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991.
992.
993.
Samples were collected at stations located in the mesohaline, oligohaline, and tidal fresh regions of the Potomac River, Maryland,
between April 1998 and December 1999 to evaluate the seasonal distribution of bacterioplankton and microbially labile organic
carbon (MLOC) in relation to hydrodynamic parameters (dissolved oxygen, salinity, and temperature). Bacterioplankton abundance
(BA) averaged 13 × 106 cells ml−1 at all stations, a value that is higher than the average observed in many other temperate estuaries around the world, and
were almost exclusively free-living. During the summer of 1998, BA often exceeded 30 × 106 cells ml−1 in the mesohaline region during periods of anoxia in subpycnocline waters. Dissolved MLOC typically accounted for 40% of
total MLOC and on some occasions during summer and autumn accounted for 80%. A significant positive relationship between dissolved
MLOC and BA was evident in the mesohaline Potomac River, the region where anoxia occurs each summer, but the regressions of
particulate MLOC and chla on BA were not significant at this location. In the mesohaline Potomac River, BA regressed negatively and significantly on
dissolved oxygen (r2=0.50, p<0.001). BA may be an important indicator of ecosystem health in this and other eutrophied estuaries, because of the
relationships between BA, dissolved MLOC, and dissolved oxygen in the salinity stratified Potomac River and because free-living
BA was elevated along the length of the river. 相似文献
994.
This paper presents the results of an experimental investigation of the complete sequence of sediment behaviour beneath progressive waves. The sediment was silty with d 50 = 0.060 mm. Two kinds of measurements were carried out: pore-water pressure measurements (across the sediment depth), and water-surface elevation measurements. The process of liquefaction/compaction was videotaped from the side simultaneously with the pressure and water-surface elevation measurements. The video records were then analysed to measure: (i) the time development of the mudline, (ii) the time development of liquefaction and compaction fronts in the sediment and (iii) the characteristics of the orbital motion of the liquefied sediment including the motion of the interface between the water body and the sediment. The ranges of the various quantities in the tests were: wave height, H = 9–17 cm, wave period, T = 1.6 sec, water depth = 42 cm, and the Shields parameter = 0.34–0.59. The experiments reveal that, with the introduction of waves, excess pore pressure builds up, which is followed by liquefaction during which internal waves are experienced at the interface of the water body and the liquefied sediment, the sequence of processes known from a previous investigation. This sequence of processes is followed by dissipation of the accumulated excess pore pressure and compaction of the sediment which is followed by the formation of bed ripples. The present results regarding the dissipation and compaction appear to be in agreement with recent centrifuge wave-tank experiments. As for the final stage of the sequence of processes (formation of ripples), the ripple steepness (normalized with the angle of repose) for sediment with liquefaction history is found to be the same as that in sediment with no liquefaction history. 相似文献
995.
Robert Shcherbakov Jordan Van Aalsburg John B. Rundle Donald L. Turcotte 《Tectonophysics》2006,413(1-2):53
Correlations in space and time play a fundamental role in earthquake processes. One direct manifestation of the effects of correlations is the occurrence of aftershocks due to the stress transfer in the vicinity of a main shock. Less obvious and more speculative changes in correlations may occur in the background seismicity before large earthquakes. Using statistical physics it is possible to introduce a measure of spatial correlations through a correlation length. This quantity characterizes how local fluctuations can influence the occurrence of earthquakes over distances comparable with the correlation length. In this work, the physical basis of spatial correlations of earthquakes is discussed in the context of critical phenomena and the percolation problem. The method of two-point correlation function is applied to the seismicity of California. Well defined variations in time of the correlation length are found for aftershock sequences and background seismicity. The scaling properties of our obtained distributions are analyzed with respect to changes in several scaling parameters such as lower magnitude cutoff of earthquakes, the maximum time interval between earthquakes, and the spatial size of the area considered. This scaling behavior can be described in a unified manner by utilizing the multifractal fit. Utilizing the percolation approach the time evolution of clusters of earthquakes is studied with the correlation length defined in terms of the radius of gyration of clusters. This method is applied to the seismicity of California. 相似文献
996.
lvaro Gonzlez Miguel Vzquez-Prada Javier B. Gmez Amalio F. Pacheco 《Tectonophysics》2006,424(3-4):319
Numerical models are starting to be used for determining the future behaviour of seismic faults and fault networks. Their final goal would be to forecast future large earthquakes. In order to use them for this task, it is necessary to synchronize each model with the current status of the actual fault or fault network it simulates (just as, for example, meteorologists synchronize their models with the atmosphere by incorporating current atmospheric data in them). However, lithospheric dynamics is largely unobservable: important parameters cannot (or can rarely) be measured in Nature. Earthquakes, though, provide indirect but measurable clues of the stress and strain status in the lithosphere, which should be helpful for the synchronization of the models.The rupture area is one of the measurable parameters of earthquakes. Here we explore how it can be used to at least synchronize fault models between themselves and forecast synthetic earthquakes. Our purpose here is to forecast synthetic earthquakes in a simple but stochastic (random) fault model. By imposing the rupture area of the synthetic earthquakes of this model on other models, the latter become partially synchronized with the first one. We use these partially synchronized models to successfully forecast most of the largest earthquakes generated by the first model. This forecasting strategy outperforms others that only take into account the earthquake series. Our results suggest that probably a good way to synchronize more detailed models with real faults is to force them to reproduce the sequence of previous earthquake ruptures on the faults. This hypothesis could be tested in the future with more detailed models and actual seismic data. 相似文献
997.
Bérengère Laslandes Florence Sylvestre Abdelfettah Sifeddine Bruno Turcq Ana Luiza S. Albuquerque Jorge Abrão 《Comptes Rendus Geoscience》2006,338(10):667-675
This study concerns a core collected in Brejo do Espinho's lagoon from Cabo Frio littoral (Brazil) submitted to dry influence of local upwelling controlled by north-east trade winds from the South Atlantic and particularly strengthened during El Niño events. Diatoms study supported by sedimentological and isotopic analyses shows dry phases infrequent before 4000 yr, a highly variable climatic phase between 3600 and 2900 yr and from 2400 yr onward a dryness enhancement. To cite this article: B. Laslandes et al., C. R. Geoscience 338 (2006). 相似文献
998.
999.
Discrete numerical modelling of rockfill dams 总被引:1,自引:0,他引:1
The aim of this study is to obtain quantitative information on the behaviour of rockfill used in embankment dams, and particularly on the influence of block breakage on the displacement field, from a numerical analysis using the Distinct element method. A methodology is set up to define the resistance of the 2D particles so that the same probability of breaking blocks may be reproduced as in a 3D material. The model uses the discrete element code PFC2D (Itasca Consulting Group Inc., PFC2D (Particle Flow Code in Two Dimensions), Version 3.0, 2002) and considers breakable clusters of 2D balls. The different parameters are determined from experimental data obtained from laboratory tests performed on rock blocks. The model is validated by comparing the results of the simulation of shearing tests with actual triaxial tests on rockfill material published in the literature. The numerical analysis of block crushing in an actual dam is proposed in the last part of this paper. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
1000.
P. González-Sampériz B.L. Valero-Garcés G. Jalut C. Martí-Bono A. Navas J.J. Dedoubat 《Quaternary Research》2006,66(1):38-52
Palynological, sedimentological and stable isotopic analyses of carbonates and organic matter performed on the El Portalet sequence (1802 m a.s.l., 42°48′00?N, 0°23′52?W) reflect the paleoclimatic evolution and vegetation history in the central-western Spanish Pyrenees over the last 30,000 yr, and provide a high-resolution record for the late glacial period. Our results confirm previous observations that deglaciation occurred earlier in the Pyrenees than in northern European and Alpine sites and point to a glacial readvance from 22,500 to 18,000 cal yr BP, coinciding with the global last glacial maximum. The patterns shown by the new, high-resolution pollen data from this continental sequence, chronologically constrained by 13 AMS 14C dates, seem to correlate with the rapid climate changes recorded in Greenland ice cores during the last glacial-interglacial transition. Abrupt events observed in northern latitudes (Heinrich events 3 to 1, Oldest and Older Dryas stades, Intra-Allerød Cold Period, and 8200 cal yr BP event) were also identified for the first time in a lacustrine sequence from the central-western Pyrenees as cold and arid periods. The coherent response of the vegetation and the lake system to abrupt climate changes implies an efficient translation of climate variability from the North Atlantic to mid latitudes. 相似文献