One-dimensional Shear Velocity Structure of Northern Africa from Rayleigh Wave Group Velocity Dispersion     

S. E. Hazler  A. F. Sheehan  D. E. McNamara  W. R. Walter 《Pure and Applied Geophysics》2001,158(8):1475-1493

—?Rayleigh wave group velocity dispersion measurements from 10?s to 160?s periods have been made for paths traversing Northern Africa. Data were accumulated from the IRIS DMC, GEOSCOPE, and MEDNET seismic networks covering the years 1991–1997. The group velocity measurements are made including the effects of debiasing for instantaneous period and a single-iteration, mode-isolation (phase match) filter. The curves are grouped by tectonic province and compared to tomographic model-based curves in an effort to test and validate the tomographic models. Within each tectonic category (rift, orogenic zone, or craton) group velocity curves from various provinces are similar. Between tectonic categories, however, there are marked differences. The rift related paths exhibit the lowest group velocities observed, and cratonic paths the fastest. One-dimensional shear velocity inversions are performed, and while highly nonunique, the ranges of models show significant differences in upper mantle velocities between the tectonic provinces.¶This work is part of a larger project to determine group velocity maps for North Africa and the Middle East. The work presented here provides important tools for the validation of tomographic group velocity models. This is accomplished by comparing group velocity curves calculated from the tomographic models with carefully selected high-quality group velocity measurements. The final group velocity models will be used in M _s measurements, which will contribute to the m _b :M _s discriminant important to the Comprehensive Nuclear-Test-Ban Treaty (CTBT). The improved shear wave velocity models provided by this study also contribute to the detection, location, and identification of seismic sources.  相似文献   

The historic and artistic heritage facing the earthquake risk: The Italian case     

Antonella Huber  Michele Dragoni 《Natural Hazards》1992,5(3):269-278

The problem of protection against earthquakes in Italy is made difficult by the presence of a huge historic and artistic heritage. Such a heritage is mainly made up of ancient buildings and monuments situated in the urban centres, which are densely distributed throughout Italy. Therefore, very complex problems are met in the determination of the value parameter concurring with the determination of seismic risk, in addition to hazard and vulnerability. An indication of the monetary value of a building is not sufficient as far as the cultural heritage is concerned: different criteria are necessary in order to distinguish which are the strategic buildings. If we consider that there are more than 2000 museums in Italy, most of which are placed inside historic buildings, it appears that museums should receive the highest priority in future initiatives for seismic rehabilitation.  相似文献   

A viscoelastic shear zone model of compressional and extensional plate boundaries     

Michele Dragoni  Stefano Santini  Andrea Tallarico 《Pure and Applied Geophysics》1993,140(3):471-491

A model is proposed describing the mechanical evolution of a shear zone along compressional and extensional plate boundaries, subject to constant strain rate. The shear zones are assumed as viscoelastic with Maxwell rheology and with elastic and rheological parameters depending on temperature and petrology. Stress and strain are computed as functions of time and depth. For both kinds of boundaries the model reproduces the existence of a shallow seismogenic zone, characterized by a stress concentration. The thickness of the seismogenic layer is evaluated considering the variations of shear stress and frictional strength on faults embedded in the shear zone. Assuming that a fault dislocation takes place, the brittle-ductile transition is assumed to occur at the depth at which the time derivative of total shear stress changes from positive to negative values. The effects of different strain rates and geothermal gradients on the depth of the brittle-ductile transition are studied. The model predictions are consistent with values inferred from seismicity data of different boundary zones.  相似文献   

What controls levels of dissolved phosphate and ammonium in surface waters?   总被引：4，自引：0，他引：4  

Walter K. Dodds 《Aquatic Sciences - Research Across Boundaries》1993,55(2):132-142

Dissolved inorganic nutrient pools are small relative to particulate pools, and dissolved pools turnover rapidly. It has been observed that pools change little from day to day on the sampling scales usually employed. A simple model is presented where uptake and regeneration rates balance to cause a local steady state concentration for dissolved inorganic nutrients. Enrichment and dilution perturbation experiments with lake water support the idea of steady state nutrient concentrations. Although inorganic nutrient concentrations are often controlled by biota, the absolute concentrations present tell little about the activity of that biota.  相似文献   

Geodesic polygons on surfaces of revolution     

Walter K?hnlein 《Journal of Geodesy》1963,37(2):201-209

This paper presents a way for computing geodesic polygons on real surfaces of revolution with partly continuous curvature and gives an application to coordinate transformations in geodesic coordinate systems. The specific formulas for an ellipsoid of revolution are briefly mentioned.  相似文献   

Die Gliederung des Rotliegenden und die Stellung seiner Vulkanite am Südwestrand des Frankenwaldes     

Walter Stephan 《International Journal of Earth Sciences》1959,48(1):92-95

Zusammenfassung Auswirkungen der saalischen Phase sind nun auch im Stockheimer Becken erstmals festgestellt. In der Hauptsache ist in diesem Gebiet jedoch mit orogenetischen Bewegungen im Unterrotliegenden selbst zu rechnen. — Der Vulkanismus entwickelte sich im Becken bzw. an seinen Rändern vermutlich bereits im jüngsten Oberkarbon (1. Phase, am stärksten ausgebildet) und hält dann im Unterrotliegenden an (2. Phase, evtl. noch Gehrener Stufe). Aus dem mittleren Rotliegenden sind bisher nur Porphyr-Fanglomerate angeführt worden. Allerdings muß berücksichtigt werden, daß deren Neubearbeitung noch nicht abgeschlossen ist und das Stockheimer Becken ohnehin nur einen kleinen Ausschnitt aus der Gesamtentwicklung am Südwestrand des Frankenwaldes darstellt. Mit einem Vulkanismus im mittleren Rotliegenden (3. Phase, ? Goldlauterer Stufe) ist demnach auch hier noch zu rechnen.  相似文献   

Die kristallographischen Konstanten Yon Löweit     

Walter Schneider  Prof. Dr. Josef Zemann 《Contributions to Mineralogy and Petrology》1959,6(4):201-202

Zusammenfassung Künstlich hergestellter Löweit kristallisiert trigonal in kleinen Rhomboedern. Die Gitterkonstanten für die hexagonale Zelle sind:a = 18,96 Å,c = 13,47 Å. Die systematischen Auslöschungen führen auf die Raumgruppe R 3 oder R 3. Der Zellinhalt wird diskutiert.  相似文献   

Variangular wind spirals     

Heinz H. Lettau  Walter F. Dabberdt 《Boundary-Layer Meteorology》1970,1(1):64-79

The term variangular is introduced to emphasize a significant difference between the present and certain earlier solutions to the problem of organized airmotion within the planetary boundary layer. The latter belong to the family of equiangular wind spirals and have the characteristic that the angle () formed by the vectors of shearing stress and geostrophic departure is invariant with height; it is shown that in this spiral-family, parabolic height-dependency of the effective (eddy) diffusivity (K) alone is permitted, including the asymptotic case of constant K; the famous Ekman spiral as well as the Rossby spiral are two prominent members of the family of equiangular wind spirals. The new variangular theory, as the name implies, permits variation of with height (z) and produces more versatile profiles of wind and stress due to less restraint in K (z). As an example of comparison with observed data, monthly mean wind profiles obtained at Plateau Station, Antarctica, are selected since they exhibit a noteworthy degree of variangularity, in relatively satisfactory agreement with properties of the new theoretical model for wind spirals.National Research Council Visiting Scientist Research Associate, Regional Environments Division, Earth Sciences Laboratory.  相似文献   

Kaltzeit-Indikatoren im Unterkambrium SO-Schonens (Schweden)     

Dipl.-Geol. Walter Vortisch 《International Journal of Earth Sciences》1973,62(2):491-506

Zusammenfassung Etwa 1,5 km NW Simrisham wurden an der GrenzflÄche Hardeberga-/Norretorp-Glaukonit-Sandstein (U.-Kambrium) unregelmÄ\ig geformte Erhebungen gefunden, die erosive Bildungen darstellen (SchrÄgschichtung teilweise erhalten) und in die überlagernden Norretorp-Schichten hineinragen. Diese GrenzflÄche weist charakteristische Kriechspuren auf, die im noch lockeren Sediment entstanden sein müssen. Da die Erhebungen andererseits nur im verfestigten Zustand den erosiven KrÄften hÄtten standhalten können, und weder Quarz noch Kalzit als Zement in Frage kommen, wird angenommen, da\ die KontaktflÄche Hardeberga/Norretorp-Sandstein zeitweise gefroren war. Weiterhin wurden Schollen und Gerölle eines Glaukonit-Sandsteines (wohl Norretorp-Sandstein) beobachtet, die vermutlich im gefrorenen Zustand von Norretorp-Schichten eingebettet wurden. Die Gesamtheit der Strukturen (Strömungslineare, laminierte Schichtung usw.), sowie die regionalgeologischen und stratigraphischen ZusammenhÄnge machen es wahrscheinlich, da\ zumindest die basalen Teile der Norretorp-Folge in einem energiereichen Litoralmilieu abgelagert wurden.

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In a Lower Cambrian funnel graben, 1,5 km NW of Simrishamn (S Schweden), irregular miniature sea-stacks were found at the boundary between the Hardeberga and Norretorp Sandstone Formations. Their erosive origin is proven by preserved internal cross-laminations. The same boundary typically contains animal tracks indicating that the surface was temporarily soft. The resistivity of the erosion remnants is explained by cementation. As cement, ice appears to be the only likely alternative. Clasts of a glauconitic sandstone (probably Norretorp Sandstone) found within the Norretorp beds were probably transported in a frozen condition. The structures (parting lineation, planar lamination, etc.) together with the regional and stratigraphic setting make it probable that at least the basal part of the Norretorp Glauconitic Sandstone was deposited in a high-energy litoral environment.

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Résumé A environ 1,5 km au NW de Simrisham (Suède méridionale), on a trouvé, au contact du quartzite de Hardeberga et du grès glauconitique de Norretorp, de petites protubérances irrégulières du premier dans le second et dont la stratification entrecroisée, partiellement conservée, montre qu'elles résultent d'une action érosive. Cette surface de contact montre des traces d'animaux qui indiquent que le sédiment sous-jacent était encore mou. Comme ces protubérances n'ont pu résister aux forces érosives que gráce à la compacité de la roche, et que ni le quartz ni la calcite ne peuvent Être invoqués comme ciment, on admet que la surface de contact a été momentanément gelée. En outre des fragments et galets d'un grès glauconieux (probablement de grès Norretorp) ont été réincorporés à l'état congelé dans les couches de Norretorp. L'ensemble des structures (lignes de courant, stratification laminaire fine, etc.) comme aussi les liaisons géologiques et stratigraphiques à l'échelle de la région, rendent vraisemblable, tout au moins pour les couches inférieures du grès Norretorp, leur dépÔt dans un milieu mouvementé.

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/ Hardeberga/Norretorp, , 1,5 - Simrishamn , — — Norretorp. , . . . , , , , , , . — Norretorp. -, , Norretorp. — , -, - , , - , Norretorp .

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Zu den Kondensationskernverhältnissen von Wien     

Walter Müller 《Pure and Applied Geophysics》1967,66(1):169-182

Zusammenfassung Nach allgemeinen und quantitativen Angaben über die Ursachen der Zu-und Abnahmen der Kondensationskerndichte wird auf die natürliche und industrielle Kernproduktion verwiesen. Auf Grund einjähriger Morgen- und Mittagsbeobachtungen in Wien mit einem Pollak-Kondensationskernzähler wird besonders die Sedimentation, der Washout- und Rainouteffekt, der Einfluss von Austausch und Windadvektion sowie der Jahres- und Wochengang besprochen.

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Summary General as well as quantitative details of reasons of the variations of the density of condensation nuclei is mentioned. Based on one year's records of observations in the morning and at noon in Vienna, executed by menas of a Pollak-condensation nucleus counter especially the effects of sedimentation, washout, rainout, exchange, wind advection, annual and weekly variation is discussed.

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