Experiments on steady buoyancy transfer through turbulent fluid layers separated by density interfaces     

Harindra J.S. Fernando  Robert R. Long 《Dynamics of Atmospheres and Oceans》1988,12(3-4)

A laboratory experiment was performed to investigate mixing across a density interface which separates two turbulent fluid layers and coexists with a stabilizing buoyancy flux. It was found that the buoyancy flux (q₀) across the interface and through the turbulent layers (of depth D) becomes steady and constant in magnitude in the vertical direction, only when , where u is the horizontal r.m.s. velocity at the base of the mixed layers. The results suggest that mixing across the density interface is controlled by a dynamically important buoyancy gradient induced in the turbulent layers and that parameters such as the bulk Richardson number, , where Δb is the interfacial buoyancy jump, are of secondary importance. Measurements are used to infer the mixing mechanism at the interface, the mixing efficiency of stratified fluids and the entrainment law. Some geophysical applications of the results are also discussed.  相似文献   

Analysis of swarms of high-frequency seismic events at Nevado del Ruiz volcano, Colombia (January 1986–August 1987): Development of a procedure     

Fernando C. Muoz  Alvaro E. Nieto  Hansjürgen Meyer 《Journal of Volcanology and Geothermal Research》1990,41(1-4)

The determination of signal properties like frequency, amplitude, “signature” of the sequence of arrivals of P- and S-phases and the subsequent computation of hypocenter parameters, are the steps of a procedure in the analysis of swarms of high-frequency seismic events from Nevado del Ruiz volcano, which could be useful in other volcanic areas. The use of this procedure on Ruiz data led to the determination of various seismic source volumes, which together with hypocenters of non-swarmed events and the identification of geological features such as faults which cross the area, alignment of domes and volcanic edifices, and the record of low-frequency seismic events not centered in the area of the active vent, suggest the existence of a magma chamber of a rather complex geometry. This hypothesis would suggest that magma, emplaced in zones of weakness (faults) that affect the area, may act as the source of seismic activity. This demands a revision of the methodology used in the deformation measurements, and in the analysis of released energy and event occurrence too. The relations between hypocenters (sources), location and relative energy of the swarms, as well as the general volcanic activity (tremor, ash and gas emission, deformation) could turn out to be a possible means to assist in the identification of premonitory activity and thus be relevant to the process of monitoring and forecasting.  相似文献   

Simulations of strong ground motion in SW Iberia for the 1969 February 28 (M_s= 8.0) and the 1755 November 1 (M∼ 8.5) earthquakes – II. Strong ground motion simulations     

Raphaël Grandin  José Fernando Borges  Mourad Bezzeghoud  Bento Caldeira  Fernando Carrilho 《Geophysical Journal International》2007,171(2):807-822

This is the second paper of a series of two concerning strong ground motion in SW Iberia due to earthquakes originating from the adjacent Atlantic area. The aim of this paper is to use the velocity model that was proposed and validated in the companion paper for seismic intensity modelling of the 1969 ( M _s= 8.0) and 1755 ( M = 8.5–8.7) earthquakes.
First, we propose a regression to convert simulated values of Peak Ground Velocity (PGV) into Modified Mercalli Intensity (MMI) in SW Iberia, and using this regression, we build synthetic isoseismal maps for a large ( M _s= 8.0) earthquake that occurred in 1969. Based on information on the seismic source provided by various authors, we show that the velocity model effectively reproduces macroseismic observations in the whole region. We also confirm that seismic intensity distribution is very sensitive to a small number of source parameters: rupture directivity, fault strike and fault dimensions. Then, we extrapolate the method to the case of the great ( M = 8.5–8.7) 1755 earthquake, for a series of hypotheses recently proposed by three authors about the location of the epicentral region. The model involving a subduction-related rupture in the Gulf of Cádiz results in excessive ground motion in northern Morocco, suggesting that the source of the 1755 earthquake should be located further west. A rupture along the western coast of Portugal, compatible with an activation of the passive western Iberian margin, would imply a relatively low average slip, which, alone, would could not account for the large tsunami observed in the whole northern Atlantic ocean. A seismic source located below the Gorringe Bank seems the most likely since it is more efficient in reproducing the distribution of high intensities in SW Iberia due to the 1755 earthquake.  相似文献   

The impact of hurricanes on sedimenting particulate matter in the semi-arid Bahía de La Paz,Gulf of California     

Norman Silverberg  Evgueni Shumilin  Fernando Aguirre-Bahena  Ana Patricia Rodríguez-Castañeda  Dmitry Sapozhnikov 《Continental Shelf Research》2007

From 2002 through 2004, time-series sediment trap samples were collected from a depth of 410 m in Cuenca Alfonso, Bahía de La Paz, on the SW coast of the Gulf of California. The instrument recorded the impact of the local passage of hurricanes “Ignacio” (24–26 August) and “Marty” (21–23 September) in 2003. These two events accounted for 82% of the total rainfall measured in 2003, equivalent to the annual average precipitation in years without hurricanes. Mean total mass fluxes (TMFs) of 2.88 and 3.58 g m⁻² d⁻¹ were measured during the week of each hurricane as well as the following week. This may have been enough to produce a lamina in the underlying sediment with characteristics peculiar to such events. The terrigenous component was particularly abundant, with notably higher concentrations of Fe, Sc, Co and Cs and REEs. In contrast, TMFs throughout 2002–2004 (excluding the hurricane periods) averaged only 0.73 g m⁻² d⁻¹ and had a larger marine biogenic component. The extraordinary elemental fluxes during the 29 days of hurricane-influenced sedimentation represented a great proportion of the totals over an entire “normal” year: Co (67.8%) >Sc (62.6) >Fe (59.6) >Cs (53.4)>Lu (51.5)>La (51.3)>Yb (51.0)>Ce (49.5) >Tb (48.4) >Sm (44.7)>Cr (36.5) >Ca (31.0)>Eu (25.4%). The terrigenous fraction was calculated using (a) TMF minus the sum of CaCO₃, biogenic silica and organic matter and (b) the ratio of Sc in the trap samples to the average in the Earth's crust. The latter was consistently smaller, but the two methods offered similar results following hurricanes (78% vs. 63%, respectively). For normal sedimentation, however, the difference method yielded values twice as large as the Sc method (58% vs. 30%) This suggests that the mineralogy of the terrigenous fraction may also vary, with unsorted dessert soil being carried to sea by the powerful flash floods associated with hurricanes. Eolian supply of particles, particularly Sc-free quartz grains, possibly from beyond the limited fluvial drainage basin, apparently dominates normal sedimentation.  相似文献   

Multiple sources for ore-forming fluids in the Neves Corvo VHMS Deposit of the Iberian Pyrite Belt (Portugal): strontium, neodymium and lead isotope evidence   总被引：1，自引：0，他引：1  

Jorge M. Relvas  Colombo C. Tassinari  José Munhá  Fernando J. Barriga 《Mineralium Deposita》2001,36(5):416-427

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Distribuzione geografica dell'attivita' magnetica terrestre     

Mario Bossolasco  Fernando Dalmasso 《Pure and Applied Geophysics》1939,1(1):3-20

Riassunto Richiamate le caratteristiche secondo cui le perturbazioni regolari ed irregolari del campo terrestre concorrono a definire l'attività geomagnetica, se ne analizza la distribuzione geografica mediante le grandezze:HR _H ,ZR _Z , e (10^–4.²). Di queste sono illustrati e discussi gli andamenti in funzione della latitudine geomagnetica, dedotti dai valori medi calcolati per quasi tutte le stazioni funzionanti durante l'Anno Polare 1932–33 ed il triennio successivo 1933–36. Si rileva particolarmente il massimo assoluto di attività della zone anulare delle aurore, e quello relativo delle regioni equatoriali. Segue un'indagine, a carattere piuttosto indicativo, sulla ripartizione geografica del grado di variazione dell'attività nel tempo, introducendo a tale scopo opportuni indici ed esponendone il comportamento. Conclude la Nota un cenno sugli andamenti stagionali dell'attività e della variabilità geomagnetica.  相似文献   

An audio-magnetotelluric investigation in Terceira Island (Azores)   总被引：2，自引：0，他引：2  

Fernando A. Monteiro Santos  Antnio Trota  Antnio Soares  Rafael Luzio  Nuno Loureno  Liliana Matos  Eugnio Almeida  Joo L. Gaspar  Jorge M. Miranda 《Journal of Applied Geophysics》2006,59(4):314-323

Ten audio-magnetotelluric soundings have been carried out along a profile crossing the Serra do Cume caldera in the eastern part of the Terceira Island (Azores). The main objectives of this investigation were to detect geoelectrical features related with tectonic structures and to characterize regional hydrological and hydrothermal aspects mainly those related to geothermal fluid dynamics.Three-dimensional numerical investigation showed that the data acquired at periods shorter than 1 s are not significantly affected by ocean effect. The data was analysed using the Smith's decomposition method in order to investigate possible distortions caused by superficial structures and to estimate a global regional strike. The results suggest that in general the soundings were not distorted. A regional N55°W strike was chosen for the two-dimensional data inversion.The low-resistivity zones (10–30 ohm-m) displayed in the central part of the 2-D geoelectrical model have been interpreted as caused by hydrothermal circulation. The low-resistivity anomalies at the ends of the profile might be attributed to alteration zones with interaction of seawater intrusion. High-resistivity (> 300 ohm-m) values have been related with less permeable zones in the SW of Cinco Picos and Guilherme Moniz caldera walls.  相似文献   

Seismic Displacement of Gravity Walls by a Two-body Model     

Constantine A. Stamatopoulos  Eleni G. Velgaki  Arezou Modaressi  Fernando Lopez-Caballero 《Bulletin of Earthquake Engineering》2006,4(3):295-318

Gravity walls retaining dry soil are modeled as a system of two bodies: (a) the gravity wall that slides along the wall-foundation soil boundary and (b) the critical soil wedge in the soil behind the wall. The strength of the system is defined by both the frictional and the cohesional components of resistance. The angle of the prism of the critical soil wedge behind the wall is obtained using the limit equilibrium method. The model accounts for changes in the geometry of the backfill soil behind the wall by considering the displacements at the end of each time step under limit equilibrium. The model shows that the standard (single) block model is over-conservative for the extreme case of critical-to-applied-seismic acceleration ratios less than about 0.30, but works well for cases where this ratio ranges between 0.5 and 0.8. The model is applied to predict the seismic displacement of gravity walls (a) tested in the shaking-table and (b) studied numerically by elaborate elasto–plastic analyses.  相似文献   

Predation on the rocky shores of Patagonia,Argentina     

Fernando J. Hidalgo  Brian R. Silliman  María Cielo Bazterrica  Mark D. Bertness 《Estuaries and Coasts》2007,30(5):886-894

Rocky intertidal communities of Argentinean Patagonia are exposed to harsh physical conditions caused by dry, strong southern trade winds (mean speed 45 km h^-1, gusts up to 140 km h^-1) that result in intense desiccation of intertidal organisms. Predator distributions in these communities were evaluated from April 2003 to December 2004 at two exposed headlands and six protected bays in Cabo dos Bahias (44°50′S, 65°40′W). Crabs and sea stars dominated the predator assemblage, with occasional scavenger snails and fish also present. During low tide, predators were never found in the open but were always associated with shelter (primarily within mussel beds and coralline algae), suggesting a strong predator dependency on foundation species to buffer them from physical stress. Few predators (mostly crabs) emerged from shelter at high tide. Unlike the larger predators found on Chilean rocky shores, invertebrate predators in this system are diminutive, generally < 2 cm. The lone exception was the newly discovered invasive green crab,Carcinus mamas. Feeding trials, gut content analyses, and visual surveys indicated that native predators feed primarily on small, soft-bodied prey.C. mamas fed on slow-moving and sessile animals, including the super abundant musselPerumytilus purpuratus, with much greater voracity than native predators. Because native intertidal organisms are dependent on mussel beds and coralline algae for shelter from desiccation, successful invasion ofC. mamas may lead to a significant decrease in native diversity by consuming foundation species. This study represents a preliminary survey of predator distributions and feeding habits on the rocky shores of Argentinean Patagonia and provides important baseline data to evaluate trophic linkages and predatory effects on Patagonian rocky shores.  相似文献   

Structural framework and Mesozoic–Cenozoic evolution of Ponta Grossa Arch, Paraná Basin, southern Brazil     

Michael Strugale  Sidnei Pires Rostirolla  Fernando Mancini  Carlos Vieira Portela Filho  Francisco Jos Fonseca Ferreira  Rafael Corrêa de Freitas 《Journal of South American Earth Sciences》2007,24(2-4):203-227

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