全文获取类型
收费全文 | 15164篇 |
免费 | 2465篇 |
国内免费 | 4493篇 |
专业分类
测绘学 | 2070篇 |
大气科学 | 2926篇 |
地球物理 | 3341篇 |
地质学 | 6203篇 |
海洋学 | 2893篇 |
天文学 | 1441篇 |
综合类 | 1346篇 |
自然地理 | 1902篇 |
出版年
2024年 | 62篇 |
2023年 | 169篇 |
2022年 | 498篇 |
2021年 | 555篇 |
2020年 | 756篇 |
2019年 | 871篇 |
2018年 | 644篇 |
2017年 | 766篇 |
2016年 | 751篇 |
2015年 | 905篇 |
2014年 | 888篇 |
2013年 | 1038篇 |
2012年 | 994篇 |
2011年 | 970篇 |
2010年 | 791篇 |
2009年 | 915篇 |
2008年 | 1015篇 |
2007年 | 1196篇 |
2006年 | 1175篇 |
2005年 | 1028篇 |
2004年 | 858篇 |
2003年 | 712篇 |
2002年 | 673篇 |
2001年 | 491篇 |
2000年 | 623篇 |
1999年 | 587篇 |
1998年 | 459篇 |
1997年 | 322篇 |
1996年 | 274篇 |
1995年 | 207篇 |
1994年 | 187篇 |
1993年 | 177篇 |
1992年 | 149篇 |
1991年 | 94篇 |
1990年 | 59篇 |
1989年 | 78篇 |
1988年 | 45篇 |
1987年 | 35篇 |
1986年 | 34篇 |
1985年 | 16篇 |
1984年 | 15篇 |
1983年 | 5篇 |
1982年 | 6篇 |
1981年 | 3篇 |
1980年 | 4篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1954年 | 5篇 |
1877年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 718 毫秒
261.
The evaluation of seismic site response in the urban area of Catania was tackled by selecting test areas having peculiar lithological
and structural features, potentially favourable to large local amplifications of ground motion. The two selected areas are
located in the historical downtown and in the northern part of Catania where the presence of a fault is evident. Site response
was evaluated using spectral ratio technique taking the horizontal- to-vertical component ratio of ambient noise. Inferences
from microtremor measurements are compared with results from synthetic accelerograms and response spectra computed at all
drillings available for this area. Such method is particularly suitable in urban areas where the nature of the outcropping
geological units is masked by city growth and anthropic intervention on the surface geology. The microtremor H/V spectral
ratios evaluated at soft sites located within the downtown profile tend to be smaller than that usually reported in the literature
for such soils. A tendency for amplifications to peaks near 2 Hz is observed only in some sites located on recent alluvial
deposits. Evidences for amplifications of site effects (frequency range 4–8 Hz) were observed in the sampling sites located
on the fault, with a rapid decrease of spectral amplitude just a few tenths of metres away from the discontinuity. Numerical
simulations evidenced the importance of geolithological features at depth levels even greater than 20–30 m. Besides this,
the results strongly confirm the importance of the subsurface geological conditions, in the estimate of seismic hazard at
urban scale. 相似文献
262.
263.
Assessing the Vibrational Frequencies of the Cathedral of Cologne (Germany) by Means of Ambient Seismic Noise Analysis 总被引:1,自引:0,他引:1
Andreas Fäcke Stefano Parolai Sandra M. Richwalski Lothar Stempniewski 《Natural Hazards》2006,38(1-2):229-236
Ambient seismic noise measurements were conducted inside the Cathedral of Cologne (Germany) for assessing its frequencies
of vibration and for checking whether these occur in the range where soil amplification is expected. If this is the case,
damages may increase in case of an earthquake due to an increased structural response of the building. Analysis of the ratio
between the horizontal and vertical components of the spectra recorded at stations located inside the building as well as
the ratio between the corresponding components of the spectra recorded simultaneously inside the building and at a reference
station placed in the basement of the cathedral indicated several modes of vibration. Facilitated by these results an assessment
of the seismic vulnerability was attempted for a 2D ground motion scenario using the finite element method. 相似文献
264.
Land use change has been recognized throughout the Earth as one of the most important factors influencing the occurrence of rainfall-driven geomorphological processes. However, relating the occurrence of historical soil erosion rates is difficult because of the lack of long-term research projects in river basins. Also, complex models are not adequate to reconstruct erosion rate changes because they require significant input data not always available on long timescales. Given the problems with assessing sediment yield using complex erosion models, the objective of this study is to explore a parsimonious scale-adapted erosion model (ADT) from the original Thornes and Douglas algorithms, which aims at reconstruction of annual net erosion (ANE) upon multisecular timescales. As a test site, the Calore River basin (3015 km2 in southern Italy) provides a peculiar and unique opportunity for modelling erosion responses to climate and land cover changes, where input-data generation and interpretation results were also supported by documented hydrogeomorphological events that occurred before and after land deforestation. In this way, ANEADT-values were reconstructed for the period 1675–2004 by using precipitation indexes, complemented by recent instrumental records, and by using land cover statistics from documented agrarian sources. Pulses of natural sedimentation in the predeforestation period have been related to Vesuvius volcanic activity and changes in rainstorm frequency. After deforestation, the basin system became unstable with sudden fluctuations in the hydrogeomorphological regime contributing significantly to increased erosion and, in turn, sediment transport sequences via river drainage towards the Tyrrhenian coast. 相似文献
265.
Riparian meadows of southern Patagonia are temporally and spatially heterogeneous habitats. They are thought to play a key role in regulating the dynamics of arid grazed steppes of the region. We conducted a 2-year study with sheep to determine the influence of two grazing conditions (deferment and spring grazing) on structural and nutritional parameters of three vegetation types apparently associated with a soil moisture gradient in a riparian meadow in southern Santa Cruz (Argentina). Spring deferment allowed forage accumulation in very moist and intermediate vegetation types by the beginning of the summer and had no detrimental impact on forage quality. Deferment had no effect on biomass accumulation of the drier vegetation type at our study site. Structural attributes of the vegetation appeared to affect spatial grazing patterns of sheep. Soil-related patchiness, rather than sheep grazing, appeared to control vegetation structure and nutritional value. The proportion of more mesic plant communities in riparian habitats of Patagonia could determine the outcome of plant–animal interactions in these systems. Practical recommendations of grazing capacity in paddocks, or more theoretical considerations of ecosystem dynamics of the Patagonian steppes need to explicitly consider the internal patchiness of riparian habitats. 相似文献
266.
267.
268.
The devastating 1999 Marmara and Düzce earthquakes led to a significant increase in the earthquake studies in Turkey in geological,
engineering and financial aspects. Extreme Value Theory (EVT) has a range of applications from stock market changes to natural
disasters like floods and hurricanes. Here EVT is fitted to the ordinary and earthquake reinsurance claims of Turkey. 相似文献
269.
A gravity core collected from the upper slope of southwest of Quilon at a water depth of 776 m (Lat: 8°12′263″N, Long: 76°28′281″E)
was analysed for texture (carbonate free), calcium carbonate and organic carbon. Variation in silicic fraction seems to be
controlled by silt, i.e., enrichment from 15 ka BP to 10 ka BP and then constant in Holocene. Below 15 ka BP, the silicic
fraction gets depleted compared to the Holocene section with a minimum around 21 ka BP. Clay content remains nearly constant
except in the Holocene where it shows an enrichment. Carbonate content of less than 63 micron when computed by subtracting
coarse fraction content from the total carbonate suggests that the total carbonates are mainly concentrated in the finer fraction.
All these carbonate phases show an inverse relationship with silicic fraction except in Holocene. Below 15 ka BP, CaCO3 dominates in sediments comprising more than 65%, such an increase is also seen in the coarse fraction. Coarse fraction from
these sections contains abundant nodular type aggregates encrusting small forams. This period is marked by a high sedimentation
rate comparable to Holocene. These parameters suggest that the productivity and precipitation have increased in the Holocene
due to the intensification of the southwest monsoon. During the last glacial maximum and early deglacial period the high sedimentation
rate indicates redeposition of the carbonates from the existing carbonate lithofacies situated between Quilon and Cape Comorin
probably due to the slope instability. 相似文献
270.
p-nitrotoluene is an important organic intermediate widely used in pesticide, foamed plastics, dyestuff and medicine industries.
In this paper, Crusian carps (Carassius auratus) were exposed to dilute p-nitrotoluene solutions with different concentrations of 0, 20, 40, 80, 120, 160, 240 and 320 μg/L; the subsequent physiological
responses to the chemical were observed, and the variation characteristics of blood corpuscles, ovaries and oocytes were studied
by bio-microscopy. Results showed that even very low concentrations of p-nitrotoluene would lead to toxicosis and even the death of Carassius auratus. The bio-microscopic structures revealed that, compared with the compartment of Carassius auratus growing in the p-nitrotoluene-free water (the control group), the lower concentrations of p-nitrotoluene (for example no more than 80 μg/L) would result in the faster disintegration of erythrocytes and the significant
enlargement of oocytes, while the higher contents of p-nitrotoluene (no less than 240 μg/L in this case) would inhibit the division of cells and their further growth, even degenerate
blood corpuscles and ovaries, suggesting stronger estrogenic activities.
The study is financially supported by the Development Foundation of Science and Technology under Shandong Provincial Education
Department (No. J05D51). 相似文献