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41.
We investigated grain growth of calcite aggregates fabricated from crushed natural single crystals with different impurity
content. The total trace-element concentration of the starting powders varied from about 170 ppm to more than 930 ppm with
Mn as the major component. Samples were produced by hot-isostatic pressing of the different powders at 300 MPa confining pressure
at 600 °C for 2 h. The starting material for the anneals was dry and had a uniform microstructure with an average grain size
of about 5 μm and a porosity of <2.1%. Three disks with Mn concentrations of 10, 350, and 670 ppm, respectively, were annealed
in the same run at a confining pressure of 300 MPa, and temperatures between 700 and 900 °C for up to 20 h. Grain growth was
fastest in samples with the highest Mn concentrations. A multivariable fit to the data yields grain-growth exponents of 2.0
± 0.3 for samples with 10 ppm Mn and 2.3 ± 0.2 for those with 670 ppm Mn. The activation energies for grain growth vary from
99 ± 12 kJ mol−1 to 147 ± 14 kJ mol−1 for the respective calcite compositions.
Received: 22 August 2000 / Accepted: 12 March 2001 相似文献
42.
Active faulting in the dead sea rift 总被引:8,自引:0,他引:8
Manifestations of Late Quaternary and Holocene faulting were studied in a 500 km long segment of the Dead Sea transform (rift). Most prominent are left-slip faults, whose characteristic physiographic features are recognizable along most of the studied segment. Where these faults bend or are stepped to the left, rhomb-shaped grabens (or pull aparts) are produced, forming depressions. In the reverse situation compressional features such as pressure ridges, domes and folds form positive topographic features. Such structures are combined on a variety of scales ranging from a few hundred meters long to tens of kilometers. Normal faults, sub-parallel to the left slip faults, produce a trough-like valley along much of the Dead Sea transform, but are most prominent along the margins of the large rhomb-grabens, e.g., the Dead Sea trough. They apparently record a small component of transverse extension. Generally, their motion is slow: young slip did not occur along some segments during the last few 104 y. Elsewhere throws of 10–20 m at least occurred in this period. The Dead Sea transform is seismically active. The instrumental and historic records indicate a seismic slip rate of 0.15–0.35 cm/y during the last 1000–1500 y, while estimates of the average Pliocene—Pleistocene rate are 0.7–1.0 cm/y. Either much creep takes place, or the slip rate varies over periods of a few 103 y. 相似文献
43.
J. Dorschner C. Friedemann J. Gürtler H. Oleak K. -H. Schmidt 《Astrophysics and Space Science》1972,19(2):263-270
An attempt is made to explain the infrared radiation observed for several quasars and Seyfert galaxies as thermal radiation of a dust envelope surrounding the cores of these objects. Two kinds of dust particles (graphite and silica) are taken into consideration. It is shown that the observed spectral behaviour and the luminosity in the infrared can be introduced as thermal radiation of silica grains. In the case of 3C 273 one finds that the radius of the dust envelope is about 50 pc and the total mass of dust is about 600M ⊙. 相似文献
44.
Interplay between tectonic, fluvial and erosional processes along the Western Border Fault of the northern Upper Rhine Graben, Germany 总被引:3,自引:1,他引:3
Gwendolyn Peters Thies J. Buchmann Peter Connolly Ronald T. van Balen Friedemann Wenzel Sierd A.P.L. Cloetingh 《Tectonophysics》2005,406(1-2):39-66
The northern Upper Rhine Graben, situated in the central part of the European Cenozoic rift system, is currently characterized by low intra-plate seismicity. Historical earthquakes have not been large enough to produce surface rupturing. Moreover, the records of Quaternary surface processes and human modifications are presumably better preserved than the record of the relatively slow tectonic deformation.In order to gain information on the neotectonic activity and paleoseismicity in this setting, the geological and geomorphological records of fault movements along a segment of the Western Border Fault (WBF) were studied using an integration of techniques in paleoseismology, structural analysis and shallow geophysics. The WBF segment investigated follows a 20 km long linear scarp of unclear origin. A series of geophysical measurements were performed and the results suggested that near-surface deformation structures are present at the segments' southern end. Several trenches opened at this location revealed fault structures with consistent extensional style and a maximum vertical displacement of 0.5 m. In one trench, the deformation structures were dated between 19 and 8 ka. Assuming the deformation has been caused by an earthquake, a Mw 6.5 earthquake would be implied. Aseismic deformation would point to a fault creep rate ≥ 0.04 mm/yr.A reconstruction of the sequence of events at the trench site, from Middle Pleistocene to Present, demonstrates that the morphology at the base of the scarp is the result of interplay between tectonic activity and fluvial and erosional processes. At the regional scale, a mixed origin for the WBF scarp is proposed, combining the effects of fluvial dynamics, erosion, regional uplift and localized tectonic activity on the WBF. 相似文献
45.
Bodo Freund 《GeoJournal》1986,13(1):67-73
S European Industry is characterized by the importance of certain branches and by plant size. Traditional branches of consumer goods are dominated by the national bourgeoisie, intermediate goods by oligopolistic (state) enterprises, modern consumer and capital goods by multinational corporations. Industrialization was retarded for various social and physical reasons. For the locational pattern some factors had specific importance (diffusion, shipping traffic for supplies, deficient infrastructure, urbanisation economies, social structure). Economic nationalism and interventionism from the thirties to the late fifties had structural and locational effects. Subsequent opening led to important foreign investment with diverse locational consequences: the large majority of the market-orientated companies concentrated on big cities and induced industrial suburbanisation. Export-orientated plants, relatively often in Portugal, also decided for rural areas and provincial towns. Regional industrial policy which began weakly with fiscal allowances in the fifties and soon adopted the growth pole idea led to results remaining only far beyond expectations. International economic crisis has heavily stroken the countries which turned out to be very dependant. 相似文献
46.
The study was implemented in the framework of the DFNK (Deutsches Forschungsnetz Naturkatastrophen – German Research Network
Natural Disasters) project. The area around the city of Cologne was chosen as an object for assessment of seismic hazard and
risk. A comprehensive geo-database was compiled for the area, which allows using a computational approach for analysis of
site amplification functions and probable surface effects. Taking into account peculiarities of the local soil conditions,
the analysis of site effects included both amplification of ground motions and liquefaction potential. These phenomena, having
different nature, are interrelated and can considerably contribute to seismic hazard. This paper presents some results obtained
on the base of computational analysis using geo-modeling. 相似文献
47.
Adrien Oth Friedemann Wenzel Hillel Wust-Bloch Ellen Gottschämmer Zvi Ben-Avraham 《Pure and Applied Geophysics》2007,164(1):23-37
3-D simulations of elastic wave propagation generated by earthquakes with magnitudes between 5.5 and 7.0 are used to parameterize
strong ground motion attenuation relations for the Dead Sea Rift (DSR) graben structure. The results show that standard attenuation
relations with an isotropic distance parameter are inadequate for a graben structure with a deep sedimentary trough. A new
strategy is devised for the parameterization of attenuation relations in graben structures by looking at the statistical properties
of 53 simulated earthquakes of variable magnitudes located at various sites along the western boundary fault of the DSR graben.
An exemplary attenuation relation is designed from the synthetics for the 1 Hz spectral acceleration, modifying the Joyner-Boore-type
parametrization by adding coefficients suited for three different source-to-sensor configurations: within the graben, beyond
the graben and path unaffected by the graben structure. 相似文献
48.
49.
Nicole Kowalski Olaf Dellwig Melanie Beck Maik Grunwald Sibylle Fischer Maike Piepho Thomas Riedel Holger Freund Hans-Jürgen Brumsack Michael E. Böttcher 《Ocean Dynamics》2009,59(2):333-350
Tidal and seasonal behaviour of the redox-sensitive trace metals Mn, Fe, Mo, U, and V have been investigated in the open-water
column and shallow pore waters of the backbarrier tidal flats of the island of Spiekeroog (Southern North Sea) in 2002 and
2007. The purpose was to study the response of trace metal cycles on algae blooms, which are assumed to cause significant
changes in the redox state of the entire ecosystem. Trace metal data were complemented by measurements of nutrients and enumeration
of algae cells in 2007. Generally, Mn and V show a tidal cyclicity in the water column with maximum values during low tide
which is most pronounced in summer due to elevated microbial activity in the sediments. Mo and U behave almost conservatively
throughout the year with slightly increasing levels towards high tide. Exceptions are observed for both metals after summer
algae blooms. Thus, the seasonal behaviour of the trace metals appear to be significantly influenced by productivity in the
water column as the occurrence of algae blooms is associated with an intense release of organic matter (e.g. transparent exopolymer
particles, TEP) thereby forming larger organic-rich aggregates. Along with elevated temperatures in summer, the deposition
of such aggregates favours microbial activity within the surface sediments and release of DOC, nutrients and trace metals
(Mn, Mo and V) during the degradation of the aggregates. Additionally, pronounced reducing conditions lead to the reduction
of Mn(IV)-oxides and Fe(III)-(oxihydr)oxides, thereby releasing formerly scavenged compounds as V and phosphate. Therefore,
pore-water profiles show significant enrichments in trace metals especially from July to September. Finally, the trace metals
are released to the open water column via draining pore waters (esp. Mo, Mn, and V) and/or fixed in the sediment as sulphides
(Fe, Mo) and bound to organic matter (U). Non-conservative behaviour of Mo in oxygenated seawater, first observed in the investigation
area by Dellwig et al. (Geochim Cosmochim Acta 71:2745–2761, 2007a), was shown to be a recurrent phenomenon which is closely coupled to bacterial activity after the breakdown of algae blooms.
In addition to the postulated fixation of Mo in oxygen-depleted micro-zones of the aggregates or by freshly formed organic
matter, a direct removal of Mo from the water column by reduced sediment surfaces may also play an important role. 相似文献
50.
Friedemann T. Freund Ipek G. Kulahci Gary Cyr Julia Ling Matthew Winnick Jeremy Tregloan-Reed Minoru M. Freund 《Journal of Atmospheric and Solar》2009,71(17-18):1824-1834
Pre-earthquake signals have been widely reported, including perturbations in the ionosphere. These precursory signals, though highly diverse, may be caused by just one underlying physical process: activation of highly mobile electronic charge carriers in rocks that are subjected to ever increasing levels of stress. The charge carriers are defect electrons associated with O? in a matrix of O2?. Known as positive holes or pholes h, they flow out of the stressed rock into the unstressed rock volume, traveling meters in the laboratory, probably kilometers in the field. At the rock–air interface they cause: (i) positive surface potential, (ii) field-ionization of air molecules, (iii) corona discharges. The rate of formation of airborne ions can exceed 109 cm?2 s?1. Massive air ionization prior to major earthquakes increases the electrical conductivity in the air column and may cause ionospheric perturbations, earthquake lights, and unusual animal behavior as well as infrared emission. 相似文献