首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   384301篇
  免费   80805篇
  国内免费   101957篇
测绘学   64538篇
大气科学   74313篇
地球物理   67539篇
地质学   197188篇
海洋学   66645篇
天文学   6860篇
综合类   40137篇
自然地理   49843篇
  2024年   4490篇
  2023年   15808篇
  2022年   19203篇
  2021年   20243篇
  2020年   15631篇
  2019年   22786篇
  2018年   21582篇
  2017年   14468篇
  2016年   15930篇
  2015年   19576篇
  2014年   26416篇
  2013年   24273篇
  2012年   25941篇
  2011年   26534篇
  2010年   25490篇
  2009年   24865篇
  2008年   24453篇
  2007年   22668篇
  2006年   21748篇
  2005年   18797篇
  2004年   17665篇
  2003年   16232篇
  2002年   14449篇
  2001年   13461篇
  2000年   11025篇
  1999年   8740篇
  1998年   6186篇
  1997年   5367篇
  1996年   4862篇
  1995年   4582篇
  1994年   4313篇
  1993年   4126篇
  1992年   4104篇
  1991年   3699篇
  1990年   3504篇
  1989年   3723篇
  1988年   3895篇
  1987年   3790篇
  1986年   3217篇
  1985年   3266篇
  1984年   2951篇
  1983年   2766篇
  1982年   2279篇
  1981年   1714篇
  1980年   1273篇
  1979年   801篇
  1978年   392篇
  1959年   311篇
  1958年   332篇
  1957年   315篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
791.
我国滇西北地震活动区的活动构造与应力状态   总被引:19,自引:2,他引:17       下载免费PDF全文
本文根据野外调查和原地应力测量资料,讨论了滇西北地震活动区的活动构造型式和现今应力状态。资料表明,本区是由红河和中甸两条右旋走滑断裂所形成的拉分构造,其应力状态也与外区不同。地震在空间上的分布和在时间上的活动规律受这一构造型式及其应力状态的控制  相似文献   
792.
We have used observations of the rare isotopes of HCN and HNC to determine the relative abundance of these two chemical isomers along the central ridge of the Orion molecular cloud. The abundance ratio [HCN]/[HNC] decreases by more than an order of magnitude from the relatively warm plateau and hot core sources toward the KL nebula to the colder, more quiescent clouds to the north and south. Even in the cooler regions, however, the ratio is an order of magnitude larger than that found in previous investigations of cold dark clouds. We determine the kinetic temperature in the regions we have studied from new observations of methylacetylene (CH3CCH), together with other recent estimates of the gas temperature near KL. The results suggest that the warmer portions of the cloud are dominated by different chemical pathways than those in the general interstellar cloud material.  相似文献   
793.
There are great spatial variations in the distribution of the fast growing populations of India. About 70% of the 1981 population is concentrated in only 40% of the geographical area. Such a pattern of population concentration, with its spatio-temporal dimension, can be understood better if examined at the macro-regional level considering the physico-climatic and socio-economic diversity of the country. This is done by way of analyzing district level data for India as a whole and also for its various macro-regions. The study reveals that the Great Plains and Coastal Plains and Islands have a homogenous distribution and a higher population increase. The Deccan Plateau shows the most homogenous dispersion of population. The population is declining in the Northern Mountains and the Great Plains but accentuating in the Coastal Plains and Islands and the Deccan Plateau. The north and south of India are showing opposite trends in population concentration.  相似文献   
794.
Trace and RE element geochemistry and genesis have been studied with respect to ferromanganese nodules from the sediments of the Pacific, Atlantic and Indian oceans.  相似文献   
795.
A total of 21 elements, Si, Al, K, Na, Ca, Mg, Fe, Mn, Ti, P, Cu, Co, Ni, Zn, Pb, Cr, Rb, Sr Ba, Cd and Corg(organic carbon), was determined in the trough. Average contents of most elements were found to be higher in the trough bottom than in the western and eastern slopes. We call these elements the indicator elements because of their "transition" property. This is in keeping with the "intermediate" geographical position of the trough where the continental crust turns to oceanic crust.Though the authigenic fraction of some elements here increases, the detrital fraction .of most elements is still dominant, indicating that the influences of terrigenous and volcanic detrital materials on the trough sediments are still dominant.The abundant biogenous component in the trough is characterized by the increase of Ca, Sr and Corg contents. Ca existes mainly in the form of CaCo3 that is mostly found in various shells. Sr often easily replaces Ca within the shells.A belt-shaped distribution pattern of elements wa  相似文献   
796.
"A modification of the population potential model is used to analyze the structure of the potential of population for cities of the Moscow region, to develop a relationship between population potential and city size, and to use this procedure to forecast future population change in Moscow Oblast cities." Cities in the region with a population of 50,000 or more at the censuses of 1959, 1970, and 1979 are included.  相似文献   
797.
我国著名的水文地理学家、中国科学院地理研究所研究员郭敬辉同志于1985年4月5日溘然长逝。他的逝世是我国地理学界和水文学界的一大损失。我们怀着十分悲痛的心情对他表示沉重悼念。  相似文献   
798.
试论国外河流地貌学的进展   总被引:1,自引:0,他引:1  
本文分四个阶段论述国外河流地貌学研究的发展,阐述了现代河流地貌学在一些领域中取得的成就,以及研究方法和技术的进展。  相似文献   
799.
"The paper reports the use of Australian Resources Information System to calculate two proximate populations (people living within (i) 150 and (ii) 500 km) for the centroids of each of 3,000 half by half degree geodetic grid cells covering Australia. The use of proximate population data is exemplified by computing a map of the eighteen regions collectively containing a maximum fraction of the Australian population." The data used are from the 1981 census.  相似文献   
800.
Bahr J  Mertins G 《Erdkunde》1985,39(3):218-238
"An attempt is made in the light of an analysis of population changes occurring in several sub-districts of Greater Santiago in the period between the last population censuses of 1970 and 1982, based on small-area census units (distritos and communas) to draw attention to the effects on the spatial growth of the city resulting from changed conditions in the political and juridical sphere since the change of power in 1973." The effect of changes in government policy concerning the liberalization of the property market and the illegal occupation of land are noted. (summary in ENG)  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号