全文获取类型
收费全文 | 1477篇 |
免费 | 115篇 |
国内免费 | 104篇 |
专业分类
测绘学 | 124篇 |
大气科学 | 45篇 |
地球物理 | 294篇 |
地质学 | 252篇 |
海洋学 | 41篇 |
天文学 | 745篇 |
综合类 | 73篇 |
自然地理 | 122篇 |
出版年
2024年 | 6篇 |
2023年 | 4篇 |
2022年 | 15篇 |
2021年 | 14篇 |
2020年 | 15篇 |
2019年 | 24篇 |
2018年 | 15篇 |
2017年 | 30篇 |
2016年 | 28篇 |
2015年 | 30篇 |
2014年 | 31篇 |
2013年 | 63篇 |
2012年 | 29篇 |
2011年 | 36篇 |
2010年 | 44篇 |
2009年 | 81篇 |
2008年 | 92篇 |
2007年 | 157篇 |
2006年 | 93篇 |
2005年 | 78篇 |
2004年 | 85篇 |
2003年 | 77篇 |
2002年 | 92篇 |
2001年 | 58篇 |
2000年 | 74篇 |
1999年 | 57篇 |
1998年 | 54篇 |
1997年 | 78篇 |
1996年 | 38篇 |
1995年 | 39篇 |
1994年 | 34篇 |
1993年 | 26篇 |
1992年 | 20篇 |
1991年 | 11篇 |
1990年 | 15篇 |
1989年 | 10篇 |
1988年 | 18篇 |
1987年 | 14篇 |
1986年 | 5篇 |
1985年 | 3篇 |
1982年 | 2篇 |
1981年 | 1篇 |
排序方式: 共有1696条查询结果,搜索用时 765 毫秒
591.
592.
Soil rotations around horizontal axes, during an earthquake, are studied through records collected by closely spaced arrays of strong motion accelerometers. The cross power spectrum of accelerations at nearby stations has been generally utilized to describe the spatial distribution of the motion. A number of cross spectra have been obtained during the training of these arrays. To take profit of these elaborations, a mathematical relation is established between the cross power spectrum and the power spectrum of the rotation. Rotation data presented by Liu et al, concerning 52 earthquake records collected at a single station in Taiwan, are compared with rotation data computed according to our procedure. The two series of data are suitably normalized to the peak horizontal acceleration. The data are shown in function of the distance from epicentre. The same ratio, computed according to our procedure, is in good agreement with the average value of these data. Direct measurements and the present approach have lead to evaluations of rotation higher than those predicted by mathematical investigations on the basis of the wave propagation theory, for comparable circumstances. The relevance of this input motion for relatively tall structures is examined, with reference to the structural effects that the horizontal motion concurrently provides. Meaningful will be ranked those effects of the order of magnitude of 20% or higher than those implied by the horizontal excitation. For understanding the relevance on building structures, the procedure has two areas of concern: 1) the coherence implicit in the cross power spectra, which depends on the interpolation process of the original records, collected in the arrays of instruments, and 2) the relative importance of the vertical to the horizontal input motion. As to the second item, the relevance of the rotation component on structures largely depends on the relative importance of the vertical to the horizontal input motion. When the records in an area a few km from the epicenter are considered, the response spectrum of vertical motion is comparable and in some records even higher, than that of horizontal motion, over the entire range of frequencies. This has been observed as well for the 2009 earthquake event of L'Aquila, Italy, and that at the Christchurch (New Zealand) 2011. When the response spectrum of vertical motion is comparable to that of horizontal motion, the effects of rotational motions on most engineering structures can be meaningful. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
593.
The cyclic behaviour of plastic hinges is an essential component in tracking the behaviour of RC frames to failure, not only for monotonically increasing force/pressure loads such as under extreme wind loads but also for dynamic displacement-driven loads such as under earthquake ground motions. To describe member deformations at ultimate loading, traditional moment–curvature techniques have required the use of an empirical hinge length to predict rotations, and despite much research a definitive generic expression for this empirical hinge length is yet to be defined. To overcome this problem, a discrete rotation approach, which directly quantifies the rotation between crack faces using mechanics, has been developed for beams and been shown to be accurate under monotonic loading. In this paper, the discrete rotation approach for monotonic loads is extended to cope with cyclic loads for dynamic analyses, and this has led to the development of a new partial interaction numerical simulation capable of allowing for reversals of slip of the reinforcing bars. This numerical tool should be very useful for the nonlinear analysis of reinforced concrete beams and reinforced concrete columns with small axial loads under severe dynamic loads. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
594.
????????????仯???????????LS??AR??????????????????????????????????ERP????????????????????в???????????????в????н??в??????????????AR??????ERP?????????ERP??????1??10????????????????????????????????? 相似文献
595.
596.
M. Vavoulidis K. D. Kokkotas A. Stavridis 《Monthly notices of the Royal Astronomical Society》2008,384(4):1711-1724
We study low-amplitude crustal oscillations of slowly rotating relativistic stars consisting of a central fluid core and an outer thin solid crust. We estimate the effect of rotation on the torsional toroidal modes and on the interfacial and shear spheroidal modes. The results compared against the Newtonian ones for wide range of neutron star models and equations of state. 相似文献
597.
N. J. Dunstone G. A. J. Hussain A. Collier Cameron S. C. Marsden M. Jardine J. R. Barnes J. C. Ramirez Velez J.-F. Donati 《Monthly notices of the Royal Astronomical Society》2008,387(4):1525-1536
We present the first measurements of surface differential rotation on a pre-main-sequence binary system. Using intensity (Stokes I) and circularly polarized (Stokes V) time-series spectra, taken over 11 nights at the Anglo-Australian Telescope (AAT), we incorporate a solar-like differential rotation law into the surface imaging process. We find that both components of the young, 18 Myr, HD 155555 (V824 Ara, G5IV + K0IV) binary system show significant differential rotation. The equator–pole lap times as determined from the intensity spectra are 80 d for the primary star and 163 d for the secondary. Similarly, for the magnetic spectra we obtain equator–pole lap times of 44 and 71 d, respectively, showing that the shearing time-scale of magnetic regions is approximately half of that found for stellar spots. Both components are therefore found to have rates of differential rotation similar to those of the same spectral-type main-sequence single stars. The results for HD 155555 are therefore in contrast to those found in other, more evolved, binary systems where negligible or weak differential rotation has been discovered. We discuss two possible explanations for this: first that at the age of HD 155555 binary tidal forces have not yet had time to suppress differential rotation and secondly that the weak differential rotation previously observed on evolved binaries is a consequence of their large convection zone depths. We suggest that the latter is the more likely solution and show that both temperature and convection zone depth (from evolutionary models) are good predictors of differential rotation strength. Finally, we also examine the possible consequences of the measured differential rotation on the interaction of binary star coronae. 相似文献
598.
599.
Frans J. M. Rietmeijer 《Earth, Moon, and Planets》2008,102(1-4):461-471
Bulk compositions of aggregate meteoroids made of the originally accreted dust with its highly varied in mineral content and
chemistry and considerable grain size variations do not have a chondritic bulk composition. Deviations from CI element abundances
reflect indigenous variations within and among comet nuclei. These unmodified meteoroids that are heterogeneous in all their
properties are fundamentally different from meteoroids with a CI bulk composition that are fine-grained, equigranular materials
and chemically and mineralogically homogeneous. Collection and data reduction bias exists but the compositions of individual
fast meteors are entirely constrained by the measured main component meteor abundances. 相似文献
600.
G. A. Krasinsky 《Celestial Mechanics and Dynamical Astronomy》2008,101(4):325-336
The long-term systematic errors of the analytical theories IAU 2000 and IAU 2006 of the Earth’s precession–nutational motion
are studied making use of the VLBI data of 1984–2007. Several independent methods give indubitable evidence of the significant
quadratic error in the IAU 2000 residuals of the precessional angle while the adopted value of the secular decrease /cy of the Earth’s ellipticity e (derived from Satellite Laser Ranging data) should manifest itself in the residuals of as the negative quadratic trend . The problem with the precession of the IAU 2006 theory adopted as a new international standard and based on the precession
model P03 (Capitaine et al., Astron Astrophys 432:355–367, 2005) appears to be even more serious because the above mentioned quadratic term has already been incorporated into the P03 precession. Our analysis of the VLBI data demonstrates that the quadratic trend
of the IAU 2006 residuals does amount to the expected value (30.0 ± 3) mas/cy2. It means, first, that the theoretical precession rate of IAU 2006 should be augmented by the large secular correction and, second, that the available VLBI data have potentiality of estimating the rate . And indeed, processing these data by the numerical theory ERA of the Earth’s rotation (Krasinsky, Celest Mech Dyn Astron
96:169–217, 2006, Krasinsky and Vasilyev, Celest Mech Dyn Astron 96:219–237, 2006) yields the estimate /cy statistically in accordance with the satellite-based . On the other hand, applying IAU 2000/2006 models, the positive value /cy is found which is incompatible with the SLR estimate and, evidently, has no physical meaning. The large and steadily increasing
error of the precession motion of the IAU 2006 theory makes the task of replacing IAU 2006 by a more accurate model be most
pressing. 相似文献