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101.
C. P. Rajendran Jaishri Sanwal Kristin D. Morell Mike Sandiford B. S. Kotlia John Hellstrom Kusala Rajendran 《Journal of Seismology》2016,20(2):579-594
The central part of the Himalaya (Kumaun and Garhwal Provinces of India) is noted for its prolonged seismic quiescence, and therefore, developing a longer-term time series of past earthquakes to understand their recurrence pattern in this segment assumes importance. In addition to direct observations of offsets in stratigraphic exposures or other proxies like paleoliquefaction, deformation preserved within stalagmites (speleothems) in karst system can be analyzed to obtain continuous millennial scale time series of earthquakes. The Central Indian Himalaya hosts natural caves between major active thrusts forming potential storehouses for paleoseismological records. Here, we present results from the limestone caves in the Kumaun Himalaya and discuss the implications of growth perturbations identified in the stalagmites as possible earthquake recorders. This article focuses on three stalagmites from the Dharamjali Cave located in the eastern Kumaun Himalaya, although two other caves, one of them located in the foothills, were also examined for their suitability. The growth anomalies in stalagmites include abrupt tilting or rotation of growth axes, growth termination, and breakage followed by regrowth. The U-Th age data from three specimens allow us to constrain the intervals of growth anomalies, and these were dated at 4273?±?410 years BP (2673–1853 BC), 2782?±?79 years BP (851–693 BC), 2498?±?117 years BP (605–371 BC), 1503?±?245 years BP (262–752 AD), 1346?±?101 years BP (563–765 AD), and 687?±?147 years BP (1176–1470 AD). The dates may correspond to the timings of major/great earthquakes in the region and the youngest event (1176–1470 AD) shows chronological correspondence with either one of the great medieval earthquakes (1050–1250 and 1259–1433 AD) evident from trench excavations across the Himalayan Frontal Thrust. 相似文献
102.
103.
Wood load, channel parameters and valley parameters were surveyed in 50 contiguous stream segments each 25 m in length along 12 streams in the Colorado Front Range. Length and diameter of each piece of wood were measured, and the orientation of each piece was tallied as a ramp, buried, bridge or unattached. These data were then used to evaluate longitudinal patterns of wood distribution in forested headwater streams of the Colorado Front Range, and potential channel‐, valley‐ and watershed‐scale controls on these patterns. We hypothesized that (i) wood load decreases downstream, (ii) wood is non‐randomly distributed at channel lengths of tens to hundreds of meters as a result of the presence of wood jams and (iii) the proportion of wood clustered into jams increases with drainage area as a result of downstream increases in relative capacity of a stream to transport wood introduced from the adjacent riparian zone and valley bottom. Results indicate a progressive downstream decrease in wood load within channels, and correlations between wood load and drainage area, elevation, channel width, bed gradient and total stream power. Results support the first and second hypotheses, but are inconclusive with respect to the third hypothesis. Wood is non‐randomly distributed at lengths of tens to hundreds of meters, but the proportion of pieces in jams reaches a maximum at intermediate downstream distances within the study area. We use these results to propose a conceptual model illustrating downstream trends in wood within streams of the Colorado Front Range. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
104.
Kristin D. Morell Donald M. Fisher Thomas W. Gardner 《Earth and Planetary Science Letters》2008,265(1-2):82-95
Subduction of the right-lateral Panama Fracture Zone, along the convergent margin of Central America creates abrupt lateral variations in convergence rate, obliquity, and subducting crustal thickness at its intersection with the Middle America Trench. This intersection, known as the Panama (CO-NZ-CA) Triple Junction, is migrating to the southeast at a rate of 55 mm/yr, and currently coincides with the lateral termination of the Fila Costeña Thrust Belt in the inner forearc of the overriding plate. Mapping in the inner forearc in the area that straddles the subducting Panama Fracture Zone reveals that Cocos–Caribbean convergence west of the triple junction leads to the development of an inner forearc thrust belt inboard of the colliding Cocos Ridge, while little deformation is evident inboard of Nazca–Caribbean convergence, east of the triple junction. This results in the lateral termination of the Fila Costeña Thrust Belt in the region of the forearc that projects over the Panama Fracture Zone, where four out of five mapped thrust faults tip out and are buried by lahars. Three new balanced cross-sections indicate a steep gradient in shortening from the center of the thrust belt to its southeastern termination. The short-term history of the inner forearc recorded in the landscape and topography of the Fila Costeña is consistent with the southeastward migration of the thrust belt and the Panama Triple Junction throughout the past ~ 3 Ma, with evidence for the growth of a new topographic divide and reorganization of stream channel networks. 相似文献
105.
Haranas Ioannis Gkigkitzis Ioannis Cobbett Kristin Hovesen Gregory Shah Kay Gauthier Ryan Cavan Eli 《Astrophysics and Space Science》2021,366(9):1-16
Astrophysics and Space Science - We performed a detailed timing study of the Atoll source 4U 1705-44 in order to understand the accretion disk geometry. Cross correlation function (CCF) studies... 相似文献
106.
Cameron M. Mercer Kip V. Hodges Bradley L. Jolliff Matthijs C. Van Soest Jo‐Anne Wartho John R. Weirich 《Meteoritics & planetary science》2019,54(4):721-739
40Ar/39Ar incremental heating experiments on whole‐rock lunar samples commonly provide evidence of varying degrees of radiogenic 40Ar (40Ar*) loss. However, these experiments provide limited information about whether or not 40Ar* is preferentially lost from specific glasses, minerals, or polyphase domains. Ultraviolet laser ablation microprobe (UVLAMP) 40Ar/39Ar dating and electron probe microanalysis of mineral clasts and polyphase melt assemblages in Apollo 17 poikilitic impact melt rock 77135 show evidence of geochemical controls on 40Ar/39Ar dates. Potassium‐rich glass and K‐feldspar in the mesostasis are the dominant sources for Ar released during low‐temperature steps of published 40Ar/39Ar release spectra for this rock, while pyroxene oikocrysts with enclosed plagioclase chadacrysts contribute Ar predominantly to intermediate‐ to high‐temperature steps. Additionally, UVLAMP analysis of a mm‐scale plagioclase clast demonstrates the potential to use stranded 40Ar* diffusive loss profiles to constrain the thermal evolution of lunar impact melt deposits and indicates that the melt component of 77135 cooled quickly. While some submillimeter clasts of plagioclase are distinctly older than the melt, other small clasts yield dates younger than the oldest melt components in 77135, plausibly due to subgrain fast diffusion pathways and/or 40Ar* loss during brief episodes of reheating at high temperatures. Our data suggest that integrated petrologic and microanalytical geochronologic studies are necessary complements to bulk sample geochronologic studies in order to fully evaluate competing models for the impactor flux during the first billion years of the Moon's evolution. 相似文献
107.
Homogeneous, nonrotating flow over a backward-facing rounded step is simulated using the 2D vertical version of two general
circulation models, a z-coordinate model—the Massachusetts Institute of Technology general circulation model (MITgcm)—and a σ-coordinate model—the Bergen Ocean Model (BOM). The backward-facing step is a well-known testcase since it is geometrically
simple but still embodies important flow characteristics such as separation point, reattachment length, and recirculation
of the flow. The study compares the core of the two models and uses constant eddy viscosities and diffusivities. The Reynolds
numbers ranges from 2·102 to 2·106. The results correspond with previously published results having a relatively stationary separation point and a fluctuating
reattachment length due to downslope propagating eddies released from the reattachment zone for Reynolds numbers higher than
or equal to 2 · 104. For Reynolds number within the laminar regime, the flow is stationary. The discrepancies between the models increase by
enhancing Reynolds numbers. The σ-coordinate model experiences a reduction in eddy sizes with increasing resolution and Reynolds numbers in correspondence
with published experiments, while the size of the eddies are independent of the Reynolds number using the MITgcm. Due to mixing
generated by the staircase topography, the z-coordinate model gives a better convergence of the separation point and reattachment length compared with the BOM; however,
this conclusion might change with the inclusion of a relevant turbulence scheme. 相似文献
108.
Joyce Maschinski Michael S. Ross Hong Liu Joe O��Brien Eric J. von Wettberg Kristin E. Haskins 《Climatic change》2011,107(1-2):147-167
Low-elevation islands face threats from sea level rise (SLR) and increased storm intensity. Evidence of endangered species?? population declines and shifts in vegetation communities are already underway in the Florida Keys. SLR predictions indicate large areas of these habitats may be eliminated in the next century. Using the Florida Keys as a model system, we present a process for evaluating conservation options for rare and endemic taxa. Considering species characteristics and habitat, we assess central issues that influence conservation options. We contrast traditional and controversial options for two animal and two plant species giving special emphasis to perceptions of ecological risk and safety from SLR and suggest courses of action. Multiple strategies will be required to spread extinction risk and will be effective for different time periods. Global climate change presents an uncertain, perhaps no-analog future that will challenge land managers and practitioners to re-evaluate equilibrium-state-conceived laws and policies not only for these taxa, but for many facing similar threats. To embrace conservation in a changing world will require a new dialogue that includes controversial ideas, a review of existing laws and policies, and preparation for the oncoming change. 相似文献
109.
A systems dynamics model was developed to assess the planetary boundary for P supply in relation to use by human society. It is concluded that present day use rates and poor recycling rates of P are unsustainable at timescales beyond 100+ a. The predictions made suggest that P will become a scarce and expensive material in the future. The study shows clearly that market mechanisms alone will not be able to secure an efficient use before a large part of the resource will have been allowed to dissipate into the natural environment. It is suggested that population size management and effective recycling measures must be planned long term to avoid unpleasant consequences of hunger and necessary corrections imposed on society by mass balance and thermodynamics. 相似文献
110.
Donald P. Hauber Kristin Saltonstall David A. White Craig S. Hood 《Estuaries and Coasts》2011,34(4):851-862
Multiple introductions are believed to play an important role in increasing genetic diversity and adaptability of invasive
species, but there are few well-documented examples. The common reed, Phragmites australis, has dramatically increased in tidal wetlands throughout the USA in the past century due primarily to the introduction of
a Eurasian lineage. In the Mississippi River “Balize” delta, P. australis is the dominant vegetation where monotypic stands of an introduced form blanket the outer marshes. The delta’s interior marshes,
on the other hand, are more vegetatively diverse, serving as important waterfowl foraging habitat. Recent encroachment by
various phenotypic forms of P. australis into the interior marshes led to this study examining genetic variation in these stands. Our results revealed four chloroplast
DNA haplotypes that also segregate based on microsatellite variation. Three of these are closely related and introduced, but
differ relative to time and likely mode of introduction. The “Delta” type (haplotype M1), which is unique to the region and
the most common lineage, displays considerable microsatellite diversity. The Eurasian introduced lineage of P. australis (haplotype M), which is invasive elsewhere in North America, is increasing its distribution in the delta. A novel haplotype,
AD, was also identified which is phenotypically and genetically similar to haplotype M. Despite the close relatedness, we
found no evidence for inter-haplotype gene exchange at the nuclear level, suggesting that intraspecific hybridization is not
a contributing factor to these invasions. The site provides a unique opportunity for researchers to understand the dynamics
of multiple P. australis invasions. 相似文献