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41.
The Boconó fault system is a major NE-SW, right-lateral strike-slip tectonic feature whose trace extends northeastward for 500 km, from the Tachira depression at the Colombian-Venezuelan border (near the city of Cúcuta) to the town of Morón (located on the Caribbean coast of Venezuela), within the Venezuelan (or Mérida) Andes, and slightly oblique to its main axis. The Boconó fault is morphologically expressed by a continuous straight alignment of longitudinal valleys, linear depressions, pull-apart basins, fault scarps, trenches, sag-ponds, linear ridges and saddles that suggest that this major tectonic feature is active. Moreover, several destructive earthquakes (e.g., 1610, 1812, 1894, 1932 and 1950) affecting the Andean region have been usually attributed to it, without any geological confirmation. Therefore, exploratory trenching on this major fault, the only reliable means of corroborating seismotectonic associations, were carried out at two different sites: slightly north of La Grita and few kilometres north of Cordero (Fundo Mis Delirios); both villages being located between the cities of Mérida and San Cristobal, in the southern part of the Venezuelan Andes. Both trenches revealed that the Boconó fault system has been active during Holocene time. On the one hand, the La Grita trench has particularly demonstrated that: a) the 1610 and 1894 earthquakes occurred along the single trace of the Boconó fault in this region; b) the magnitude of those two earthquakes can be estimated between M = 7.1 and 7.3; c) their return period is about 300 yr; and d) the Holocene oblique-slip rate ranges between 4.3 and 6.1 mm/yr (5.2 ± 0.9 mm/yr) along this segment of the fault system. On the other hand, the Boconó fault at the Mis Delirios trench does not show any deformation associated with the 1610, 1894 or any other historical earthquakes. The complexity of the Boconó fault trace—three active strands have been mapped around the Mis Delrios trench—may account for the lack of such recent ruptures on the excavated strand. However, the occurrence of two to three previous Holocene earthquakes on this fault strand is recorded in the alluvial deposits dug at the trench site. 相似文献
42.
Within the framework of a model of liquid immiscibility in the outer core, we calculate a stably stratified layer about 11 km thick near the core-mantle boundary and discuss its reflection and scattering properties for seismic waves. 相似文献
43.
For over 50 years, several models based on diverse geologic concepts and variable quality of data have been proposed to explain the major structure and history of the Mérida Andes (MA), in western Venezuela. Lately, this chain growth and associated flexural basins deepening have been related to incipient type-A subductions of either polarity, accounting for the across-chain asymmetry. However, these recent models have not well integrated the present tectonically active setting driven by neighboring major plate interactions. At present, this chain exhibits ongoing strain partitioning where cumulative right-lateral slip along chain axis is as much as half of, or about the same, as the transverse shortening since late Miocene, thus implying that the NNE-directed Maracaibo block extrusion with respect to the South America (SA) plate is not a secondary feature. Consequently, this paper discusses some limitations exhibited by the SE-directed continental subduction models—Maracaibo crust underthrusting the Mérida Andes—in the light of available geological and geophysical data. Besides, it is herein proposed that the Mérida Andes structuration is related to a NW-directed, gently dipping, incipient type-A subduction, where chain growth and evolution are similar to those of a sedimentary accretionary wedge (i.e., Barbados), but at crustal scale and with ongoing strain partitioning. This continental subduction is the SE portion of a major orogenic float that also comprises the Perijá range and the Santa Marta block. 相似文献
44.
Pulleniatina obliquiloculata shells from 16 core-top samples from the tropical Indo-Pacific Oceans are analyzed for the ratios of boron and cadmium to calcium(B/Ca and Cd/Ca). The B/Ca ratios show a very weak positive relationship with [B(OH)_4~-] and the dissolved carbonate species at the apparent calcification depth of P.obliquiloculata. The boron partition coefficients(K_D) between P. obliquiloculata B/Ca and seawater[B(OH)_4~-]/[HCO_3~-] distribute around 1.1×10~(-3)-1.3×10~(-3) with a mean value of(1.19±0.12)×10~(-3),and are significantly related to the nutrient concentration, especially phosphate. The lack of any clear correlation between the P. obliquiloculata B/Ca and seawater carbonate chemical parameters suggests that the physiochemical controls on boron incorporation are masked by the complexity of natural seawater condition. But the significant dependence of Ku on nutrient may likely be explained by a nutrient related growth-rate effect. Cd/Ca of P.obliquiloculata shows significant correlation with seawater phosphate concentration, and its partition coefficients(D_(cd)) are significantly related to temperature. A first-principle methodology of P. obliquiloculata B/Ca is applied;with the aid of Cd/Ca as a phosphate proxy and a constraint on K_D, to estimating sea water carbonate chemistry(e.g., pH). The results are fairly promising and allow us to propose the possibility to apply the combination of B/Ca and Cd/Ca proxies(and also Mg/Ca and δ~(18)O for estimating temperature and salinity) for the paleoreconstruction of seawater carbonate chemistry. 相似文献
45.
A worldwide database of liquefaction-induced sand blows has been compiled to generate empirical relationships between earthquake
parameters and sand blow geometries. Curves resulting from these empirical relationships can be used afterwards for the study
of historic and pre-historic earthquakes that formed sand blows. This database only incorporates instrumental earthquakes
inducing sand blows, characterised in terms of magnitude (Mw), hypocentral location and focal mechanism solution. Two relationships
are herein presented. The first curve, minimum likelihood of liquefaction occurrence (magnitude/epicentral distance), displays
a logarithmic behaviour, as others already proposed. The second curve of sand-blow diameter vs epicentral distance is the
first proposal of this kind, whose shape follows a negative power-law. Magnitude estimations of (pre-)historic earthquakes
then may be attempted through these empirical relationships. Resulting magnitudes derived from these curves should be mostly
considered as underestimated. The curves will reflect actual magnitudes only if, correspondingly, the farthest and largest
blow has ever been reported during the after-earthquake survey. Following the same principle, a magnitude estimation resulting
from the measuring of a sand blow can only be considered as minimal because it is almost impossible to ascertain that the
measured feature is the biggest one produced by the earthquake under evaluation. Finally, these results call for thorough
surveys of induced effects after every future moderate-to-large earthquake, as any empirical relationship simply improves
by incorporating new data. 相似文献
46.
Franck Humbert Guy Libourel Christian France-Lanord Laurent Zimmermann Bernard Marty 《Geostandards and Geoanalytical Research》2000,24(2):255-260
A new installation for the analysis of picomole quantities of nitrogen has been designed and constructed. It permits the simultaneous analysis of N and rare gases extracted from silicates by heating. The extraction procedure involves the use of a CO2 laser as a heater, and the analysis is made by static mass spectrometry using a high sensitivity, high resolution mass spectrometer. Procedural blanks of 2 picomoles N2 (60 picogram N) allow us to analyse routinely N in mg-sized samples such as mantle-derived and meteoritic minerals, and Lunar soil grains. 相似文献
47.
Although iron isotopes provide a new powerful tool for tracing a variety of geochemical processes, the unambiguous interpretation of iron isotope ratios in natural systems and the development of predictive theoretical models require accurate data on equilibrium isotope fractionation between fluids and minerals. We investigated Fe isotope fractionation between hematite (Fe2O3) and aqueous acidic NaCl fluids via hematite dissolution and precipitation experiments at temperatures from 200 to 450 °C and pressures from saturated vapor pressure (Psat) to 600 bar. Precipitation experiments at 200 °C and Psat from aqueous solution, in which Fe aqueous speciation is dominated by ferric iron (FeIII) chloride complexes, show no detectable Fe isotope fractionation between hematite and fluid, Δ57Fefluid-hematite = δ57Fefluid − δ57Fehematite = 0.01 ± 0.08‰ (2 × standard error, 2SE). In contrast, experiments at 300 °C and Psat, where ferrous iron chloride species (FeCl2 and FeCl+) dominate in the fluid, yield significant fluid enrichment in the light isotope, with identical values of Δ57Fefluid-hematite = −0.54 ± 0.15‰ (2SE) both for dissolution and precipitation runs. Hematite dissolution experiments at 450 °C and 600 bar, in which Fe speciation is also dominated by ferrous chloride species, yield Δ57Fefluid-hematite values close to zero within errors, 0.15 ± 0.17‰ (2SE). In most experiments, chemical, redox, and isotopic equilibrium was attained, as shown by constancy over time of total dissolved Fe concentrations, aqueous FeII and FeIII fractions, and Fe isotope ratios in solution, and identical Δ57Fe values from dissolution and precipitation runs. Our measured equilibrium Δ57Fefluid-hematite values at different temperatures, fluid compositions and iron redox state are within the range of fractionations in the system fluid-hematite estimated using reported theoretical β-factors for hematite and aqueous Fe species and the distribution of Fe aqueous complexes in solution. These theoretical predictions are however affected by large discrepancies among different studies, typically ±1‰ for the Δ57Fe Fe(aq)-hematite value at 200 °C. Our data may thus help to refine theoretical models for β-factors of aqueous iron species. This study provides the first experimental calibration of Fe isotope fractionation in the system hematite-saline aqueous fluid at elevated temperatures; it demonstrates the importance of redox control on Fe isotope fractionation at hydrothermal conditions. 相似文献
48.
A HF‐free sample preparation method was used to purify silicon in twelve geological RMs. Silicon isotope compositions were determined using a Neptune instrument multi‐collector‐ICP‐MS in high‐resolution mode, which allowed separation of the silicon isotope plateaus from their interferences. A 1 μg g‐1 Mg spike was added to each sample and standard solution for online mass bias drift correction. δ30Si and δ29Si values are expressed in per mil (‰), relative to the NIST SRM 8546 (NBS‐28) international isotopic RM. The total variation of δ30Si in the geological reference samples analysed in this study ranged from ‐0.13‰ to ‐0.29‰. Comparison with δ29Si values shows that these isotopic fractionations were mass dependent. IRMM‐17 yielded a δ30Si value of ‐1.41 ± 0.07‰ (2s, n = 12) in agreement with previous data. The long‐term reproducibility for natural samples obtained on BHVO‐2 yielded δ30Si = ‐0.27 ± 0.08‰ (2s, n = 42) on a 12 month time scale. An in‐house Si reference sample was produced to check for the long‐term reproducibility of a mono‐elemental sample solution; this yielded a comparable uncertainty of ± 0.07‰ (2s, n = 24) over 5 months. 相似文献
49.
50.
J.F. Perez-Quezada C.A. DelpianoK.A. Snyder D.A. JohnsonN. Franck 《Journal of Arid Environments》2011,75(1):29-37
The pattern of carbon (C) allocation among the different pools is an important ecosystem structural feature, which can be modified as a result of changes in environmental conditions that can occur gradually (e.g., climatic change) or abruptly (e.g., management practices). This study quantified the C pools of plant biomass, litter and soil in an arid shrubland in Chile, comparing the natural condition (moderately disturbed by grazing) vs. the afforested condition (two-year-old plantation with Acacia saligna (Labill.) H.L. Wendl.), each represented by a 60 ha plot. To estimate plant biomass, allometric functions were constructed for the four dominant woody species, based on the volume according to their shape, which showed high correlation (R2 > 0.73). The soil was the largest C pool in both natural and afforested conditions (89% and 94%, respectively) and was significantly lower in the afforested than natural condition at all five soil depths. The natural condition had in total 36.5 ton (t) C ha−1 compared to 21.1 t C ha−1 in the afforested condition, mainly due to C loss during soil preparation, prior to plantation of A. saligna. These measurements serve as an important baseline to assess long-term effects of afforestation on ecosystem C pools. 相似文献