全文获取类型
收费全文 | 444篇 |
免费 | 11篇 |
国内免费 | 3篇 |
专业分类
测绘学 | 3篇 |
大气科学 | 19篇 |
地球物理 | 115篇 |
地质学 | 110篇 |
海洋学 | 79篇 |
天文学 | 114篇 |
综合类 | 2篇 |
自然地理 | 16篇 |
出版年
2021年 | 6篇 |
2020年 | 6篇 |
2019年 | 12篇 |
2018年 | 2篇 |
2017年 | 12篇 |
2016年 | 8篇 |
2015年 | 8篇 |
2014年 | 23篇 |
2013年 | 14篇 |
2012年 | 15篇 |
2011年 | 16篇 |
2010年 | 24篇 |
2009年 | 25篇 |
2008年 | 24篇 |
2007年 | 19篇 |
2006年 | 26篇 |
2005年 | 14篇 |
2004年 | 16篇 |
2003年 | 10篇 |
2002年 | 19篇 |
2001年 | 13篇 |
2000年 | 10篇 |
1999年 | 10篇 |
1998年 | 10篇 |
1997年 | 8篇 |
1996年 | 4篇 |
1995年 | 6篇 |
1994年 | 6篇 |
1993年 | 9篇 |
1992年 | 4篇 |
1991年 | 2篇 |
1990年 | 6篇 |
1989年 | 2篇 |
1988年 | 4篇 |
1987年 | 9篇 |
1986年 | 5篇 |
1985年 | 3篇 |
1984年 | 5篇 |
1983年 | 5篇 |
1982年 | 3篇 |
1981年 | 5篇 |
1977年 | 6篇 |
1975年 | 2篇 |
1974年 | 4篇 |
1973年 | 5篇 |
1972年 | 3篇 |
1971年 | 2篇 |
1970年 | 2篇 |
1966年 | 1篇 |
1963年 | 1篇 |
排序方式: 共有458条查询结果,搜索用时 420 毫秒
351.
Effects of coarse woody debris on morphology and sediment storage of a mountain stream system in western Oregon 总被引:1,自引:0,他引:1
Effects of coarse woody debris (CWD) on channel morphology and sediment storage were investigated at five sites, representative of first-order to fifth-order streams. In the steep and bedrock-confined stream (first-second order), interaction between the channel and CWD was limited, except where breakage upon falling produced CWD pieces shorter than channel width. Channel widening, steepening and sediment storage associated with CWD were observed predominantly in third- to fifth-order streams. Variation in channel width and gradient was regulated by CWD. In the fifth-order stream, most of the CWD pieces derived from the riparian forest interacted directly with the channel without being suspended by sideslopes. In this system CWD promoted lateral channel migration, but sediment storage was temporary, with annual release and capture. 相似文献
352.
High-beta plasma blobs in the morningside plasma sheet 总被引:1,自引:0,他引:1
G. Haerendel W. Baumjohann E. Georgescu R. Nakamura L. M. Kistler B. Klecker H. Kucharek A. Vaivads T. Mukai S. Kokubun 《Annales Geophysicae》1999,17(12):1592-1601
Equator-S frequently encountered, i.e. on 30%0of the orbits between 1 March and 17 April 1998, strong variations of the magnetic field strength of typically 5–15-min duration outside about 9RE during the late-night/early-morning hours. Very high-plasma beta values were found, varying between 1 and 10 or more. Close conjunctions between Equator-S and Geotail revealed the spatial structure of these “plasma blobs” and their lifetime. They are typically 5–10° wide in longitude and have an antisymmetric plasma or magnetic pressure distribution with respect to the equator, while being altogether low-latitude phenomena (<15°). They drift slowly sunward, exchange plasma across the equator and have a lifetime of at least 15–30 min. While their spatial structure may be due to some sort of mirror instability, little is known about the origin of the high-beta plasma. It is speculated that the morningside boundary layer somewhat further tailward may be the source of this plasma. This would be consistent with the preference of the plasma blobs to occur during quiet conditions, although they are also found during substorm periods. The relation to auroral phenomena in the morningside oval is uncertain. The energy deposition may be mostly too weak to generate a visible signature. However, patchy aurora remains a candidate for more disturbed periods. 相似文献
353.
Masako Nogami Takeshi Matsuno Takehiro Nakamura Tadashi Fukumoto 《Journal of Oceanography》2000,56(3):319-329
We have observed the temporal variation of oxygen deficient water with short time scale (less than a few days) in the central area of Ohmura Bay, Kyushu, Japan, in summer, 1995 and 1996. The vertical profiles of temperature were similar to those of dissolved oxygen. We noticed a linear relation between temperature and dissolved oxygen in the bottom layer, and applied the T-DO relation to estimate the net oxygen consumption rate, rather than direct evaluation of the advection and diffusion. Oxygen consumption rate just above the bottom was estimated to be about 0.21 g O2 m–3day–1 in July 1995, and about 0.28 g O2 m–3day–1 in August 1996. The net oxygen consumption rate estimated for the bottom layer below the second thermocline was about 0.61 g O2 m–3day–1 with variability from 0.55 to 0.66 g O2 m–3day–1 during July 25 to 29, 1995. This is was about 0.64 g O2 m–3day–1 with variability from 0.18 to 1.4 g O2 m–3day–1 during August 22 to 30, 1996. The net oxygen consumption rates are about half of those measured with a closed system in the Seto Inland Sea. 相似文献
354.
Shigehisa Nakamura 《Marine Geodesy》2000,23(1):55-61
Specific properties of the interannual sea level variations and annual tides in the Northwestern Pacific were studied. Several tide stations were monitored. The monthly mean sea level for the year of 1995 was analyzed at each tide station. A seismic event in 1995, some tectonic activity around the subject area, and the Kuroshio (the oceanic western boundary current) may possibly contaminate results which would have occurred from the astronomical annual tide alone. 相似文献
355.
Festus T. Aka Tetsuya Yokoyama Minoru Kusakabe Eizo Nakamura Gregory Tanyileke Bekoa Ateba Vincent Ngako Joseph Nnange Joseph Hell 《Journal of Volcanology and Geothermal Research》2008
From previously published 14C and K–Ar data, the age of formation of Lake Nyos maar in Cameroon is still in dispute. Lake Nyos exploded in 1986, releasing CO2 that killed 1750 people and over 3000 cattle. Here we report results of the first measurements of major elements, trace elements and U-series disequilibria in ten basanites/trachy-basalts and two olivine tholeiites from Lake Nyos. It is the first time tholeiites are described in Lake Nyos. But for the tholeiites which are in 238U–230Th equilibrium, all the other samples possess 238U–230Th disequilibrium with 15 to 28% enrichment of 230Th over 238U. The (226Ra/230Th) activity ratios of these samples indicate small (2 to 4%) but significant 226Ra excesses. U–Th systematics and evidence from oxygen isotopes of the basalts and Lake Nyos granitic quartz separates show that the U-series disequilibria in these samples are source-based and not due to crustal contamination or post-eruptive alteration. Enrichment of 230Th is strong prima facie evidence that Lake Nyos is younger than 350 ka. The 230Th–226Ra age of Nyos samples calculated with the (226Ra/230Th) ratio for zero-age Mt. Cameroon samples is 3.7 ± 0.5 ka, although this is a lower limit as the actual age is estimated to be older than 5 ka, based on the measured mean 230Th/238U activity ratio. The general stability of the Lake Nyos pyroclastic dam is a cause for concern, but judging from its 230Th–226Ra formation age, we do not think that in the absence of a big rock fall or landslide into the lake, a big earthquake or volcanic eruption close to the lake, collapse of the dam from erosion alone is as imminent and alarming as has been suggested. 相似文献
356.
Virginia Ruiz-Villanueva Bruno Mazzorana Ernest Bladé Livia Bürkli Pablo Iribarren-Anacona Luca Mao Futoshi Nakamura Diego Ravazzolo Dieter Rickenmann Marcos Sanz-Ramos Markus Stoffel Ellen Wohl 《地球表面变化过程与地形》2019,44(9):1694-1709
Inorganic sediment is not the only solid-fraction component of river flows; flows may also carry significant amounts of large organic material (i.e. large wood), but the characteristics of these wood-laden flows (WLFs) are not well understood yet. With the aim to shed light on these relatively unexamined phenomena, we collected home videos showing natural flows with wood as the main solid component. Analyses of these videos as well as the watersheds and streams where the videos were recorded allowed us to define for the first time WLFs, describe the main characteristics of these flows and broaden the definition of wood transport regimes (adding a new regime called here hypercongested wood transport). According to our results, WLFs may occur repeatedly, in a large range of catchment sizes, generally in steep, highly confined single thread channels in mountain areas. WLFs are typically highly unsteady and the log motion is non-uniform, as described for other inorganic sediment-laden flows (e.g. debris flows). The conceptual integration of wood into our understanding of flow phenomena is illustrated by a novel classification defining the transition from clear water to hypercongested, wood and sediment-laden flows, according to the composition of the mixture (sediment, wood, and water). We define the relevant metrics for the quantification and modelling of WLFs, including an exhaustive discussion of different modelling approaches (i.e. Voellmy, Bingham and Manning) and provide a first attempt to simulate WLFs. We draw attention to WLF phenomena to encourage further field, theoretical, and experimental investigations that may contribute to a better understanding of flows in river basins, leading to more accurate predictions, and better hazard mitigation and management strategies. © 2019 John Wiley & Sons, Ltd. 相似文献
357.
Masashi Watanabe Kazuhisa Goto Fumihiko Imamura Andrew Kennedy Daisuke Sugawara Norihiro Nakamura Takayuki Tonosaki 《地球表面变化过程与地形》2019,44(15):2939-2956
Clifftop coastal boulders transported by storm waves or tsunamis have been reported around the world. Although numerical calculation of boulder transport is a strong tool for the identification of tsunami or storm boulders, and for estimation of the wave size emplacing boulders, models which can reasonably solve boulder transport from below a cliff or from a cliff-edge onto a cliff-top do not yet exist. In this study, we developed a new numerical formulation for cliff-top deposition of boulders from the cliff edge or below the cliff, with validation from laboratory tests. We then applied the model using storm and tsunami wave forcing to simulate the observed boulder deposits at the northwest coast of Hachijo Island, Japan. Using the model, the actual distribution of boulders was explained well using a reasonable storm wave height without assumption of anomalously high-water level by storm surge. Results show that boulder transport from the cliff edge or under the cliff onto the cliff-top was possible from a tsunami with periods of 5~10 min or storm waves with no storm surge. However, the actual distribution of boulders on the cliff was explained only by storm waves, but not by tsunami. Therefore, the boulders distributed at this site are likely of storm wave origin. Our developed model for the boulder transport calculation can be useful for identifying a boulder's origin and can reasonably calculate cliff-top deposition of boulders by tsunami and storm waves. © 2019 John Wiley & Sons, Ltd. © 2019 John Wiley & Sons, Ltd. 相似文献
358.
M. Hayakawa Y. Kasahara T. Nakamura Y. Hobara A. Rozhnoi M. Solovieva O.A. Molchanov 《Journal of Atmospheric and Solar》2010,72(13):982-987
The long-term data during seven years from January 2001 to December 2007, as observed by the Pacific VLF/LF network consisting of several Japanese stations and one station in Kamchatka, are extensively utilized to perform a statistical correlation study between the lower ionospheric perturbations as detected by subionospheric propagation and earthquakes (EQs). In this paper, we adopt a physical parameter, the maximum seismicity intensity observed (I) to define the strength of an EQ unlike the previously and conventionally used EQ magnitude and depth, which is a combined effect of EQ magnitude and depth, together with the Earth’s surface information and geological condition around the EQ epicenter. After considering EQs only take place on the land because of the use of seismicity and by using the superimposed epoch analysis, it is found for the larger EQs with I from 5 to 7 (we feel serious trembling and we expect serious damage) that the most important VLF/LF parameter, trend (nighttime average amplitude), shows a definite decrease about 10 days before the EQ by exceeding 2σ (σ: standard deviation) criterion; the dispersion shows a maximum about 10 days before the EQ but not exceeding 2σ line and finally the nighttime fluctuation shows an enhancement about 10 days before the EQ (with exceeding the 2σ level). A definite statistical correlation is confirmed between the ionospheric perturbations and I when I is strong enough in a range from 5 to 7. Whereas, there is no significant correlation between the two when I is in a range from 3 to 4. Finally, together with the corresponding results for EQs in the sea, but close to the land, these results are discussed in the light of lithosphere–ionosphere coupling mechanism. 相似文献
359.
Mitsuru Nakamura Sami F. Masri Anastassios G. Chassiakos Thomas K. Caughey 《地震工程与结构动力学》1998,27(9):997-1010
A neural network-based approach is presented for the detection of changes in the characteristics of structure-unknown systems. The approach relies on the use of vibration measurements from a ‘healthy’ system to train a neural network for identification purposes. Subsequently, the trained network is fed comparable vibration measurements from the same structure under different episodes of response in order to monitor the health of the structure. The methodology is applied to actual data obtained from ambient vibration measurements on a steel building structure that was damaged under strong seismic motion during the Hyogo-Ken Nanbu Earthquake of 17 January 1995. The measurements were done before and after repairs to the damaged frame were made. A neural network is trained with data after the repairs, which represents ‘healthy’ condition of the building. The trained network, which is subsequently fed data before the repairs, successfully identified the difference between the damaged storey and the undamaged storey. Through this study, it is shown that the proposed approach has the potential of being a practical tool for a damage detection methodology applied to smart civil structures. © 1998 John Wiley & Sons, Ltd. 相似文献
360.
A practical method is described of initiating a water mass curtain (a jet flow of water partitioning another body of water) to be used to control fish movements, along with its theoretical analysis. In this paper, equations to evaluate the change of density and upward flow rates in a water curtain due to entrainment are presented, and the accuracy of the equations is verified experimentally. The discontinuity of the density on the boundary between jet and ambient fluid was assumed in the analysis, since our experiments showed that the instantaneous density distribution was discontinuous. The equations are much simpler than the conventional formulae deduced by assuming a Gaussian curve, and are amenable to practical usage. The control efficiency of the water curtain is very good for young red sea bream. 相似文献