New Evidence of Azimuthal Variations at Campi Flegrei: Attenuation and Predominant Frequency     

J. Vila  A. M. Correig  R. Carbonell 《Pure and Applied Geophysics》1997,150(2):285-304

—In this paper we have determined apparent Q from body-wave dispersion and a spectral ratio method for the Campi Flegrei area. A contour map of azimuthal variations of apparent Q (ranging from Q = 4 to Q = 40) reveals that the lower Q (higher attenuation) corresponds to an area that includes the Solfatara Crater (with active fumaroles), near the site of maximum uplift produced during the bradyseismic crisis of 1984, and just over the site evidencing the presence of a magmatic body. A strong azimuthal variation of the predominant frequency of the first pulse of the P wave has been found and is interpreted in terms of a vertical travel time in a surficial resonant layer (～ 1 km thick) over a halfspace. The attenuation maps, the interpretation of the resonant frequencies, and the additional information provided by the P-wave velocity suggest the presence of a circular structure in the area. Not only does the distribution of the thickness of the resonant layer (obtained through the estimation of the contribution of different harmonics) show the circular structure, but it also agrees with the presence of a thin layer of lighter sediments, determined by means of a microgravimetric study, and with the total magnetic map, previously obtained for this area. The results of the azimuthal variation provide new evidence of the structure that can be used to improve the knowledge of the area.  相似文献   

HF radar observations of equatorial spread-F over West Africa     

J.-F. Cécile  P. Vila  E. Blanc 《Annales Geophysicae》1996,14(4):411-418

New experimental data depicting equatorial spread-F were taken during an HF radar sounding campaign in Korhogo (Ivory Coast, 9°24N, 5°37W, dip 4°S). Range-time-intensity maps of the radar echoes have been analyzed to identify the signatures of density depletions and bottomside spread-F. Density depletions are well known features of equatorial spread-F, and are believed to emerge after the development of a Rayleigh-Taylor instability on the bottomside F-layer. A simple model is developed and used to simulate the flow of density depletions over the radar field of view. The simulation permits an interpretation of the data that yields the zonal flow velocity as a function of local time. Comparisons with previous measurements are undertaken to assess the consistency of the computational results, and qualitative arguments are presented to identify bottomside spread-F. Using the computational results as reference, a morphological study of ionograms showing spread-F is undertaken which reveals the specific signature of bottomside spread-F on ionograms recorded just after sunset.  相似文献   

KARSYS hydrogeological 3D modeling of alpine karst aquifers developed in geologically complex areas: Picos de Europa National Park (Spain)     

Daniel Ballesteros  Arnauld Malard  Pierre-Yves Jeannin  Montserrat Jiménez-Sánchez  Joaquín García-Sansegundo  Mónica Meléndez-Asensio  Gemma Sendra 《Environmental Earth Sciences》2015,74(12):7699-7714

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New morpho-bathymetric data on marine hazard in the offshore of Gulf of Naples (Southern Italy)     

Aiello  Gemma  Sacchi  Marco 《Natural Hazards》2022,111(3):2881-2908

Natural Hazards - Morpho-bathymetric data have been analyzed in order to provide new geological evidence on the Gulf of Naples, i.e., on the southern Ischia slope, on the Naples canyons and on the...  相似文献   

Solemys (Chelonii,Solemydidae) remains from the Maastrichtian of Pyrenees: evidence for a semi-aquatic lifestyle     

Josep Marmi  Bernat Vila  Àngel Galobart 《Cretaceous Research》2009,30(5):1307-1312

We report on a turtle from the Mina Esquirol site (Vallcebre basin), a new locality of early Maastrichtian age in the south-eastern Pyrenees. Fossils were located in the basal Tremp Formation, which was deposited in a littoral marsh. The material consists of a cast of a carapace including peripheral fragments and partial neural plates. The carapace exhibits a vermiculate ornamentation that is characteristic of genus Solemys and a histological bone structure similar to that of terrestrial taxa. However, taphonomic data indicates little transport and a short biostratinomic history, whereas palaeontological and sedimentological context indicates that the specimen was preserved in a shallow brackish water environment. Based on this taphonomic and sedimentological evidence, we suggest that at least some species of genus Solemys had a lifestyle similar to extant fresh or brackish water turtles (terrapins) and that the histological evidence alone for a terrestrial lifestyle is misleading.  相似文献   

Hurricane flood risk assessment for the Yucatan and Campeche State coastal area     

Rey  Wilmer  Mendoza  E. Tonatiuh  Salles  Paulo  Zhang  Keqi  Teng  Yi-Chen  Trejo-Rangel  Miguel A.  Franklin  Gemma L. 《Natural Hazards》2019,96(3):1041-1065

Natural Hazards - In this study, the first ever Sea, Lake, Overland Surges from Hurricanes (SLOSH) grid was built for the Yucatan Peninsula. The SLOSH model was used to simulate storm surges in the...  相似文献   

Invasive crayfish as drivers of fine sediment dynamics in rivers: field and laboratory evidence          下载免费PDF全文

Gemma L Harvey  Alexander James Henshaw  Tom P Moorhouse  Nicholas J Clifford  Helen Holah  Jonathan Grey  David W Macdonald 《地球表面变化过程与地形》2014,39(2):259-271

Despite increasing recognition of the potential of aquatic biota to act as ‘geomorphic agents’, key knowledge gaps exist in relation to biotic drivers of fine sediment dynamics at microscales and particularly the role of invasive species. This study explores the impacts of invasive signal crayfish on suspended sediment dynamics at the patch scale through laboratory and field study. Three hypotheses are presented and tested: (1) that signal crayfish generate pulses of fine sediment mobilisation through burrowing and movement that are detectable in the flow field; (2) that such pulses may be more frequent during nocturnal periods when signal crayfish are known to be most active; and (3) that cumulatively the pulses would be sufficient to drive an overall increase in turbidity. Laboratory mesocosm experiments were used to explore crayfish impacts on suspended sediment concentrations for two treatments: clay banks and clay bed substrate. For the field study, high frequency near‐bed and mid‐flow turbidity time series from a lowland river with known high densities of signal crayfish were examined. Laboratory data demonstrate the direct influence of signal crayfish on mobilisation of pulses of fine sediment through burrowing into banks and fine bed material, with evidence of enhanced activity levels around the mid‐point of the nocturnal period. Similar patterns of pulsed fine sediment mobilisation identified under field conditions follow a clear nocturnal trend and appear capable of driving an increase in ambient turbidity levels. The findings indicate that signal crayfish have the potential to influence suspended sediment yields, with implications for morphological change, physical habitat quality and the transfer of nutrients and contaminants. This is particularly important given the spread of signal crayfish across Europe and their presence in extremely high densities in many catchments. Further process‐based studies are required to develop a full understanding of impacts across a range of river styles. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

Simple large wood structures promote hydromorphological heterogeneity and benthic macroinvertebrate diversity in low-gradient rivers     

Francesca Pilotto  Gemma L. Harvey  Geraldene Wharton  Martin T. Pusch 《Aquatic Sciences - Research Across Boundaries》2016,78(4):755-766

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Running safety assessment of trains moving over bridges subjected to moderate earthquakes          下载免费PDF全文

P. A. Montenegro  R. Calçada  N. Vila Pouca  M. Tanabe 《地震工程与结构动力学》2016,45(3):483-504

A study about the running safety of trains moving over bridges subjected to earthquakes is presented. The study focuses on moderate earthquakes with relatively small return periods and high probability of occurrence. The analyses are performed using a nonlinear train‐bridge interaction method proposed by the authors, being the running safety evaluated with safety criteria existent in the literature. The influence on the train running safety of the seismic intensity levels, train running speed, and track quality is evaluated. Because no significant nonlinearity is likely to be exhibited in the columns for moderate levels of seismicity, the analyses are performed in the elastic domain. However, the reduction in the columns stiffness due to cracking is accounted, and a methodology to compute their effective stiffness is proposed. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

Morphology and sedimentary systems in the Central Bransfield Basin, Antarctic Peninsula: sedimentary dynamics from shelf to basin     

Marga García  Gemma Ercilla  Belén Alonso 《Basin Research》2009,21(3):295-314

A detailed regional characterization of the physiography, morphology and sedimentary systems of the Central Bransfield Basin (CBB) was carried out using swath bathymetry and high‐ and very high‐resolution seismic profiles. The basin margins show continental shelves with numerous glacial troughs, and continental slopes where relatively wide and flat slope platforms represent the middle domain in an atypical physiographic scenario in glaciated margins. Although the CBB is tectonically active, most of the morphologic features are sedimentary in origin, and can be classified into four sedimentary systems: (1) glacial‐glaciomarine, composed of erosional surfaces, glacial troughs, furrows and draping sheets; (2) slope‐basin, formed by trough mouth fans, slope aprons, the Gebra‐Magia instability complex and turbidity systems; (3) seabed fluid outflow system composed of pockmark fields; and (4) contourite, composed of drifts and moats. The sedimentary systems show a clear zonation from shelf to basin and their dynamics reflects the complex interplay among glacial, glaciomarine, marine and oceanographic processes involved in the entire shelf‐to‐basin sediment distribution. The CBB morphology is primarily controlled by glacial/interglacial cyclicity and physiography and to a lesser extent by tectonics and oceanography. These factors have affected the South Shetland Islands (SSI) and Antarctic Peninsula (AP) margins differently, creating a relatively starved SSI margin and a more constructional AP margin. They have also created two entire sediment‐dispersal domains: the shelf‐to‐slope, which records the glaciation history of the CBB; and the lower slope‐to‐basin, which records the imprint of local factors. This study provides a ‘source‐to‐sink’ sedimentary scheme for glaciated margins, which may be applied to the basin research in other margins, based on the characterization of sedimentary systems, their boundaries and the linkages among them. This approach proves to be adequate for the identification of global and local factors governing the CBB and may therefore be applied to other study areas.  相似文献