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11.
Regional studies of Sq variations measured at five permanent observatories in Argentina were performed to estimate the distribution of the electrical conductivity of the upper and mid-mantle, reaching a depth of approximately 800 km. After separating the total field into its parts of external and internal origin, the in duced field was modeled, applying two-dimensional techniques. Additionally, induction arrows and an equivalent apparent resistivity curve were calculated A north-south heterogeneity was detected, showing an increase of the conductivity of the shallower layers north of the profile, which may be related to the presence of high volcanic activity in the area.Also at CONICET (Consejo Nacional de Investigaciones Científicas y Técnicas). 相似文献
12.
Rainer Wolf Alicia B. Woodrow Richard A.F. Grieve 《Geochimica et cosmochimica acta》1980,44(7):1015-1022
Eleven impact melt and 6 basement rock samples from 4 craters were analyzed by neutron activation for Au, Co, Cr, Fe, Ge, Ir, Ni, Os, Pd, Re and Se. Wanapitei Lake, Ontario: the impact melts show uniform enrichments corresponding to 1–2% C1-chondrite material. Interelement ratios (, , ) suggest that the impacting body was a Cl-, C2-, or LL-chondrite. Nicholson Lake, North West Territory: Ni, Cr and Co are distinctly more enriched than Ir and Au which tentatively suggests an olivine-rich achondrite (nakhlite or ureilite). Gow Lake, Saskatchewan and Mistastin, Labrador: small enrichments in Ir and Ni; both the low ratios and low Cr content suggest iron meteorites, but the signals are too weak for conclusive identification.A tentative comparison of meteoritic signatures at 10 large, ≥4km craters and their presumed celestial counterparts (13 Apollo and Amor asteroids) shows more irons and achondrites among known projectile types, and a preponderance of S-type objects, having no known meteoritic equivalent, among asteroids. It is not yet clear that these differences are significant, in view of the tentative nature of the crater identifications (achondrites in particular), and the limited statistics. 相似文献
13.
Jorge Ramón Alicia Correa Edison Timbe Giovanny M. Mosquera Enma Mora Patricio Crespo 《水文研究》2021,35(6):e14209
Hydrogeochemical based mixing models have been successfully used to investigate the composition and source identification of streamflow. The applicability of these models is limited due to the high costs associated with data collection and the hydrogeochemical analysis of water samples. Fortunately, a variety of mixing models exist, requiting different amount of data as input, and in data scarce regions it is likely that preference will be given to models with the lowest requirement of input data. An unanswered question is if models with high or low input requirement are equally accurate. To this end, the performance of two mixing models with different input requirement, the mixing model analysis (MMA) and the end-member mixing analysis (EMMA), were verified on a tropical montane headwater catchment (21.7 km2) in the Ecuadorian Andes. Nineteen hydrogeochemical tracers were measured on water samples collected weekly during 3 years in streamflow and eight potential water sources or end-members (precipitation, lake water, soil water from different horizons and springs). Results based on 6 conservative tracers, revealed that EMMA (using all tracers) and MMA (using pair-combinations out of the 6 conservative ones), identified the same end-members: rainfall, soil water and spring water., as well as, similar contribution fractions to streamflow from rainfall 21.9% and 21.4%, soil water 52.7% and 52.3%, and spring water 26.1% and 28.7%, respectively. Our findings show that a hydrogeochemical mixing model requiring a few tracers can provide similar outcomes than models demanding more tracers as input data. This underlines the value of a preliminary detailed hydrogeochemical characterization as basis to derive the most cost-efficient monitoring strategy. 相似文献
14.
A new method to calculate volcanic susceptibility, i.e. the spatial probability of vent opening, is presented. Determination of volcanic susceptibility should constitute the first step in the elaboration of volcanic hazard maps of active volcanic fields. Our method considers different criteria as possible indicators for the location of future vents, based on the assumption that these locations should correspond to the surface expressions of the most likely pathways for magma ascent. Thus, two groups of criteria have been considered depending on the time scale (short or long term) of our approach. The first one accounts for long-term hazard assessment and corresponds to structural criteria that provide direct information on the internal structure of the volcanic field, including its past and present stress field, location of structural lineations (fractures and dikes), and location of past eruptions. The second group of criteria concerns to the computation of susceptibility for short term analyses (from days to a few months) during unrest episodes, and includes those structural and dynamical aspects that can be inferred from volcano monitoring. Thus, a specific layer of information is obtained for each of the criteria used. The specific weight of each criterion on the overall analysis depends on its relative significance to indicate pathways for magma ascent, on the quality of data and on their degree of confidence. The combination of the different data layers allows to create a map of the spatial probability of future eruptions based on objective criteria, thus constituting the first step to obtain the corresponding volcanic hazards map. The method has been used to calculate long-term volcanic susceptibility on Tenerife (Canary Islands), and the results obtained are also presented. 相似文献
15.
Servando De la Cruz-Reyna Izumi Yokoyama Alicia Martínez-Bringas Esteban Ramos 《Bulletin of Volcanology》2008,70(6):753-767
Popocatépetl Volcano is located in the central Mexican Volcanic Belt, within a densely populated region inhabited by over
20 million people. The eruptive history of this volcano indicates that it is capable of producing a wide range of eruptions,
including Plinian events. After nearly 70 years of quiescence, Popocatépetl reawakened in December 21, 1994. The eruptive
activity has continued up until the date of this submission and has been characterized by a succession of lava dome growth-and-destruction
episodes, similar to events that have apparently been typical for Popocatépetl since the fourteenth century. In this regime,
the episodes of effusive and moderately explosive activity alternate with long periods of almost total quiescence. In this
paper we analyze five years of volcano-tectonic seismicity preceding the initial eruption of the current episode. The evolution
of the V-T seismicity shows four distinct stages, which we interpret in terms of the internal processes which precede an eruption
after a long period of quiescence. The thermal effects of a magma intrusion at depth, the fracturing related to the slow development
of magma-related fluid pathways, the concentration of stress causing a protracted acceleration of this process, and a final
relaxation or redistribution of the stress shortly before the initial eruption are reflected in the rates of V-T seismic energy
release. A hindsight analysis of this activity shows that the acceleration of the seismicity in the third stage asymptotically
forecast the time of the eruption. The total seismic energy release needed to produce an eruption after a long period of quiescence
is related to the volume of rock that must be fractured so imposing a characteristic threshold limit for polygenetic volcanoes,
limit that was reached by Popocatépetl before the eruption. 相似文献
16.
Trevor I. Allen David J. Wald Paul S. Earle Kristin D. Marano Alicia J. Hotovec Kuowan Lin Michael G. Hearne 《Bulletin of Earthquake Engineering》2009,7(3):701-718
We present an Atlas of ShakeMaps and a catalog of human population exposures to moderate-to-strong ground shaking (EXPO-CAT)
for recent historical earthquakes (1973–2007). The common purpose of the Atlas and exposure catalog is to calibrate earthquake
loss models to be used in the US Geological Survey’s Prompt Assessment of Global Earthquakes for Response (PAGER). The full
ShakeMap Atlas currently comprises over 5,600 earthquakes from January 1973 through December 2007, with almost 500 of these
maps constrained—to varying degrees—by instrumental ground motions, macroseismic intensity data, community internet intensity
observations, and published earthquake rupture models. The catalog of human exposures is derived using current PAGER methodologies.
Exposure to discrete levels of shaking intensity is obtained by correlating Atlas ShakeMaps with a global population database.
Combining this population exposure dataset with historical earthquake loss data, such as PAGER-CAT, provides a useful resource
for calibrating loss methodologies against a systematically-derived set of ShakeMap hazard outputs. We illustrate two example
uses for EXPO-CAT; (1) simple objective ranking of country vulnerability to earthquakes, and; (2) the influence of time-of-day
on earthquake mortality. In general, we observe that countries in similar geographic regions with similar construction practices
tend to cluster spatially in terms of relative vulnerability. We also find little quantitative evidence to suggest that time-of-day
is a significant factor in earthquake mortality. Moreover, earthquake mortality appears to be more systematically linked to
the population exposed to severe ground shaking (Modified Mercalli Intensity VIII+). Finally, equipped with the full Atlas
of ShakeMaps, we merge each of these maps and find the maximum estimated peak ground acceleration at any grid point in the
world for the past 35 years. We subsequently compare this “composite ShakeMap” with existing global hazard models, calculating
the spatial area of the existing hazard maps exceeded by the combined ShakeMap ground motions. In general, these analyses
suggest that existing global, and regional, hazard maps tend to overestimate hazard. Both the Atlas of ShakeMaps and EXPO-CAT
have many potential uses for examining earthquake risk and epidemiology. All of the datasets discussed herein are available
for download on the PAGER Web page ().
T. I. Allen and M. G. Hearne—contracted through Synergetics Incorporated. 相似文献
17.
This paper deals with a general discussion of the subject matter. Recent literature is analyzed including some incorrect treatments of particular cases. Finally, a variational solution is obtained for the case where a linear distribution of stresses is applied to the plate boundaries. The lower natural frequencies are then tabulated as a function of the governing geometric and mechanical parameters. 相似文献
18.
Tom Bradwell Derek Fabel Chris D. Clark Richard C. Chiverrell David Small Rachel K. Smedley Margot H. Saher Steven G. Moreton Dayton Dove S. Louise Callard Geoff A. T. Duller Alicia Medialdea Mark D. Bateman Matthew J. Burke Neil McDonald Sean Gilgannon Sally Morgan David H. Roberts Colm ó Cofaigh 《第四纪科学杂志》2021,36(5):871-933
Predicting the future response of ice sheets to climate warming and rising global sea level is important but difficult. This is especially so when fast-flowing glaciers or ice streams, buffered by ice shelves, are grounded on beds below sea level. What happens when these ice shelves are removed? And how do the ice stream and the surrounding ice sheet respond to the abruptly altered boundary conditions? To address these questions and others we present new geological, geomorphological, geophysical and geochronological data from the ice-stream-dominated NW sector of the last British–Irish Ice Sheet (BIIS). The study area covers around 45 000 km2 of NW Scotland and the surrounding continental shelf. Alongside seabed geomorphological mapping and Quaternary sediment analysis, we use a suite of over 100 new absolute ages (including cosmogenic-nuclide exposure ages, optically stimulated luminescence ages and radiocarbon dates) collected from onshore and offshore, to build a sector-wide ice-sheet reconstruction combining all available evidence with Bayesian chronosequence modelling. Using this information we present a detailed assessment of ice-sheet advance/retreat history, and the glaciological connections between different areas of the NW BIIS sector, at different times during the last glacial cycle. The results show a highly dynamic, partly marine, partly terrestrial, ice-sheet sector undergoing large size variations in response to sub-millennial-scale climatic (Dansgaard–Oeschger) cycles over the last 45 000 years. Superimposed on these trends we identify internally driven instabilities, operating at higher frequency, conditioned by local topographic factors, tidewater dynamics and glaciological feedbacks during deglaciation. Specifically, our new evidence indicates extensive marine-terminating ice-sheet glaciation of the NW BIIS sector during Greenland Stadials 12 to 9 – prior to the main ‘Late Weichselian’ ice-sheet glaciation. After a period of restricted glaciation, in Greenland Interstadials 8 to 6, we find good evidence for rapid renewed ice-sheet build-up in NW Scotland, with the Minch ice-stream terminus reaching the continental shelf edge in Greenland Stadial 5, perhaps only briefly. Deglaciation of the NW sector took place in numerous stages. Several grounding-zone wedges and moraines on the mid- and inner continental shelf attest to significant stabilizations of the ice-sheet grounding line, or ice margin, during overall retreat in Greenland Stadials 3 and 2, and to the development of ice shelves. NW Lewis was the first substantial present-day land area to deglaciate, in the first half of Greenland Stadial 3 at a time of globally reduced sea-level c. 26 kabp , followed by Cape Wrath at c. 24 kabp. The topographic confinement of the Minch straits probably promoted ice-shelf development in early Greenland Stadial 2, providing the ice stream with additional support and buffering it somewhat from external drivers. However, c. 20–19 kabp , as the grounding-line migrated into shoreward deepening water, coinciding with a marked change in marine geology and bed strength, the ice stream became unstable. We find that, once underway, grounding-line retreat proceeded in an uninterrupted fashion with the rapid loss of fronting ice shelves – first in the west, then the east troughs – before eventual glacier stabilization at fjord mouths in NW Scotland by ~17 kabp. Around the same time, ~19–17 kabp , ice-sheet lobes readvanced into the East Minch – possibly a glaciological response to the marine-instability-triggered loss of adjacent ice stream (and/or ice shelf) support in the Minch trough. An independent ice cap on Lewis also experienced margin oscillations during mid-Greenland Stadial 2, with an ice-accumulation centre in West Lewis existing into the latter part of Heinrich Stadial 1. Final ice-sheet deglaciation of NW mainland Scotland was punctuated by at least one other coherent readvance at c. 15.5 kabp , before significant ice-mass losses thereafter. At the glacial termination, c. 14.5 kabp , glaciers fed outwash sediment to now-abandoned coastal deltas in NW mainland Scotland around the time of global Meltwater Pulse 1A. Overall, this work on the BIIS NW sector reconstructs a highly dynamic ice-sheet oscillating in extent and volume for much of the last 45 000 years. Periods of expansive ice-sheet glaciation dominated by ice-streaming were interspersed with periods of much more restricted ice-cap or tidewater/fjordic glaciation. Finally, this work indicates that the role of ice streams in ice-sheet evolution is complex but mechanistically important throughout the lifetime of an ice sheet – with ice streams contributing to the regulation of ice-sheet health but also to the acceleration of ice-sheet demise via marine ice-sheet instabilities. 相似文献
19.
20.
Guiomar Rotllant Carles G. Simeó Guillermo Guerao Marta Sastre Daniel F. R. Cleary Ricardo Calado Alicia Estévez 《Marine Ecology》2014,35(3):298-307
The spider crab Maja brachydactyla is an important commercial species in Europe and supports intensive fisheries in the NE Atlantic. A field survey was performed to assess long‐term and consecutive interannual (2005–2010) variation of the biochemical composition of newly hatched larvae of M. brachydactyla. Larval biochemical profiles differed significantly among years, with pronounced differences being recorded in 2010. Differences among batches of newly hatched larvae were mainly explained by the contribution of triacylglycerols and, to a lesser degree, by protein and lipid content. The use of different nutrition indices is discussed. The biochemical composition of newly hatched larvae from M. brachydactyla was highly variable, even though surveyed broodstock was obtained from the same local population and was always sampled during the same season. The unpredictable biochemical profiles of newly hatched larvae may condition their survival and recruitment. This largely overlooked aspect of larval variability should be considered in future fisheries management strategies and captive production of marine organisms that still rely on wild seeds. 相似文献