全文获取类型
收费全文 | 4614篇 |
免费 | 925篇 |
国内免费 | 1285篇 |
专业分类
测绘学 | 309篇 |
大气科学 | 820篇 |
地球物理 | 1185篇 |
地质学 | 2228篇 |
海洋学 | 943篇 |
天文学 | 238篇 |
综合类 | 617篇 |
自然地理 | 484篇 |
出版年
2024年 | 49篇 |
2023年 | 108篇 |
2022年 | 226篇 |
2021年 | 261篇 |
2020年 | 241篇 |
2019年 | 261篇 |
2018年 | 320篇 |
2017年 | 282篇 |
2016年 | 299篇 |
2015年 | 221篇 |
2014年 | 280篇 |
2013年 | 262篇 |
2012年 | 357篇 |
2011年 | 315篇 |
2010年 | 300篇 |
2009年 | 279篇 |
2008年 | 227篇 |
2007年 | 255篇 |
2006年 | 206篇 |
2005年 | 190篇 |
2004年 | 131篇 |
2003年 | 127篇 |
2002年 | 96篇 |
2001年 | 112篇 |
2000年 | 157篇 |
1999年 | 214篇 |
1998年 | 147篇 |
1997年 | 172篇 |
1996年 | 121篇 |
1995年 | 111篇 |
1994年 | 104篇 |
1993年 | 100篇 |
1992年 | 69篇 |
1991年 | 53篇 |
1990年 | 45篇 |
1989年 | 30篇 |
1988年 | 20篇 |
1987年 | 19篇 |
1986年 | 8篇 |
1985年 | 6篇 |
1984年 | 6篇 |
1983年 | 12篇 |
1982年 | 6篇 |
1981年 | 7篇 |
1980年 | 2篇 |
1979年 | 3篇 |
1977年 | 1篇 |
1958年 | 2篇 |
1957年 | 1篇 |
1954年 | 2篇 |
排序方式: 共有6824条查询结果,搜索用时 15 毫秒
91.
92.
邛崃地震发生后,成都市地震局很快做出反应,及时开展了地震考察并做出了正确的震后趋势判断,取得了较好的社会效益。 相似文献
93.
We use 23298 Pn arrival-time data from Chinese national and provincial earthquake bulletins to invert fine structure of Pn velocity and anisotropy at the top of the mantle beneath the Sichuan-Yunnan and its adjacent region. The results suggest that the Pn velocity in this region shows significant lateral variation; the Pn velocity varies from 7.7 to 8.3 km/s. The Pn-velocity variation correlates well with the tectonic activity and heat flow of the region. Low Pn velocity is observed in southwest Yunnan, Tengchong volcano area, and the Panxi tectonic area. These areas have very active seismicity and tectonic activity with high surface heat flow. On the other hand, high Pn velocity is observed in some stable regions, such as the central region of the Yangtze Platform; the most pronounced high velocity area is located in the Sichuan Basin, south of Chengdu. Pn anisotropy shows a complex pattern of regional deformation. The Pn fast direction shows a prominent clockwise rotation pattern from east of the Tibetan block to the Sichuan-Yunnan diamond block to southwest Yunnan, which may be related to southeastward escape of the Tibetan Plateau material due to the collision of the Indian Plate to the Eurasia Plate. Thus there appears to be strong correlation between the crustal deformation and the upper mantle structure in the region. The delay times of events and stations show that the crust thickness decreases from the Tibetan Plateau to eastern China, which is consistent with the results from deep seismic sounding. 相似文献
94.
95.
The Vincent Thomas Bridge in the Los Angeles metropolitan area, is a critical artery for commercial traffic flow in and out of the Los Angeles Harbor, and is at risk in the seismically active Southern California region, particularly because it straddles the Palos Verdes fault zone. A combination of linear and non‐linear system identification techniques is employed to obtain a complete reduced‐order, multi‐input–multi‐output (MIMO) dynamic model of the Vincent Thomas Bridge based on the dynamic response of the structure to the 1987 Whittier and 1994 Northridge earthquakes. Starting with the available acceleration measurements (which consists of 15 accelerometers on the bridge structure and 10 accelerometers at various locations on its base), an efficient least‐squares‐based time‐domain identification procedure is applied to the data set to develop a reduced‐order, equivalent linear, multi‐degree‐of‐freedom model. Although not the main focus of this study, the linear system identification method is also combined with a non‐parametric identification technique, to generate a reduced‐order non‐linear mathematical model suitable for use in subsequent studies to predict, with good fidelity, the total response of the bridge under arbitrary dynamic environments. Results of this study yield measurements of the equivalent linear modal properties (frequencies, mode shapes and non‐proportional damping) as well as quantitative measures of the extent and nature of non‐linear interaction forces arising from strong ground shaking. It is shown that, for the particular subset of observations used in the identification procedure, the apparent non‐linearities in the system restoring forces are quite significant, and they contribute substantially to the improved fidelity of the model. Also shown is the potential of the identification technique under discussion to detect slight changes in the structure's influence coefficients, which may be indicators of damage and degradation in the structure being monitored. Difficulties associated with accurately estimating damping for lightly damped long‐span structures from their earthquake response are discussed. The technical issues raised in this paper indicate the need for added spatial resolution in sensor instrumentation to obtain identified mathematical models of structural systems with the broadest range of validity. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
96.
Petrology, geochemistry and isotopic ages of eclogites from the Dulan UHPM Terrane, the North Qaidam, NW China 总被引:35,自引:0,他引:35
The Dulan eclogite–gneiss region is located in the eastern part of the North Qaidam eclogite belt, NW China. Widespread evidence demonstrates that this region is a typical ultrahigh-pressure (UHP) metamorphic terrane. Eclogites occur as lenses or layers in both granitic and pelitic gneisses. Two distinguished sub-belts can be recognized and differ in mineralogy, petrology and geochemistry. The North Dulan Belt (NDB) has tholeiitic protoliths with high TiO2 and lower Al2O3 and MgO contents. REE patterns and trace element contents resemble those of N-type and E-type MORB. In contrast, eclogites in the South Dulan Belt (SDB) are of island arc protoliths with low TiO2, high Al2O3 and show LREE-enriched and HFSE-depleted patterns. Sm–Nd isotope analyses give isochron ages of 458–497 Ma for eclogite-facies metamorphism for the two sub-belts. The ages are similar to those of Yuka and Altun eclogites in the western extension of the North Qaidam-Altun eclogite belt. The Dulan UHP metamorphic terrane, together with several other recently recognized eclogite-bearing terrenes within the North Qaidam-Altun HP-UHP belt, constitute the key to the understanding of the tectonic evolution of the northern Tibetan Plateau. The entire UHP belt extends for more than 1000 km from the Dulan UHP terrane in the southeast to the Altun eclogite–gneiss terrane in the west. This super-belt marks an early Paleozoic continental collision zone between the Qaidam Massif and the Qilian Massif. 相似文献
97.
Origin and tectonic significance of a Mesozoic multi-layer over-thrust system within the Yangtze Block (South China) 总被引:21,自引:0,他引:21
Dan-Ping Yan Mei-Fu Zhou Hong-Lin Song Xin-Wen Wang John Malpas 《Tectonophysics》2003,361(3-4):239-254
In the Yangtze Block (South China), a well-developed Mesozoic thrust system extends through the Xuefeng and Wuling mountains in the southeast to the Sichuan basin in the northwest. The system comprises both thin- and thick-skinned thrust units separated by a boundary detachment fault, the Dayin fault. To the northwest, the thin-skinned belt is characterized by either chevron anticlines and box synclines to the northwest or chevron synclines to the southeast. The former structural style displays narrow exposures for the cores of anticlines and wider exposures for the cores of synclines. Thrust detachments occur along Silurian (Fs) and Lower Cambrian (Fc) strata and are dominantly associated with the anticlines. To the southeast, this style of deformation passes gradually into one characterized by chevron synclines with associated principal detachment faults along Silurian (Fs), Cambrian (Fc) and Lower Sinian (Fz) strata. There are, however, numerous secondary back thrusts. Therefore, the thin-skinned belt is like the Valley and Ridge Province of the North American Applachian Mountains. The thick-skinned belt structurally overlies the thin-skinned belt and is characterized by a number of klippen including the Xuefeng and Wuling nappes. It is thus comparable to the Blue Ridge Province of Appalachia.The structural pattern of this thrust system in South China can be explained by a model involving detachment faulting along various stratigraphic layers at different stages of its evolution. The system was developed through a northwest stepwise progression of deformation with the earliest delamination along Lower Sinian strata (Fz). Analyses of balanced geological cross-sections yield about 18.1–21% (total 88 km) shortening for the thin-skinned unit and at least this amount of shortening for the thick-skinned unit. The compressional deformation from southeast to northwest during Late Jurassic to Cretaceous time occurred after the westward progressive collision of the Yangtze Block with the North China Block and suggests that the orogenic event was intracontinental in nature. 相似文献
98.
云南地壳形变与丽江7.0级地震 总被引:9,自引:5,他引:9
本利用滇西、滇中地区大地垂直形变和跨断层短水准、短基线的监测资料进行综合分析论证,其结果表明:1996年2月滇西丽江7.0级地震前,该区应力场方向自1990年以来产生了由北东-北西的转变,震源区出现高达109mm的垂直差异运动1993-1996年有75%的断层形变测点相继出现不同程度的中期、短期和临震异常。大地垂直形变与断层形变的共性演变特性,为强震的预测提供了一个较好的震例。 相似文献
99.
用Pg波走时重建华北地区结晶基底速度及时间项图像 总被引:3,自引:0,他引:3
应用层析成象以投影方法,对华北地区28条深地震剖剖面地震测深剖面Pg波走时进行处理,作出该区结晶基底速度及时间项成像。并作了方法的数值模型合成数据的敏感性分析,证明所采用有效性及成象结果的可靠性。计算中应用LSQR算法求解大型稀疏超定方程。并采用只存储矩阵非零元素技术,使存储量减小到原矩阵百分之一,同时零元素不参数加计算,提高了运算速度。华北地区结昌基底速度为5.9-6.35km/s,时间最大为2 相似文献
100.
玛纳斯县城房屋建筑地震损失预测及抗震设防效益分析 总被引:2,自引:0,他引:2
以玛纳斯县城现有房屋建筑调查资料为基础,对该县城的房屋建筑进行了分类。作了抗震分析,建立了相应的易损性矩阵。根据地震危险性分析结果及县城房屋建筑的发展情况,给出了未来10年房屋建筑的震害预测结果,并对房屋建筑抗震设防和加固的经济效益进行了初步分析。 相似文献