全文获取类型
收费全文 | 8197篇 |
免费 | 1620篇 |
国内免费 | 777篇 |
专业分类
测绘学 | 287篇 |
大气科学 | 256篇 |
地球物理 | 6896篇 |
地质学 | 1809篇 |
海洋学 | 633篇 |
天文学 | 21篇 |
综合类 | 251篇 |
自然地理 | 441篇 |
出版年
2024年 | 45篇 |
2023年 | 121篇 |
2022年 | 220篇 |
2021年 | 247篇 |
2020年 | 327篇 |
2019年 | 343篇 |
2018年 | 304篇 |
2017年 | 316篇 |
2016年 | 305篇 |
2015年 | 391篇 |
2014年 | 511篇 |
2013年 | 440篇 |
2012年 | 453篇 |
2011年 | 485篇 |
2010年 | 442篇 |
2009年 | 497篇 |
2008年 | 525篇 |
2007年 | 482篇 |
2006年 | 458篇 |
2005年 | 392篇 |
2004年 | 388篇 |
2003年 | 308篇 |
2002年 | 306篇 |
2001年 | 239篇 |
2000年 | 253篇 |
1999年 | 254篇 |
1998年 | 267篇 |
1997年 | 222篇 |
1996年 | 224篇 |
1995年 | 204篇 |
1994年 | 126篇 |
1993年 | 117篇 |
1992年 | 79篇 |
1991年 | 58篇 |
1990年 | 49篇 |
1989年 | 40篇 |
1988年 | 55篇 |
1987年 | 17篇 |
1986年 | 5篇 |
1985年 | 6篇 |
1984年 | 23篇 |
1983年 | 4篇 |
1982年 | 4篇 |
1981年 | 1篇 |
1980年 | 4篇 |
1979年 | 14篇 |
1978年 | 6篇 |
1977年 | 10篇 |
1954年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
91.
92.
93.
岩石破裂实验,地球岩石层断裂构造运动,地震孕育、发生和发展过程等研究表明,地震活动的时间、空间和强度分布特征具有分形结构。作为一种分维理论的应用,本文利用厦门数字化地震遥测台网的数据,采用信息的分析方法,对福建省的地震多发区-漳州、华安、龙岩地区的地震活动进行了分析和研究,讨论了信息维随时间变化的特征。 相似文献
94.
95.
96.
This paper is the first in a two-part series that discusses the principal axes of M-DOF structures subjected to static and dynamic loads. The primary purpose of this series is to understand the magnitude of the dynamic response of structures to enable better design of structures and control modification devices/systems. Under idealized design conditions, the structural responses are obtained by using single direction input ground motions in the direction of the intended control devices/systems,and by assuming that the responses of the structure is decoupleable in three mutually perpendicular directions. This standard practice has been applied to both new and retrofitted structures using various seismic protective systems. Very limited information is available on the effects of neglecting the impact of directional couplings (cross effects - of which torsion is a component) of the dynamic response of structures. In order to quantify such effects, it is necessary to examine the principal axes of structures under both static and dynamic loading.This first paper deals with quantitative definitions of principal axes and "cross effects" of three-dimensional structures under static load by using linear algebra. It shows theoretically that, for three-dimensional structures, such principal axes rarely exist. Under static loading conditions, the cross effect is typically small and negligible from the viewpoint of engineering applications. However, it provides the theoretical base for subsequent quantification of the response couplings under dynamic loads, which is reported in part Ⅱ of this series. 相似文献
97.
Horizontal and vertical components of earthquake ground motions at liquefiable sites 总被引:1,自引:0,他引:1
Field observations on ground motions from recent earthquakes imply that current knowledge is limited with regard to relating vertical and horizontal motions at liquefiable sites. This paper describes a study with the purpose of clarifying this emerging issue to some extent. A series of numerical analyses is carried out on a liquefiable soil deposit with a verified, fully coupled, nonlinear procedure. It is shown that the transformation of vertical motions in the deposit differs considerably from the transformation of horizontal motions. Both the amplitude and frequency content of the horizontal motions are strongly dependent on the shaking level or the associated nonlinear soil behavior. The transfer function for vertical motions is however likely to be independent of the intensity of input motions; no reduction in the amplitude occurs even in the case of strong shaking. The results are shown to be in consistence with the laboratory observations on shaking table tests and recent field observations that less nonlinearity exists for vertical motions. It is also shown that the possibility exists for using information on spectral ratios between the horizontal and vertical surface motions to quickly identify in situ soil behavior and liquefaction that are not readily covered by conventional field or laboratory experimentation procedures. 相似文献
98.
A comprehensive analytical solution is developed to examine the torsional vibration of an elastic foundation on a semi-infinite saturated elastic medium for the first time. First, the governing equations of saturated media are solved by use of Hankel transform techniques. Then, based on the assumption that the contact between the foundation and the half-space is perfectly bonded, this dynamic mixed boundary-value problem can lead to dual integral equations, which are further reduced to the standard Fredholm integral equations of the second kind and solved by numerical procedures. Numerical examples are given at the end of the paper. The numerical results indicate that the response of the elastic foundation strongly depends on the material and geometrical properties of both the saturated soil-foundation system and the load acting on the foundation. In most of the cases, the dynamic behavior of an elastic foundation on saturated media significantly differs from that of a rigid plate bearing on the elastic half-space. 相似文献
99.
Under seismic excitation, liquefied clean medium to dense cohesionless soils may regain a high level of shear resistance at large shear strain excursions. This pattern of response, known as a form of cyclic mobility, has been documented by a large body of laboratory sample tests and centrifuge experiments. A plasticity-based constitutive model is developed with emphasis on simulating the cyclic mobility response mechanism and associated pattern of shear strain accumulation. This constitutive model is incorporated into a two-phase (solid–fluid), fully coupled finite element code. Calibration of the constitutive model is described, based on a unique set of laboratory triaxial tests (monotonic and cyclic) and dynamic centrifuge experiments. In this experimental series, Nevada sand at a relative density of about 40% is employed. The calibration effort focused on reproducing the salient characteristics of dynamic site response as dictated by the cyclic mobility mechanism. Finally, using the calibrated model, a numerical simulation is conducted to highlight the effect of excitation frequency content on post-liquefaction ground deformations. 相似文献
100.
It is generally accepted that both deterministic and statistical approaches are useful for the characterization of earthquake hazard. Although the most reliable estimates of seismic hazard can only be based on an improved understanding of the earthquake mechanism, efficient utilization of the appropriate methods provided by recent statistical theories is also important in seismic risk analysis. This is especially true in regions where the connection between seismicity and geologic structure is tenuous at best. We are particularly interested in developing better statistical treatments of data for regions with little known seismic activity. To this end, we have applied three statistical methods to the historical record of seismicity in relatively quiet regions of eastern North America. These are: (1) the threshold method for tail inference, a new theory for modeling earthquakes with sizes above a given threshold, (2) the ‘bootstrap’ technique in which the characteristics of an unknown population are simulated by replacing the true population by an estimated one, and (3) a technique to estimate the number of earthquakes below a given size, in order to compensate for the under-reporting of small earthquakes in most catalogs. A combination of these techniques has been used to estimate the probabilities of future large earthquakes for the regions studied. Because of limitations imposed by existing catalogs, the size estimate used has been maximum intensity. 相似文献