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731.
Novelaria, a new genus of rhagionid of late Albian age with three new species, is the first record of this family from Charentes amber (southwestern France). The new genus is probably closely related to the recent genus Chrysopilus. However its relationship with the other fossils in amber is discussed. A key for separation of the new species is provided and the diversity of the family during the Cretaceous is also briefly discussed. 相似文献
732.
733.
A seismic source model is presented for use in probabilistic seismic hazard analyses to be conducted for sites within the
Buller–NW Nelson region of New Zealand. The application of common probabilistic seismic hazard analysis (PSHA) methodology
for sites in this region has been complicated by the long-held suspicion that the observed rates of seismic activity are high
and not representative of long-term earthquake activity. However, recent analyses of geological, seismicity and geodetic data
indicate that the extent of this anomaly may have been overestimated and that current rates of seismic activity within this
region are likely to continue into the foreseeable future. Probable bounds for the most appropriate long-term rates of seismic
activity are estimated after considering all available sources of constraint. These include geodetic analyses, plate-motion
modelling, finite element modelling, structural geological considerations, paleoseismic information, tree-ring analyses, precarious
rock information, observed seismicity and fundamental mechanics. A suite of fault sources is identified, and the observed
seismicity is partitioned between these sources and a background source using Bayesian inference, and then analysed to obtain
a magnitude–frequency distribution for each seismic source. The annual moment release rate for the region, resulting from
the identified and characterised sources, is shown to be consistent with available constraints. Consequently, it is demonstrated
that the observed seismicity in the Buller–NW Nelson region can be used to model future earthquake occurrence within the region
and that standard PSHA may therefore be implemented within the region. 相似文献
734.
735.
Michael B. Turner Shane J. Cronin Mark S. Bebbington Thomas Platz 《Bulletin of Volcanology》2008,70(4):507-515
The majority of continental arc volcanoes go through decades or centuries of inactivity, thus, communities become inured to
their threat. Here we demonstrate a method to quantify hazard from sporadically active volcanoes and to develop probabilistic
eruption forecasts. We compiled an eruption-event record for the last c. 9,500 years at Mt Taranaki, New Zealand through detailed
radiocarbon dating of recent deposits and a sediment core from a nearby lake. This is the highest-precision record ever collected
from the volcano, but it still probably underestimates the frequency of eruptions, which will only be better approximated
by adding data from more sediment core sites in different tephra-dispersal directions. A mixture of Weibull distributions
provided the best fit to the inter-event period data for the 123 events. Depending on which date is accepted for the last
event, the mixture-of-Weibulls model probability is at least 0.37–0.48 for a new eruption from Mt Taranaki in the next 50 years.
A polymodal distribution of inter-event periods indicates that a range of nested processes control eruption recurrence at
this type of arc volcano. These could possibly be related by further statistical analysis to intrinsic factors such as step-wise
processes of magma rise, assembly and storage. 相似文献
736.
闽南—台湾浅滩渔场微型硅藻的分类研究 总被引:2,自引:0,他引:2
利用透射电镜(TEM)研究闽南-台湾浅滩渔场的微型硅藻时,鉴定出14个在我国首次的种类,它们是:极小曲壳藻(Achnanthes minutissima),分岐双壁藻(Diploneis divergens)、槌棒脆杆藻(Fragilatia opephoroides)、微细脆杆藻(Fragilaria atomus)、折断舟形藻(Navicula fracta)、虫瘿舟形藻(Naviculaga 相似文献
737.
垂直集成采样法在234Th—238U不平衡研究海洋新生产力中的应用 总被引:4,自引:0,他引:4
对基于梯形积分原理的集成采样法的可行性在理论上进行了阐述,并在厦门湾和南海北部的5个站位分别采集不同深度的水样和由这些水样按一定比例混合而成的集成水样进行了现场研究,结果表明两种采样方法经不可逆稳态清除模型得出的溶解态234Th的清除通量、颗粒态234Th的输出通量及溶解态、颗粒态234Th的停留时间基本一致,证实了垂直集成采样法的可行性与可靠性。这种新的采样方案为今后充分发挥234Th法的优势,在更大空间尺度上进行新生产力的研究提供了可靠的保证。 相似文献
738.
Lead-210 methods have been used to establish a chronology of sedimentation extending back almost 80 years in Black Mountain Lagoon near Guyra on the New England Tablelands of north-eastern New South Wales, Australia. Estimates of the direct atmospheric fallout of unsupported 210Pb and historical records of the pattern of phosphorus input to the lake provide support for the 210Pb chronology. The sediments in the lake record an episode of disturbance which took place prior to c .1916, but after c .1790-1860. Since then, however, the site-specific rate of minerogenic sedimentation has been maintained at a relatively low and constant level of 0.93 kg m-2 a-1. [Note: the following symbols are used in this paper: a = year (annum), and d min-1 g-1 = radioactivity (spontaneous nuclear disintegrations per minute per gram of material).] This is despite major shifts in land use and intensification of agriculture, despite the occurrence of significant floods and droughts, and in the absence until recent times of the application of soil conservation practices. These rates may be contrasted with rates of 0.023 kg m-2 a-1 from the period 12.3-5.6 ka. These are at least an order of magnitude lower than those of the twentieth century. The lake sediments preserve evidence of the use of agricultural chemicals on the catchment during the twentieth century. These include phosphorus, zinc, arsenic and lead. Soils and sediments may act as long-term toxic stores for such chemicals, a problem often overlooked in rural areas. 相似文献
739.
A three-dimensional image of shallow subduction: crustal structure of the Raukumara Peninsula, New Zealand 总被引:1,自引:0,他引:1
Martin Reyners Donna Eberhart-Phillips & Graham Stuart 《Geophysical Journal International》1999,137(3):873-890
Earthquake arrival time data from a 36-station deployment of portable seismographs on the Raukumara Peninsula have been used to determine the 3-D Vp and Vp/Vs structure of this region of shallow subduction. A series of inversions have been performed, starting with an inversion for 1-D structure, then 2-D, and finally 3-D. This procedure ensures a smooth regional model in places of low resolution. The subducted plate is imaged as a northwest-dipping feature, with Vp consistently greater than 8.5 km s−1 in the uppermost mantle of the plate. Structure in the overlying plate changes significantly along strike. In the northeast, there is an extensive low-velocity zone in the lower crust underlying the most rapidly rising part of the Raukumara Range. It is bounded on its arcward side by an upwarp of high velocity. A viable explanation for the low-velocity zone is that it represents an accumulation of underplated subducted sediment, while serpentinization of the uppermost mantle may be responsible for the adjacent high-velocity region. The low-velocity zone decreases and the adjacent high-velocity region is less extensive in the southwest. This change is interpreted to be related to a change in the thickness of the crust of the overlying plate. In the northeast the crust is thinner, and subducted sediment ponds against relatively strong uppermost mantle, while in the southwest the crust is thicker, and the relatively weak lower crust allows sediment subduction to greater depths. A narrow zone of high Vp/Vs parallels the shallow part of the plate interface. This suggests elevated fluid pressures, with the distribution of earthquakes about this zone further suggesting that these pressures may be close to lithostatic. The plate interface at 20 km depth beneath the Raukumara Peninsula may thus be a closed system for fluid flow, similar to that seen at much shallower depths in other subduction décollements. 相似文献
740.