全文获取类型
收费全文 | 24403篇 |
免费 | 173篇 |
国内免费 | 916篇 |
专业分类
测绘学 | 1410篇 |
大气科学 | 1977篇 |
地球物理 | 4501篇 |
地质学 | 11588篇 |
海洋学 | 1003篇 |
天文学 | 1633篇 |
综合类 | 2161篇 |
自然地理 | 1219篇 |
出版年
2020年 | 1篇 |
2018年 | 4762篇 |
2017年 | 4037篇 |
2016年 | 2577篇 |
2015年 | 234篇 |
2014年 | 81篇 |
2013年 | 25篇 |
2012年 | 989篇 |
2011年 | 2729篇 |
2010年 | 2014篇 |
2009年 | 2310篇 |
2008年 | 1889篇 |
2007年 | 2360篇 |
2006年 | 52篇 |
2005年 | 194篇 |
2004年 | 402篇 |
2003年 | 409篇 |
2002年 | 249篇 |
2001年 | 47篇 |
2000年 | 51篇 |
1999年 | 13篇 |
1998年 | 21篇 |
1981年 | 21篇 |
1980年 | 19篇 |
1976年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
Iunio Iervolino Carmine Galasso Edoardo Cosenza 《Bulletin of Earthquake Engineering》2010,8(2):339-362
In code-based seismic design and assessment it is often allowed the use of real records as an input for nonlinear dynamic
analysis. On the other hand, international seismic guidelines, concerning this issue, have been found hardly applicable by
practitioners. This is related to both the difficulty in rationally relating the ground motions to the hazard at the site
and the required selection criteria, which do not favor the use of real records, but rather various types of spectrum matching
signals. To overcome some of these obstacles a software tool for code-based real records selection was developed. REXEL, freely
available at the website of the Italian network of earthquake engineering university labs (), allows to search for suites of waveforms, currently from the European Strong-motion Database, compatible to reference spectra
being either user-defined or automatically generated according to Eurocode 8 and the recently released new Italian seismic
code. The selection reflects the provisions of the considered codes and others found to be important by recent research on
the topic. In the paper, record selection criteria are briefly reviewed first, and then the algorithms implemented in the
software are discussed. Finally, via some examples, it is shown how REXEL can effectively be a contribution to code-based
real records selection for seismic structural analysis. 相似文献
102.
Giuseppe D. Chirico Massimiliano Favalli Paolo Papale Enzo Boschi Maria Teresa Pareschi Arthur Mamou-Mani 《Bulletin of Volcanology》2009,71(4):375-387
Mt. Nyiragongo is one of the most dangerous volcanoes in the world for the risk associated with the propagation of lava flows.
In 2002 several vents opened along a huge system of fractures, pouring out lava which reached and destroyed a considerable
part of Goma, a town of about 500,000 inhabitants on the shore of Lake Kivu. In a companion paper (Favalli et al. in Bull
Volcanol, this issue, 2008) we employed numerical simulations of probable lava flow paths to evaluate the lava flow hazard on the flanks of the volcano,
including the neighbouring towns of Goma (DRC) and Gisenyi (Rwanda). In this paper we use numerical simulations to investigate
the possibility of significantly reducing the lava flow hazard in the city through the construction of protective barriers.
These barriers are added to the DEM of the area as additional morphological elements, and their effect is evaluated by repeating
numerical simulations with and without the presence of barriers. A parametric study on barrier location, size, shape and orientation
led to the identification of barriers which maximize protection while minimizing their impact. This study shows that the highest
hazard area corresponding to eastern Goma, which was largely destroyed by lava flows in 2002, cannot be effectively protected
from future lava flows towards Lake Kivu and should be abandoned. On the contrary, the rest of the town can be sheltered from
lava flows by means of two barriers that deviate or contain the lava within the East Goma sector. A proposal for the future
development of the town is formulated, whereby “new” Goma is completely safe from the arrival of lava flows originating from
vents outside its boundaries. The proposal minimizes the risk of further destruction in town due to future lava flows. 相似文献
103.
The seasonal nature of Australia’s tropical rivers means that connected groundwater aquifers are an important source of both
consumptive and non-consumptive water, particularly during the dry season. The management of these common pool groundwater
resources is one of the predominant water issues facing northern Australia. A national program of water reform stipulates
the expansion of water trading as a key instrument for water allocation. The effectiveness of new institutional arrangements
such as water markets will be determined mostly by how well they coordinate with local environmental requirements, local institutions
and local norms. This paper describes a novel application of combined field work, institutional analysis, experimental economics
and agent-based modeling to the analysis of a potential water market in the Katherine region of the Northern Territory, Australia.
The effectiveness of different versions of the policy instrument is assessed in light of local conditions. Instruments that
enable personal relationships and local institutions and norms to play a role in water management are found to be more effective
in terms of both farming income and environmental impact. 相似文献
104.
Probabilistic aftershock hazard assessment (PAHA) has been introduced by Wiemer (Geophys Res Lett 27:3405–3408, 2000). The method, in its original form, utilizes attenuation relations in evaluating peak ground velocity (PGV) exceedence probability.
We substitute the attenuation relations together with their uncertainties by strong ground motion simulations for a set of
scenarios. The main advantage of such an approach is that the simulations account for specific details of the aftershock source
effects (faulting style, slip distribution, position of the nucleation point, etc.). Mean PGVs and their standard deviations
are retrieved from the simulation results obtained by the new hybrid k-squared source model, and they are used for the PAHA analysis at a station under study. The model chosen for the testing
purposes is inspired by the Izmit A25 aftershock (M
w
= 5.8) that occurred 26 days after the mainshock. The PAHA maps are compared with (1) those obtained by the use of attenuation
relations and (2) the peak values of ten selected strong-motion recordings written by the aftershock at epicentral distances
<50 km. We conclude that, although the overall hazard decay with increasing fault distance is similar, the PAHA maps obtained
by the use of simulations exhibit remanent radiation pattern effect and prolongation in the strike direction due to the directivity
effect pronounced for some of the scenarios. As regard the comparison with real data, we conclude that the PAHA maps agree
with observed peak values due to appropriate attenuation model adopted in the analysis. 相似文献
105.
Truncation of the distribution of ground-motion residuals 总被引:1,自引:3,他引:1
Recent studies to assess very long-term seismic hazard in the USA and in Europe have highlighted the importance of the upper
tail of the ground-motion distribution at the very low annual frequencies of exceedance required by these projects. In particular,
the use of an unbounded lognormal distribution to represent the aleatory variability of ground motions leads to very high
and potentially unphysical estimates of the expected level of shaking. Current practice in seismic hazard analysis consists
of truncating the ground-motion distribution at a fixed number (ε
max) of standard deviations (σ). However, there is a general lack of consensus regarding the truncation level to adopt. This paper investigates whether
a physical basis for choosing ε
max can be found, by examining records with large positive residuals from the dataset used to derive one of the ground-motion
models of the Next Generation Attenuation (NGA) project. In particular, interpretations of the selected records in terms of
causative physical mechanisms are reviewed. This leads to the conclusion that even in well-documented cases, it is not possible
to establish a robust correlation between specific physical mechanisms and large values of the residuals, and thus obtain
direct physical constraints on ε
max. Alternative approaches based on absolute levels of ground motion and numerical simulations are discussed. However, the choice
of ε
max is likely to remain a matter of judgment for the foreseeable future, in view of the large epistemic uncertainties associated
with these alternatives. Additional issues arise from the coupling between ε
max and σ, which causes the truncation level in terms of absolute ground motion to be dependent on the predictive equation used. Furthermore,
the absolute truncation level implied by ε
max will also be affected if σ is reduced significantly. These factors contribute to rendering a truncation scheme based on a single ε
max value impractical. 相似文献
106.
107.
The Gray Fossil Site (GFS) includes a small (<2 ha) paleosinkhole lake fill with an exceptionally well-preserved record of
sedimentation and fossils from the latest Miocene to earliest Pliocene. The uppermost lacustrine stratigraphy is characterized
by rhythmites that regularly alternate between coarse-grained and organic-rich (A) laminae and fine-grained, silty clay (B)
laminae. Both the A and B components are almost exclusively comprised of exogenic sediment (including organic matter). Periodicities
of 24 and 4.4 are recorded within a continuous 96 interpreted year sequence of rhythmite sediment. In a small lake with a
poorly oxygenated bottom, the presence of laterally continuous laminated sediment that includes well-known periodicities in
rhythmite thickness is interpreted as representing annually generated varves that correspond to seasonal variations in sedimentation.
The distinctly larger fraction of medium sand-size quartz grains present within the A laminae, as well as the abrupt transitions
between A and B components suggest that the rhythmites represent deposition during alternating high-energy and lower-energy
seasons, which is consistent with a monsoonal precipitation pattern. The seasonal climate may relate to changes in the ocean
circulation pattern prior to 4.6 Ma that resulted in an increased temperature and atmospheric pressure gradient between the
east coast of North America and the Atlantic Ocean, but this climate phase seems to be only a temporary condition, as underlying
and overlying sediment are both consistent with drier conditions. The periodicity at 24 interpreted years is consistent with
the well-known Hale solar cycle. The 4.4 interpreted-year periodicity occurs within the ENSO frequency band, and if this documentation
of ENSO-like interannual climate change is correct, then it suggests that ENSO operated at times during the warm Earth conditions
characterizing the late Tertiary. 相似文献
108.
Epiphytic diatoms as flood indicators 总被引:1,自引:0,他引:1
Johan A. Wiklund Natalie Bozinovski Roland I. Hall Brent B. Wolfe 《Journal of Paleolimnology》2010,44(1):25-42
The hydroecology of floodplain lakes is strongly regulated by flood events. The threat of climate warming and increasing human
activities requires development of scientific methods to quantify changes in the frequency of short-lived flood events, because
they remain difficult to identify using conventional paleolimnological and monitoring approaches. We developed an approach
to detect floods in sediment records by comparing the abundance and composition of epiphytic diatom communities in flooded
and non-flooded ponds of the Peace-Athabasca Delta (PAD), Canada, that grew on submerged macrophytes (Potamogeton zosteriformis, P.
perfoliatus) and an artificial substrate (polypropylene sheets) during the open-water season of 2005. Analysis of similarity tests showed
that epiphytic diatom community composition differs significantly between flooded and non-flooded ponds. After accounting
for the “pond effect,” paired comparisons of the three substrates determined that variation in community composition between
the artificial substrate and macrophytes was similar to that between the macrophyte taxa. Similarity percentage analysis identified
diatom taxa that discriminate between flooded and non-flooded ponds. The relative abundance of ‘strong flood indicator taxa’
was used to construct an event-scale flood record spanning the past 180 years using analyses of sedimentary diatom assemblages
from a closed-drainage pond (PAD 5). Results were verified by close agreement with an independent paleoflood record from a
nearby flood-prone oxbow pond (PAD 54) and historical records. Comparison of epiphytic diatoms in flooded and non-flooded
lakes in this study provides a promising approach to detect changes in flood frequency, and may have applications for reconstructing
other pulse-type disturbances such as hurricanes and pollutant spills. 相似文献
109.
The Geology and Petroleum Resources of Kansas: A Review From Alpha to Omega or From the Pleistocene to the Precambrian 总被引:1,自引:0,他引:1
Daniel F. Merriam 《Natural Resources Research》2010,19(4):293-316
The geological story of Kansas is told through the rocks that are present. It is a simple story in generalities but complex
in detail. Knowing the story, gives insight into understanding the occurrence and location of possible economic valuable minerals,
such as petroleum. This is a brief review of Kansas geology with respect to the known occurrence of oil and gas. Kansas is
part of the Midcontinent oil province with oil having been discovered 150 years ago and commercial production commencing in
1873. Although many prospects remain in Kansas, the state has gone from the number 1 producer in the U.S. in 1916 to 8th today.
Exploration for new oil and gas production therefore is going to have to be more imaginative and utilize new approaches and
techniques to find the elusive petroleum. There are possibilities however for the prospector who can search diligently. Although
the big fields probably have been discovered, the prospects today are deeper, in more undetectable traps, and in essentially
untested places. 相似文献
110.
Sofia U. Holmgren Christian Bigler Ólafur Ingólfsson Alexander P. Wolfe 《Journal of Paleolimnology》2010,43(2):393-412
Lake sediments from four small lakes on western Spitsbergen (Svalbard Archipelago, Norwegian High Arctic) preserve biostratigraphic and isotopic evidence for a complex suite of twentieth century environmental changes. At Lake Skardtjørna and Lake Tjørnskardet on Nordenskiöldkysten, there is a marked diatom floristic change coupled to increased diatom concentrations beginning around 1920. At Lake Istjørna and Lake Istjørnelva, 25 km southwest of Longyearbyen, both diatom total valve and chrysophyte stomatocyst concentrations have increased dramatically since the beginning of the 1900s. The early twentieth century changes are probably related to climate warming after the Little Ice Age. However, the most pronounced changes in diatom assemblages seem to have occurred in the last few decades. At the same time, nitrogen stable isotopes in sediment organic matter in two of the lakes became progressively depleted by ~2‰, which is consistent with diffuse atmospheric inputs from anthropogenic sources and attendant fertilization. These data suggest that climate change and nitrogen deposition may be acting together in driving these lakes towards new ecological states that are unique in the context of the Holocene. 相似文献