Showing papers on "Discontinuity (geotechnical engineering) published in 1973"

Use of reflection phases on microearthquake seismograms to map an unusual discontinuity beneath the Rio Grande rift

Abstract: Microearthquake seismograms recorded by stations located in or bordering the Rio Grande rift near Socorro, New Mexico, frequently have two sharp impulsive phases following direct S . These phases have been identified as S x P and S x S reflections from a sharp discontinuity that has a depth beneath Socorro of 18 km and dips northward at an angle near 6° for a distance of 30 km. Farther north, the dip steepens so that at a distance of 60 km from Socorro the depth is about 30 km. Ratios of S x P to S x S amplitudes in conjunction with plane-wave reflection theory indicate a zone of very low rigidity beneath the discontinuity. Large S x S amplitudes are believed to be the result of the large velocity contrast across the discontinuity and a fault mechanism that radiates more S -wave energy downward than outward from the focus.

Variations of the occurrence rate of discontinuities in the interplanetary magnetic field

Abstract: A statistical study of magnetic field discontinuities for the interval from February to October 1968 using data from the NASA Goddard Space Flight Center/University of Rome experiment on Pioneer 8 has been conducted. High-resolution data (10-sec averages) have been employed, stringent criteria being utilized to select more than 16,000 events, about 50 per cent being identified as disturbances that are probably tangential discontinuities (TD). Near 1 AU the average occurrence rate of discontinuities is about 3.6/hr, whereas that of tangential-like discontinuities is about 1.6/hr. The rate of occurrence of discontinuities is found to be correlated with the directional change of the field across the discontinuity surface. The normal to the TD discontinuity plane is most often perpendicular to the Archimedean spiral field configuration. A decrease in the rate of occurrence of TD is found with increasing heliocentric distance and/or with decreasing heliocentric latitude. Possible interpretations of the effect are discussed.

Discontinuities in the dorsal lateral geniculate nucleus corresponding to the optic disc: A comparative study

Abstract: A cylindrical cellular discontinuity commonly occurs in lateral geniculate layers that are innervated by the contralateral eye. Such a discontinuity has been found in a variety of mammalian species, including carnivores, primates, a rodent and a marsupial. Electrophysiological evidence obtained from some of these species shows that the discontinuity corresponds to the blind spot. It is concluded that the representation of the retina within the lateral geniculate nucleus is extremely accurate, since the retinal receptor layer and the geniculate layers have corresponding holes. Two possible mechanisms that would demand such a discontinuity are considered. One is an intralaminar mechanism in which the cells in each lamina accurately reflect the distribution of retinal receptors; the other is an interlaminar mechanism in which the representations of the homonymous hemiretinae are so accurately aligned that the optic disc must be represented by a cellular discontinuity.

Flow near a pumped well in an unconfined aquifer under nonsteady conditions

Abstract: The problem of unsteady radial flow in an unconfined aquifer is considered as a boundary value problem with discontinuous initial conditions at the surface of the well The discontinuity of heads occurs at the surface of the well as pumping commences; it dies down in time exponentially and thus represents the delay of the transitional process of reestablishing equilibrium in time The downward variable movement of water in the vicinity of the well resulting from this discontinuity is proportional to the difference between the gradually falling water table and the average head and is a cause of the slow draining of the unconfined aquifer The contribution of the unsaturated zone of the aquifer in the process of slow drainage is usually of minor importance and subordinate as far as most drainage problems are concerned Experimental verification and model design are presented

The 25-km Discontinuity: Implications for Lunar History.

Abstract: The lunar velocity profile and laboratory data on terrestrial and lunar rocks are constraints on models of lunar history. They show that shock-induced microcracks are absent from the rocks present in the moon today at depths of 25 to 60 kilometers. All possible causes of this observation are examined, and the most likely explanations are that either the rocks at depths of 25 to 60 kilometers formed after the major impacts ceased or the microcracks have annealed at temperatures of about 600 C over geologically long times.

Dislocation Theory for Geophysical Applications

Abstract: A fault plane which has undergone slip over a limited area, a thin intrusion or a crack whose faces have been caused to slide over one another or separate by the action of an applied stress are all physical realizations of a dislocation, that is, an internal surface in an elastic solid across which there is a discontinuity of displacement. Since this discontinuity varies from point to point of the internal surface it is actually a so-called Somigliana dislocation. It can, however, be built up from the more familiar dislocations of crystal physics which have a constant displacement discontinuity. Methods of finding the elastic displacement field around a dislocation in a solid with free surfaces will be outlined. The field of an infinitesimal dislocated area in a semi-infinite solid can be found quite simply, and from it the field of a general dislocation can be obtained by integration. The energy associated with a dislocation is discussed in connexion with energy release in earthquakes.

Water movement in archaeocyathids; evidence and implications of passive flow in models

Abstract: Large excurrent opening: passive flow, models, experiments, no functional discontinuity between sponges and archaeocyathids

Sets of points of discontinuity

Abstract: In order that a subset F of a topological space coincide with the set of points of discontinuity of a real-valued function on the space, it is necessary that F be an Fff-set devoid of isolated points. It is shown that this condition is also sufficient if the space is "almost-resolvable", and in particular if the space is either separable, first countable, locally compact Hausdorff, or topological linear.

Analysis of the dynamics of strike slip faulting

Abstract: Strike slip generated by wave motions and wave motions generated by strike slip are analyzed for vertical surface faults on which motion is opposed by a frictional shear stress, which is assumed to increase linearly with depth. The fault is a plane of discontinuity in a homogeneous, isotropic, linearly elastic half-space. The half-space is subjected to normal stresses that increase linearly with depth and to spatially uniform antiplane shear stresses. The horizontally polarized shear motions occurring in these two problems can be treated by a single mathematical analysis, which is presented in some detail. The results of this paper include an expression for the rate of advance of the leading edge of the zone of sliding in the initial stages of the sliding process and an expression for the maximum depth of penetration of the zone of sliding. Shear stresses in the fault plane have also been computed, and an upper bound has been established for the relative displacement at the surface trace of the fault.

Upper-mantle discontinuity from amplitude data of P′P′ and its precursors

Abstract: Recent observations of P′P′ and its precursors, identified as reflections from within the Earth9s upper mantle, are used to examine the structure of the uppermantle discontinuities with specific reference to the density, the S velocity, and the Q variations. The Haskell-Thomson matrix method is used to generate the complex reflection spectrum, which is then Fourier synthesized for a variety of upper-mantle velocity-density and Q models. Surface displacements are obtained for the appropriate recording instrument, permitting a direct comparison with the actual seismograms. If the identifications of the P′P′ precursors are correct, our proposed method yields the following: (1) a structure of Gutenberg-Bullen A type is not likely to produce observable P′P′ upper-mantle reflections, (2) in order that a P′P′ upper-mantle reflection is strong enough to be observed, first-order density and S -velocity discontinuities together with a P -wave discontinuity are needed at a depth of about 650 km, and (3) corresponding to a given uppermantle velocity-density model, an estimate can be made of the Q in the upper mantle for short-period seismic body waves.

Boundaries and Fractures in Finite Element Models of Geological Structures

Abstract: Summary The finite element method provides a useful tool for setting limits to the forces which form crustal structures. In this paper, we present methods of obtaining realistic boundary forces and of simulating and predicting fractures within a model. The use of edge forces, representing the horizontal, gravitational and residual tectonic forces, is found to be a satisfactory boundary condition for the ends of models as they simulate the model extending to infinity. Airy isostasy is used to constrain the base of crustal models but no universal boundary conditions exist for intracrustal situations. The Murrell-Griffith fracture criteria are shown to provide a method of the predicting fracture initiation and the subsequent development into a major discontinuity. Three models containing discontinuities are described to illustrate the application of the techniques described above.

Diffraction by Anomalous Regions in the Earth's Mantle

Abstract: Summary Asymptotic results are derived for the travel times and amplitudes of waves diffracted by surfaces of discontinuity in the Earth's mantle. A canonical problem, defined to be that of diffraction by a uniform elastic or visco-elastic sphere in a uniform elastic medium, is first solved asymptotically and the result applied, by means of simple ray theory, to an Earth model which is spherically symmetric except for the diffracting discontinuity. Some calculations are carried out for comparison with the signals from the Bukhara explosion discussed by Douglas et al. and an attempt is made to identify their arrival PH, with a diffracted arrival from a mantle discontinuity. To explain the low amplitude of the direct wave a dissipative region in the mantle, which is avoided by the diffracted wave, is postulated and the approximate values of Q needed to equate the amplitudes of direct and diffracted waves are calculated.

Apparatus and method for using the same to ascertain the angular position of a discontinuity in the medium surrounding a test bore

Abstract: After the depth of a discontinuity in the medium surrounding a test bore has been ascertained using a probe carrying an omnidirectional radiation source and a detector responsive to back scattering from the medium, the aximuthal location (i.e., the aximuth of the discontinuity relative to the axis of the bore) is ascertained by limiting the field of view of the detector with a shield having a radiation transparent window that extends circumferentially around the probe up to 180* of arc, and angularly displacing the shielded probe through 360* at the predetermined depth of the discontinuity. By noting the output of the detector as a function of azimuthal location, the angular position of the discontinuity can be determined. The invention includes a shield, the combination of a shield with the probe, and the method of using such combination.

A Study on the Nonstoichiometric Compounds V2O3+x

Abstract: Magnetic susceptibility, density and lattice constant of V2O3+x fired at 1600°K and 1700°K were measured.When x (in V2O3+x) reaches to 0.03, a discontinuity was found in the curve of susceptibility vs. nonstoichiometric parameter x. The similar discontinuity was also found in the density curve at x≅0.04. At x≅0.05 an abrupt change in the lattice constant was detected.Magnetic susceptibility measurement seems to be the most sensitive method for detecting nonstoichiometry in the V2O3+x compound.

A numerical model for plugging failure

Abstract: A numerical model for predicting and calculating the material flow and separation associated with the plugging failure of plates subjected to normal impact has been developed. The basis for the plugging model is the two-dimensional, multi-material code, HELP (Hydrodynamic Elastic-Plastic). The code was modified to allow a slip discontinuity to propagate through the calculational mesh in the direction of maximum shearing stress. For the case of plugging failure, the slip discontinuity is allowed to propagate when the specific plastic work associated with the material ahead of the discontinuity attains a critical value. The model is briefly described and the results of a calculation are presented and compared with experiment.

On Set Points of Discontinuity

Abstract: (1973). On Set Points of Discontinuity. The American Mathematical Monthly: Vol. 80, No. 2, pp. 186-187.

Observation of the disturbances occurring in a discontinuity layer of Lake Biwa-Ko

Abstract: Short-period temperature fluctuations were observed in the uppermost region of the seasonal thermocline in Lake Biwa-Ko, under the existence of the strong wind-stirring. In the observation period, the temperature profile had a sharp discontinuity at the bottom of the surface mixed layer, and a large gradient in the discontinuity layer of about 2-m thickness. The most dominant disturbances occurred in the discontinuity layer had the period of 2 to 3 minutes and the amplitude of about 1 m. They occurred intermittently with 5-to 15-minute intervals, and the growth and decay cycles were repeated locally. On the basis of these results, it is suggested that they were caused by the shear instability, and that such disturbances may control the erosion process of the seasonal thermocline.

Magnetic field dissipation in D-sheets

Abstract: The effects of magnetic field annihilation at a tangential or rotational discontinuity in a resistive plasma are examined. The magnetic field intensity profile depends on (1) the field intensities far from the current sheet (+ and - infinity), (2) the angle between the two intensities, and (3) the electrical resistivity. For a tangential discontinuity, the theory predicts a depression in B, centered at the discontinuity, and it predicts a monotonic transition. The theory provides satisfactory fits to the magnetic field intensity and proton temperature profiles observed for two extremely broad D-sheets in the solar wind. Assuming a diffusion time 10 days, one obtains effective resistivities or approximately = 3 x 10 to the 12th power and 2 x 10 to the 13th power emu for the D-sheets. Either resistivity at directional discontinuities is much lower than 10 to the 12th power emu or annihilation does not always occur at discontinuities.