Numerical experiments on the generation of long ocean waves in coastal waters of the Buenos Aires province,Argentina

A numerical model (two horizontal dimensions, vertically integrated) is used to investigate the generation of long ocean waves, ranging from 20 min to almost 2 h, at Buenos Aires continental shelf. The domain includes the Río de la Plata estuary and the continental shelf together and extends from 33.5° to 40.5°S latitude, and from 51° to 63°W longitude. Sea-level oscillations are modeled by forcing with passage of atmospheric cold fronts and atmospheric gravity waves. Both forcing mechanisms, which have been present during high activity lapses of long ocean waves, are mathematically implemented. After several numerical simulations, it is concluded that the pressure and wind fields associated to cold fronts do not generate long ocean waves in the area, though they do produce disturbances with periods longer than the tidal ones. On the other hand, it is so concluded that atmospheric gravity waves are an effective mechanism to force long ocean waves. Results obtained show that generation of long ocean waves is highly sensitive depending on the propagation direction and the phase speed of the atmospheric gravity waves. The long ocean wave event detected during the large-amplitude gravity-wave event of 13 October 1985 is successfully simulated. Finally, all our results suggest that atmospheric gravity waves are a highly effective mechanism forcing for the generation of long ocean waves in Buenos Aires coastal waters.     

A Surface Wave Dispersion Study of the Middle East and North Africa for Monitoring the Comprehensive Nuclear-Test-Ban Treaty

—?We present results from a large-scale study of surface-wave group velocity dispersion across the Middle East, North Africa, southern Eurasia and the Mediterranean. Our database for the region is populated with seismic data from regional events recorded at permanent and portable broadband, three-component digital stations. We have measured the group velocity using a multiple narrow-band filter on deconvolved displacement data. Overall, we have examined more than 13,500 seismograms and made good quality dispersion measurements for 6817 Rayleigh- and 3806 Love-wave paths. We use a conjugate gradient method to perform a group-velocity tomography. Our current results include both Love- and Rayleigh-wave inversions across the region for periods from 10 to 60 seconds. Our findings indicate that short-period structure is sensitive to slow velocities associated with large sedimentary features such as the Mediterranean Sea and Persian Gulf. We find our long-period Rayleigh-wave inversion is sensitive to crustal thickness, such as fast velocities under the oceans and slow along the relatively thick Zagros Mts. and Turkish-Iranian Plateau. We also find slow upper mantle velocities along known rift systems. Accurate group velocity maps can be used to construct phase-matched filters along any given path. The filters can improve weak surface wave signals by compressing the dispersed signal. The signals can then be used to calculate regionally determined M _S measurements, which we hope can be used to extend the threshold of m _b :M _S discriminants down to lower magnitude levels. Other applications include using the group velocities in the creation of a suitable background model for forming station calibration maps, and using the group velocities to model the velocity structure of the crust and upper mantle.     

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

—?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.     

Geodesic polygons on surfaces of revolution

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.     

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

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.     

Die kristallographischen Konstanten Yon Löweit

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.     

Variangular wind spirals

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.     

Equilibrium coexistence of three amphiboles

Electron probe and wet chemical analyses of amphibole pairs from the sillimanite zone of central Massachusetts and adjacent New Hampshire indicated that for a particular metamorphic grade there should be a restricted composition range in which three amphiboles can coexist stably. An unequivocal example of such an equilibrium three amphibole rock has been found in the sillimanite-orthoclase zone. It contains a colorless primitive clinoamphibole, space group P2₁/m, optically and chemically like cummingtonite with blue-green hornblende exsolution lamellae on (100) and (¯101) of the host; blue-green hornblende, space group C2/m, with primitive cummingtonite exsolution lamellae on (100) and (¯101) of the host; and pale pinkish tan anthophyllite, space group Pnma, that is free of visible exsolution lamellae but is a submicroscopic intergrowth of two orthorhombic amphiboles. Mutual contacts and coarse, oriented intergrowths of two and three host amphiboles indicate the three grew as an equilibrium assemblage prior to exsolution. Electron probe analyses at mutual three-amphibole contacts showed little variation in the composition of each amphibole. Analyses believed to represent most closely the primary amphibole compositions gave atomic proportions on the basis of 23 oxygens per formula unit as follows: for primitive cummingtonite (Na_0.02Ca_0.21 ^– Mn_0.06Fe²⁺ _2.28Mg_4.12Al_0.28) (Al_0.17Si_7.83), for hornblende (Na_0.35Ca_1.56Mn_0.02Fe_1.71Mg_2.85Al_0.92) (Al_1.37Si_6.63), and for anthophyllite (Na_0.10Ca_0.06Mn_0.06Fe_2.25Mg_4.11Al_0.47) (Al_0.47Si_7.53). The reflections violating C-symmetry, on X-ray single crystal photographs of the primitive cummingtonite, are weak and diffuse, and suggest a partial inversion from a C-centered to a primitive clinoamphibole. Single crystal photographs of the anthophyllite show split reflections indicating it is an intergrowth of about 80% anthophyllite and about 20% gedrite which differ in their b crystallographic dimensions. Split reflections are characteristic of all analyzed orthorhombic amphiboles so far examined from Massachusetts and New Hampshire except the most aluminous gedrites, and the relative intensity of the gedrite reflections is roughly proportional to the degree of Na and Al substitution. Thin sections of a few of these anthophyllite specimens show lamellae parallel to (010) that are just resolved with a high power objective.Publication approved by the Director, U.S. Geological Survey.     

Kaltzeit-Indikatoren im Unterkambrium SO-Schonens (Schweden)

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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