全文获取类型
收费全文 | 3954篇 |
免费 | 1387篇 |
国内免费 | 204篇 |
专业分类
测绘学 | 99篇 |
大气科学 | 215篇 |
地球物理 | 2234篇 |
地质学 | 1676篇 |
海洋学 | 382篇 |
天文学 | 573篇 |
综合类 | 46篇 |
自然地理 | 320篇 |
出版年
2024年 | 3篇 |
2023年 | 7篇 |
2022年 | 39篇 |
2021年 | 70篇 |
2020年 | 94篇 |
2019年 | 248篇 |
2018年 | 257篇 |
2017年 | 361篇 |
2016年 | 404篇 |
2015年 | 382篇 |
2014年 | 429篇 |
2013年 | 482篇 |
2012年 | 336篇 |
2011年 | 343篇 |
2010年 | 321篇 |
2009年 | 228篇 |
2008年 | 282篇 |
2007年 | 215篇 |
2006年 | 149篇 |
2005年 | 152篇 |
2004年 | 122篇 |
2003年 | 138篇 |
2002年 | 121篇 |
2001年 | 108篇 |
2000年 | 113篇 |
1999年 | 31篇 |
1998年 | 12篇 |
1997年 | 21篇 |
1996年 | 11篇 |
1995年 | 10篇 |
1994年 | 13篇 |
1993年 | 4篇 |
1992年 | 3篇 |
1991年 | 5篇 |
1990年 | 2篇 |
1989年 | 5篇 |
1988年 | 2篇 |
1987年 | 2篇 |
1986年 | 4篇 |
1985年 | 3篇 |
1984年 | 2篇 |
1979年 | 1篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1974年 | 2篇 |
1973年 | 1篇 |
1971年 | 1篇 |
1969年 | 1篇 |
1962年 | 1篇 |
1958年 | 1篇 |
排序方式: 共有5545条查询结果,搜索用时 546 毫秒
51.
Jean‐Jacques Macaire Saïda Bellemlih Christian Di‐Giovanni Patrick De Luca Lionel Visset Jacques Bernard 《地球表面变化过程与地形》2002,27(9):991-1009
In the Négron River catchment area (162 km2), surface‐sediment stores are composed of periglacial calcareous ‘grèze’ (5 × 106 t) and loess (21 × 106 t), and Holocene alluvium (12·6 × 106 t), peat (0·6 × 106 t) and colluvium (18·5 × 106 t). Seventy‐five per cent of the Holocene sediments is stored along the thalwegs. Present net sediment yield, calculated from solid discharge at the Négron outlet, is low (0·6 t km?2 a?1) due to the dominance of carbonate rocks in the catchment. Mean sediment yield during the Holocene period is 7·0 t km?2 a?1 from alluvium stores and 7·6 t km?2 a?1 from colluvium stores. Thus, the gross sediment yield during the Holocene period is about 18·7 t km?2 a?1 and the sediment delivery ratio 3 per cent. The yield considerably varies from one sub‐basin to another (3·9 to 24·5 t km?2 a?1) according to lithology: about 25 per cent and 50 per cent of initial stores of periglacial grèze and loess respectively were reworked during the Holocene period. Sediment yield has increased by a factor of 6 in the last 1000 years, due to the development of agriculture. The very high rate of sediment storage on the slope during that period (88 per cent of the yield) can be accounted for by the formation of cultivation steps (‘rideaux’). It is predicted that the current destruction of these steps will result in a sediment wave reaching the valley floors in the coming decades. Subboreal and Subatlantic sediments and pollen assemblages in the Taligny marsh, where one‐third of the alluvium is stored, show the predominant influence of human activity during these periods in the Négron catchment. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
52.
Shin Okamoto Nagahide Kani Masahiko Higashino Norihide Koshika Masayuki Kimizuka Mitsumasa Midorikawa Masanori Iiba 《地震工程与工程振动(英文版)》2002,1(2):213-225
The Building Standard Law of Japan and related Enforcement Order and Notifications have been substantially revised since the
year 2000 to introduce a performance-based regulatory and deregulation system for building control systems. Up to then, time-history
analyses were mandatory for isolated buildings and had to be specially approved by the Minster of the Ministry of Construction
(MOC). Simplified design procedures based on the equivalent linear method for seismically isolated buildings have been issued
as “Notification 2009 — Structural calculation procedure for buildings with seismic isolation” from MOC, and are now integrated
into the Ministry of Land, Infrastructure, and Transportation (MLIT). Along with Notification 2009, “Notification 1446 of
year 2000 — Standard for specifications and test methods for seismic isolation devices” was also issued. Buildings with heights
equal to or less than 60m and that are designed according to these Notifications, including base isolated buildings, only
need approval from local building officials, and no longer require the special approval of the Minister of MLIT. This paper
summarizes: 1) some statistics related to buildings with seismic isolation completed up to the end of 2001; 2) simplified
design procedures required by Notification 2009 of year 2000; and 3) performance of seismic isolation devices required by
Notification 1446 of year 2000. 相似文献
53.
54.
东南极格罗夫山存在一套经历了峰期麻粒岩相变质作用的镁铁质麻粒岩和斜长角闪岩。对含石榴石的镁铁质麻粒岩的详细研究则显示了近等温降压 (ITD)的顺时针PT演化轨迹 ,与拉斯曼丘陵有相似的演化历史。岩石的主量元素组成和玄武岩一致 ,并且具有拉斑玄武岩演化趋势。进一步的地球化学研究表明 ,这套玄武岩为洋岛型玄武岩 (OIB)和洋中脊型玄武岩(MORB)的组合。OIB型具有大体类似的地球化学性质 ,它们均富集Ti(TiO2 =2 .68% )、REE( =2 0 2 μg/g)、LREE[(La/Yb) N=4.8]、Ti/Y( =343)、Zr/Y( =3.1 ) ,具洋岛玄武岩的特征 ,推测岩浆来源于富集地幔源区 (EM)。而MORB型以低Ti (TiO2 =1 .1 %— 1 .31 % ) ,明显低于OIB的P的含量 (P2 O5=0 .1 %— 0 .2 % ) ,低REE ( 4 7— 93μg/g)、LREE/HREE( 2 .2 7— 2 .5 4 )、(La/Yb) N( =1 .30— 1 .62 )为特征 ,具洋中脊玄武岩的特征。MORB和OIB组合的出现说明在泛非期该区可能存在过洋盆。 相似文献
55.
56.
Investigated is the accuracy in estimating the response of asymmetric one‐storey systems with non‐linear viscoelastic (VE) dampers by analysing the corresponding linear viscous system wherein all non‐linear VE dampers are replaced by their energy‐equivalent linear viscous dampers. The response of the corresponding linear viscous system is determined by response history analysis (RHA) and by response spectrum analysis (RSA) extended for non‐classically damped systems. The flexible and stiff edge deformations and plan rotation of the corresponding linear viscous system determined by the extended RSA procedure is shown to be sufficiently accurate for design applications with errors generally between 10 and 20%. Although similar accuracy is also shown for the ‘pseudo‐velocity’ of non‐linear VE dampers, the peak force of the non‐linear VE damper cannot be estimated directly from the peak damper force of the corresponding linear viscous system. A simple correction for damper force is proposed and shown to be accurate (with errors not exceeding 15%). For practical applications, an iterative linear analysis procedure is developed for determining the amplitude‐ and frequency‐dependent supplemental damping properties of the corresponding linear viscous system and for estimating the response of asymmetric one‐storey systems with non‐linear VE dampers from the earthquake design (or response) spectrum. Finally, a procedure is developed for designing non‐linear supplemental damping systems that satisfy given design criteria for a given design spectrum. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
57.
Results of a detailed statistical study of constant relative strength inelastic displacement ratios to estimate maximum lateral inelastic displacement demands on existing structures from maximum lateral elastic displacement demands are presented. These ratios were computed for single‐degree‐of‐freedom systems with different levels of lateral strength normalized to the strength required to remain elastic when subjected to a relatively large ensemble of recorded earthquake ground motions. Three groups of soil conditions with shear wave velocities higher than 180m/s are considered. The influence of period of vibration, level of lateral yielding strength, site conditions, earthquake magnitude, distance to the source, and strain‐hardening ratio are evaluated and discussed. Mean inelastic displacement ratios and those associated with various percentiles are presented. A special emphasis is given to the dispersion of these ratios. It is concluded that distance to the source has a negligible influence on constant relative strength inelastic displacement ratios. However, for periods smaller than 1s earthquake magnitude and soil conditions have a moderate influence on these ratios. Strain hardening decreases maximum inelastic displacement at a fairly constant rate depending on the level of relative strength for periods of vibration longer than about 1.0s while it decreases maximum inelastic displacement non‐linearly as the period of vibration shortens and as the relative‐strength ratio increases for periods of vibration shorter than 1.0s. Finally, results from non‐linear regression analyses are presented that provide a simplified expression to be used to approximate mean inelastic displacement ratios during the evaluation of existing structures built on firm sites. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
58.
S. H. Ju 《地震工程与结构动力学》2003,32(9):1431-1442
This paper discusses how to use the three‐dimensional (3D) time‐domain finite‐element method incorporating the least‐squares method to calculate the equivalent foundation mass, damping and stiffness matrices. Numerical simulations indicate that the accuracy of these equivalent matrices is acceptable when the applied harmonic force of 1+sine is used. Moreover, the accuracy of the least‐squares method using the 1+sine force is not sensitive to the first time step for inclusion of data. Since the finite‐element method can model problems flexibly, the equivalent mass, damping and stiffness matrices of very complicated soil profiles and foundations can be established without difficulty using this least‐squares method. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
59.
The purpose of this study is to develop landslide susceptibility analysis techniques using an arti?cial neural network and to apply the newly developed techniques to the study area of Yongin in Korea. Landslide locations were identi?ed in the study area from interpretation of aerial photographs, ?eld survey data, and a spatial database of the topography, soil type and timber cover. The landslide‐related factors (slope, curvature, soil texture, soil drainage, soil effective thickness, timber age, and timber diameter) were extracted from the spatial database. Using those factors, landslide susceptibility was analysed by arti?cial neural network methods. The landslide susceptibility index was calculated by the back‐propagation method, which is a type of arti?cial neural network method, and the susceptibility map was made with a geographic information system (GIS) program. The results of the landslide susceptibility analysis were veri?ed using landslide location data. The validation results showed satisfactory agreement between the susceptibility map and the existing data on landslide location. A GIS was used to ef?ciently analyse the vast amount of data, and an arti?cial neural network to be an effective tool to maintain precision and accuracy. The results can be used to reduce hazards associated with landslides and to plan land use and construction. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
60.
Energy serves as an alternative index to response quantities like force or displacement to include the duration‐related seismic damage effect. A procedure to evaluate the absorbed energy in a multistorey frame from energy spectra was developed. For low‐ to medium‐rise frames, it required a static pushover analysis of the structure to determine the modal yield force and ductility factor of an equivalent single‐degree‐of‐freedom system for the first two modes. The energy spectra were then used to determine the energy contribution of each mode. A procedure was also developed to distribute the energy along the frame height based on energy shapes. This study showed that the second‐mode response in some cases needs to be considered to reflect the energy (or damage) concentration in the upper floors. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献