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DEFORMATIONAL BEHAVIOURS OF ALLUVIAL UNITS DETECTED BY ADVANCED RADAR INTERFEROMETRY IN THE VEGA MEDIA OF THE SEGURA RIVER,SOUTHEAST SPAIN 
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We study the evolution of finite perturbations in the Lorenz '96 model, a meteorological toy model of the atmosphere. The initial perturbations are chosen to be aligned along different dynamic vectors: bred, Lyapunov, and singular vectors. Using a particular vector determines not only the amplification rate of the perturbation but also the spatial structure of the perturbation and its stability under the evolution of the flow. The evolution of perturbations is systematically studied by means of the so-called mean-variance of logarithms diagram that provides in a very compact way the basic information to analyse the spatial structure. We discuss the corresponding advantages of using those different vectors for preparing initial perturbations to be used in ensemble prediction systems, focusing on key properties: dynamic adaptation to the flow, robustness, equivalence between members of the ensemble, etc. Among all the vectors considered here, the so-called characteristic Lyapunov vectors are possibly optimal, in the sense that they are both perfectly adapted to the flow and extremely robust. 相似文献
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PATRICK MEISTER CAROLINA REYES WILL BEAUMONT MIGUEL RINCON LISA COLLINS WILLIAM BERELSON LOWELL STOTT FRANK CORSETTI KENNETH H. NEALSON 《Sedimentology》2011,58(7):1810-1830
Dolomite [Ca,Mg(CO3)2] precipitation from supersaturated ionic solutions at Earth surface temperatures is considered kinetically inhibited because of the difficulties experienced in experimentally reproducing such a process. Nevertheless, recent dolomite is observed to form in hypersaline and alkaline environments. Such recent dolomite precipitation is commonly attributed to microbial mediation because dolomite has been demonstrated to form in vitro in microbial cultures. The mechanism of microbially mediated dolomite precipitation is, however, poorly understood and it remains unclear what role microbial mediation plays in natural environments. In the study presented here, simple geochemical methods were used to assess the limitations and controls of dolomite formation in Deep Springs Lake, a highly alkaline playa lake in eastern California showing ongoing dolomite authigenesis. The sediments of Deep Springs Lake consist of unlithified, clay‐fraction dolomite ooze. Based on δ18O equilibria and textural observations, dolomite precipitates from oxygenated and agitated surface brine. The Na‐SO4‐dominated brine contains up to 500 mm dissolved inorganic carbon whereas Mg2+ and Ca2+ concentrations are ca 1 and 0·3 mm , respectively. Precipitation in the subsurface probably is not significant because of the lack of Ca2+ (below 0·01 mm ). Under such highly alkaline conditions, the effect of microbial metabolism on supersaturation by pH and alkalinity increase is negligible. A putative microbial effect could, however, support dolomite nucleation or support crystal growth by overcoming a kinetic barrier. An essential limitation on crystal growth rates imposed by the low Ca2+ and Mg2+ concentrations could favour the thermodynamically more stable carbonate phase (which is dolomite) to precipitate. This mode of unlithified dolomite ooze formation showing δ13C values near to equilibrium with atmospheric CO2 (ca 3‰) contrasts the formation of isotopically light (organically derived), hard‐lithified dolomite layers in the subsurface of some less alkaline environments. Inferred physicochemical controls on dolomite formation under highly alkaline conditions observed in Deep Springs Lake may shed light on conditions that favoured extensive dolomite formation in alkaline Precambrian oceans, as opposed to modern oceans where dolomites only form diagenetically in organic C‐rich sediments. 相似文献
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Existing formulations for bed sediment entrainment under steady flow are incapable of explaining two well-documented observational facts: (i) water flow requires considerably higher dimensionless shear stresses to move the bed grains than air flow; and (ii) under open channel flow, steep granular beds are more stable than beds with milder slopes. These two facts, together with recent direct measurements of forces acting on bed grains giving time-mean negative drags ( Schmeeckle et al. , 2007 ), question the conventional models of forces used so far. Here, fluid forces acting on bed particles are treated in a new way in order to take into consideration the fundamental interference effects, thus obtaining appropriate magnitude estimates that exhibit good agreement with direct force measurements by Schmeeckle et al. (2007) . Impulsive pressure fluctuations generated by turbulence are shown to be capable of dislodging the bed grains by saltation under air flow, whereas they can only produce a rocking effect under water flow, thus explaining the first anomaly. On the other hand, previous work by the authors allows a direct estimate of space averaged time-mean drag and lift forces exerted on bed grains. Both components have the same order of magnitude but, contrary to the common belief, the mean lift is downward, which provides an explanation for the second anomaly. Finally, spatial disturbances of pressure, both positive and negative, appear to generate maximum, persistent, local forces considerably greater than mean forces, thus allowing an explanation for the observed negative time-mean drag. A new formula for predicting incipient motion of sediment under open channel flow is derived, which incorporates all dynamically significant effects and gives very good agreement with observation for the entire range of bed slopes. 相似文献
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MICHELLE FERRANDINI JEAN FERRANDINI MIGUEL SANTIAGO ISABELLE THINON JEAN‐PIERRE ANDRÉ YVES BARTHET POL GUENNOC BERNADETTE TESSIER 《Sedimentology》2013,60(2):599-623
This study documents a change from a non‐tidal to tide‐dominated shelf system that occurred between Corsica and Sardinia (the Bonifacio Basin, Western Mediterranean) during the early to middle Miocene. The non‐tidal deposits formed on a low‐energy siliciclastic shelf surrounded by progradational coralline algal ramps at full highstand. The tidal deposits consist of an up to 200 m thick succession of siliciclastic to coralline‐rich cross‐beds formed by large sub‐tidal dunes. Based on outcrop and sub‐surface data, it is possible to conclude that the tidal currents were amplified as a consequence of the rapid subsidence of the basin centre due to tectonic activity. It is suggested that this tectonic event initiated the strait between Corsica and Sardinia. The strait was deep enough to allow the tidal flux to be significantly increased, generating a localized strong tidal current at the junction between the Western Mediterranean and the East Corsica Basin. 相似文献
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ESPERANZA MUÑOZ‐SALINAS MIGUEL CASTILLO‐RODRÍGUEZ VLAD MANEA MARINA MANEA DAVID PALACIOS 《Geografiska Annaler: Series A, Physical Geography》2010,92(3):311-328
Lahars are hazardous events that can cause serious damage to people who live close to volcanic areas; several were registered at different times in the last century, such as at Mt St Helens (USA) in 1980, Nevado del Ruiz (Colombia) in 1985 and Mt Pinatubo (Philippines) in 1990. Risk maps are currently used by decision‐makers to help them plan to mitigate the hazard‐risk of lahars. Risk maps are acquired based on a series of tenets that take into account the distribution and chronology of past lahar deposits, and basically two approaches have been used: (1) The use of Flow Simulation Software (FSS), which simulates flows along channels in a Digital Elevation Model and (2) The Geochronological Method (GM), in which the mapping is based on the evaluation of lahar magnitude and frequency. This study addresses the production of a lahar risk map using the two approaches (FSS and GM) for a study area located at Popocatépetl volcano – Central Mexico. Santiago Xalitzintla, a town located on the northern flank of Popocatépetl volcano, where volcanic activity in recent centuries has triggered numerous lahars that have endangered local inhabitants, has been used for the case study. Results from FSS did not provide satisfactory findings because they were not consistent with lahar sediment observations made during fieldwork. By contrast, the GM produced results consistent with these observations, and therefore we use them to assess the hazard and produce the risk map for the study area. 相似文献
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The quantification and prediction of damage due to different seismic actions to structure types of different strength is an important problem not yet solved in the Earthquake Engineering field. In addition, owing to the fact that macroseismic information cannot be used directly in dynamic calculations, a new problem appears when these are the only kind of data available. Thus, there is a need to estimate a parameter to relate the energy of the ground motion and the damage occurrence, and eventually achieve a better seismic risk assessment. After the study and review of some representative potential damage parameters, attention has been paid to the Arias intensity (unfiltered and filtered in certain frequency ranges) and the Cumulative Absolute velocity (CAV) as the parameters to evaluate the energy of movement, and to relate them with the observed damage. The data used to infer these correlations have been provided by the ENEA-ENEL (Italy). The information consists of strong motion records from the Campano Lucano (1980), Umbria (1984) and Lazio-Abruzzo (1984) earthquakes, and data of damage to buildings in the vicinity of recording instruments (within a maximum radius of 300 m, where the soil conditions remain constant). In this paper, some relations have been obtained to quantify the damage level for different seismic inputs. The results suggest that unfiltered Arias intensity and CAV (for calculation threshold 20 cm/s2) correlate well with the macroseismic information used. Best fits are obtained between the quoted parameters and the observed damage in type A structures. © 1997 by John Wiley & Sons, Ltd. 相似文献
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