1,720,961 research outputs found
Shear-velocity structure, radial anisotropy and dynamics of the Tibetan crust
No full textGeophysical and geological data suggest that Tibetan middle crust is a partially molten, mechanically weak layer, but it is debated whether this low-viscosity layer is present beneath the entire plateau, what its properties are, how it deforms, and what role it has played in the plateau's evolution. Broad-band seismic surface waves yield resolution in the entire depth range of the Tibetan crust and can be used to constrain its shear-wave velocity structure (indicative of crustal composition, temperature and partial melting) and radial anisotropy (indicative of the patterns of deformation). We measured Love- and Rayleigh-wave phase-velocity curves in broad period ranges (up to 7-200 s) for a few tens of pairs and groups of stations across Tibet, combining, in each case, hundreds of interstation measurements, made with cross-correlation and waveform-inversion methods. Shear-velocity profiles were then determined by extensive series of non-linear inversions of the data, designed to constrain the depth-dependent ranges of isotropic-average shear speeds and radial anisotropy. Shear wave speeds within the Tibetan middle crust are anomalously low and, also, show strong lateral variations across the plateau. The lowest mid-crustal shear speeds are found in the north and west of the plateau (˜3.1-3.2 km s-1), within a pronounced low-velocity zone. In southeastern Tibet, crustal shear wave speeds increase gradually towards southeast, whereas in the north, the change across the Kunlun Fault is relatively sharp. The lateral variations of shear speeds within the crust are indicative of those in temperature. A mid-crustal temperature of 800 °C, reported previously, can account for the low shear velocities across Lhasa. In the north, the temperature is higher and exceeds the solidus, resulting in partial melting that we estimate at 3-6 per cent. Strong radial anisotropy is required by the data in western-central Tibet (>5 per cent) but not in northeastern Tibet. The amplitude of radial anisotropy in the crust does not correlate with isotropic-average shear speed (and, by inference, with crustal rock viscosity) or with surface elevation. Instead, radial anisotropy is related to the deformation pattern and is the strongest in regions experiencing extension (crustal flattening), as noted previously. The growth of Tibet by the addition of Indian crustal rocks into its crust from the south is reflected in the higher crustal seismic velocities (and, thus, lower temperatures) in the southern compared to northern parts of the plateau (more recently added rocks having had less time to undergo radioactive heating within the thickened Tibetan crust). Gravity-driven flattening-the basic cause of extension and normal faulting in the southern, western and central Tibet-is evidenced by pervasive radial anisotropy in the middle crust beneath the regions undergoing extension; the overall eastward flow of the crust is directed by the boundaries and motions of the lithospheric blocks that surround Tibet
A single-station automated earthquake location system at Wied Dalam Station, Malta
No full textThe seismicity of the Sicily Channel, bordered by the Sicilian, Tunisian, and Libyan coastlines, is mainly controlled by active faults of the Sicily Channel rift zone (SCRZ). This region is characterized by a moderate level of earthquake activity with magnitudes generally below 5.0. However, most seismicity, especially south of the Maltese islands, has, to date, either gone unreported or been poorly located owing to difficulties in instrumental coverage. Since many earthquakes are recorded only on a single station on Malta (broadband station WDD), it was deemed necessary to develop a routine procedure for detecting and locating earthquakes using three-component polarization analysis. Such a system, nicknamed LESSLA (Local Earthquake Single-Station Location Analyser) has been successfully implemented since 2005. It uses an automated method of recognizing local/regional events based on a weighting scheme applied to triggers in different sampling streams. LESSLA has allowed a lower detection threshold for earthquakes in the Sicily Channel, and as a result provided new insights into the pattern of seismicity on the rift zone. LESSLA has also had a good success rate at rapidly and accurately reporting larger events as far as the Greek subduction zone. The description, performance, and limitations of the system are here discussed
Analysis of working fluids applicable in Enhanced Geothermal Systems: nitrous oxide as an alternative working fluid
No full textThis document presents a study into the working fluids that can be used at an Enhanced Geothermal System (EGS) plant as a way of making efficient, large-scale use of the enormous resources offered by geothermal energy.First, we investigate the two working fluids most used in such plants: water (H2O) and carbon dioxide (CO2). The comparative analysis brings to light the advantages of each one, making it possible to assess their beneficial properties. This leads us to establish what properties any alternative working fluid should have.Then, we analyse fluids included in the database of the Engineering Equation Solver (EES) program. This entails a study of their thermodynamic properties in the working conditions established. Based on the properties of each alternative working fluid and the results obtained from the EES, we seek to determine which working fluid has the best performance.Finally, the results obtained after the analysis leads us to conclude that single supercritical phase nitrous oxide seems to be an alternative to the two working fluids used to date
Lithospheric structure in the Baikal-central Mongolia region from integrated geophysical-petrological inversion of surface-wave data and topographic elevation
No full textRecent advances in computational petrological modeling provide accurate methods for computing seismic velocities and density within the lithospheric and sub-lithospheric mantle, given the bulk composition, temperature, and pressure within them. Here, we test an integrated geophysical-petrological inversion of Rayleigh- and Love-wave phase-velocity curves for fine-scale lithospheric structure. The main parameters of the grid-search inversion are the lithospheric and crustal thicknesses, mantle composition, and bulk density and seismic velocities within the crust. Conductive lithospheric geotherms are computed using P-T-dependent thermal conductivity. Radial anisotropy and seismic attenuation have a substantial effect on the results and are modeled explicitly. Surface topography provides information on the integrated density of the crust, poorly constrained by surface waves alone. Investigating parameter inter-dependencies, we show that accurate surface-wave data and topography can constrain robust lithospheric models. We apply the inversion to central Mongolia, south of the Baikal Rift Zone, a key area of deformation in Asia with debated lithosphere-asthenosphere structure and rifting mechanism, and detect an 80–90 km thick lithosphere with a dense, mafic lower crust and a relatively fertile mantle composition (Mg# < 90.2). Published measurements on crustal and mantle Miocene and Pleistocene xenoliths are consistent with both the geotherms and the crustal and lithospheric mantle composition derived from our inversion. Topography can be fully accounted for by local isostasy, with no dynamic support required. The mantle structure constrained by the inversion indicates no major thermal anomalies in the shallow sub-lithospheric mantle, consistent with passive rifting in the Baikal Rift Zone
Saltwater intrusion in Friuli Low Plain
No full textIn coastal areas, the interaction between seawater and fresh water is in a dynamic equilibrium and occurs both in surface bodies (saltwater ingression) and in groundwater (saltwater intrusion in aquifers). Human actions and climatic changes, especially sea level rising, can alter this equilibrium leading to a contamination of the fresh water and soils by seawater.
The shift of salt wedge towards inland can be a consequence of both natural and anthropogenic processes. Natural processes have generally slow effects, except in the case of tsunami, as in Sri Lanka (Villholth and Neupane, 2011). The sea level rising due to climatic changes (Melloul and Collin, 2006) and local subsidence along coastal areas cause a slow salt wedge shifting. Instead, human actions have rapid effects on the coast system. Examples of the human actions are the overexploitation of aquifer, the incorrect and extreme watercourse regimentation, the excavation of floodway canal and dredging for navigation.
The salt wedge intrusion can lead to freshwater contamination and consequently to a decrease of water reserve, to coastal area desertification by ground salinization and to a loss of faunistic and floristic species
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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