Showing papers on "Discontinuity (geotechnical engineering) published in 1997"

Mantle discontinuity structure from midpoint stacks of converted P to S waves across the Yellowstone hotspot track

Abstract: Analysis of a deployment of broadband sensors along a 500-km-long line crossing the Yellowstone hotspot track (YHT) has provided 423 in-plane receiver functions with which to image lateral variations in mantle discontinuity structure. Imaging is accomplished by performing the converted wave equivalent of a common midpoint stack, which significantly improves resolution of mantle discontinuity structure with respect to single-station stacks. Timing corrections are calculated from locally derived tomographic P and S wave velocity images and applied to the Pds (where d is the depth of the conversion) ray set in order to isolate true discontinuity topography. Using the one-dimensional TNA velocity model and a Vp/Vs ratio of 1.82 to map our Pds times to depth, the average depths of the 410- and 660-km discontinuities are 423 and 664 km, respectively, giving an average transition zone thickness of 241 km. Our most robust observation is provided by comparing the stack of all NW back-azimuth arrivals versus all SE back-azimuth arrivals. This shows that the transition zone thickness varies between 261 and 232 km, between the NW and SE portions of our line. More spatially resolved images show that this transition zone thickness variation results from the occurrence of 20–30 km of topography over 200–300 lateral scale lengths on the 410- and 660-km discontinuities. The topography on the 410- and 660-km discontinuities is not correlated either positively or negatively beneath the 600-km-long transect, albeit correlation could be present for wavelengths larger than the length of our transect. If this discontinuity topography is controlled exclusively by thermal effects, then uncorrelated 250° lateral temperature variations are required at the 410- and 660-km discontinuities. However, other sources of discontinuity topography such as the effects of garnet-pyroxene phase transformations, chemical layering, or variations in mantle hydration may contribute. The most obvious correlation between the discontinuity structure and the track of the Yellowstone hotspot is the downward dip of the 410-km discontinuity from 415 km beneath the NW margin of the YHT to 435 km beneath the easternmost extent of Basin and Range faulting. Assuming this topography is thermally controlled, the warmest mantle resides not beneath the Yellowstone hotspot track, but 150 km to the SE along the easternmost edge of the active Basin and Range faulting.

The Seismic 8° Discontinuity and Partial Melting in Continental Mantle

Abstract: Strong, scattered reflections beyond 8 degrees (8°) offset are characteristic features of all high-resolution seismic sections from the continents. The reflections identify a low-velocity zone below approximately 100 kilometers depth beneath generally stratified mantle. This zone may be caused by partial melting, globally initiated at equal depth in the continental mantle. Solid state is again attained at the Lehmann discontinuity in cold, stable areas, whereas the zone extends to near the 400-kilometer discontinuity in hot, tectonically active areas. Thus, the depth to the Lehmann discontinuity may be an indicator of the thermal state of the continental mantle.

Role of a covalent bonding interaction in noncontact-mode atomic-force microscopy on Si(111)7×7

Abstract: We experimentally investigated a force interaction between a Si tip and the Si(111)7\ifmmode\times\else\texttimes\fi{}7 reconstructed surface using a noncontact-mode atomic-force microscope (AFM) in ultrahigh vacuum. Two types of force gradient curves with and without discontinuity were found. Furthermore, the correlation between the force gradient curves and the noncontact AFM image contrasts has been clarified. The image contrast was very weak in the case of the force gradient curve without discontinuity, while it was clearly enhanced and the contrast between inequivalent adatoms was obtained in the case of the force gradient curve with discontinuity. The discontinuity of the force gradient curve can be explained by a model which considers a crossover between physical bonding and chemical bonding interactions between tip and sample. This model suggests that the weak contrast image reflects the variation in van der Waals and/or electrostatic force interactions, while the strong contrast image reflects the variation in a chemical reactivity on Si adatoms.

Sensitivity of SS precursors to topography on the upper-mantle 660-km discontinuity

Abstract: The sensitivity of SS precursors to the presence of topography on the 660-km discontinuity is addressed using an axisymmetric finite difference approximation to the SH wave propagation in the Earth mantle. Numerical experiments are lead to quantify the bias in both wavelength and amplitude committed in estimating depth information on the upper-mantle discontinuities from ray inversion of SdS arrival times. It is further shown that long period S660S arrival-times do not provide information on the deepening of the ‘660‧ in presence of a velocity increase expected as the thermal signature of a subducting slab.

Gross errors in upper‐mantle discontinuity topography from underside reflection data

Abstract: SUMMARY Currnet models of upper-mantle discontinuity topography derived from traveltime variations of long-period underside reflections (PdP or SdS waves) may contain gross errors due to the use of geometrical optics in relating observed traveltimes to depth variations of the discontinuity near the reflection point. The use of geometrical optics is not valid if variations in the depth of the discontinuities exist on a lateral scale smaller than the size of the Fresnel zone in the data. Geometrical optics does not take into account the large size and the complex structure of the traveltime surface of underside reflections and may introduce spurious structure when used in inversions. Examples of synthetic long-period PdP waveforms for reflections near subducting lithosphere show that scattering from expected small-scale depth variations of the discontinuities inside slabs causes complex waveform variations. For a 15–20s PdP wave these small-scale depth variations (with a scale-length of 4) are smaller than the Fresnel zone (roughly 20°) but larger than the PdP wavelength at 420 km depth (about 150–200 km). The synthetic waveforms are processed to obtain PdP traveltimes, which are in turn converted to apparent discontinuity depth at the reflection point using geometrical optics. The apparent discontinuity-depth variations are not only of the same order of magnitude as those obtained from real data but also exhibit artefacts that closely resemble features observed in current models. It is shown that large-scale (wavelengths of the order of 1000 km) downwarping of the 670-km discontinuity near subduction zones derived from underside reflection data can be explained as the long-wavelength manifestation of a single, small-scale (wavelengths of several hundreds of kilometres) deflection of the discontinuity inside and near subducting slabs.

Partial regularity of free discontinuity sets, I

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Depth variation of the mid-mantle seismic discontinuity

Abstract: Short-period array seismograms of deep events that occurred in the Indonesia, Japan and Izu-Bonin arcs are stacked and beam-formed to identify the near-source S-P converted waves that result from the mantle transition discontinuities. Most of the resulting images reveal the existence of a mid-mantle seismic discontinuity (“920 km discontinuity”) in these regions. Of the 15 events analyzed, three that occurred at the western end of the Indonesia arc show clear S-P arrivals observable even in individual seismograms. The mid-mantle discontinuity is characterized by large depth variation (900 ∼ 1080 km) and velocity contrast variation in different subduction zones. Especially, the depth variation of the mid-mantle discontinuity beneath the Indonesia arc, where the discontinuity deepens from 940 km at the eastern end to 1080 km at the western end, appears to be well correlated with the location of the high-velocity anomalies in recent tomographic models. However, the mid-mantle discontinuity cannot be simply coincided with the bottom of the high-velocity anomalies, because a velocity increase at the discontinuity is observed from the waveform analysis.

Loss of Ellipticity in Plane Deformation of a Simple Directionally Reinforced Incompressible Nonlinearly Elastic Solid

Abstract: Change of type in the governing equations of equilibrium is examined in the context of a base neo-Hookean response augmented with unidirectional reinforcing that is characterized by a single additional constitutive parameter for strength of reinforcement. Plane deformations interpreted in terms of both local and global plane strain are considered. Loss of ordinary ellipticity is found to occur for sufficiently large strength of reinforcement under sufficiently severe deformation which necessarily involves contraction in the reinforcing direction. Loss of ellipticity in local plane strain is easily characterized, and its incipient breakdown is associated with the possible emergence of surfaces of weak discontinuity with orientation normals in the reinforcing direction. Loss of ellipticity in global plane strain is given a two-dimensional manifold characterization in a space involving two deformation parameters and the strength of reinforcing parameter. Orientation normals for the associated surfaces of weak discontinuity at incipient breakdown do not in general conform to the reinforcing direction.

A spectral method for numerical elastodynamic fracture analysis without spatial replication of the rupture event

Abstract: Perrin et al . (1995) and Geubelle and Rice (1995) have introduced a spectral method for numerical solution of two- and three-dimensional elastodynamic fracture problems. The method applies for ruptures confined to a plane separating homogeneous elastic half spaces. In this method, the physical variables, such as the traction components of stress and displacement discontinuity on the rupture plane, are represented as Fourier series in space with time-dependent coefficients. An analytical solution is found for each Fourier mode, in that each Fourier coefficient for stress is expressed by the time convolution of the corresponding coefficient for displacement with a convolution kernel specific to the rupture mode. Once the 2D formulation of the method is known, the method is readily generalizable to 3D problems in that it involves only linear combinations of the convolution kernels found for each rupture mode in 2D. This conceptual simplicity has, however, a major drawback : due to the Fourier series representations of the physical variables, the problem solved is in fact an infinite and periodic replication of rupture events on the fracture plane. So, in order to study the evolution of a single rupture, one has to use a spatial period large enough in order that the waves coming from the replication cracks do not enter the zone of interest during the time duration studied, or provide negligible stress alteration when they do arrive. We show here how to rigorously offset this defect while retaining the modal independence. Once expressed in the spatial domain, the method amounts to truncating in space the space-time convolution kernels, in a manner that provides an exact evaluation for all positions within the rupture domain (where the constitutive law between stress and displacement discontinuity is to be imposed), but not outside. In order for the method to be identical in structure to the method of Perrin et al . (1995) and Geubelle and Rice (1995), the period of the Fourier series is requested to be only twice as large as the rupture domain of interest. The only difference, then, to the original spectral method is that the convolution kernels in the Fourier domain require more elaborate calculations to be established, but this has to be done only once to allow simulations on a given domain.

Evaluating rock mass geometry from photographic images

Abstract: The importance of discontinuities in controlling rock mass behaviour in any engineering project involving excavations calls for a sound and spatial characterization of the discontinuity structure present. Such a characterization necessitates field work that requires sufficient rock exposures in order to obtain an adequate number of data, time and considerable cost. Photoanalysis techniques can help in overcoming the above difficulties equally well or better than other techniques. This paper refers to simple photographic techniques and their implementation for computer aided analyses for the characterization of the rock mass fracturing features. In particular attention is focused on scale problems and on reconstruction of fracture density stereoplots on the basis of data collected from one or two images according to different lithologies and outcropping conditions. The methodology for evaluating the volumetric fracture intensity follows in a slightly modified way a technique previously suggested in the literature. Certain reported examples allow to validate the photoanalytical technique used and the proposed method of analysis. Furthermore, analyses on planar density, spacing, frequency, terminations in solid rock or against other discontinuities and spatial correlation have been implemented in a software to yield a more complete rock mass characterization. At the same time input data and analysis results are produced in data files available as input for numerical analyses.

Seismic evidence against a mantle chemical discontinuity near 660 km depth beneath Izu-Bonin

Abstract: We analyze P-wave codas from 12 deep Izu-Bonin earthquakes recorded by hundreds of Western United States seismograph stations for evidence of small-amplitude phases caused by near-source mantle discontinuities. For nearly every event, the dominant phase in the coda is the result of an S-to-P conversion from a nearly horizontal discontinuity ranging in depth from 650 to 690 km. This is interpreted as a thermally depressed spinel to perovskite and magnesiowustite phase transition. If the 660-km seismic discontinuity is also associated with a change in chemistry, it would be dynamically depressed by a subducting slab. We consistently see that there is no nearly horizontal discontinuity between 700 and 1000 km with shear wave velocity contrast exceeding 1%; this observation places constraints on the properties of a postulated chemical discontinuity separating the upper and lower mantles.

Short-period observation of the 520 km discontinuity in northern Eurasia

Abstract: Short-period three-component recordings of the seismic wave field of peaceful nuclear explosions (PNEs) along several long-range profiles in northern Eurasia are analyzed to study the structure of the upper mantle. We use the data to search for evidence of the disputed 520 km discontinuity in the mantle transition zone. Here we present short-period evidence for the existence of this proposed mantle discontinuity. On three long-range profiles in Siberia we observe in the distance range from 2400 to 2600 km a typical change of the apparent velocity of the first arrivals and the appearance of two characteristic travel time branches. We interpret these phenomena as being caused by the proposed upper mantle discontinuity. To determine the optimum one-dimensional P wave velocity model, synthetic seismograms were calculated with the reflectivity method. By comparing them with the observed data, we estimate the P wave velocity contrast across the 520 km discontinuity to be 0.25 ± 0.05 km/s, while the velocity gradient below the boundary is 0.001 ± 0.0005 1/s. This result indicates that in the mantle transition zone both phase transitions and variations in chemical composition can occur.

Perception of Accelerating Self-Motion: Global Optical Flow Rate Dominates Discontinuity Rate

Abstract: Owen, Wolpert, and Warren (1984) proposed that egospeed may be perceived from global optical flow rate, discontinuity rate, or both Previous psychophysical research found that both sources of information influence judgments of acceleration and control of egospeed, but that discontinuity rate dominated However, the validity of these results is questionable due to problems with the visual stimuli used, such as confounding of discontinuity rate with proximal flow rate and low frame rates The current study examined the relative contributions of global optical flow rate and discontinuity rate to perception of accelerating self-motion with stimuli that lacked these problems I found that global optical flow rate accounted for 60% of the variability in acceleration judgments, compared with 086% for discontinuity rate This result indicates that discontinuity rate exerts only a minor influence, and global optical flow rate is the primary basis for perception of accelerating self-motion, and hence, egospeed

Crustal Structure in the Southern Kanto-Tokai Region Derived from Tomographic Method for Seismic Explosion Survey

Abstract: The detailed P-wave velocity structure of the crust in the southern Kanto-Tokai region was analyzed using the tomographic method for seismic refraction survey in this paper. A total of 332 P-wave arrival times received from 13 seismic explosion surveys were used in the analysis. The results indicate that analyses of travel-time curves are probably useful for the evaluation of inverted structures. The lateral heterogeneity of the velocity structure is obviously related to tectonics. The crust in the eastern region is thinner than that in the western region. The Conrad discontinuity obviously fluctuates. The granitic layer is thinner beneath the oceanic region to the east of Oshima. The layer becomes about 16 km thick beneath Suruga Bay. The Conrad discontinuity drops nearly 17 km in depth beneath Suruga Bay, and velocity is relatively low there. The Conrad discontinuity rises 6 km beneath MTL and its vicinity. The Moho discontinuity is located at a depth of around 34 km beneath the region to the west of ISTL and roughly coincides with the upper boundary of the seismic zone due to subduction of the Philippine Sea Plate under the Eurasian Plate. It becomes shallow across the Suruga trough toward the eastern region. The discontinuity is located about 27 km in depth beneath the oceanic region east of Oshima.

Observations of double discontinuities in the magnetotail

Abstract: Observations of slow shocks in the Earth's magnetotail at the plasma sheet-lobe boundaries have been well documented. We restudy the magnetic field data of two slow shocks: one was observed from Geotail on January 17, 1994 at XGSE = −92 RE, and another was observed from ISEE-3 on February 2, 1983 at XGSE = −220 RE. In both cases, the slow shock layer was followed by an adjoining rotational discontinuity layer on the postshock side. Compound structures each composed of a slow shock layer and an adjoining rotational discontinuity layer have been recently observed in interplanetary space from Wind, Geotail and Imp-8. Because the two successive discontinuities are very close to each other, the compound structure looks like a new kind of MHD discontinuity. It may be called a double discontinuity. Since double discontinuities exist not only in interplanetary space but also in the magnetotail region, they could be a general MHD structure in space plasma.

A comparative investigation of micro-flaw models for the simulation of brittle fracture in rock

Abstract: The search for a numerical method to model fracture formation around deep level gold mine excavations had led to the development of the DIGS (Discontinuity Interaction and Growth Simulation) boundary element code to simulate the incremental growth of fractures. However, the need to develop constitutive models of basic failure processes resulted in the adoption of a tessellation approach to simulate grain interaction and breakage. Linear variation displacement discontinuity elements are arranged in structures which simulate the microstructure of the rock by applying either a Voronoi (polygonal) or Delaunay (triangular) tessellation procedure. The tessellation approach has been applied to investigate the role of micromechanical mechanisms such as pre-existing pores and sliding flaws on the macroscopic failure patterns at a scale that is representative of realistic rock microstructures. Procedures for calculating the crack density tensors and the average stress and strain in a sample permit comparison of the results with alternative models of brittle fracture such as continuum damage mechanics. Simulations of laboratory tests have revealed that the tessellation approach can represent experimentally observed macroscopic failure modes such as splitting in uniaxial compression and shear band formation in biaxial compression, as well as the dependence of strength and inelastic deformation on the flaw density.

The shear strength of clay-filled bedding planes in limestones - back-analysis of a slope failure in a phosphate mine, Israel

Abstract: The failure of a slope in a phosphate mine by shear-sliding along a clay-filled bedding plane in limestone, and by separation across a tension crack at the back, is back-analysed. The failure cannot be explained using laboratory measured values of the shear strength parameters. In order to simulate field conditions better two ‘physical models’ of the bedding plane were prepared for testing under triaxial compression. Cylindrical cores with an inclined saw-cut discontinuity were filled with remoulded montmorillonite. It is shown that failure in the models initiates along the contacts between the clay infilling and the limestone boundaries, and not through the clay itself, as would be intuitively expected. Furthermore, it is argued that in the analysis of rock slope stability in general, and particularly in the case of clay-filled discontinuities, the influence of paleo-overburden stress on frictional resistance must be resolved before the appropriate constitutive law can be established for analysis.

Investigation of the Heat-Island Effect for Heat-Flux Measurements in Short-Duration Facilities

Abstract: This paper presents the results of an experimental investigation designed to determine the possible influence of a surface-temperature discontinuity on the heat-flux history inferred from the temperature history of a miniature thin-film heat-flux gage (painted on an insulator) embedded in the surface of a metallic component. The resulting surface material discontinuity has the potential of causing a heat-island effect. Simultaneous measurements of the heat-flux history at selected locations along the midspan of a representative turbine airfoil (metal) were performed using continuous strip-type contoured inserts (without temperature discontinuity) and button-type inserts (with temperature discontinuity). Measurements are reported for two Reynolds number conditions. The experimental results suggest that for the experimental conditions of interest to gas turbine research in short-duration facilities, the influence of a potential heat-island on the measured heat flux is insignificant.

On the possibility of accurate public prediction

Abstract: Henshel's (1995) challenge to practical applicability of the theorem demonstrating the possibility of public predictions of social behavior fails, for his examples do not show discontinuity of the public behavior to the magnitude of the prediction of behavior.

Instability of a Tangential Discontinuity in an Axisymmetric Flow with a Stagnation Point

Abstract: The linear problem of the stability of a plane tangential discontinuity occurring at the interface of two counter-streaming inviscid incompressible axisymmetric flows and including a stagnation point is considered. Using the integral Hankel transform, the problem was reduced to the solution of a single elliptic differential equation governing the discontinuity shape. An analysis of this equation by the normal-mode technique leads to a dispersion relation from which there follows the instability of the discontinuity. A similar problem for the plane-symmetric case has previously been studied by the author.