1,721,036 research outputs found
Global postseismic deformation: Deep earthquakes
No full textWe study the global viscoelastic deformations associated with a shear dislocation on a fault embedded in a viscoelastic mantle. To address this problem, we extend the theory of the quasi-static deformations of a Maxwell, spherical, N-layer incompressible Earth, previously limited to the modeling of the effects of earthquakes occurring only within the elastic lithosphere. Permanent postseismic deformations, which can be generated by lithospheric sources, cannot be sustained if the source region is viscoelastic; however, the transient response of the mantle strongly depends on the viscosity of the source region. We use the technique developed here to investigate thoroughly the quasi-static surface deformations induced by seismic events of variable depth. We show that owing to the combined effect of sphericity and viscoelastic mantle relaxation, the surface displacements do not systematically decrease as the depth of the source increases. Instead, with increasing source depth we predict an increasing efficiency of mantle relaxation in triggering postseismic deformations of large size. Most of our results are based on a simple four-layer model, with a 100-km-thick lithosphère, upper and lower mantle separated by the 670-km discontinuity, and a fluid core; the last part of this work is devoted to a study of the effects of a low-viscosity asthenosphere on the rates of deformation detected at the Earth's surface. The findings reported here may be useful for the interpretation of the transient motions of the Earth's surface in response to deep-focus earthquakes
Using the Post-Widder formula to compute the Earth's viscoelastic Love numbers
No full textThe post-glacial or post-seismic relaxation of a Maxwell viscoelastic earth, 1-D or slightly
laterally heterogeneous, can be calculated in a normal-mode approach, based on an application
of the propagator technique. This semi-analytical approach, widely documented in the liter-
ature, allows to compute the response of an earth model whose rheological parameters vary
quite strongly with depth, at least as accurately and efficiently as by 1-D numerical integra-
tion (Runge–Kutta). Its main drawback resides in the need to identify the roots of a secular
polynomial, introduced after reformulating the problem in the Laplace domain, and required
to transform the solution back to the time domain. Root finding becomes increasingly difficult,
and ultimately unaffordable, as the complexity of rheological profiles grows: the secular poly-
nomial gradually gets more ill behaved, and a larger number of more and more closely spaced
roots is to be found. Here, we apply the propagator method to solve the Earth’s viscoelastic
momentum equation, like in the above-mentioned normal-mode framework, but bypass root
finding, using, instead, the Post–Widder formula to transform the solution, found again in the
Laplace domain, back to the time domain. We test our method against earlier normal-mode
results, and prove its effectiveness in modelling the relaxation of earth models with extremely
complex rheological profiles
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
Time-dependent residual deformations associated with the June 9, 1994 Bolivia earthquake
No full textBy means of a spherical Earth model with Maxwell rheology we compute the coseismic and the postseismic displacements associated with the June 9, 1994 Bolivia earthquake. Our results are compared with the coseismic uplift detected by an array of broadband seismic stations in Southern Bolivia and with previous theoretical predictions. Our solution method has been recently employed to study the global deformations due to large lithospheric earthquakes. In contrast to purely elastic Earth models, our method includes the time‐dependent effects associated with mantle relaxation. The results presented here show that the surface deformation due to a deep‐focus earthquake are sensibly magnified by the delayed viscoelastic mantle relaxation. The time‐scales which characterize mantle relaxation and the time evolution of surface observables depend on the rheological profile of the upper mantle, and particularly on the thickness of the viscoelastic asthenosphere
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