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531.
Areas of low strain rate are typically characterized by low to moderate seismicity. The earthquake catalogs for these regions
do not usually include large earthquakes because of their long recurrence periods. In cases where the recurrence period of
large earthquakes is much longer than the catalog time span, probabilistic seismic hazard is underestimated. The information
provided by geological and paleo-seismological studies can potentially improve seismic hazard estimation through renewal models,
which assume characteristic earthquakes. In this work, we compare the differences produced when active faults in the northwestern
margin of the València trough are introduced in hazard analysis. The differences between the models demonstrate that the introduction
of faults in zones characterized by low seismic activity can give rise to significant changes in the hazard values and location.
The earthquake and fault seismic parameters (recurrence interval, segmentation or fault length that controls the maximum magnitude
earthquake and time elapsed since the last event or Te) were studied to ascertain their effect on the final hazard results. The most critical parameter is the recurrence interval,
where shorter recurrences produce higher hazard values. The next most important parameter is the fault segmentation. Higher
hazard values are obtained when the fault has segments capable of producing big earthquakes. Finally, the least critical parameter
is the time elapsed since the last event (Te), when longer Te produces higher hazard values. 相似文献
532.
Yang Tao Jiang Shaoyong Ge Lu Yang Jinghong Ling Hongfei Wu Nengyou Zhang Guangxue Liu Jian Chen Daohua 《Frontiers of Earth Science》2007,1(2):212-217
Gas hydrate is a recently-found new source of energy that mostly exists in marine sediments. In recent years, we have conducted
gas hydrate exploration in the South China Sea. The Xisha trough, one of the promising target areas for gas hydrate, is located
in the northern margin of the South China Sea, adjacent to several large oil and gas fields. The Xisha trough extends 420
km long with the water depth of 1 500 m in the west part and 3 400 m in the east part and deposits thick sediments with organic
matter content of 0.41%–1.02%. Previous studies on topographical features, geological P-T conditions, structural geology,
sedimentary geology and geophysical bottom simulating reflectors (BSR) in the Xisha trough suggest that this area is favorable
for the formation and accumulation of gas hydrate. In this paper, we present geochemical analyses for the sediment and pore
water from a piston core at Site XS-01 in the Xisha trough. Seven pore water samples were analyzed for their anion (Cl−, SO4
2−, Br−, I−) contents, cation (Na, K, Ca, Mg) contents and trace element (Li, B, Sr, Ba, Rb, Mn) contents. Eight sediment samples were
analyzed for stable carbon and oxygen isotopic compositions. A number of geochemical anomalies such as anions (e.g. Cl−, SO4
2−), cations (e.g. Ca, Mg) and trace elements (e.g. Sr, Ba, B) were found in this study. For example, the concentrations of
Cl− and SO4
2− in pore water show a decreasing trend with depth. The estimated sulfate/methane interface (SMI) is only 18 m, which is quite
similar to the SMI value of 23 m in the ODP164 Leg 997 at Blake Ridge. The Ca, Mg and Sr concentrations of pore water also
decrease with depth, but concentrations of Ba, and Mg/Ca and Sr/Ca ratios increase with depth. These geochemical anomalies
are quite similar to those found in gas hydrate locations in the world such as the Blake Ridge and may be related to the formation
and dissociation of gas hydrates. The salt exclusion effect during the gas hydrate formation will cause an increase in major
ion concentrations in the pore waters that diffused upward such as Cl. The anaerobic methane oxidation (AMO) may lead to the
change of SO4
2− and other cations such as Ca, Mg, Sr and Ba in pore water. Low δ
13C value of authigenic carbonates is a good indicator for gas hydrate occurrence. However, the bulk sediment samples we analyzed
all show normal δ
13C values similar to biogenic marine carbonates, and this may also suggest that no gas hydrate-related authigenic carbonates
exist or their amount is so small that they are not detectable by using this bulk analytical method. In conclusion, we suggest
that the Site XS-01 in the Xisha trough of the northern margin of the South China Sea is a potential target for further gas
hydrate exploration.
Translated from Quaternary Sciences, 2006, 26(3): 442–448 [译自: 第四纪研究] 相似文献
533.
534.
535.
文章分析了2002年7月16日浙江省北部地区的一次飑线天气过程。分析表明:高空前倾槽是这次飑线发生的大尺度环流背景,飑线发生在副热带高压边缘的不稳定区域。浙江省北部地区低层暖湿平流和地面的持续强烈升温,一方面为其积蓄了大量不稳定能量;另一方面高空冷平流形成并大大促进了这一地区的不稳定层结,这一不稳定层结主要是通过高空、地面的温度差动平流来实现的。水汽分析表明:低层850hPa的水汽输送是通过暖湿西南气流来实现的,强对流发生的浙江省北部地区正好处于低层850hPa干湿区交界中的露点锋区,而高层700hPa和500hPa的水汽输送却主要是通过西北气流来实现的。 相似文献
536.
利用天气学暴雨天气形势概念模式,根据KCMWF数值预报产品,提前2天预报出7月22和30日的暴雨过程;日本传真图预报的降水量和物理量,也满足多年使用日本传真图总结出的强降水指标。 相似文献
537.
速流结构防治泥石流的理论及应用 总被引:17,自引:4,他引:17
作为一种公路沿线发育的地质灾害,泥石流是路基、路面及相关建、构筑物最强烈的毁损动力。为了防治泥石流公路灾害,作者开发了速流结构。该结构由汇流槽和速流槽组成。其中汇流槽的侧墙长度、高度’、厚度等参数通过泥石流沟形态及其冲击力综合确定;而速流槽的宽度、侧墙及纵横断面则由泥石流排泄量、流速、锁固桩及流体性质综合确定。根据公路穿越速流结构的方式,将速流结构分为底越式和顶越式2种。速流结构对于解决具有大冲大淤特性的冲淤变动型沟谷泥石流灾害具有重要应用价值。据此可以确保穿越泥石流沟的公路构、建筑物的安全与稳定.确保公路交通有序进行。此外,文章对速流结构的一些子结构,如速流槽侧墙及锁固桩进行了力学分析。位于川西南西昌-木里干线公路的平川泥石流属于典型的冲淤变动型沟谷泥石流。该沟长度8.2km,沟床平均比降0.181,平均流速9.0m/s,沉积区宽度500m左右。为了确保该段公路交通的有序进行,作者干1999年实施了平川泥石流速流结构。通过2000年以来的研究及现场测试,显示了该防治结构的优越性。但是,作为一种特殊的水工结构,泥石流对速流结构的磨蚀作用是比较强烈的。因此,开展速流结构的抗冲、抗撞研究是进一步研究具有重要价值。 相似文献
538.
The bathymetry and abrupt changes in earthquake seismicity around the eastern end of the Java Trench suggest it is now blocked south–east of Sumba by the Australian, Jurassic-rifted, continental margin forming the largely submarine Roti–Savu Ridge. Plate reconstructions have demonstrated that from at least 45 Ma the Java Trench continued far to the east of Sumba. From about 12 Ma the eastern part of the Java Trench (called Banda Trench) continued as the active plate boundary, located between what was to become Timor Island, then part of the Australian proximal continental slope, and the Banda Volcanic Arc. This Banda Trench began to be obliterated by continental margin-arc collision between about 3.5 and 2 Ma.The present position of the defunct Banda Trench can be located by use of plate reconstructions, earthquake seismology, deep reflection seismology, DSDP 262 results and geological mapping as being buried under the para-autochthon below the foothills of southern Timor. Locating the former trench guides the location of the apparently missing large southern part of the Banda forearc that was carried over the Australian continental margin during the final stage of the period of subduction of that continental margin that lasted from about 12 Ma to about 3.5 Ma.Tectonic collision is defined and distinguished from subduction and rollback. Collision in the southern part of the Banda Arc was initiated when the overriding forearc basement of the upper plate reached the proximal part of the Australian continental slope of the lower plate, and subduction stopped. Collision is characterised by fold and thrust deformation associated with the development of structurally high decollements. This collision deformed the basement and cover of the forearc accretionary prism of the upper plate with part of the unsubducted Australian cover rock sequences from the lower plate. Together with parts of the forearc basement they now form the exposed Banda orogen. The conversion of the northern flank of the Timor Trough from being the distal part of the Banda forearc accretionary prism, carried over the Australian continental margin, into a foreland basin was initiated by the cessation of subduction and simultaneous onset of collisional tectonics.This reinterpretation of the locked eastern end of the Java Trench proposes that, from its termination south of Sumba to at least as far east as Timor, and probably far beyond, the Java-Banda Trench and forearc overrode the subducting Australian proximal continental slope, locally to within 60 km of the shelf break. Part of the proximal forearc's accretionary prism together with part of the proximal continental slope cover sequence were detached and thrust northwards over the Java-Banda Trench and forearc by up to 80 km along the southwards dipping Savu Thrust and Wetar Suture. These reinterpretations explain the present absence of any discernible subduction ocean trench in the southern Banda Arc and the narrowness of the forearc, reduced to 30 km at Atauro, north of East Timor. 相似文献
539.
LI Xusheng YANG Dayuan KE Xiankun ZHOU Lufu CHEN Deji YANG Tianmin XUE Guofu 《地理学报》2003,13(4):507-512
This paper describes valley bottom troughs of the Changjiang River and infers the geomorphologic development of troughs. Based on the morphology of the troughs, the following conclusions are drawn. (1) The deep troughs on the Three Gorges valley bottom are formed by river downcutting along the structural zones on the background of regional tectonic uplift at about 40-30 ka BP. (2) When river downcutting occurred in the river bed of Changjiang, the jets current (particularly eddy current) with a large number of pebbles ground and eroded the valley bottom, resulting in trough formation and deepening. Meanwhile, water currents with gravels and pebbles eroded the bank and the left wall of No.76 trough as well as the right wall of No.77 trough by striking, scouring, horizontal and vertical grinding. (3) The depth of the trough is mainly determined by the intensity of the water current and the consistency of bedrock against erosion, and is not controlled by the altitude of the sea level as the base level of erosion. 相似文献
540.
Hydrometeorological Aspects of Floods in India 总被引:1,自引:1,他引:1
The Indian sub-continent being located in the heart of the summermonsoon belt, receives in most parts more than 75% of its annual rainfall during the fourmonsoon months of June to September. As the bulk of summer monsoon rainfall occurs withina period of four months, naturally majority of floods occur in Indian rivers during thisseason only. The ground conditions also help in generating high percentage of run-offbecause of the antecedent wet conditions caused by rainy spells occurring within the monsoonperiod itself. Besides mentioning different weather systems, which cause heavy rainfall and consequentfloods, a detailed discussion of 15 years' floods in different river systems has alsobeen given in the article. This study has shown that the flood problem in India is mostly confinedto the states located in the Indo-Gangetic plains, northeast India and occasionally in therivers of Central India. 相似文献