known cross-sectional configuration from known stress boundary conditions on the upper surface. Basal surface tractions are derived without basal boundary conditions. The geometry of the computed slip line field can then be used to constrain the lower limit of the yield stress. The known velocity boundary conditions are insufficient for calculation of a velocity field because the coupling between the wedge and the subducting plate is unknown. Consequently, basal velocity distributions must be assumed to permit the construction of velocity fields. These assumed distributions are constrained by known rates of surface uplift and rates of tilt as well as inferred patterns of instantaneous strain rates within a wedge. The Sunda accretionary wedge west of central Sumatra is used to illustrate the plasticity approach. A minimum average yield stress of 20–30 MPa and a weaker basal layer of variable effective strength are indicated for this example. Surface uplift rates may be affected significantly by regions of high basal strain rate, finite velocity discontinuities, and underplating. The known pattern of uplift rates near Nias Island, located on the outer arc ridge, is consistent with the constraint that 90% of the total incremental shortening of the wedge is concentrated within 20 km of the trench. Given a sufficient number of uplift and tilt rate observations across a particular wedge, the relative influence of underplating, tectonic erosion, and basal strain rate variations could be assessed using the perfectly plastic model.

The reflection of a symmetric Rayleigh-Lamb wave at the fixed or free edge of a plate

Abstract: A semi-infinite plate of homogeneous isotropic, linearly elastic material occupies the region x≥0, |y|≤1, -∞

A Griffith crack shielded by a dislocation pile-up

Abstract: The dislocation free zone at the tip of a mode III shear crack is analyzed. A pile-up of screw dislocations parallel to the crack front, in anti-plane shear, in the stress field of a crack has been solved using a continuous distribution of dislocations. The crack tip remains sharp and is assumed to satisfy Griffith's fracture criteria using the local crack tip stress intensity factor. The dislocation pile-up shield the sharp crack tip from the applied stress intensity factor by simple addition of each dislocation's negative contribution to the applied stress intensity value. The analysis differs substantially from the well known BCS theory in that the local crack tip fracture criteria enters into the dislocation distributions found.

Elastic moduli of a cracked body

Abstract: Statistical solutions for the internal stress and displacement fields are constructed for a body in an external applied stress field, and containing a prescribed distribution of penny shaped cracks. From the fields, a stress-strain relationship is calculated for the body, and effective elastic moduli are derived. The fields are constructed as expansions in the parameter $$\overline {Na^3 }$$ wich is assumed small (N is the crack density, anda is the crack radius). Angular integrals over the distribution function are performed for two special cases that represent a bedded crack distribution, and an isotropic crack distribution. A criterion for the validity of the use of these quasistatic results in dynamical calculations is presented.

Estimation of Low-Frequency Wind Stress Fluctuations over the Open Ocean

Abstract: A simple, approximate formula for mean wind stress is given in terms of the mean and variance of the wind fluctuations over the averaging period. The formula is nonlinear with respect to the mean wind speed. The formula is tested using 3 h wind observations from eight North Atlantic Ocean Weather Ships. Mean wind stress is calculated 1) by vector averaging the 3 h wind stresses and 2) by applying the approximate formula. For an averaging period of 4 months the two methods agree to within ±0.025 Pa, 95% of the time. For an averaging period of 1 month the approximate formula slightly overestimates the stress. This is due to skewness in the probability density function of the observed 3 h wind fluctuations. An expression for the modification of the mean stress due to skewness is given. A straightforward method is described for the estimation of vector mean wind and variance fields, and thus mean stress fields, over the open ocean. To cheek the method, the long-term stress field of the North Atlantic...

The stress intensity factor for the double cantilever beam

Abstract: Fourier transforms and the Wiener-Hopf technique are used in conjunction with plane elastostatics to examine the singular crack tip stress field in the Double Cantilever Beam (DCB) specimen With terms of orderh 2/a 2 retained in the series expansion, the dimensionless stress intensity factor is found to be $$Kh^{\tfrac{1}{2}} /P = (12)^{\tfrac{1}{2}} (a/h + 06728 + 00377h^2 /a^2 )$$ , in whichP is the magnitude of the concentrated forces per unit thickness, a is the distance from the crack tip to the points of load application, andh is the height of each cantilever beam This result is quite similar to the expression $$Kh^{\tfrac{1}{2}} /P = 346a/h + 238$$ , which Gross and Srawley obtained by fitting a line to discrete results from their boundary collocation analysis Still another expression, $$Kh^{\tfrac{1}{2}} /P = [12\{ (a/h + 06)^2 + \tfrac{1}{3}\} ]^{\tfrac{1}{2}}$$ , obtained by Ripling, Mostovoy and Patrick from a strength of materials approach combined with compliance measurements, although somewhat different in form from the present results, also yields accurate values ofK for thea/h-range of practical interest (2 ⩽a/h ⩽ 10) The present result serves as both a confirmation and a refinement of the Gross and Srawley formula For this reason, and because of its simplicity, the present result should be useful in the derivation of other simple and parametrically appropriate equations for the analysis of data from DCB specimen tests

General Pattern of Seismotectonic Dislocation and the Earthquake - Generating Stress Field in Central Europe Between the Alps and the North Sea

Abstract: We have used 30 reliable fault-plane solutions from earthquakes which occured during 1975 to 1982 in Central Europe between the Alps and the North Sea to derive the general pattern of seismotec-tonic dislocations and the characteristic features of the earthquake generating stress field in the Rhenish Massif area and surrounding region. Stress directions from fault-plane solutions are in good agreement with the results of in situ stress measurements. Although the maximum compressive stress shows an average direction of about 145° with respect to north in the whole area, regional differences in the stress directions and in the seismotectonic regime are recognizable. In the southern part of the Upper Rhine Graben, the Swabian Jura, and in the western Vosges mountains, strike slip mechanisms are predominantly, whereas in the northernmost part of the Upper Rhine Graben, in the Rhenish Massif, and the Lower Rhine Embayment tensional dip-slip dislocations along NW-SE trending fault planes are typical. Seismotectonic evidence strongly suggests that an active rift zone separates the Rhenish Massif along a line between the Lower Rhine Embayment and the Upper Rhine Graben.

On the stability of shear cracks and the calculation of compressive strength

Abstract: Griffith's 1921 theory of crack stability is extended to account for the frictional energy dissipation associated with interfacial sliding when the crack faces are in contact. This global energy balance approach is essentially different from Griffith's 1924 local theory of compression cracks, which assumes that instability occurs when the stress at any point exceeds the intrinsic strength of the material. An explicit expression for critical crack size in compression is obtained. On the basis of this expression we find the most critical crack orientation and define theoretical strength as the critical stress for that orientation. This uniaxial compression strength significantly exceeds the tensile strength. For example, when the coefficient of friction is unity, the ratio of compressive to tensile strength is 8.7, and the crack plane lies at 13° to the loading direction. Following this, the heating due to interfacial sliding is found by a one-dimensional thermoelastic analysis. It is concluded that under normal conditions the stress field acting on the crack faces is only slightly affected by frictional heating, though the temperature rise may be very high, and melting may occur within microseconds.

Stress-induced transformations in composite zirconia ceramics.

Abstract: A martensitic transformation may be induced in confined tetragonal zirconia particles by the stress field in front of a crack tip, resulting in an increase in toughness. The microstructure, transformation mechanisms, elastic strain distribution, and in-situ studies of the interaction of the confined particles with the stress field of the crack tip have been studied by means of high voltage transmission electron microscopy techniques. Facetted particles below a critical size dc are retained metastably down to room temperature. — The transformation is predominantly controlled by a nucleation barrier. A model of nucleation by inhomogeneity of internal strains is postulated, in which the total resolved strain must exceed 0.25 of the unconstrained transformation strain over the volume of the nucleus for initiation of the martensitic transformation. The “critical size effect” may then be explained in terms of irregular-shaped particles inducing an inhomogeneous internal strain field which scales with particle size and has a well-defined dc. In contrast, regular-shaped spherical particles have a homogeneous strain field with no well-defined critical size.

Stress Field and Strain Release in the Rhenish Massif

Abstract: The stress conditions prevailing in the Rhenish Massif were determined at 14 sites by means of the doorstopper method. The mean direction of the maximum horizontal principal stress is 140° to 150°, thus not differing from the rest of the western Alpine foreland. Tensile stresses are the characteristic differences of the stress field of the Rhenish Massif with respect to its surroundings. These tensile stresses are due to the diverging σ1-jtrajectories, but besides that they are probably promoted by thermal effects at depth or by diapiric doming. The special kind of strain release found in the Rhenish Massif, if compared with that in the bordering rift segments of the Rhine Graben and the Lower Rhine Embayment, is mainly due to differing mechanical behavior of the crust in these areas.

Fault plane solutions for microearthquakes induced at the Fenton Hill hot dry rock geothermal site: Implications for the state of stress near a quaternary volcanic center

Abstract: Fault plane solutions of earthquakes induced during attempts to stimulate two hot dry rock reservoirs at Fenton Hill have significantly different patterns of first motions. The fault plane solution for the lower reservoir indicates strike slip, either left lateral strike slip on a N-S vertical plane or right lateral slip on an E-W vertical plane. In contrast, the solution for the upper zone includes largely vertical slip on a N-S nearly vertical plane, or oblique slip on a nearly horizontal plane. Because the N-S nodal plane is common in both solutions we infer that this represents the true fracture plane. Faulting thus seems to occur on a series of parallel faults or joints that intersect both reservoirs but a change in the slip vector indicates a major change in the state of stress between the upper and lower reservoirs. This latter conclusion is surprising because the two reservoirs are separated by less than 1 km. We suggest that this rapid change in the stress field may be related to the structure and subsidence of the nearby Valles Caldera.

Optimization of Stimulation Design Through the Use of In-Situ Stress Determination

Abstract: This paper describes the results and applications of several research programs carried out over the past few years to optimize the design of hydraulic-fracture stimulation treatments using information pertaining to in-situ stress action within the reservoir. Begun as fracture-mechanics-based theoretical studies of propagation and containment of hydraulically induced fractures, these programs have grown into full-scale field demonstrations of the deduced principles. A review is provided of field-measured in-situ stresses in the pay and confining formations. The existence of in-situ stress contrast between the pay zone and the bounding layers has been demonstrated in these field demonstrations. Furthermore, the results also showed the significant role of the in-situ contrasts in fracture containment. Unfortunately, however, great variability in the stress contrast from site to site has been observed. The field programs have been performed in both openhole and cased wells. Laboratory studies of hydraulically fractured large block samples have been carried out. The programs described in this paper indicate that successful stimulation design requires a knowledge of the in-situ stress field and contrasts within relatively narrow ranges at well depth where the stimulation treatment is performed. To minimize the costs of in-situ stress determinations on a well-by-well basis, a wireline-operated hydraulic-fracturing toolmore » has been designed. The tool does not require a rig on the well; and because it is entirely self-contained, considerable cost savings will be possible compared with the costs of standard techniques of stress determination by hydraulic fracturing.« less

Elastic interaction of two precipitates subjected to an applied stress field

Abstract: Using the Born approximation to an integral equation, analytic approximations are obtained for the strain fields and interaction energies associated with two elastic inhomogeneities subjected to a uniformly applied stress field. Solutions are obtained for two spheres of unequal size and two rectangular parallelepipeds also of different dimensions in isotropic media for an applied tensile stress. The method employed can be extended to systems of more than two inhomogeneities and other precipitate morphologies.

Dynamic stress drops of moderate earthquakes of the Eastern Aleutians and their relation to a great earthquake

Abstract: Short-period WWSSN records are used to calculate dynamic stress drops of moderate earthquakes (m/sub b/ = 5.5 to 6.1) in the Eastern Aleutians, Alaska. For short-period pulses where scattering effects often distort the pulse shape, estimates of dynamic stress drop using the formula of Boatwright (1980) have fewer sources of uncertainty than does the formula for static stress drop given by Brune (1970, 1971). Earthquakes of several tectonic settings within the subduction zone were studied: shallow (5 to 8 km) normal faulting events near the trench; earthquakes in the main thrust zone (30- to 50-km depth), and intermediate-depth (100 to 200 km) earthquakes in the Benioff zone. Dynamic stress drops of the intermediate-depth earthquakes ranged from 20 to 70 bars while the stress drop of earthquakes in the main thrust zone varied over a considerably larger range, 10 to 300 bars. The two normal events had stress drops around 100 bars. Evidence from the great 1964 Alaska earthquake and its aftershocks also suggests an inhomogeneous stress field in the main thrust zone which may produce large amounts of high-frequency radiation when ruptured by a large earthquake. 27 references, 16 figures, 4 tables.