Showing papers on "Discontinuity (geotechnical engineering) published in 1991"

Constraints on upper mantle discontinuities from observations of long-period reflected and converted phases

Abstract: Stacked images combining over 5 years of long-period Global Digital Seismograph Network data reveal many phases associated with reflections and conversions from upper mantle discontinuities. These images show travel time and amplitude relative to a reference phase which is aligned and normalized on all seismograms prior to stacking. Results obtained for P, S, SS, and PP reference arrivals resolve numerous phases from discontinuities near 410 and 660 km, while some of the stacks also show evidence for a weaker discontinuity near 520 km. Phases of particular interest include P and SH multiples resulting from topside reflections, precursors to SS from underside reflections, and P-to-SV converted phases. These phases can be clearly seen both in the waveform stacks and in cross-correlation analysis of individual seismograms. Travel times for these arrivals are converted to discontinuity depths relative to velocities in the Preliminary Reference Earth Model, and empirical corrections are applied for the effects of lateral velocity variations in the upper mantle. Average apparent depths to the discontinuities for the different phases agree to within ±3 km for the 410-km discontinuity, to within ±4 km for the 520-km discontinuity, and to within ±8 km for the 660-km discontinuity. The best global averages are obtained from the SS precursor data which indicate discontinuities at 415, 519, and 659 km. Discontinuity depths obtained from the P-to-SV converted phases at over 35 individual seismic stations exhibit variations of less than ±20 km. Apparent depths to the 660-km discontinuity consistently show greater variability than depths to the 410-km discontinuity, supporting recent laboratory results which indicate that the Clapeyron slope for the 660-km discontinuity is significantly larger in magnitude than the slope for the 410-km discontinuity. Precursors to SS (seen between 110° and 180°) are particularly useful for mapping possible lateral variations in discontinuity depths since each arrival can be associated with a single underside reflection point Apparent discontinuity depths computed from SS precursors for different tectonic regions agree to within about ±5 km. The SS precursors have especially good coverage near the subducting slabs in the northwest Pacific. Analysis of apparent discontinuity depths in this area suggests the presence of a broad 1000- to 1500-km-wide region near the slab in which the 660-km discontinuity is depressed by about 20 km. Measuring absolute amplitudes for these phases is difficult due to the large corrections required to compensate for the effects of incoherent stacking. Relative amplitude analysis suggests that the P and S wave impedance changes at 410 km are about 0.8–1.1 times the size of the changes at 660 km and that the contrasts at 520 km are between 0.3 and 0.6 of the changes at 410 km.

Mantle layering from ScS reverberations: 2. The transition zone

Abstract: This is the second paper in a four-part sequence that examines the nature of mantle layering using the ScSn phases and internal reflections observed within the reverberative interval of SH-polarized seismograms. Mantle reflectivity profiles for 18 seismic corridors sampling portions of Indonesia, Australia, and the western and central Pacific are constructed from low-frequency (25-mHz) ScS reverberations; i.e, ScSn-type phases reflected once from internal mantle discontinuities. Modeling the reflectivity profiles using synthetic seismograms yields precise, path-specific estimates of the travel times to the 410-km and 660-km discontinuities and their reflection coefficients. The data are consistent with an olivine-dominated model of the transition zone, marked by two major phase transitions having average apparent depths of 414±2 km and 660±2 km and impedance contrasts of 0.046±0.010 and 0.072±0.010, respectively. The interpath variations observed in the reverberation travel times imply that the topography on the two discontinuities is of the order of 20 km (peak to peak) and is negatively correlated, consistent with an endothermic transition at 660 km whose Clapeyron slope magnitude is comparable to that of the 410-km (exothermic) transition. In the case of the 660-km discontinuity, we have been able to supplement these long-wavelength (1500–5000 km) observations with an estimate of intrapath topography, inferred from the frequency dependence of the 660-km reflection coefficient, which gives peak-to-peak variations less than 40 km on scale lengths of 500–1500 km. For the 660-km discontinuity, the observed topography is significantly less than the dynamic topography expected for a simple compositional interface in regions of subduction. However, we do observed an intriguing negative correlation between the apparent depth and the reflection coefficient of the 660-km discontinuity which may involve small compositional variations. Plausible explanations for this correlation include heating of an initially cool chemical boundary layer gravitationally trapped above an endothermic phase transition, loss of reflected energy due to local curvature (or roughness) of the reflector, and/or extreme topography occurring on a small compositional component to the discontinuity (≤40% of the total impedance contrast). By stripping the 410-km and 660-km peaks from the composite path reflectivity profile, we identify three minor reflectors at mean depths of 520, 710, and 900 km. The shallowest may mark the β-phase → γ-spinel phase transition. The 710-km discontinuity can be attributed to the ilmenite (gamet) → perovskite phase transition, although changes in perovskite symmetry can potentially provide explanations for both the 710-km and 900-km features.

Second-order ridge axis discontinuities in the South Atlantic: morphology, structure, and evolution

Abstract: Continuous along-axis Sea Beam coverage of the slow-intermediate spreading (34–38 mm yr−1 full rate) southern Mid-Atlantic Ridge (25°–27°30′S and 31°–38° S) shows that the ridge axis is segmented by both rigid and non-rigid discontinuities. Following the model of Macdonald et al. (1988b), a hierarchy of four orders is proposed for ridge axis discontinuities based on a continuum of relative age and distance offset across the discontinuites. This paper discusses the characteristics associated with five second-order discontinuities found in the areas surveyed. First-order discontinuities represent rigid offsets, transform faults, whereas non-rigid discontinuities fall into the second, third and fourth orders. Like transform fault boundaries, second-order discontinuities have distinctive morphologic signatures both on and off-axis-discordant zones — and therefore are better defined than third- or fourth-order discontinuities. Second-order discontinuities are offsets that range in distance from less than 10 km to approximately 30 km and vary in age offset from 0.5 to approximately 2.0 m.y. The variable morphotectonic geometries associated with these discontinuities indicate that horizontal shear strains are accommodated by both extensional and strike-slip tectonism and that the geometries are unstable in time. Three characteristic geometries are recognized: (1)en echelon jog in the plate boundary where ridge axis tips overlap slightly, (2)en echelon jog in the plate boundary where ridge axes are separated by an extensional basin whose long axis is oriented parallel to the strike of the adjoining ridge axes, and (3) oblique offset characterized by a large extensional basin that is oriented approximately 45° to the strike of the ridge axes. In the case of the third type, evidence for short strands of strike-slip tectonism that link an obliquely oriented extensional basin flanking ridge tips is often apparent. Analysis of the detailed bathymetric and magnetic data collected over the second-order discontinuities and their off axis terrain out to 5–7 m.y. documents that second-order discontinuities can follow several evolutionary paths: they can evolve from transform fault boundaries through prolonged asymmetric spreading, they may migrate along strike leaving a V-shaped wake, and they may remain in approximately the same position but oscillate slightly back and forth. In addition, a small change in the pole of relative motion occurring 4–5 Ma is thought to have resulted in the initiation of at least one second-order discontinuity in the survey area. A geologic model is proposed which involves the interplay of lithospheric thickness, asymmetric spreading, temporal and spatial variability of along-axis magmatic input and changes in the poles of relative motion to explain the origin, morphology and evolution of second-order ridge axis discontinuities.

Spinel elasticity and seismic structure of the transition zone of the mantle

Abstract: THERE is continuing debate about whether the upper mantle is chemically stratified, and whether the seismic discontinuity at 400 km depth represents a chemical boundary, a phase change of the (Mg, Fe)2SiO4 component from the β phase (olivine) to the p phase, or a combination of both. Recent developments in high-pressure synthesis1,2 and ultrasonic interferometry3,4 have made possible measurements of elastic-wave velocities in small, poly-crystalline samples of high-pressure phases. By combining our new acoustic measurements on the spinel (γ) phase of Mg2 SiO4 with existing data for the α and β phases, we present here velocity profiles for the M2SiO4 component of the mantle (where M represents Mg or Fe) to depths of about 600 km. We find it to be unlikely that any seismologically observable velocity discontinuity at about 520 km depth can be attributed to this component, although the contrast in impedance (the product of density and velocity) might be sufficient for the β→γtransformation to be observed in long-period seismic reflection studies at near-normal incidence5. The velocity gradients in the transition zone, particularly for shear waves, are steeper than would be expected for simple adiabatic compression of likely mantle compositions, suggesting that alternative explanations including chemical heterogeneity and anelastic relaxation need to be explored.

Acceleration waves, flutter instabilities and stationary discontinuities in inelastic porous media

Abstract: coupled constitutive equations for a saturated porous medium are developed in the frame of the theories of mixtures : the fluid constituent is elastic and the solid skeleton behaves as a rate-independent elastic-plastic solid. Then the existence of real acceleration wave-speeds is considered : actually the analysis centers on the modes in which these wave-speeds cease to be real. An explicit criterion indicating the critical value of the plastic modulus at the onset of a stationary discontinuity (one wave-speed is zero) is derived in both cases where the fluid and solid constituents are compressible and where they are not. Furthermore, it is shown that in some circumstances, some wave-speeds cease to be real in the very early stages of the inelastic deformation process due to the incipience of a flutter instability (two wave-speeds are complex conjugate). When it is not excluded, this mode of loss of hyperbolicity of the dynamic equilibrium equations usually precedes the onset of a stationary discontinuity and may occur right at the inception of plastic loading, that is for an infinitely large plastic modulus. Flutter instability is excluded when the plastic behavior of the solid skeleton is associative and its existence depends strongly on the relative positions of the shear and longitudinal elastic wave-speeds. It is not likely to occur if the shear wave-speed is the smallest elastic wave-speed and then a stationary discontinuity prevails.

Mantle layering from ScS reverberations: 4. The lower mantle and core-mantle boundary

Abstract: This is the final installment in a four-part sequence that examines the nature of mantle layering required by the multiple-ScS phases and internal reflections observed within the reverberative interval of SH-polarized seismograms. In this paper, long-period, SH-polarized, multiple-ScS phases reflected once from a mantle discontinuity (first-order reverberations) are used to search for abrupt shear-wave impedance contrasts in the lower mantle. Beneath the geographic regions sampled, the depth interval of 1000–2600 km (Bullen's region D′) appears free of any distinct, radial layering, in agreement with the majority of recent seismic models and the notion of near-adiabatic compression of a compositionally homogeneous lower mantle. To pass undetected, discontinuities in D′ must be either small (SH-polarized reflection coefficient R0<1.0%), highly intermittent or subject to lateral depth variations well in excess of 50 km. At greater depths, corresponding to the D″ region of the lowermost mantle, we find evidence for a reflector of long-period seismic energy situated an average of 325 km above the core-mantle boundary (CMB), similar to the discontinuity proposed by T. Lay and coworkers. Our results indicate a 4.4% increase in shear wave impedance and a 1.7% increase in density if the 2.75% increase in shear velocity proposed by Lay and Helmberger (1983) holds true for our study area. The discontinuity is observed beneath less than one-third of the 18 seismic corridors examined and does not appear to be a ubiquitous feature of the lower mantle. This, plus an observed correlation of discontinuity depth and lower-mantle velocity heterogeneity, and the lack of experimental evidence for deep-mantle phase transformations strongly favor a compositional origin. Mantle-side layering nearer the CMB, perhaps associated with a thin chemical boundary layer, would mimic the effects of the surface crust, producing a complex wave train of first- and higher-order reverberations superposed upon all other reverberative interval arrivals. If present and unaccounted for, CMB structure would drive estimates of crustal structure and QScS obtained by multiple-ScS waveform inversion away from their true values. Compared to independent estimates, our results admit little bias. Monte Carlo tests are used to assess the sensitivity of this technique to CMB layering, which decreases with decreasing boundary layer thickness. Nonetheless, the results limit CMB transition zones in excess of 20 km to impedance contrasts less than 12% (8% for a 40-km transition) beneath much of the western Pacific, Australia, Melanesia, and Indonesia.

Predictions of block size distribution for quarrying

Abstract: The in situ block size and shape parameters of a rock mass with discontinuities are of signifcance for determining whether an existing or virgin quarry is suitable for armourstone production and for designing an appropriate blast pattern. A method for predicting these parameters is presented in this paper, which includes a detailed discontinuity survey technique and computer determination of the blocks dissected by the discontinuities. The applications of the method to simulated discontinuity data and natural joint data are discussed.

Dynamics of spreading of a liquid drop across a surface chemical discontinuity

Abstract: HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. Dynamics of spreading of a liquid drop across a surface chemical discontinuity Thierry Ondarçuhu, M. Veyssié

Upper mantle models and the thickness of the continental lithosphere

Abstract: SUMMARY Analysis of the Early Rise traveltimes in the United States shows that on several of the profiles there are features which can be interpreted as indicating a low-velocity zone above 200km, i.e. that the continental lithosphere thickness is less than 200 km. The explosion data from the Nevada Test Site, GNOME and Early Rise are all consistent with Lehmann's suggestion that there is a discontinuity at a depth of about 200 km. Comparison of the observed data between 15" and 23" with model calculations shows that the major differences between the western and centraleastern United States upper mantles occur above 200 km. It appears that the only reasonable explanation for the 200 km discontinuity is that it represents the termination of a zone of 'partial melting.

Phase diagrams of the FeO-MgO-SiO 2 system and the structure of the mantle discontinuities

Abstract: Internally consistent thermodynamic computation of equilibria in the FeO-MgO-SiO2 system up to 300 kbar is carried out and phase diagrams and profiles of the elastic properties and density are constructed at the depths of 300–800 km. Comparisons of calculated thermodynamic properties for different petrological models with seismic velocity profiles have been used to constrain the mineralogy of the mantle discontinuities. The 400-km discontinuity may represent the univariant or divariant transition in the olivine component of pyrolite as well as a chemical boundary. For the pyrolite composition at the depth of 650 km there are two different spinel + perovskite + stishovite (640 km) and magnesiowustite + spinel + perovskite (650 km) divariant loops (1–2 km wide) separated by a Invariant zone spinel + perovskite (4–6 km wide). The results indicate that phase changes in pyrolite do not explain the 650-km discontinuity. It is also shown that it is impossible to match the seismic properties observed at the depths of 600–800 km and through the discontinuity with any isochemical petrological model considered in the FMS system. However, increasing the iron content or silica and iron contents across the 650-km discontinuity can produce thermodynamic properties in the lower mantle that are more consistent with those inferred from seismic observations. Constraints on the SiO2 and iron contents in the mantle are inferred from the comparison of thermodynamic and seismological data.

Analysis of shielded coplanar waveguide step discontinuity considering the finite metallization thickness effect

Abstract: The mode-matching technique is applied to analyze the shielded coplanar waveguide (CPW) step discontinuity. The effect of the finite thickness of the CPW center strip and ground planes is also considered. Results on the frequency-dependent scattering parameters of the shielded CPW step discontinuity incorporating the finite metallization thickness effect are presented. >

Development of a preliminary rock mass classification scheme for near-surface excavation

Abstract: In rock excavation work, the resistance of a rock mass depends partially on the mode of loading, shape of the loading instrument, rock moisture conditions and the orientations of the significant flaws relative to that of the applied force. Flaws or discontinuities in rocks may be cracks, bedding planes, weak grain boundaries and faults. Laboratory tests on rocks are often conducted on small cores of reasonably intact material. Generally, rocks decrease in strength with increase in sample size because the probability of the presence of large strength-controlling flaws increases with sample size. This is a plausible explanation for the disparity between laboratory and field specific energies often observed in rock excavation. Since the synergistic effects of discontinuity properties, scale of excavation and moisture condition can not be effectively evaluated using laboratory test results solely, it is necessary to use judgement along with such results in rating rock masses. Such a rating may involv...

Liner for a van or like vehicle

Abstract: There is provided a new and useful liner for a van or the like vehicle, the liner comprising at least one panel member, the panel member comprising a face sheet having a series of grooves therein, each groove having at least one area of discontinuity or of reduced depth therein, at least one area of discontinuity aligned with a similar area in each groove of the series whereby the areas of discontinuity may be secured to a stud located behind the panel member.

Apparatus and method for detecting the presence of a discontinuity on a glass surface

Abstract: The present invention is directed to an apparatus and method for detecting the presence or absence of a discontinuity on a glass surface. The apparatus includes a transmitting means for directing and receiving infrared electromagnetic radiation relative to the glass surface, a positioning means for positioning the transmitting means in a fixed angular and distance relation with respect to the glass surface, and a sensing means for determining the relative magnitude of the received radiation, the magnitude being indicative of the presence or absence of a discontinuity. The apparatus may be translated relative to the glass surface to detect discontinuities therealong.

The interaction of subducting slabs and the 670 kilometer discontinuity

Abstract: The subduction of oceanic lithosphere plays a major role in the dynamics of the Earth. The dynamics of subduction are influenced both by variations in density (due to phase changes or compositional changes), and variations in viscosity encountered by the slab; the rheology of the slab and the coupling of the slab with the oceanic lithosphere also play important roles. Geoid and topography place fundamental constraints on subduction, and observations can be used to test various mantle models. The effects of the rheology of slabs are considered using finite element convection calculations with Newtonian (linear) and non-Newtonian (power-law) temperature-dependent rheology. Newtonian temperature-dependent fluids do not exhibit slab-like features without weakening the thermal boundary layer. Weak zones are imposed at the trench and ridge, and the effects of varying the size, location, and strength of the weak zones are studied. Non-Newtonian rheology provides a self-consistent mechanism for weakening the thermal boundary layer without imposing a weak zone at the trench. This self-consistency is not proof or confirmation of the importance of power-law deformation in the Earth. Even with non-Newtonian rheology, a weak zone at the ridge is necessary for plate-like behavior. The geoid and topography for slabs with a density discontinuity and a viscosity discontinuity are compared. Weak slabs deform rapidly by spreading out along the density discontinuity with little deformation of the boundary, while strong slabs deform slowly and locally depress the density boundary. However, the long wavelength components of the geoid and topography are independent of the lateral variations in viscosity from the slab. Finite deformations of a compositional boundary are compared with an undeformable boundary approximation; the long wavelength components of the geoid and topography are indistinguishable for boundary deformations up to several hundred kilometers. Subduction calculations are computationally intensive and high resolution is required to resolve deformation at the trench. The solutions are time-dependent, and a temperature-dependent rheology is required. Faster and more powerful numerical techniques are needed. A fast implementation of the finite element method is presented. Applied to creeping flow, this formulation allows large viscosity variations, but is still efficient on a vector supercomputer.

Vehicle leaf spring with a longitudinal discontinuity for crack propagation

Abstract: A vehicle suspension member such as a leaf spring (50) is provided comprising at least one composite leaf or plate (2) having a second layer (12) sandwiched within a first layer (10) in such a manner as to provide at least one discontinuity therein effective to divert crack propagation in a direction generally parallel to a longitudinal axis ("L/A") extending between opposite ends of plate (2).

Application de l'analyse de stabilité d'une perturbation à la localisation de la déformation dans un matériau dilatable adoucissant

Abstract: Fracture may proceed from strain localization on a surface. A linear perturbation stability analysis is applied to rigid-plastic incompressible or dilatant materials. According to material constitutive relations, one or two modes of localization are obtained, related to shear fracture and to void damage fracture. A comparison is made with results derived from the conditions for the existence of strain rate discontinuity surfaces.

The displacement discontinuity method in open crack analysis

Abstract: The application of the displacement discontinuity numerical technique to the solution of some problems of fracture mechanics is demonstrated in the hypothesis of homogeneous and elastic material. The fracture is supposed to be free from traction and is represented by a set of constant displacement discontinuity elements, except for two parabolic elements, located at each crack tip, in order to simulate the singularity of the solution near the crack tips. On the basis of the stress and displacement field determined by the displacement discontinuity method, the stress intensity factors for mode I and II are computed according to the method of the displacements. Three examples are provided to verify the validity of the formulation.

General analysis of discontinuity waves in thermoviscoelastic solids of integral type

Abstract: In this paper we study the propagation of general discontinuity waves of orderN≥1 through a homogeneous anisotropic linear thermoviscoelastic solid whose heat flux vector depends upon the past history of the temperature gradient. After determining the normal speeds of propagation we state the evolution law of the discontinuities along the rays associated with the wave front. The results are independent ofN.

"Regression-Discontinuity Design" by Any Other Name Might Be Less Problematic.

Abstract: In "Random Measurement Error Does Not Bias the Treatment Estimate in the Regression- Discontinuity Design : II. When an Interaction Effect Is Present, " Trochim, Cappelleri, and Reichardt claim to ...

First instabilities in the wake past a circular cylinder. Comparison of transient regimes with landau's model

Abstract: Velocity measurements were carried out in the flow past a circular cylinder at Reynolds number ranging from 30 to 180 to study the genesis and the evolution of the first instabilities in the wake. Since the occurrence of aeroelastic coupling between flow and cylinder has been cause of debate about the results obtained in recent experiments on the subject, a great deal of care has been taken to avoid that coupling. We focused our attention on two points: the onset of the first instability and the transition from the ‘low-speed’ to the ‘high-speed’ oscillation mode. With regard to the second topic we can confirm the existence of a discontinuity in the relation between St and Re at Re≈90 even in the absence of any aeroelastic coupling. However, no band enlargements in the velocity spectrum, indicating the appearance of highly disorganized motion, were observed for Re varying through that discontinuity interval. To investigate the onset of the first periodic oscillation we performed measurements with a slowly varying Reynolds number by means of very small positive or negative accelerations. Our results indicate that in the neighborhood of the onset of the first oscillation the amplitude of the transient velocity fluctuations does not obey the so-called Landau model.