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951.
根据地震前震源及周围地区出现的空区的时空特征, 提出了一种估计ldquo;区域-时间-长度rdquo;算法中特征参数(特征距离和特征时间)的方法, 并介绍了2006年一些学者提出的用相关系数估计特征参数的方法. 用这两种方法研究了近年来发生在中国大陆西北和西南地区的4次中强地震前的地震活动异常特征. 结果表明, 本文提出的估计特征参数的方法是一种具有物理意义、 使用简单的方法, 对中国大陆西北和西南地区的4次中强震的应用都有较好的效果. 相似文献
952.
953.
In many geoscientific applications, one needs to recover the quantities of interest from indirect observations blurred by
colored noise. Such quantities often correspond to the values of bounded linear functionals acting on the solution of some
observation equation. For example, various quantities are derived from harmonic coefficients of the Earth’s gravity potential.
Each such coefficient is the value of the corresponding linear functional. The goal of this paper is to discuss new means
to use information about the noise covariance structure, which allows order-optimal estimation of the functionals of interest
and does not involve a covariance operator directly in the estimation process. It is done on the basis of a balancing principle
for the choice of the regularization parameter, which is new in geoscientific applications. A number of tests demonstrate
its applicability. In particular, we could find appropriate regularization parameters by knowing a small part of the gravitational
field on the Earth’s surface with high precision and reconstructing the rest globally by downward continuation from satellite
data. 相似文献
954.
Liu Lifang 《中国地震研究》2007,21(1):62-73
Based on horizontal-component digital seismograms recorded on 6 stations of the Yunnan Regional Digital Network, we inversed the inelastic attenuation in the source region of the Shidian swarm using the Atkinson method and the site responses of the 6 stations 200km around the Shidian epicenters using the Moya method. The observational seismic waveform data were corrected by removing the propagation, instrument and site effects before the source parameters of the Shidian swarm in 2001 were determined using genetic algorithms. The results are as follows: (1) There is a linear relation between seismic moment and local magnitude. The seismic moment is between 1×10~ 12 and 10~ 14 N·m. The rupture radius of the seismic focus varies from 157m to 973m. The seismic moment and the rupture radius maintain a linear correlation. (2) The corner frequency increases as the seismic moment decreases. Based on the expression between corner frequency and seismic moment using least squares fitting, we can obtain the estimated value of the corner frequency. The time-varying value of the calculated corner frequency minus the estimated corner frequency shows that there were continuous high and low anomalies before the strong aftershocks. (3) The seismic stress drop is in the range of 0.07~1.55MPa. The stress drop seems independent of the local magnitude. The variation of stress drops is high before the occurrence of the strong aftershocks. (4) The depth of aftershocks is mostly in a range from 5km to 10km, which means that energy release of aftershocks is mainly concentrated in this range of depth. 相似文献
955.
On the basis of gap's temporal-spatial characteristics in and around source area before an earthquake, we propose a method to estimate characteristic parameters (characteristic distance and time) in the region-time-length (RTL) algorithm and introduce the method of correlation coefficient developed by some authors in 2006 to determine the characteristic parameters. The anomalous seismic activities before four moderately strong earthquakes occurred in the northwestern and southwestern China in recent years are studied by the two methods. The results show that the method to estimate characteristic parameters advanced in this paper is a simple one, which possesses a physical meaning and is well applicable to the four moderately strong earthquakes studied. 相似文献
956.
Marian Ivan 《Journal of Seismology》2007,11(1):73-85
The seismic attenuation in the Vrancea region (Romania) is investigated from teleseismic recordings of P and pP waves during
the four major, intermediate-depth Romanian events that occurred since the onset of digital instrumentation. Most stations
are located in Canada and in the United States, being equipped with a variety of sensors, especially short-period ones. The
amplitude spectral ratio method is used, assuming no frequency dependence of the Q
P factor in the range 0.2–2 Hz. No apparent correlation between the derived attenuation value and the type of recording sensor
is observed. Lateral variations of the attenuation are obtained, with a very low Q
P area (values down to 33) located in the northwestern part of the Vrancea seismogenic volume. For the stations with different
azimuth angles in relation to the epicentral area, Q
P values routinely exceed 200. Most likely, the low attenuation values are related to an upwelling mantle material located
immediately beneath the crust, but limited in depth to at least 100 km. 相似文献
957.
A worldwide database of liquefaction-induced sand blows has been compiled to generate empirical relationships between earthquake
parameters and sand blow geometries. Curves resulting from these empirical relationships can be used afterwards for the study
of historic and pre-historic earthquakes that formed sand blows. This database only incorporates instrumental earthquakes
inducing sand blows, characterised in terms of magnitude (Mw), hypocentral location and focal mechanism solution. Two relationships
are herein presented. The first curve, minimum likelihood of liquefaction occurrence (magnitude/epicentral distance), displays
a logarithmic behaviour, as others already proposed. The second curve of sand-blow diameter vs epicentral distance is the
first proposal of this kind, whose shape follows a negative power-law. Magnitude estimations of (pre-)historic earthquakes
then may be attempted through these empirical relationships. Resulting magnitudes derived from these curves should be mostly
considered as underestimated. The curves will reflect actual magnitudes only if, correspondingly, the farthest and largest
blow has ever been reported during the after-earthquake survey. Following the same principle, a magnitude estimation resulting
from the measuring of a sand blow can only be considered as minimal because it is almost impossible to ascertain that the
measured feature is the biggest one produced by the earthquake under evaluation. Finally, these results call for thorough
surveys of induced effects after every future moderate-to-large earthquake, as any empirical relationship simply improves
by incorporating new data. 相似文献
958.
Based on spectral ratio method, a joint inversion method was used to obtain parameters of Lg wave attenuation and site response. The inversion method allows simple and direct (two-parameter) determination of Lg wave attenua- tion and site response from sparse spectral data, which are not affected by radiation pattern factor and different response of same instrument after geometrical spreading. The method was used successfully for estimating site re- sponse of stations of Zhejiang Seismic Network and measuring Lg wave attenuation. The study is based on 20 earth- quakes occurred in northeast of Taiwan with magnitude MS5.0~6.7 and 960 seismic wave records from 16 stations in Zhejiang area from 2002 to 2005. The parameters of site response and Lg attenuation were calculated with a fre- quency interval of 0.2 Hz in the range of 0.5 Hz to 10 Hz. Lg wave attenuation coefficient corresponding to U-D, E-W and N-S components are γ ( f )=0.00175 f 0.43485, γ ( f )=0.00145f 0.48467 and γ ( f )=0.0021f 0.41241, respectively. It is found that the site response is component-independent. It is also found that the site response of QIY station is significant above the frequency of 1.5 Hz, and that the site response of NIB station is low for most frequency 相似文献
959.
960.
The quadrant technique, a conditional sampling approach that allocates Reynolds stresses into four different types of events
(ejections or bursts, sweeps, inward interactions and outward interactions), is applied to stable marine atmospheric boundary-layer
data, collected in the framework of the Coupled Boundary Layer Air—Sea Transfer, Low wind component experiment at Nantucket
Island, Massachusetts, USA. The general properties of both scalar and momentum transport are analyzed under the scope of quadrant
analysis experimentally and theoretically. It is shown that the third-order Gram–Charlier series is necessary and even sufficient
in most of the cases, in describing the experimental time and flux contributions of each quadrant to the total transfer, for
both scalar and momentum transport, while the ability of the Gaussian distribution is limited to outlining the general pattern
of these quantities. Moreover, a threshold value is applied to the conditional analysis, separating the most important events
from the less significant ones and the sensitivity of the flux and especially the time fraction of each quadrant on the choice
of this value is presented and discussed. Also, a set of numerically extracted equations, completing the analytical relations,
is derived, enabling the prediction of the time and flux fractions of each quadrant, for a wide range of correlation coefficient
and threshold values. Finally, the sensitivity of the analysis to the atmospheric stability and the Reynolds averaging scales
showed that correlated and uncorrelated motions tend to balance for increasingly stable conditions and/or for large time scales. 相似文献