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71.
72.
F. Kaabouben A. I. Brahim E. Toto M. A. Baptista J. M. Miranda P. Soares J. F. Luis 《Journal of Seismology》2008,12(4):575-583
The 1975 May 26 earthquake, of magnitude Ms = 7.9, occurred in the North Atlantic close to the Azores Archipelago. Its epicentre,
as given by US Geological Survey, was 17.5° W, 35.9° N, 200 km south of the Gloria Fault. Several authors determined the focal
mechanism as a dextral strike-slip event with no significant dip-slip component, compatible with the relative motion between
Eurasia and Nubia plates but away from the presumed plate boundary. The 1975 earthquake generated a tsunami of small amplitude,
recorded at the Portuguese tide-gauge network, in Spain and Northern Africa. The peculiar location of the earthquake and tsunami
source and the generation of a noticeable tsunami were already discussed by several authors, but up to now, no direct modelling
of the tsunami generation and propagation was made to judge the set of source solutions obtained by seismological analysis.
In this paper, we present tsunami simulations, backward ray tracing and forward non-linear shallow water simulations using
data from Iberia and Azores and Northern Africa. We show that a good fit between observed data and synthetic waveforms can
be obtained with a focal mechanism with no significant dip-slip component, favouring its interpretation as almost pure dextral
strike-slip event located in an old fracture zone south of Gloria Fault. 相似文献
73.
An integrated geochemical and toxicological assessment of environmental mercury contamination and attendant human exposure
in Honda Bay, Palawan was undertaken in 1995 following a nationally reported pollution scare centered on a coastal jetty,
Sitio Honda Bay, constructed using approximately 1 million tons of tailings and beneficiation waste from a cinnabar mine.
Mercury (Hg) data for marine and fluvial sediments, fish tissues and human hair indicate that the toxicological hazard is
considerably lower than initially reported by state environment and health officials. Typical Hg concentrations in surficial
Honda Bay sediments were found to lie within the global background range (<60 μg/kg). Downcore profiles provide no evidence
of enhanced Hg fluxes coincident with the onset of mining and/or coastal tailings disposal. The mean and median Hg concentrations
recorded in tissues of six species of Honda Bay fish are compliant with thresholds established by the US Environmental Protection
Agency (US-EPA) for marketable stocks. Earlier reports of 'Minamata range' Hg concentrations in fish and shellfish from Honda
Bay remain unsubstantiated. Geochemical analyses of samples of the Sitio Honda Bay substrate have confirmed the prevalence
of solid-phase Hg concentrations to ca. 340 mg/kg. The speciation of Hg is, however, dominated by secondary oxides of low
bioavailability. The mean Hg concentration in hair from Sitio Honda Bay residents (4.41 mg/kg) was found to be statistically
analogous to that for a neighbouring coastal community unimpacted by the coastal disposal of mine waste. A negligible residential
exposure factor is thus inferred for the former. Relatively high hair Hg burdens prevail throughout the coastal Honda Bay
population, consistent with significant methyl Hg ingestion through daily fish consumption. The data presented provide no
environmental or toxicological justification for immediate remedial action.
Received: 14 May 1998/Accepted: 1 September 1998 相似文献
74.
Morpho-tectonic analysis of the Azores Volcanic Plateau from a new bathymetric compilation of the area 总被引:13,自引:0,他引:13
Lourenço N. Miranda J.M. Luis J.F. Ribeiro A. Victor L.A. Mendes Madeira J. Needham H.D. 《Marine Geophysical Researches》1998,20(3):141-156
The existing studies of the Azores triple junction, although based on specific geological or geophysical data, largely rely upon morphological considerations. However, there is no systematic bathymetric coverage of this area, and the quality of the available bathymetric charts does not allow consistent morpho-structural analysis.In this work we present a new bathymetric grid elaborated with all the available data sources in an area comprised between 24° W to 32° W and 36° N to 41° N. The basic data set corresponds to the merge of NGDC data with new swath profiles. This new map, included as an Appendix, combined with other results from seismology and neotectonics, is the basis for the study of the morpho-structural pattern of the Azores area, the present day stress field and its implications on the current view of the Azores geodynamics.As a major result, we conclude that the Azores region is controlled by two sets of conjugated faults with 120° and 150° strikes that establish the framework for the onset of volcanism, expressing as linear volcanic ridges or as point source volcanism. This interaction develops what can be considered as the morphological signature of the Azores Spreading axis segmentation. We argue that the Azores domain, presently in a broad transtensional regime, is acting simultaneously as a ultra slow spreading centre and as a transfer zone between the MAR and the dextral Gloria Fault, as it accommodates the differential shear movement between the Eurasian and African plates. 相似文献
75.
Total dissolved gas pressure (PTDG) measurements are useful to measure accurate in situ dissolved gas concentrations in groundwater, but challenged by in-well degassing. Although in-well degassing has been widely observed, its cause(s) are not clear. We investigated the mechanism(s) by which gas-charged groundwater in a recently pumped well becomes degassed. Vertical PTDG and dissolved gas concentration profiles were monitored in the standing water column (SWC) of a groundwater well screened in a gas-charged aquifer for 7 days before and 15 days after pumping. Prior to pumping, PTDG values remained relatively constant and below calculated bubbling pressure (PBUB) at all depths. In contrast, significant increases in PTDG were observed at all depths after pumping was initiated, as fresh groundwater with elevated in situ PTDG values was pumped through the well screen. After pumping ceased, PTDG values decreased to below PBUB at all depths over the 15-day post-pumping period, indicating well degassing was active over this time frame. Vertical profiles of estimated dissolved gas concentrations before and after pumping provided insight into the mechanism(s) by which in-well degassing occurred in the SWC. During both monitoring periods, downward mixing of dominant atmospheric and/or tracer gases, and upwards mixing of dominant groundwater gases were observed in the SWC. The key mechanisms responsible for in-well degassing were (i) bubble exsolution when PTDG exceeded PBUB as gas-charged well water moves upwards in the SWC during recovery (i.e., hydraulic gradient driven convection), (ii) microadvection caused by the upward migration of bubbles under buoyancy, and (iii) long-term, thermally driven vertical convection. 相似文献
76.
Evaluation of seismic displacement demands from the September 19, 2017 Puebla‐Morelos (Mw = 7.1) earthquake in Mexico City 下载免费PDF全文
This short communication presents the assessment of seismic inelastic and elastic displacement demands computed from earthquake ground motions (EQGMs) recorded in Mexico City during the intermediate‐depth intraslab Puebla‐Morelos earthquake on 19 September 2017 (Mw = 7.1). Evaluation is conducted by means of peak elastic and inelastic displacement demand spectra, inelastic displacement ratio, CR, spectra, and generalized interstory drift spectra computed for selected recording stations located in different soil sites of Mexico City, including those located in areas of reported collapsed buildings. Results of this study confirm previous observations made from interplate (subduction) EQGMs that peak inelastic displacement demands are greater than corresponding elastic counterparts for short‐to‐medium period structures, while the opposite is true for medium‐to‐long period structures. Possible basin site effects were identified from generalized interstory drift spectra. It is also shown that an equation introduced in the literature to obtain estimates of CR developed from interplate EQGMs provides also a good estimate for mean CR computed from the intermediate‐depth intraslab EQGMs. 相似文献
77.
The effective application of normal moveout correction processes mainly depends on four factors: the chosen traveltime approximation, the stretching associated with the given traveltime, crossing events and phase changes, the last two being inherent to the seismic data. In this context, we conduct a quantitative analysis on stretching considering a general traveltime expression depending on half-offset and midpoint coordinates. Through this analysis, we propose a mathematically proven procedure to eliminate stretching, which can be applied to any traveltime approximation. The proposed method is applied to synthetic and real data sets, considering different traveltime approximations and achieved complete elimination of stretching. 相似文献
78.
Although for many years it was thought that amplitude scaling of acceleration time series to reach a target intensity did not introduce any bias in the results of nonlinear response history analyses, recent studies have showed that scaling can lead to an overestimation of deformation demands with increasing scale factors. Some studies have suggested that the bias can be explained by differences in spectral shape between the response spectra of unscaled and scaled records. On the basis of these studies, some record selection procedures assume that if records are selected using spectral-shape-matching procedures, amplitude scaling does not induce any bias on the structural response. This study evaluates if bias is introduced on lateral displacement demands and seismic collapse risk estimates even when spectral shape is carefully taken into consideration when selecting ground motions. Several single-degree-of-freedom and multiple-degree-of-freedom systems are analyzed when subjected to unscaled and scaled ground motions selected to approximately match the mean and the variance of the conditional spectrum at the target level of intensity. Results show that an explicit consideration of spectral shape is not enough to avoid a systematic overestimation of lateral displacement demands and collapse probabilities as the scale factor increases. Moreover, the bias is observed in practically all cases for systems with strength degradation and it increases with decreasing period and decreasing lateral strength relative to the strength required to remain elastic. Key reasons behind the bias are presented by evaluating input energy, causal parameters, and damaging pulse distributions in unscaled and scaled ground motion sets. 相似文献
79.
The buildings’ capacity to maintain minimum structural safety levels during natural disasters, such as earthquakes, is recognisably one of the aspects that most influence urban resilience. Moreover, the public investment in risk mitigation strategies is fundamental, not only to promote social and urban and resilience, but also to limit consequent material, human and environmental losses. Despite the growing awareness of this issue, there is still a vast number of traditional masonry buildings spread throughout many European old city centres that lacks of adequate seismic resistance, requiring therefore urgent retrofitting interventions in order to both reduce their seismic vulnerability and to cope with the increased seismic requirements of recent code standards. Thus, this paper aims at contributing to mitigate the social and economic impacts of earthquake damage scenarios through the development of vulnerability-based comparative analysis of some of the most popular retrofitting techniques applied after the 1998 Azores earthquake. The influence of each technique individually and globally studied resorting to a seismic vulnerability index methodology integrated into a GIS tool and damage and loss scenarios are constructed and critically discussed. Finally, the economic balance resulting from the implementation of that techniques are also examined. 相似文献
80.
Strength-reduction factors that reduce ordinates of floor spectra acceleration due to nonlinearity in the secondary system are investigated. In exchange for permitting some inelastic deformation to occur in the secondary system or its supports, these strength reduction factors allow to design the nonstructural elements or their supports for lateral forces that are smaller than those that would be required to maintain them elastically during earthquakes. This paper presents the results of a statistical analysis on component strength-reduction factors that were computed considering floor motions recorded on instrumented buildings in California during various earthquakes. The effect of yielding in the component or its anchorage/bracing in offering protection against excessive component acceleration demands is investigated. It is shown that strength-reduction factors computed from floor motions are significantly different from those computed from ground motions recorded on rock or on firm soils. In particular, they exhibit much larger reductions for periods tuned or nearly tuned to the dominant modal periods of the building response. This is due to the large differences in frequency content of ground motions and floor motions, with the former typically characterized by wide-band spectra whereas the latter are characterized by narrow-band spectra near periods of dominant modes in the response of the building. Finally, the study provides approximate equations to estimate component strength-reduction factors computed through nonlinear regression analyses. 相似文献