scispace - formally typeset
Search or ask a question

Showing papers on "Discontinuity (geotechnical engineering) published in 1996"


Journal ArticleDOI
15 Nov 1996-Science
TL;DR: The model deemphasizes the role of Lamé's constant λ with regard to the shear modulus and constrains the mineralogical composition across the discontinuity.
Abstract: Global Seismic Network data were used to image upper-mantle seismic discontinuities. Stacks of phases that precede the PP phase, thought to be underside reflections from the upper-mantle discontinuities at depths of 410 and 660 kilometers, show that the reflection from 410 kilometers is present, but the reflection from 660 kilometers is not observed. A continuous Lame's constant lambda and seismic parameter at the 660-kilometer discontinuity explain the missing underside P reflections and lead to a P-wave velocity jump of only 2 percent, whereas the S-wave velocity and density remain unchanged with respect to previous global models. The model deemphasizes the role of Lame's constant lambda with regard to the shear modulus and constrains the mineralogical composition across the discontinuity.

237 citations


Journal ArticleDOI
TL;DR: In this article, the seismic structure of the Tonga-Hawaii corridor has been investigated by combining two data sets: Revenaugh and Jordan's reflectivity profile from ScS reverberations, which provides travel times to and impedance contrasts across the major mantle discontinuities, and 1500 new observations of frequency-dependent phase delays for the three-component S, SS, and SSS body waves and the R1 and G1 surface waves, which constrain the velocity structure within this layered framework.
Abstract: The seismic structure of the Tonga-Hawaii corridor has been investigated by combining two data sets: Revenaugh and Jordan's reflectivity profile from ScS reverberations, which provides travel times to and impedance contrasts across the major mantle discontinuities, and 1500 new observations of frequency-dependent phase delays for the three-component S, SS, and SSS body waves and the R1 and G1 surface waves, which constrain the velocity structure within this layered framework. The shear waves turning in the upper mantle showed significant splitting of the SH and SV components, indicative of shallow polarization anisotropy. The data set was inverted in conjunction with attenuation and mineralogical constraints to obtain a complete spherically symmetric, radially anisotropic structure. The final model, PA5, is characterized by a high-velocity, anisotropic lid, bounded at 68 km depth by a large (negative) G discontinuity; a low-velocity, anisotropic layer below G, extending to a small L discontinuity at 166 km; an isotropic, steep-gradient region between 166 km and 415 km; and transition-zone discontinuities at 415, 507, and 651 km. The depth of the radial anisotropy in PA5 is shallower than in most previous studies based on surface waves and higher modes. The average value of radial shear anisotropy in the lid, +3.7%, is consistent with the magnitude expected from the spreading-controlled models of olivine orientation, while anisotropy in the low-velocity zone, which is required by our data set, could be induced either by paleostrains that took place near the ridge crest or by shearing in the asthenosphere as a result of present-day plate motions. On the basis of recent work by Hirth and Kohlstedt, we suggest that the G discontinuity is caused by a rapid increase in the water content of mantle minerals with depth, marking the fossilized lower boundary of the melt separation zone active during crust formation. The high-gradient zone between 200 and 400 km is a characteristic feature of convecting oceanic upper mantle and is probably controlled by a steady decrease in the homologous temperature over this depth interval. The average shear-velocity gradient in the transition zone is lower than in most previous seismic models, in better agreement with the predictions for a pyrolitic composition.

186 citations


Journal ArticleDOI
TL;DR: In this paper, the authors used geometrical ray theory and assuming a constant velocity versus density scaling relationship, fit this reflectivity profile with velocity models of the upper mantle using both forward modeling and direct inversion.
Abstract: Stacks of long-period Global Digital Seismograph Network (GDSN) seismograms at 110° to 180° epicentral distance reveal precursors to SS that result from underside reflections off upper mantle seismic discontinuities. The 410- and 660-km discontinuities are obvious in these stacks, but identification and modeling of other transition zone discontinuities are complicated by sidelobes from the 410- and 660-km reflections. These sidelobes result from the limited bandwidth of the GDSN instrument responses and the effect of crustal reverberations on the SS reference phase. The crustal effects can be minimized by restricting the records to oceanic bounce points where the ∼6-km-thick crust has little effect on the long-period waveforms. Over 2000 long-period, transverse-component seismograms with oceanic SS bounce points recorded by the GDSN from 1976 to 1991 are manually edited, aligned on SS, and then stacked using a new procedure that weights the records by data quality. The resulting image shows a clear reflection from a 520-km discontinuity that cannot be explained as a sidelobe artifact, confirming earlier results of Shearer [1990, 1991] and Revenaugh and Jordan [1991]. By stacking along the expected travel time curves for discontinuity phases, the time versus range image of the precursor wave field is reduced to a single trace that measures upper mantle reflectivity versus time. The features in this reflectivity profile are sensitive to the brightness and depth of the transition zone discontinuities and to the steepness of the velocity gradients between the interfaces. Using geometrical ray theory and assuming a constant velocity versus density scaling relationship, I fit this reflectivity profile with velocity models of the upper mantle using both forward modeling and direct inversion. The inverse problem is addressed by performing a deconvolution of the profile with the SS reference phase (after a correction for attenuation), followed by a direct mapping of reflectivity versus time into velocity versus depth. Velocity-depth profiles resulting from these procedures are roughly in agreement with standard upper mantle velocity models, except that the SS precursor data require a minor discontinuity near 520 km and a steeper gradient just below the 660-km discontinuity. Estimated discontinuity shear impedance changes are 6.7 ± 1.1% at 420 km, 2.9 ± 0.7% at 519 km, and 9.9 ± 1.5% at 663 km. The impedance change near 520 km is consistent with current mineral physics results for the olivine β to γ phase change and places constraints on the fraction of olivine in the transition zone.

128 citations


Journal ArticleDOI
TL;DR: For the first time, an irreversibility line well below the vortex melting line has been achieved, indicating a first order transition in magnetization on clean and untwinned YBa 2Cu3O6.95 single crystals with extremely low pinning.
Abstract: We report magnetization measurements on clean and untwinned YBa 2Cu3O6.95 single crystals with extremely low pinning. For the first time, an irreversibility line well below the vortex melting line has been achieved. A discontinuity in the magnetization at the vortex melting transition is observed, indicating a first order transition. The entropy change at the phase transition is 0.8kB per vortex per CuO2 double layer at 10 kOe and 0.6kB per vortex per CuO2 double layer at 40 kOe, in reasonable agreement with theoretical calculations.

128 citations


Journal ArticleDOI
TL;DR: In this paper, a 5 km linear velocity gradient yields an average reflection coefficient identical to that of a 10 km transition interval based on olivine phase-diagram features, and alternative forms of the phase diagram, equally consistent with experimentally determined iron-magnesium partitioning, can yield true transition intervals as narrow as 4 km.
Abstract: SUMMARY Estimates of the thickness of the 410 km seismic discontinuity, believed to be due to the α-olivineβ-modified-spinel transformation in olivine, are as low as 4 km based on discontinuity reflectivity. The seismically estimated thickness is, however, biased to values narrower than the true transformation interval if linear interpolation of properties is used for modelling. A 5 km linear velocity gradient yields an average reflection coefficient identical to that of a 10 km transition interval based on olivine phase-diagram features. Moreover, alternative forms of the phase diagram, equally consistent with experimentally determined iron-magnesium partitioning, can yield true transition intervals as narrow as 4 km. This reconciles a discrepancy between phase equilibrium and seismic measures of discontinuity thickness in two ways: (1) seismic thickness estimates are too narrow; and (2) narrow transition intervals are permissible given existing phase-equilibrium constraints. Incorporating recent results on the influence of H2O on discontinuity properties, it appears that 410 km discontinuity reflectivity is much more sensitive to varying H2O concentration than to temperature, suggesting that discontinuity reflectivity variations reflect changes in mantle chemistry.

81 citations


Journal ArticleDOI
TL;DR: In this paper, an inversion scheme is developed to determine a discontinuity response function at each station by fitting observed waveform data with superpositions of the P-to-S converted waves.
Abstract: Broadband seismic waveform data recorded at stations in the western Pacific region are analyzed to investigate mantle discontinuities. By using teleseismic deep events, we observe unambiguous P-to-S conversion waves associated with the mid-mantle discontinuities in many of the individual seismograms. The commonly used method of stacking receiver functions is not so effective in this case due to the limited number of deep events. An inversion scheme is developed to determine a discontinuity response function at each station by fitting observed waveform data with superpositions of the P-to-S converted waves. Beneath the station in northeast China (MDJ) where the subducted Pacific plate appears to stagnate along the "660-km" discontinuity, the discontinuity response function has more complicated features than those of other stations. The preliminary results indicate no depression of the "660-km" discontinuity at the tip of the subducting slab beneath MDJ; instead a multiple-discontinuity structure down to a depth of 780 km is observed.

74 citations



Journal ArticleDOI
TL;DR: In this paper, a modified displacement discontinuity model (MDD) is proposed to describe the effect of fractures on seismic-wave velocity and attenuation, where fractures are treated as transmission lines for the passage of seismic waves.
Abstract: The effects of fractures on the seismic velocity and attenuation of a rock are investigated using theoretical results and experimental data. Fractures in a rock mass influence the traveltimes and amplitudes of seismic waves that have propagated through them. The displacement discontinuity model, recently employed in fracture investigations, is modified to describe the effect of fractures on seismic-wave velocity and attenuation. This new model, the modified displacement discontinuity model (MDD), is formulated in a way analogous to transmission-line analysis. The fractures are treated as transmission lines for the passage of seismic waves. The MDD takes into consideration realistic fracture parameters which include the fracture length, the fractional area of a fracture surface in contact, and the nature of the infilling material. A single fracture of varying geometric and material properties is shown to affect dramatically the transmission properties of a propagating waveform, and hence the seismic velocity and attenuation. These effects have been shown to result in a frequency-dependent velocity and attenuation. The sensitivity of the fracture parameters to seismic-wave velocity and attenuation was investigated and interesting results were obtained. Fracture parameters used in designing experimental models consisting of synthetically manufactured cracks were fed into the MDD and a well-known crack model, Hudson's model, for comparison. Velocities as a function of the incident-wave angle were obtained from both numerical models and were compared with the results from the experimental modelling. For P waves, the MDD model results show better agreement with those of the experimental model for all crack densities investigated than those from Hudson's model.

67 citations


Journal ArticleDOI
TL;DR: In this paper, a two-dimensional velocity structure for the mantle down to a depth of 700 km derived from super long-range nuclear explosion seismic data recorded along the Quartz profile in northern Eurasia is presented.
Abstract: A two-dimensional velocity structure for the mantle down to a depth of 700 km derived from super long-range nuclear explosion seismic data recorded along the Quartz profile in northern Eurasia is presented. The profile extends from the Russian Platform, across Western Siberia to the Altai mountains. A detailed structural image of the lithosphere and asthenosphere shows strong lateral variations along the profile. The subcrustal lithosphere to a depth of 100 km is characterized by lateral variations of the P -wave velocity between 7.7 and 8.7 km/sec. The seismic data are consistent with the Ural mountains having a crustal root. Another important finding in the data is an additional discontinuity in the mantle transition zone at 530 km. This discontinuity could represent one or more phase transitions known to occur at about this depth. At present, the nuclear-seismic records are believed to represent a key data set to prove the existence of this much debated upper mantle discontinuity.

67 citations


Journal Article
TL;DR: In this paper, the authors investigated the relation between the roughness profiles and fractal dimension determined by the divider method, and showed that the relation can lead to erroneous relations.

61 citations


Journal ArticleDOI
TL;DR: In this article, a 400 m tunnel was used to characterize the discontinuity geometry of the rock mass around the shiplock area, and traces of 39 major discontinuities (faults and dykes) were found on the tunnel exposures.


Journal ArticleDOI
TL;DR: In this paper, the authors detected transform phase converted phases in the records of 11 seismograph stations in easternmost Russia and China and found that the average thickness of the mantle transition zone (MTZ) in Eurasia is about 10 kilometres more than beneath the surrounding oceans.
Abstract: Mantle Pds (Fig. 1) converted phases are detected in the records of 11 seismograph stations in easternmost Russia and China. These data reveal neither a strong depression on the 660-km discontinuity nor a layer of partial melting atop the 410-km discontinuity that were found previously beneath this region in studies using long-period underside SH reflections and multiple ScS reverberations. Apparently, a significant deepening of the 660-km discontinuity occurs only if the subducted plate penetrates the discontinuity, which is not the case in the study region. Most estimates of thickness of the mantle transition zone (MTZ) in Eurasia based on the Pds data are a few kilometres larger than the standard value (250 km), and suggest that the average thickness of the MTZ beneath Eurasia is about 10 kilometres more than beneath the surrounding oceans.


Journal ArticleDOI
Filip Neele1
TL;DR: In this paper, pre-critical reflections from the 410-km discontinuity below the Gulf of California, appearing as a clear and continuous feature in the data to epicentral distances as small as 10°, are used to estimate the width of the gradient zone near 410 km depth, constraining the velocity jump across the discontinuity using critically refracted waves.
Abstract: P-wave data from the short-period (around 1 Hz) Southern California Seismic Network reveal pre-critical reflections from the 410-km discontinuity below the Gulf of California, appearing as a clear and continuous feature in the data to epicentral distances as small as 10°. Deconvolution and stacking techniques are used to extract these weak arrivals from the coda of earlier arriving P waves. The reflections are used to estimate the width of the gradient zone near 410 km depth, constraining the velocity jump across the discontinuity using critically refracted waves. The velocity jump across the 410-km discontinuity is 6±2.5 %, which is in agreement with existing estimates of the velocity jump for this region. The clear and uninterrupted branch of reflected waves suggests laterally homogeneous discontinuity properties on a scale of several hundred km. Synthetic seismogram modelling of the reflected waves using realistic models of the phase transition occurring at this depth shows that the transition must be as thin as 10 km, with most of the velocity increase occurring over about 4 km or less, for the reflections to be visible in short-period data. Applying this result to recent thermodynamic models of upper mantle composition suggests that the high temperatures associated with the spreading centre continue down to at least 410 km depth.

Journal ArticleDOI
TL;DR: In this paper, the scattering of guided elastic SH waves from step discontinuities in the material properties and/or geometry of otherwise uniform flat layer waveguides is analytically/numerically analyzed.
Abstract: The scattering of guided elastic SH waves from step discontinuities in the material properties and/or geometry of otherwise uniform flat layer waveguides is analytically/numerically analyzed. Analytical results for the case of a step discontinuity in material properties in a layer of uniform thickness are presented for comparison with later numerical results. The case of a sudden change in thickness of the waveguide, possibly in conjunction with a change in material properties, is solved numerically. The solution is based on an expansion of the wave fields on either side of the discontinuity in terms of the normal modes permissible in the analogous infinite layer and subsequent determination of the modal expansion coefficients. A ‘‘non‐coupling’’ condition is proposed and proven which explains why particular modes are not generated in the scattering process. A ‘‘perfect‐coupling’’ condition is also introduced, and the exact transmission amplitude of any mode satisfying the condition is given explicitly.

Journal ArticleDOI
TL;DR: In this article, a boundary integral equation method for modeling elastic wave propagation in a rock mass with nonwelded discontinuities, such as fractures, joints, and faults, is presented.
Abstract: This paper describes a boundary integral equation method for simulating two-dimensional elastic wave propagation in a rock mass with nonwelded discontinuities, such as fractures, joints, and faults. The numerical formulation is based on the three-dimensional boundary integral equations that are reduced to two dimensions by numerical integration along the axis orthogonal to the plane of interest. The numerical technique requires the assembly and solution of the coefficient matrix only for the first time step, resulting in a significant reduction in computational time. Nonwelded discontinuities are each treated as an elastic contact between blocks of a fractured rock mass. Across such an elastic contact, seismic stresses are continuous and particle displacements are discontinuous by an amount which is proportional to the stress on the discontinuity and inversely to the specific stiffness of the discontinuity. Simulations demonstrate that such formulated boundary element method successfully models elastic wave propagation along and across a single fracture generated by a line source.



Journal ArticleDOI
TL;DR: In this paper, the authors showed that discontinuity traces on inaccessible steep rock slopes can be studied in detail by means of a recently developed computerized electronic tacheometer (total station) and shows a field application.

Patent
05 Jan 1996
TL;DR: In this paper, information for forming a predicted image having at least two blocks sharing a common border is evaluated to determine whether a discontinuity exists at the common border, and the predicted image is filtered in the vicinity of the border.
Abstract: Information for forming a predicted image having at least two blocks sharing a common border is evaluated to determine whether a discontinuity exists at the common border If it is determined that a discontinuity exists at the border, the predicted image is filtered in the vicinity of the border

Journal ArticleDOI
TL;DR: A feasibility-level geotechnical evaluation carried out at Anamur dam site, in terms of stability analysis of dam foundation, excavation slopes and a diversion tunnel, is described in this paper.

Journal Article
TL;DR: In this article, the authors investigated the discontinuity properties in low altitude corotating stream regions which are encountered during Ulyssses traveling from the south to north heliographic poles in 1995.
Abstract: In this study, we investigate the discontinuity properties in the low altitude corotating stream regions which are encountered during Ulyssses traveling from the south to north heliographic poles in 1995. Through the occurrence rates of directional discontinuites and tangential discontinuities, we find that there are three different regions around a high speed stream.

Journal ArticleDOI
TL;DR: A boundary element model for stress/stability analysis of underground excavations in the vicinity of faults is presented in this article, which adopts the fictitious stress method for the simulation of excavation boundaries and the displacement discontinuity method for representation of faults.
Abstract: A boundary element model for stress/stability analysis of underground excavations in the vicinity of faults is presented. The boundary element formulation adopts the fictitious stress method for the simulation of excavation boundaries and the displacement discontinuity method for the representation of faults. The numerical model employs the Barton-Bandis non-linearjoint model for the modelling of the fault behaviour and linear elastic behaviour for the rock. An incremental-iterative in situ stress relaxation algorithm is implemented for the non-linear analysis of the faults. Both deformation and peak strength models of Barton-Bandis are incorporated for modelling the mechanical behaviour of the fault. The non-linear deformation of fault considers the effects of coupling between shear and normal stresses and displacement, joint closure, joint separation, hardening followed by post-peak or residual behaviour. The peak strength model employs a mobilized non-linear shear strength envelope. The differences between linear and non-linear simulation of the fault models are discussed. A comparison of model predictions with the classical Mohr-Coulomb peak strength model with constant joint stiffness is presented. The numerical model is used for a case study of Canadian hard rock underground mine. The shear and normal displacements along the fault during four mining sequences with backfill simulation are presented and discussed.

Journal ArticleDOI
TL;DR: In this paper, the authors proposed a numerical-analytical method based on the partial inversion of diffraction problem operator in the following way: the simple part of the semi-infinite obstacle for which the scattering operator is known has to be extracted, then the desired total operator has been constructed using the specific symmetry of the semidefinite structure on the basis of the above mentioned operator allowing for the interaction of waves scattered by the obstacle components.
Abstract: Wave diffraction problem by semi-infinite equidistant succession of identical obstacles (such as semi-transparent screens, dielectric layers, waveguide steps and etc.) is classical fundamental problem and it has great significance in modern radiophysics and acoustics branches. The numerical-analytical method of this problem solution has been suggested [1]. It is based on the idea of the partial inversion of diffraction problem operator in the following way: the simple part of the semi-infinite obstacle for which the scattering operator is known has to be extracted. Then the desired total operator has been constructed using the specific symmetry of the semi-infinite structure on the basis of the above mentioned operator allowing for the interaction of waves scattered by the obstacle c.omponents. And it is very important that suggested method allows to get interesting physical results structure about using information single element only.

Journal ArticleDOI
TL;DR: In this article, the finite difference time-domain (FDTD) method has been applied to the analysis of a double step microstrip discontinuity having thickness changes in the longitudinal direction, which occurs in patch antenna feeds or interconnections between microwave planar circuit modules.
Abstract: The finite-difference time-domain (FDTD) method has been applied to the analysis of a double step microstrip discontinuity having thickness changes in the longitudinal direction. The discontinuity occurs in patch antenna feeds or interconnections between microwave planar circuit modules. The simulation results are compared with those computed by HFSS to show a good agreement. An equivalent circuit for the double step discontinuity is developed from the scattering parameters computed by the FDTD method.

Journal ArticleDOI
Asghar Ali1
TL;DR: In this article, a finite element method (FEM) for a body containing displacement discontinuity is used for the investigation of tensile fracture behavior under mode-I and mixed-mode loading conditions in concrete structures.


Journal ArticleDOI
TL;DR: In this article, Isakov et al. showed that the results of a single boundary measurement implemented on an arbitrarily small part Γ of the lateral boundary uniquely determine the part of the discontinuity surface which is projected onto Γ.
Abstract: We deal with recovering the discontinuity surface of the speed of the threedimensional wave propagation from (single) boundary observation. We prove a sharp uniqueness result assuming that the surface is the graph of a function of the depth and that the reference domain is a half-space. We make use of (new) exact results on the uniqueness of the continuation for hyperbolic equations, orthogonality relations, and of careful geometrical study of waves propagation. We consider the identification problem for a leading coefficient of a second order hyperbolic equation from an overdetermined lateral boundary data. This problem is a theoretical basis for prospecting by seismic method the interior of the Earth. The leading coefficient determines the speed of propagation and characterizes very important properties of a medium. In case of smooth coefficients and many lateral boundary measurements uniqueness theorem for this inverse problem was recently established by Belishev [1]. Uniqueness in case of one set of boundary measurements and non-zero initial data (or interior sources) was proved by Bukhgeim and Klibanov [2], [5]. However for the most important case of zero initial data the question of uniqueness is not yet resolved. Discontinuity of a leading coefficient also provides with an important information about the medium, in particular it can indicate location of underground oil. There are few theoretical results on discontinuity surfaces. We can mention the paper by Hansen [4] where there is a discussion of a linearization and the recent paper of Rakesh [9] where a special discontinuity relevant to acoustic waves is considered. In the both papers [4] and [9] they recover only convex hull of the discontinuity surface but the data are given on a finite interval of time. So the inverse problem is genuinely hyperbolic. If the boundary measurements are available for all times then by using the Fourier-Laplace transform in t one can obtain the complete scattering data, so uniqueness will follow from the paper of Isakov [6] on the inverse transmision scattering problem (see also the book of Colton and Kress [3]). It is realistic that by using recent results of Robbiano [10] and Tataru [11] one can show that the data on (large) finite interval of time uniquely determine the boundary data for all times, but it is not proven. In this paper we show that the results of a special single boundary measurement implemented on an arbitrarily small part Γ of the lateral boundary uniquely determine the part of the discontinuity surface which is "projected" onto Γ. To achieve this goal we will use * Department of Mathematics and Statistics, Wichita State University, Wichita, KS 67260-0033. E-mail: isakov@twsuvm.bitnet