Observations of vertical ground accelerations exceeding gravity during the 1997 Umbria-Marche (central Italy) earthquakes

We found extensive evidence that the vertical ground accelerations produced during the largest shock (M = 6.0) of the 1997 Umbria-Marche earthquake sequence exceeded 1g in two areas close to the heavily-damaged villages of Annifo and Colle Croce. This evidence comes from the striking observation of thousands of freshly fractured and broken rocks and stones in these areas. Some of the broken stones lie isolated on soft detritic soil while others had been previously piled up, probably a long time agoto clear the fields for farming. The freshness of the cuts and fractures and the consistency of the observations for thousands of rocks and stones in these areas indicate that these rocks were thrown upwards during the earthquake, with breakage occurring at the time of impact. Ground motion calculations consistent with the static deformation inferred from GPS and interferometry data, show that the broken stones and rocks are found in the zone where the strongest shaking took place during the earthquake and that most of the shaking there was vertical.     

Einfluß der Gezeitenstromturbulenz auf die hydrographische Schichtung der Nordsee

Zusammenfassung Es wird das kritische vertikale Dichtegefälle ermittelt, das die Gezeitenstromturbulenz, die vom Meeresboden ausgeht, zum Erliegen bringt (Abb. 2). Ihm wird jenes vertikale Dichtegefälle gegenübergestellt, das der tägliche Wärmestrom durch die Turbulenz der winderzeugten Oberflächenströmungen bewirkt (Abb. 3). In derjeningen Tiefe, in der beide Gefällswerte gleich werden, liegt die Obergrenze der bodennahen Mischungsschicht, deren Verteilung in der Nordsee auf zwei ozeanographischen Schnitten verfolgt wird, die mit dem Vermessungs- und Forschungsschiff Gauss im August 1953 ausgeführt wurden (Abb. 1). Der eine Schnitt verläuft vom Nordausgang der Nordsee zu den westfriesischen Inseln, der andere von der schottischen zur norwegischen Küste (Abb. 4). Durch diese Anordnung konnten alle charakteristischen sommerlichen Lagen in der Nordsee erfaßt werden: der Übergang von der unbedeutenden bodennahen Mischungsschicht in der nördlichen Nordsee zur mächtigen Mischungsschicht der mittleren Nordsee, die dort in den Gebieten mit höheren Gezeitenströmen die normale sommerliche Sprungschicht zu einer wenige Dezimeter dicken Schicht zusammendrängt und die in den Gebieten mit starken Gezeitenströmen in die oberflächennahe Mischungsschicht übergreift. Als Folge davon wird in ausgedehnten Gebieten Schichtungslosigkeit während des ganzen Jahres aufrechterhalten. Die weitreichenden, verschiedenartigen Einflüsse dieser Mischungsvorgänge machen sich unter anderem auf den Jahresgang der Oberflächentemperatur der Nordsee deutlich bemerkbar (Abb. 5 und 6).

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Summary The critical vertical density gradient for suppressing the tidal current turbulence which originates from the bottom of the sea is determined (fig. 2). It is compared with the vertical density gradient caused by the daily heat transport in consequence of the turbulence of the wind-produced surface currents (fig. 3). At the depth where the two gradients become equal there lies the upper limit of the mixed ground layer. Its distribution in the North Sea is shown by two oceanographic profiles on the basis of measurements carried out by the survey and research ship Gauss in April 1953 (fig. 1). One profile extends from the northern exit of the North Sea to the West-Frisian islands, the other one from the Scotch to the Norwegian coast (fig. 4). By this arrangement all characteristic summer situations in the North Sea could be included: the change from the small mixed ground layer in the northern North Sea to the mighty mixed ground layer of the central North Sea compressing there, in regions with higher tidal currents, the normal summer discontinuity layer to a few decimeters and, in regions with strong tidal currents, reaching into the mixed surface layer. As a consequence in vast regions a state without stratification is maintained during the whole year. The far-reaching and various influences of these mixing processes, e. g. on the annual variation of the surface temperature of the North Sea, are evident (fig. 5 and 6).

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Résumé On recherche le gradient vertical critique de densité capable d'annuller le remous dû à la marée au fond de la mer (fig. 2) et on le compare au même gradient produit par le réchauffement diurne combiné avec la turbulence des courants de surface dûs au vent (fig. 3). La profondeur à laquelle ces deux gradients sont identiques fixe la limite supérieure de la couche de mélange voisine du fond. La distribution de cette couche dans la Mer du Nord ressort de deux profils relevés par le navire océanographique Gauss en août 1953 (fig. 1). L'un des profils va de l'extrémité septentrionale de cette mer jusqu'aux îles occidentales de la Frise; l'autre s'étend de la côte écossaise à celle de la Norvège (fig. 4). De cette façon une étude de tous les caractères estivaux de la Mer du Nord met en évidence le passage de la faible couche de mélange dans le Nord à la couche très importante dans la région moyenne; cette dernière couche a pour effet de réduire à quelques décimètres la couche de transition estivale normale dans les régions à courants de marée profonds tandis qu'elle pénètre dans la couche de mélange de surface dans les régions à forts courants de marée. En vertu de ces processus, l'absence de stratification se maintient pendant toute l'année dans de vastes régions. Les effets variés et lointains de ces phénomènes de mélange se font nettement sentir, entre autres, sur la variation annuelle de la température de surface dans la Mer du Nord (fig. 5 et 6).

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Instruments, Detectors and the Future of Astronomy with Large Ground Based Telescopes

Results of a survey of instrumentation and detector systems, either currently deployed or planned for use at telescopes larger than 3.5 m, in ground based observatories world-wide, are presented. This survey revealed a number of instrumentation design trends at optical, near, and mid-infrared wavelengths. Some of the most prominent trends include the development of vastly larger optical detector systems (> 10⁹ pixels) than anything built to date, and the frequent use of mosaics of near-infrared detectors – something that was quite rare only a decade ago in astronomy. Some future science applications for detectors are then explored, in an attempt to build a bridge between current detectors and what will be needed to support the research ambitions of astronomers in the future.     

Cosmic-ray positrons: are there primary sources?

Galactic cosmic rays consist of primary and secondary particles. Primary cosmic rays are thought to be energized by first order Fermi acceleration processes at supernova shock fronts within our Galaxy. The cosmic rays that eventually reach the Earth from this source are mainly protons and atomic nuclei, but also include electrons. Secondary cosmic rays are created in collisions of primary particles with the diffuse interstellar gas. They are relatively rare but carry important information on the Galactic propagation of the primary particles. The secondary component includes a small fraction of antimatter particles, positrons and antiprotons. In addition, positrons and antiprotons may also come from unusual sources and possibly provide insight into new physics. For instance, the annihilation of heavy supersymmetric dark matter particles within the Galactic halo could lead to positrons or antiprotons with distinctive energy signatures. With the High-Energy Antimatter Telescope (HEAT) balloon-borne instrument, we have measured the abundances of positrons and electrons at energies between 1 and 50 GeV. The data suggest that indeed a small additional antimatter component may be present that cannot be explained by a purely secondary production mechanism. Here we describe the signature of the effect and discuss its possible origin.     

The Visible Solar Spectral Irradiance from 350 to 850 nm As Measured by the SOLSPEC Spectrometer During the ATLAS I Mission

The SOLSPEC instrument has been built to carry out solar spectral irradiance measurements from 200 to 3000 nm. It consists of three spectrometers designed to measure the solar spectral irradiance in ultraviolet, visible, and infrared domains. It flew with the ATLAS I mission in March 1992. This paper is dedicated to the visible part of the solar spectrum. Comparisons with recent data are shown and differences below 450 nm are discussed.     

Asymmetry and variations of solar limb darkening along the diameter defined by diurnal motion in April 1981

Unexpected asymmetries and variations, which showed up in the first, preliminary reductions of new limb-darkening observations made in June 1986, near the present minimum of solar activity, stimulated a re-analysis of our limb-darkening observations made in April 1981 at Kitt Peak (Neckel and Labs, 1984). The results seem to indicate rather definitely that the intensity distribution across the disk varies at all observed wavelengths between 3300 and 6600 Å with amplitudes in the order of 1–2 % and time-scales from minutes to hours. Asymmetries in the intensity distribution with respect to the disk center are a frequent phenomenon. There can be no doubt, that also the absolute disk center intensity undergoes variations with comparable size and modulation, as they were in fact recently observed by Koutchmy and Lebecq (1986). Presumably, the solar oscillations contribute a significant part to the observed effects. However, due to the 5–7 min periodicity of our observations no definite conclusions can be drawn.Examples for widely differing limb darkening curves - at the same wavelength! - are given in Figure 7. Figure 8 illustrates the variations in time by exhibiting the differences between (north-) western and (south-) eastern radius for a larger number of selected, successive scans, regardless of wavelength. Figure 9 displays some differences between observed limb-to-limb distributions and special, symmetric reference curves.     

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