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Slabs

Distribution by Scientific Domains

Engineering	35%
Earth and Environmental Science	18%
Chemistry	12%
Physics and Astronomy	8%
Medical Sciences	8%
Polymers and Materials Science	5%
Humanities and Social Sciences	4%
Mathematics and Statistics	3%
2 Other Domains	7%

Distribution within Engineering

General & Introductory Architecture	37%
Communication Technology	14%
Numerical Methods & Algorithms	11%
General & Introductory Civil Engineering & Construction	5%
7 Other Subdomains	33%

Kinds of Slabs

Selected Abstracts

THE SECTILIA PANELS OF FARAGOLA (ASCOLI SATRIANO, SOUTHERN ITALY): A MULTI-ANALYTICAL STUDY OF THE GREEN, MARBLED (GREEN AND YELLOW), BLUE AND BLACKISH GLASS SLABS

ARCHAEOMETRY, Issue 3 2010
E. GLIOZZO
Analyses at the Cu,K, Fe,K and Mn,K edge were performed to study the green, marbled (green and yellow), blue and blackish (deep greyish olive green) glass slabs decorating three sectilia panels from the archaeological site of Faragola. Results indicate that all slabs were made by mixing siliceous sand with natron, sometimes probably mixed with small percentages of plant ash. Cu2+ and Pb antimonates should be responsible for the opaque green colours. The dark green and yellow portions of the marbled slabs are respectively comparable to the slabs comprising only one of these colours. Cu2+ together with Ca antimonates probably produced light blue slabs, whereas cobalt was used to produce dark blue slabs. We consider it possible that the abundance ratio of Fe2+/Fe3+ and the complex Fe3+S2, would have an effect on the blackish slabs. The contribution of Mn cannot be ascertained even if it could have played a role in darkening glass colour. The comparison between the chemical composition of Faragola samples and several glass reference groups provided no conclusive evidence of provenance; whereas, the presence of a secondary local workshop can be hypothesized. [source]

Multi-generational carbonate assemblages in martian meteorite Allan Hills 84001: Implications for nucleation, growth, and alteration

METEORITICS & PLANETARY SCIENCE, Issue 1 2004
Catherine M. CORRIGAN
These regions contain familiar forms of carbonate, as well as textural forms previously unreported including carbonate rosettes, planiform "slab" carbonates, distinct "post-slab" magnesites, and carbonates interstitial to feldspathic glass and orthopyroxene. Slab carbonates reveal portions of the carbonate growth sequence not seen in the rosettes and suggest that initial nucleating compositions were calcite-rich. The kinetically controlled growth of rosettes and slab carbonates was followed by an alteration event that formed the magnesite-siderite layers on the exterior surfaces of the carbonate. Post-slab magnesite, intimately associated with silica glass, is compositionally similar to the magnesite in these exterior layers but represents a later generation of carbonate growth. Feldspathic glasses had little or no thermal effect on carbonates, as indicated by the lack of thermal decomposition or any compositional changes associated with glass/carbonate contacts. [source]

MoS2 Single Slab as a Model for Active Component of Hydrodesulfuration Catalyst: A Quantum Chemical Study.

CHEMINFORM, Issue 29 2006
Part 1.
Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF. [source]

Wittgenstein's Builders and Perry's Objection to Sentence Priority

DIALECTICA, Issue 1 2002
Eli Dresner
In the first section of this paper I present a view of linguistic meaning that I label 'Sentence Priority'(SP): the position that semantically primitive language-world contact is made at the level of complete sentences (rather than the level of sentence parts). Then, in the main part of the paper, I consider and reject an objection against Sentence Priority raised by John Perry, an objection that appeals to Wittgenstein's builders parable. Perry argues that the builder's utterances (,Slab',,Pillar', etc.) are utterances of self-standing nouns, and that therefore they constitute a counter-example to SP. A sound assessment of Perry's argument, however, depends on a clear distinction between two cases: one in which the four expressions mentioned in Wittgenstein's example exhaust the builders'expressive powers, and one in which they do not. Once these cases are distinguished it can be seen that in neither does Perry's argument go through. [source]

Subducted slabs and lateral viscosity variations: effects on the long-wavelength geoid

GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 2 2009
Nicola Tosi
SUMMARY The characteristic broad local maxima exhibited by the long-wavelength geoid over subduction zones are investigated with a numerical model of mantle flow. In a spherical axisymmetric geometry, a synthetic model of buoyancy driven subduction is used to test the effects on the geoid caused by the depth of penetration of the lithosphere into the mantle, by the viscosity stratification and by lateral viscosity variations (LVV) in the lithosphere, upper and lower mantle. The presence of anomalous slab density in the lower mantle guarantees geoid amplitudes comparable with the observations, favouring the picture of slabs that penetrate the transition zone and sink into the deep mantle. The viscosity of the lower mantle controls the long-wavelength geoid to the first order, ensuring a clear positive signal when it is at least 30-times greater than the upper-mantle viscosity. The presence of LVV in the lithosphere, in the form of weak plate margins, helps to increase the contribution of the surface topography, causing a pronounced reduction of the geoid. Localized LVV associated with the cold slab play a secondary role if they are in the upper mantle. On the other hand, highly viscous slabs in the lower mantle exert a large influence on the geoid. They cause its amplitude to increase dramatically, way beyond the values typically observed over subduction zones. Long-wavelength flow becomes less vigorous as the slab viscosity increases. Deformation in the upper mantle becomes more localized and power is transferred to short wavelengths, causing the long-wavelength surface topography to diminish and the total geoid to increase. Slabs may be then weakened in the lower mantle or retain their high viscosity while other mechanisms act to lower the geoid. It is shown that a phase change from perovskite to post-perovskite above the core,mantle boundary can cause the geoid to reduce significantly, thereby helping to reconcile models and observations. [source]

Tragverhalten von Flachdecken aus Stahlfaserbeton im negativen Momentenbereich und Bemessungsmodell für das Gesamtsystem

BETON- UND STAHLBETONBAU, Issue 8 2010
Julien Michels Dr.
Baustoffe; Berechnungs- und Bemessungsverfahren; Versuche Abstract Der vorliegende Artikel befasst sich mit dem Tragverhalten von Flachdecken aus Stahlfaserbeton im negativen Momentenbereich. Zugkräfte werden allein von den in der Betonmatrix eingebetteten Stahlfasern aufgenommen, letztere ermöglichen in einer Dosierung von 100 kg/m3 (1.3 % des Volumens) eine hohe Rotationsfähigkeit bei gleichzeitig hoher Tragfähigkeit. In einer ersten Phase kann anhand von Laborversuchen an Deckenausschnitten gezeigt werden, dass bei Flachdecken mit reiner Faserbewehrung und unter rotationssymetrischer Belastung kein Durchstanzen auftritt und sich ein Biegeversagen mit deutlicher Bruchlinienbildung einstellt. Neben den erhaltenen Informationen über Versagensart und -last konnte ebenfalls eine abfallende Faserwirksamkeit mit steigender Plattendicke festgestellt werden. Kombiniert mit den experimentellen Untersuchungen kann anhand nicht-linearer FE-Simulationen der Einfluss von Aussparungen bei lokalem Plattenversagen im Stützenbereich auf die Traglast beschrieben werden. Anhand der Festigkeitsstreuungen und einem semi-probabilistischen Sicherheitskonzept wird ein Sicherheitsfaktor hergeleitet, welcher es ermöglicht, einen Bemessungswert der Querschnittstragfähigkeit zu berechnen. Wegen dem experimentell festgestellten Biegeversagen wird die Bruchlinientheorie als Rechenmodell angewendet. Bearing Capacity of Steel Fiber Reinforced Concrete (SFRC) Flat Slabs in the Negative Bending Moment Area and Design Model for the Complete System The present paper deals with the bearing behaviour of steel fibre reinforced concrete (SFRC) flat slabs in the negative bending moment area. Tensile forces are carried only by steel fibers. The latter allow, due to a dosage of 100 kg/m3 (1.3% in volume), a high rotation capacity with simultaneously high bearing capacities. In a first step experimental analysis showed that under symmetrical loading no punching shear failure occurred. A bending behaviour with creation of yield lines was observed for all test specimens. Furthermore, a decreasing fibre orientation with growing plate height could be noticed. The effect of openings in the column area on the bearing capacity loss in a local failure were evaluated with the use of non-linear finite element software. Scatter in bending tensile strengths was used to calculate a safety factor by the means of a semi-probabilistic safety concept. Eventually, slab design is performed by using yield line theory. [source]

Experimentelle und numerische Untersuchungen zum Tragverhalten von Stahlbetondecken mit kugelförmigen Hohlkörpern

BETON- UND STAHLBETONBAU, Issue 6 2010
Marcin Abramski Dr.-Ing.
Abstract Massive Flachdecken erweisen sich in Hoch- und Industriebau häufig als insgesamt überlegenes Deckensystem. Die hohe Eigenlast dieser Decken kann ihren Anwendungsbereich jedoch einschränken. Eine sinnvolle Abhilfe schafft dann die Integration von Hohlkörpern in der neutralen Zone, weil hierdurch eine spürbare Gewichtsreduktion eintritt. Neben der Eigenlast wird auch der Verbrauch der unter ökologischen Gesichtspunkten wesentlichen Ressourcen Betonstahl und Zement deutlich verringert. Die Hohlräume beeinflussen vor allem die Querkrafttragfähigkeit solcher Decken. Vier Versuchsserien, die mit kugelförmigen Hohlkörperdecken des Systems "cobiax" durchgeführt wurden, werden in diesem Beitrag beschrieben. Aus den Versuchen wird ein Abminderungsfaktor zur Beschreibung des Querkrafttragverhaltens abgeleitet, der auch in die kürzlich erteilte allgemeine bauaufsichtliche Zulassung des Deckensystems eingegangen ist. Die Nachrechnung der Versuche mit der Finite-Elemente-Methode zeigt eine gute Übereinstimmung von Numerik und Experiment, so dass in der Weiterentwicklung Parameterstudien auf dieser Grundlage zur Ergänzung von Versuchen hilfreich sein werden. Experimental and Numerical Investigation of the Bearing Behaviour of Hollow Core Slabs Massive flat slabs have proven to be the most appropriate ceiling system for structural and industrial engineering. However the high dead load of these slabs might reduce their field of application. This problem can be solved by void formers that are placed in the neutral zone. These block-outs cause a noticeable reduction of weight. Furthermore, the masses of ecologically important resources like reinforcement steel and cement can be reduced as well. The shear bearing capacity is significantly influenced by these block-outs. This article describes four test series with spherical void former floors of the system "cobiax". A reduction factor for the shear bearing capacity is derived from these tests. That factor was also introduced in the technical approval for this slab system which has been issued recently. A comparison to a calculation with the finite element method shows that the numerical results and the results of the tests are quite similar. In future research, parameter studies based on the finite element method might be helpful additional to further tests. [source]

Größere Spannweiten durch Flachdecken aus Konstruktions-Leichtbeton

BETON- UND STAHLBETONBAU, Issue 5 2006
Julia Stracke Dr.-Ing.
Konstruktions-Leichtbeton weist im Vergleich zu Normalbeton ein deutlich günstigeres Verhältnis von Eigengewicht zu Festigkeit auf. Es werden die hieraus resultierenden Möglichkeiten zur Optimierung von Flachdecken gerade im Hinblick auf größere Spannweiten aufgezeigt. Unter Berücksichtigung der baustoffspezifischen Eigenschaften des Leichtbetons und deren Auswirkungen auf das Tragsystem einer Flachdecke wird die Lösung einer Hohlkörperdecke entwickelt, bei der Leicht- und Normalbeton kombiniert eingesetzt werden. Damit können deutlich größere Spannweiten erreicht werden als mit herkömmlichen, nicht vorgespannten Normalbetondecken. Wide spans due to Flat Slabs with Structural lightweight aggregate Concrete Structural lightweight aggregate concrete (SLWAC) shows a much more favourable ration of dead weight to strength than normal weight concrete. The present report indicates the possibilities resulting out of this for the optimisation of flat slabs in view of wider spans. Considering the specific material properties of SLWAC and their consequences on the load bearing system of flat slabs the solution of a hollow body slab is developed in which SLWAC and normal weight concrete are used combined. Those slab systems reach clearly wider spans than conventional nonprestressed normal weight concrete slabs. [source]

Einfluß der Elementfugen auf die Durchbiegung von Flachdecken

BETON- UND STAHLBETONBAU, Issue 3 2006
Manfred Curbach Prof. Dr.-Ing.
Das Verformungsverhalten von Flachdecken in Elementbauweise unterscheidet sich von einer reinen Ortbetondecke aufgrund der vorhandenen Elementfugen. Die damit verbundenen lokalen Querschnittsschwächungen erfordern schon auf niedrigem Lastniveau eine Umlagerung der Belastung in angrenzende Feld- und Stützbereiche. Die Folge der Lastumlagerung sind anfänglich höher beanspruchte Deckenbereiche und eine frühere Rißbildung im Vergleich zu einer reinen Ortbetondecke. Mit steigender Belastung kommt es jedoch zur Annäherung der Rißzustände von Fertigteil- und Ortbetondecken, wie anhand nichtlinearer Berechnungen nachgewiesen werden konnte. Der relative Unterschied in der Durchbiegung wird deutlich reduziert. Auf Gebrauchslastniveau liegen die Verformungen der Fertigteildecken etwa 5% über denen der reinen Ortbetondecke. Influence of Bond Joints on Deformation of precast Slabs Precast slabs and in-situ concrete slabs vary in their deflection behaviour due to the bond joints between the precast concrete units. The diminished cross section of the bond joints require a redistribution of internal forces to adjoining field and support areas already at a small load. The consequence of this redistribution are higher demands in these slab areas and an earlier crack initiation in comparison to the in-situ concrete slabs. With an increase of the loading the cracking pattern of the precast slabs and the in-situ concrete slabs converge as the nonlinear FE-calculation have shown. The relative difference in deformation is clearly reduced. At the service load the deformation of the precast slabs is only 5% higher than the deformation of the in-situ concrete slabs. [source]

[Cd2(Te6O13)][Cd2Cl6] and Cd7Cl8(Te7O17): Novel Tellurium(IV) Oxide Slabs and Unusual Cadmium Chloride Architectures.

CHEMINFORM, Issue 15 2006
Hai-Long Jiang
Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF. [source]

Ln2MAl6Si4 (Ln: Gd, Tb, Dy; M: Au, Pt): Layered Quaternary Intermetallics Featuring CaAl2Si2 -Type and YNiAl4Ge2 -Type Slabs Grown from Aluminum Flux.

CHEMINFORM, Issue 6 2004
Susan E. Latturner
Abstract For Abstract see ChemInform Abstract in Full Text. [source]

Earthquake-induced interaction between adjacent reinforced concrete structures with non-equal heights

EARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICS, Issue 1 2005
Chris G. Karayannis
Abstract The influence of the structural pounding on the ductility requirements and the seismic behaviour of reinforced concrete structures designed to EC2 and EC8 with non-equal heights is investigated. Special purpose elements of distributed plasticity are employed for the study of the columns. Two distinct types of the problem are identified: Type A, where collisions may occur only between storey masses; and Type B, where the slabs of the first structure hit the columns of the other (72 Type A and 36 Type B pounding cases are examined). Type A cases yielded critical ductility requirements for the columns in the pounding area mainly for the cases where the structures were in contact from the beginning of the excitation. In both pounding types the ductility requirements of the columns of the taller building are substantially increased for the floors above the highest contact storey level probably due to a whiplash behaviour. The most important issue in the pounding type B is the local response of the column of the tall structure that suffers the hit of the upper floor slab of the adjacent shorter structure. In all the examined cases this column was in a critical condition due to shear action and in the cases where the structures were in contact from the beginning of the excitation, this column was also critical due to high ductility demands. It can be summarized that in situations of potential pounding, neglecting its possible effects leads to non-conservative building design or evaluation that may become critical in some cases. Copyright © 2004 John Wiley & Sons, Ltd. [source]

Electrochemical Detection of Anions on an Electrophoresis Microchip with Integrated Silver Electrode

ELECTROANALYSIS, Issue 13 2005
Jilin Yan
Abstract A poly(dimethylsiloxane)/glass hybrid microfluidic chip with integrated silver electrode is described for electrochemical detection of anions. The working electrode was directly fabricated on a glass slide and the chip formed by reversibly sealing of a PDMS slab with microchannels to the slide. Under an alkaline phosphate condition, thiocyanide and three halides were sensitively detected. Factors influencing the separation and detection procedure were discussed and optimized. Linear responses over two magnitudes were obtained with limit of detection at the micromolar level. [source]

What are the Categories in Sein und Zeit?

EUROPEAN JOURNAL OF PHILOSOPHY, Issue 2 2007
Brandom on Heidegger on Zuhandenheit
In his essay, ,Heidegger's Categories in Sein und Zeit', Robert Brandom argues that Heidegger, particularly in the notion of Zuhandenheit, anticipates his own normatively pragmatist conception of intentionality. He attempts to demonstrate this by marshalling short passages from right across the relevant sections of Sein und Zeit in such a way that they do seem to say what Brandom claims. But does one reach the same conclusion when one examines, more or less in sentence-by-sentence fashion, the large slab of text in which Heidegger introduces the notion of Zuhandenheit? I believe not. First, however, let us look at how Brandom reads Heidegger, in particular, how he interprets the notion of Zuhandenheit, which, in contrast both to Macquarrie and Robinson and to Brandom, I shall translate as ready-to-handedness.1 [source]

Experimental and numerical studies on dynamic crack growth in layered slate rock under wedge impact loads: part II , non-plane strain problem

FATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 10 2007
M. R. ALAM
ABSTRACT Dynamic crack propagation in non-plane strain (or 3D) slate blocks under wedge impact loads was investigated numerically in this part of the paper. A parabolic-shaped crack trajectory was taken into consideration to model the crack propagation in slate blocks for analyzing the impact splitting of layered slate rock. Major and minor axes of the parabola were determined from the condition of equal mode I stress intensity factors (SIFs) along the crack front. Mode I SIFs were determined for experimental breaking loads for each increment of crack growth in a manner similar to that mentioned in part I of this paper. These values were compared with the plane strain material fracture toughness value obtained from experimental studies and very good agreement was obtained between them, for the case of actual load applied on the specimen. Numerical analysis of a field problem, i.e., separation of a large-sized slate slab from the rock strata in a slate quarry using wedge impacting, was also carried out in this paper. It can be observed that a large magnitude of load is required to break large-sized slate blocks; but this load is applied through a number of smaller load-capacity actuators-in-parallel, requiring large power capacity for the hydraulic pumps. However, this required power could be reduced considerably if the load applied on the line of hydraulic actuators is cascaded across the (line of) actuators (starting from centrally placed actuators) with a small time delay (equal to the initial crushing time in slate rock). [source]

Restraint of fire-exposed concrete floor systems

FIRE AND MATERIALS, Issue 2-4 2004
Linus Lim
Abstract This paper describes the numerical analyses of restrained concrete floor slabs exposed to fire. The analyses of the slabs were carried out with the SAFIR finite element program considering a 200 mm thick slab, spanning 5 m between two end supports. The slabs were exposed to the ISO standard fire for up to 4 h and were analysed with pinned and rotationally restrained supports. Different heights of the line of thrust at the supports and different levels of axial restraint were also investigated. The analyses show that fully restrained pin-supported slabs can survive the 4 h ISO fire without collapse if the position of the line of thrust is located near the soffit of the slab. If the position of the line of thrust is located much above the soffit of the slab, the slabs will rapidly undergo large deformations and sag into a catenary, imposing axial tensile forces at the supports. The analyses have shown that even if the line of thrust is located close to the soffit, the slab can still deform into a catenary if there is insufficient horizontal axial restraint. In this study, rotationally restrained slabs experience much smaller vertical deflections than pin-supported slabs when exposed to fires. Rotationally restrained slabs with low levels of horizontal restraint do not collapse, due to the beneficial effects of moment redistribution. However, high levels of horizontal restraint can be detrimental, causing slabs to collapse at advanced stages of the fire. Copyright © 2004 John Wiley & Sons, Ltd. [source]

Characterization and provenance of the slabs of the Puigseslloses Megalith, Barcelona, Spain

GEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL, Issue 2 2010
Enric Vicens
The Puigseslloses megalith tomb (Barcelona, Spain) is located on the summit of a hill in the Vic Plain where the sedimentary rocks that were used in its construction do not crop out. A sedimentological and petrological characterization of the monument's slabs and a multidisciplinary analysis of the outcropping materials in nearby areas has allowed the identification of three possible source areas associated with the Paleogene Banyoles Marls Formation and the Folgueroles Sandstone Formation and allows us to propose possible routes by which the slabs were transported from the catchment site to the place where the monument was built. In addition, the lithology and position of different slabs within the megalith denote the intention to separate the chamber from the corridor by using different types of slab. The position of the monument also reveals that tomb location was more of a priority than proximity to the slab sources. © 2010 Wiley Periodicals, Inc. [source]

Geomechanics and Tunnelling 2/2010

GEOMECHANICS AND TUNNELLING, Issue 1 2010
Article first published online: 24 FEB 2010
Semmering and Koralm tunnels Neue Südbahn, Semmering und Koralm Tunnel F. Bauer: Investment in the Austrian rail network , overview of new and upgrading projects , Investitionen in das österreichische Schienennetz , Überblick Neu- und Ausbauvorhaben G. Gobiet and D. Haas: New Semmering Base Tunnel project , an interdisciplinary challenge , Semmering Basistunnel Neu , eine interdisziplinäre Herausforderung A. Fasching, R. Vanek, Th. Stadlmann, P. Reichl, G. Domberger, G. Forstinger and O. Wagner: New Semmering Base Tunnel , the investigation programme 2008/2009 and the knowledge gained in the areas of geology, hydrogeology and geotechnical engineering , Semmering-Basistunnel Neu , Das Erkundungsprogramm 2008/2009 und die daraus gewonnenen Erkenntnisse auf den Gebieten der Geologie, Hydrogeologie und Geotechnik J. Daller, A. Vigl and O. Wagner: New Semmering Base Tunnel , the current state of tunnel design taking the newest investigation results into account , Semmering-Basistunnel Neu , Aktueller Stand der Tunnelplanung unter Berücksichtigung der neuesten Erkundungsergebnisse R. Bopp, V. Langer, Ch. Neumann and O. Wagner: The ventilation and tunnel safety concept for the New Semmering Base Tunnel , Das Lüftungs- und Tunnelsicherheitskonzept für den Semmering-Basistunnel Neu G. Harer and J. Koinig: Current state of design, investigation and construction work at the Koralm Tunnel , Aktueller Stand der Planungs-, Erkundungs- und Ausführungsarbeiten beim Koralmtunnel P. Schubert, H. Hölzl, P. Sellner and F. Fasching: Geomechanical knowledge gained from the Paierdorf Investigation Tunnel in the section through the Lavanttal main fault zone , Geomechanischer Wissenszuwachs durch den Erkundungstunnel Paierdorf im Abschnitt der Lavanttaler Hauptstörungszone D. Handke, M. Nolden, K. Mussger and A. Steidl: Solution methods for the design and construction of contract KAT 3 , Lösungsansätze für Planung und Bauausführung des Bauloses KAT 3 M. Vill, H. Wagner, A. Schweighofer, H. Huber, W. Pichler and J. Kollegger: New development of a crack-limited invert slab , Neuentwicklung der rissebeschränkten Sohlplatte M. Stopka: Hengsberg Tunnel , the construction contract as seen by the contractor , Hengsbergtunnel , der Bauvertrag aus Sicht des Auftragnehmers T. Schachinger, H. Gaube and G. Krainer: Results from the Untersammelsdorf test field for the planning of the tunnelling work , Erkenntnisse aus dem Versuchsfeld Untersammelsdorf für die Gestaltung der Vortriebsmaßnahmen K. Chmelina and K. Rabensteiner: Improvement of the safety and profitability of tunnel drives through the use of automated measurement and alarm systems , examples in practice , Verbesserung der Sicherheit und Wirtschaftlichkeit von Tunnelvortrieben durch den Einsatz automatisierter Mess- und Alarm-systeme , Ausführungsbeispiele [source]

Subducted slabs and lateral viscosity variations: effects on the long-wavelength geoid

P - and S -wave reflectors in D, beneath southeast Asia

GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 2 2009
Jeremy W. Chaloner
SUMMARY Earthquakes from the South Pacific region recorded at the Kyrgyz Seismic Network (KNET) and the GHENGIS network (both part of the Tien Shan Continental Dynamics study) have been analysed to search for evidence of reflections from the D, region. The reflection points of this source,receiver combination are in a seismically fast area beneath Southeast Asia. Employing several array methods, we find evidence for reflections from a D, reflector in both P and S waves. The reflector depth generally agrees for both wave types. There is some evidence for a second, deeper reflector in this region; migrating the data suggests a lower-seismic reflector beneath the northeastern end of the study region. The waveforms of P and S waves imply that the upper reflector has a negative impedance contrast for P but a positive impedance contrast for S. Several possibilities are discussed for generating the observed reflector(s), with the post-perovskite phase transition in combination with a cold slab being the preferred explanation. [source]

Regional earthquake location using empirical traveltimes in a region of strong lateral velocity heterogeneity

GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 2 2008
T. Nicholson
SUMMARY The unmodelled effects of lateral heterogeneity are a primary cause of earthquake mislocation using routine methods. Approaches that account for the traveltime effects of lateral heterogeneity are a common feature of specialized studies, but some earthquake location methods have been developed that are also suitable for routine location. Here we investigate the use of one such method (3-D empirical traveltimes, ETTs) for local and regional earthquake location in an area of pronounced lateral velocity heterogeneity. We focus on New Zealand and the surrounding region due to the presence of strongly heterogeneous subduction zone environments. The New Zealand example poses significant challenges since most of the seismicity occurs outside the national seismograph network, and many of the observed ray paths traverse the high-velocity subducting slab. We show that in this environment, >60 per cent of the P and S residuals observed when a 1-D model velocity model is used stem from the resolvable lateral heterogeneity signal. For stations that have been in operation for more than five years, 85 per cent of observed residuals can be ascribed to this resolvable signal. We produce consistent 3-D traveltime corrections for all of New Zealand and the surrounding region and demonstrate their application to clusters of earthquakes occurring off the east coast of the North Island. [source]

Seismic evidence for a sharp lithospheric base persisting to the lowermost mantle beneath the Caribbean

GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2008
Tadashi Kito
SUMMARY Broad-band data from South American earthquakes recorded by Californian seismic networks are analysed using a newly developed seismic wave migration method,the slowness backazimuth weighted migration (SBWM). Using the SBWM, out-of-plane seismic P -wave reflections have been observed. The reflection locations extend throughout the Earth's lower mantle, down to the core,mantle boundary (CMB) and coincide with the edges of tomographically mapped high seismic velocities. Modelling using synthetic seismograms suggests that a narrow (10,15 km) low- or high-velocity lamella with about 2 per cent velocity contrast can reproduce the observed reflected waveforms, but other explanations may exist. Considering the reflection locations and synthetic modelling, the observed out-of-plane energy is well explained by underside reflections off a sharp reflector at the base of the subducted lithosphere. We also detect weaker reflections corresponding to the tomographically mapped top of the slab, which may arise from the boundary between the Nazca plate and the overlying former basaltic oceanic crust. The joint interpretation of the waveform modelling and geodynamic considerations indicate mass flux of the former oceanic lithosphere and basaltic crust across the 660 km discontinuity, linking processes and structure at the top and bottom of the Earth's mantle, supporting the idea of whole mantle convection. [source]

Guided waves at subduction zones: dependencies on slab geometry, receiver locations and earthquake sources

GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 2 2006
S. Martin
SUMMARY We investigate the geometry of deep subduction zone waveguides (depth >100 km). The wavefield characteristics for up-dip profiles are described and compared with data recorded at the Chile,Peru subduction zone. Observed distorted P onsets at stations in northern Chile near 21°S can be matched by 2-D finite difference simulations of a thin low-velocity layer (LVL) atop the slab in an IASP91 velocity model. The replacement of the LVL by simple random velocity undulations in the slab in the same model cannot explain the observations. Varying slab geometries are investigated and the distribution of guided wave onsets originating in deep waveguides is predicted relative to the slab surface. Further, double couple source position and orientation is explored and found to be closely limited by the guided wave observations. Sources situated above the layer and at distances more than 2 layer widths below the subducted Moho are not suitable. For the remaining favourable source locations, a strong link between pulse shapes and fault plane dip angle is evident. We conclude that up-dip guided wave observations at subduction zones follow a simple pattern given by slab geometry and modified by source position. The resulting onsets are shaped by layer thickness and velocity contrast and further influenced by the shape of the slab surface. [source]

P - and S -velocity images of the lithosphere,asthenosphere system in the Central Andes from local-source tomographic inversion

GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 1 2006
Ivan Koulakov
SUMMARY About 50 000 P and S arrival times and 25 000 values of t* recorded at seismic arrays operated in the Central Andes between 20°S and 25°S in the time period from 1994 to 1997 have been used for locating more than 1500 deep and crustal earthquakes and creating 3-D P, S velocity and Qp models. The study volume in the reference model is subdivided into three domains: slab, continental crust and mantle wedge. A starting velocity distribution in each domain is set from a priori information: in the crust it is based on the controlled sources seismic studies; in slab and mantle wedge it is defined using relations between P and S velocities, temperature and composition given by mineral physics. Each iteration of tomographic inversion consists of the following steps: (1) absolute location of sources in 3-D velocity model using P and S arrival times; (2) double-difference relocation of the sources and (3) simultaneous determination of P and S velocity anomalies, P and S station corrections and source parameters by inverting one matrix. Velocity parameters are computed in a mesh with the density of nodes proportional to the ray density with double-sided nodes at the domain boundaries. The next iteration is repeated with the updated velocity model and source parameters obtained at the previous step. Different tests aimed at checking the reliability of the obtained velocity models are presented. In addition, we present the results of inversion for Vp and Vp/Vs parameters, which appear to be practically equivalent to Vp and Vs inversion. A separate inversion for Qp has been performed using the ray paths and source locations in the final velocity model. The resulting Vp, Vs and Qp distributions show complicated, essentially 3-D structure in the lithosphere and asthenosphere. P and S velocities appear to be well correlated, suggesting the important role of variations of composition, temperature, water content and degree of partial melting. [source]

A practical grid-based method for tracking multiple refraction and reflection phases in three-dimensional heterogeneous media

GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 1 2006
M. De Kool
SUMMARY We present a practical grid-based method in 3-D spherical coordinates for computing multiple phases comprising any number of reflection and transmission branches in heterogeneous layered media. The new scheme is based on a multistage approach which treats each layer that the wave front enters as a separate computational domain. A finite-difference eikonal solver known as the fast-marching method (FMM) is reinitialized at each interface to track the evolving wave front as either a reflection back into the incident layer or a transmission through to the adjacent layer. Unlike the standard FMM, which only finds first arrivals, this multistage approach can track those later arriving phases explicitly caused by the presence of discontinuities. Notably, the method does not require an irregular mesh to be constructed in order to connect interface nodes to neighbouring velocity nodes which lie on a regular grid. To improve accuracy, local grid refinement is used in the neighbourhood of a source point where wave front curvature is high. The method also provides a way to trace reflections from an interface that are not the first arrival (e.g. the global PP phase). These are computed by initializing the multistage FMM from both the source and receiver, propagating the two wave fronts to the reflecting interface, and finding stationary points of the sum of the two traveltime fields on the reflecting interface. A series of examples are presented to test the efficiency, accuracy and robustness of the new scheme. As well as efficiently computing various global phases to an acceptable accuracy through the ak135 model, we also demonstrate the ability of the scheme to track complex crustal phases that may be encountered in coincident reflection, wide-angle reflection/refraction or local earthquake surveys. In one example, a variety of phases are computed in the presence of a realistic subduction zone, which includes several layer pinch-outs and a subducting slab. Our numerical tests show that the new scheme is a practical and robust alternative to conventional ray tracing for finding various phases in layered media at a variety of scales. [source]

Role of lateral mantle flow in the evolution of subduction systems: insights from laboratory experiments

GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2004
Francesca Funiciello
SUMMARY We present 3-D laboratory experiments constructed to investigate the pattern of mantle flow around a subducting slab under different boundary conditions. In particular we present a set of experiments, characterized by different conditions imposed at the trailing edge of the subducting plate (that is, plate fixed in the far field, plate detached in the far field, imposed plate motion). Experiments have been performed using a silicone slab floating inside a honey tank to simulate a thin viscous lithosphere subducting in a viscous mantle. For each set, we show differences between models that do or do not include the possibility of out-of-plane lateral flow in the mantle by varying the lateral boundary conditions. Our results illustrate how a subducting slab vertically confined over a 660-km equivalent depth can be influenced in its geometry and in its kinematics by the presence or absence of possible lateral pathways. On the basis of these results we show implications for natural subduction systems and we highlight the importance of suitable simulations of lateral viscosity variations to obtain a realistic simulation of the history of subduction. [source]

A complex, young subduction zone imaged by three-dimensional seismic velocity, Fiordland, New Zealand

GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2001
Donna Eberhart-Phillips
Summary The Fiordland subduction zone, where subduction developed in the late Miocene, has been imaged with P and S,P arrival-time data from 311 earthquakes in a simultaneous inversion for hypocentres and 3-D VP and VP/VS models. The three-month microearthquake survey, recorded with 24 portable seismographs, provides excellent coverage, and, since earthquakes to depths of 130 km are included, parts of the model are well-resolved to depths of 100 km. The crustal features are generally consistent with geology. The low velocity in the upper 10 km is associated with the Te Anau and Waiau basins. The Western Fiordland Orthogneiss is associated with a prominent feature from near-surface to over 40 km depth, which includes the residue from the basaltic source rocks. It is defined by high VP (7.4 km s,1 at 15 km depth) and slightly low VP/VS, and has distinct boundaries on its southern and eastern margins. Adjacent to the deepest earthquakes, there is high-velocity Pacific mantle below 80 km depth, inferred to be the mantle expression of ongoing shortening since the early Miocene. As the subducting slab moves down and northeast, it is hindered by the high-velocity body and bends to near-vertical. Bending is accommodated by distributed fracturing evidenced by high VP/VS and persistent deep earthquake activity. Buckling of the subducted plate pushes up the Western Fiordland Orthogneiss. In the transition to the Alpine fault in northern Fiordland, a prominent low-velocity crustal root is consistent with ductile thickening in combination with downwarp of the subducted plate. [source]

Deformation and stress change associated with plate interaction at subduction zones: a kinematic modelling

GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 2 2000
Shaorong Zhao
The interseismic deformation associated with plate coupling at a subduction zone is commonly simulated by the steady-slip model in which a reverse dip-slip is imposed on the down-dip extension of the locked plate interface, or by the backslip model in which a normal slip is imposed on the locked plate interface. It is found that these two models, although totally different in principle, produce similar patterns for the vertical deformation at a subduction zone. This suggests that it is almost impossible to distinguish between these two models by analysing only the interseismic vertical deformation observed at a subduction zone. The steady-slip model cannot correctly predict the horizontal deformation associated with plate coupling at a subduction zone, a fact that is proved by both the numerical modelling in this study and the GPS (Global Positioning System) observations near the Nankai trough, southwest Japan. It is therefore inadequate to simulate the effect of the plate coupling at a subduction zone by the steady-slip model. It is also revealed that the unphysical assumption inherent in the backslip model of imposing a normal slip on the locked plate interface makes it impossible to predict correctly the horizontal motion of the subducted plate and the stress change within the overthrust zone associated with the plate coupling during interseismic stages. If the analysis made in this work is proved to be correct, some of the previous studies on interpreting the interseismic deformation observed at several subduction zones based on these two models might need substantial revision. On the basis of the investigations on plate interaction at subduction zones made using the finite element method and the kinematic/mechanical conditions of the plate coupling implied by the present plate tectonics, a synthesized model is proposed to simulate the kinematic effect of the plate interaction during interseismic stages. A numerical analysis shows that the proposed model, designed to simulate the motion of a subducted slab, can correctly produce the deformation and the main pattern of stress concentration associated with plate coupling at a subduction zone. The validity of the synthesized model is examined and partially verified by analysing the horizontal deformation observed by GPS near the Nankai trough, southwest Japan. [source]

Vapor Intrusion in Homes over Gasoline-Contaminated Ground Water in Stafford, New Jersey

GROUND WATER MONITORING & REMEDIATION, Issue 1 2006
Paul F. Sanders
The potential for chemical vapor intrusion from contaminated ground water to the interior of homes was investigated at a site with a leaking underground gasoline storage tank in Stafford Township, New Jersey. This location exhibited conditions favorable to vapor intrusion, with sand soil and a water table depth of 3.3 m. Concentrations of volatile organic chemicals in the ground water were as high as 82 mg/L for total benzene, toluene, ethylbenzene, and xylene (BTEX) and up to 590 mg/L for methyl- t -butyl ether (MTBE). Soil vapor samples at multiple depths were taken adjacent to several homes. Inside the homes, air samples were taken on the main floor, in the basement, and under the foundation slab. Despite high ground water concentrations, only one home had measurable impacts to indoor air quality attributable to some of the ground water contaminants. In this house, the BTEX chemicals were not detected in the basement, indicating a lack of indoor air impacts from the ground water for these chemicals. Oxygen measurements suggested that degradation attenuated these chemicals as they diffused through the vadose zone. However, MTBE, 2,2,4-trimethylpentane (isooctane), and cyclohexane were found in the indoor air. The first two of these chemicals served as gasoline-specific tracers and indicated that vapor intrusion was occurring. Attenuation factors (the ratio of the indoor air concentration to a source soil vapor concentration) for the BTEX chemicals between the ground water and the indoor air were <1 × 10,5, and for MTBE was 1.2 × 10,5. Attenuation factors between the deep-soil vapor and the basement air were as follows: BTEX compounds, <1 × 10,5; MTBE, 2.2 × 10,5; 2,2,4-trimethylpentane, 3.6 × 10,4; and cyclohexane, 1.2 × 10,4. Attenuation factors between the subslab vapor and the basement air were 7 to 8 × 10,3. [source]

Wave propagation in nonlinear one-dimensional soil model

INTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 4 2009
J. Ahn
Abstract The objective of the research conducted by the authors is to explore the feasibility of determining reliable in situ values of shear modulus as a function of strain. In this paper the meaning of the material stiffness obtained from impact and harmonic excitation tests on a surface slab is discussed. A one-dimensional discrete model with the nonlinear material stiffness is used for this purpose. When a static load is applied followed by an impact excitation, if the amplitude of the impact is very small, the measured wave velocity using the cross-correlation indicates the wave velocity calculated from the tangent modulus corresponding to the state of stress caused by the applied static load. The duration of the impact affects the magnitude of the displacement and the particle velocity but has very little effect on the estimation of the wave velocity for the magnitudes considered herein. When a harmonic excitation is applied, the cross-correlation of the time histories at different depths estimates a wave velocity close to the one calculated from the secant modulus in the stress,strain loop under steady-state condition. Copyright © 2008 John Wiley & Sons, Ltd. [source]