全文获取类型
收费全文 | 16517篇 |
免费 | 3589篇 |
国内免费 | 2102篇 |
专业分类
测绘学 | 487篇 |
大气科学 | 756篇 |
地球物理 | 12607篇 |
地质学 | 4984篇 |
海洋学 | 859篇 |
天文学 | 37篇 |
综合类 | 1239篇 |
自然地理 | 1239篇 |
出版年
2024年 | 67篇 |
2023年 | 256篇 |
2022年 | 456篇 |
2021年 | 671篇 |
2020年 | 645篇 |
2019年 | 745篇 |
2018年 | 691篇 |
2017年 | 632篇 |
2016年 | 588篇 |
2015年 | 752篇 |
2014年 | 966篇 |
2013年 | 965篇 |
2012年 | 942篇 |
2011年 | 993篇 |
2010年 | 935篇 |
2009年 | 1198篇 |
2008年 | 929篇 |
2007年 | 1044篇 |
2006年 | 991篇 |
2005年 | 924篇 |
2004年 | 836篇 |
2003年 | 823篇 |
2002年 | 630篇 |
2001年 | 567篇 |
2000年 | 513篇 |
1999年 | 460篇 |
1998年 | 460篇 |
1997年 | 426篇 |
1996年 | 429篇 |
1995年 | 359篇 |
1994年 | 335篇 |
1993年 | 276篇 |
1992年 | 230篇 |
1991年 | 135篇 |
1990年 | 96篇 |
1989年 | 63篇 |
1988年 | 57篇 |
1987年 | 24篇 |
1986年 | 27篇 |
1985年 | 12篇 |
1984年 | 10篇 |
1983年 | 4篇 |
1981年 | 3篇 |
1979年 | 17篇 |
1978年 | 5篇 |
1976年 | 4篇 |
1974年 | 2篇 |
1973年 | 3篇 |
1972年 | 2篇 |
1954年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
F. Bella R. Bella P. F. Biagi M. Caputo G. Della Monica A. Ermini E. Pettinelli V. Sgrigna 《Pure and Applied Geophysics》1993,141(1):125-137
Prototype instrumentation, able to automatically measure groundwater radon content variations, is presented. The equipment is made of stainless steel and has spherical valves with automatic and pneumatic control. The deemanation of the gases from the water is obtained by evacuating a suitable expansion chamber. The instrumentation can make discrete sampling ranging from 1 per hour to 1 per 99 hours. The equipment was tested in the laboratory: the efficiency was measured by means of a266Ra solution. A mean value of (0.65±0.07) count/s/Bq was obtained. A calibration test was carried out by comparing countings from the automatic equipment with those obtained by the standard laboratory cell. Results of an operational check over a period of approximately one year indicate that variations in radon at the calibration site are attributable more to meteorological than to tectonic causes. 相似文献
122.
1983年菏泽5.9级地震前后沂水泉氡的变化 总被引:1,自引:0,他引:1
沂水泉位于山东省沂水一汤头断裂带上。多年来的观测分析表明,影响沂水泉氡值变化的最大干扰是降雨。由于这种干扰存在着“记忆”滞后影响,所以用一般的相关分析难以将这种影响排除干净。本文对最小二乘法为建立模型的判据,用动态灰箱分析法拟合并推估观测系统的变化。在分析中用了四个状态变量:长趋势成分L(t)、年周期变化成分P(t)、降雨影响部分R(t)和随机变化部分S(t)。经过分析表明,沂水泉氡的长趋势部分1 相似文献
123.
A Step Towards Evaluation of the Seismic Response Reduction Factor in Multistorey Reinforced Concrete Frames 总被引:1,自引:0,他引:1
A seismic nonlinear time-history analysis was made for four-, six-, and eight-storey reinforced concrete buildings. These buildings were made as three-dimensional space frame structures with shear walls in both orthogonal directions. They have five bays with 4.8 m spacing each in the horizontal direction, and three bays with 4.2 m spacing each in the transversal direction. The frames were designed according to the Jordanian Seismic Code of practice for Seismic Zones 4, 3, 2, and 1 as proposed for Jordan by several authors. Time-history analysis was made using the El Centro (N-S) earthquake record of May 1940 as an actual earthquake excitation. The response reduction factor (R) that primarily consists of two factors that are the ductility reduction (Rµ) and the overstrength (), is obtained. It has been seen that the seismic zoning has a slight effect on the ductility reduction factor for different buildings, since it ranges from Zone 4 to Zone 1 as 2.37 to 2.52, 1.72 to 1.78, and 1.14 to 1.18 for four-, six-, and eight-storey buildings, respectively. Moreover, it is observed that, for different buildings and different seismic zones, the ductility reduction factor (Rµ) is slightly different from the system ductility factor (µ) especially for higher values of µ (i.e., Rµ µ). The response reduction factor, called overstrength (), was evaluated. The overstrength factor was found to vary with seismic zones (Z) , number of stories, and design gravity loads. However, the dependency on seismic zones was the strongest. The average overstrength of these buildings in Zones 4 and 1 was 2.61 and 6.94, respectively. The overstrength increased as the number of storeys decreased: overstrength of a four-storey building was higher than an eight-storey building by 36% in Zone 4, and 39% in Zone 1. Furthermore, buildings of the three heights had an average overstrength 165.9% higher in Zone 1 than in Zone 4. These observations have a significant implications for the seismic design codes which currently do not take into account the variation of the response reduction factor, R (i.e., ductility reduction factor times overstrength). 相似文献
124.
本文在《中国震例》资料基础上,详细分析了华北地区17次中强以上地震前的短临异常,得到华北地区中强以上地震短临异常的三个综合特征,在归纳出短临异常综合特征的基础上,提出了两个判定孕震过程、前兆异常由中期向短期过渡的定量的综合标志。据中期异常的追踪分析和短临异常综合分析相结合的原则,定义并计算了综合预报指标S值,以此值作为是否发生中强以上地震的判据。本文的重点是在分析短临异常综合特征的基础上,结合实用 相似文献
125.
论述了适用于遥测地震台网大震速报的一套计算机处理系统,系统设计以《全国遥测台网观测技术规范》和《近震分析》理论为依据,实现了数据输入,震相选择,数据处理,地震定位,震级计算,结果输出及贮存等多项功能,达到了快速确定地震三要素,提高速报速度和精度之目的,是一个较完善的,实用的速报系统。 相似文献
126.
F. Bella P. F. Biagi M. Caputo G. Della Monica A. Ermini P. V. Manjgaladze V. Sgrigna D. O. Zilpimiani 《Pure and Applied Geophysics》1995,144(2):277-300
Daily averaged tilt component data from two sites of the Central Apennines (Italy) and of the Southern Caucasus (Georgia), respectively, revealed intermediate-term tilts as possible precursors to earthquakes (M=3.0÷4.7) which occurred in the above-mentioned seismic areas within a distance of 50 km from the sites. A good temporal correlation as well as a fair spatial correspondence between these residual tilts (with amplitude and duration of some microradians and months, respectively) and main shocks were pointed out, by removing both secular trends and seasonal thermoelastic effects from the raw tilts. An attempt was made to justify the above-mentioned results, based on the assumption that the observed intermediate-term preseismic tilts are the manifestation of aseismic creep episodes of comparable duration in the fault materials of thrust faults close to the tilt sites. The mechanism refers to a strain field slowly propagating from the preparation focal area to the tilt site, through crustal blocks separated by weak transition zones. This propagation is thought to be the cause of the local aseismic fault slip recorded by the tiltmeters. Previously, both discrete structures and strain propagation effects were revealed in the Central Apennines and are thought also to exist in the Southern Caucasus. As in the past, the rheological properties of fault materials are revealed as viscoelastic ones. In fact, creep equations obtained by applying several viscoelastic models on our data, proved to fit quite well some of the observed tilt precursors, producing viscosity and rigidity values very similar to those reported in literature.Professor Petr Viktorovich Manjgaladze died during the writing of this paper 相似文献
127.
A sequence of computer experiments is used to study questions concerning the tsunami problem as a quantitative estimate of tsunami danger, detailed geographical tsunami classification, determination of the parameters of critical tsunami waves, and the conditions of their development. We call a wave critical, if its impact on the coast is most hazardous.Using the Middle Kuril Island as an example, we present the results of a computer experiment which includes determining the wavefields on the shelf and estimating the effects connected with the deep-water Bussol and Diana Straits.Numerical simulation of tsunami waves of different sources permits the assessment of the extent of tsunami danger in different areas of the coastal zone of Simushir Island, depending on the location of the focus zone and their geometry.The major singularities of the wavefield arise in the zones of the deep-water straits. The distribution of the amplification factors is determined by both the global parameters of the wavefields and the local properties of individual harbours. The results obtained for a particular harbour in the northern part of Simushir Island, formed the basis for the quantitative estimate of tsunami danger for this area. 相似文献
128.
J. Zahradník M. Antonini G. Grünthal J. Janský D. Procházková E. Schmedes A. Špičak J. Zedník 《Pure and Applied Geophysics》1990,133(1):53-71
The main shock of the West-Bohemian earthquake swarm, Czechoslovakia, (magnitudem=4.5, depthh=10 km) exhibits an irregular areal distribution of macroseismic intensities 6° to 7° MSK-64. Four lobes of the 6° isoseismal are found and the maximum observed intensity is located at a distance of 8 km from the instrumentally determined epicentre. This distribution can be explained by the energy flux of the directS wave generated by a circular source, the hypocentral location and focal mechanism of which are taken from independent instrumental studies. The theoretical intensity, which is assumed to be logarithmically proportional to the integrated squared ground-motion velocity (i.e.,I=const+log v
2
(t)dt), fits the observed intensity with an overall root-mean-square error less than 0.5°. It is important that the present intensity data can also be equally well explained by the isotropic source. The fit was attained by means of a horizontally layered model though large fault zones and an extended sedimentary basin suggest a significant lateral heterogeneity of the epicentral region. The results encourage a broader application of the simple modelling technique used. 相似文献
129.
T. L. Chelidze H. Spetzler I. C. Getting Z. A. Avaliani 《Pure and Applied Geophysics》1990,134(1):31-43
The elastic properties of a physical model representing a damaged rock matrix were studied using a square lattice deformed under tensile stress. The elastic modulusM of such a system varies in agreement with percolation theory as|x–x
c
|
f
, wherex is the damage parameter andx
c
the threshold value of the damage parameter,f3.6. Atxx
c
the scale dependence ofM can be expressed asML
–f/v
, whereL is the size of the sample andv the correlation exponent in percolation theory.The experimental results are of interest in assessing elastic properties in earthquake focal zones and fault zones in general. 相似文献
130.