1,721,035 research outputs found
Surface-Wave Attenuation From Seismic Ambient Noise: Numerical Validation and Application
No full textWe evaluate, by numerical tests, whether surface-wave attenuation can be determined from ambient-noise data. We generate synthetic recordings of numerically simulated ambient seismic noise in several experimental setups, characterized by different source distributions and different values of attenuation coefficient. We use them to verify that the source spectrum can be reconstructed from ambient recordings (provided that the density of sources and the attenuation coefficient are known) and that true attenuation can be retrieved from normalized cross correlations of synthetic signals. We then apply the so validated method to real continuous recordings from 33 broadband receivers distributed within the Colorado Plateau and Great Basin. A preliminary analysis of the signal-to-noise ratio as a function of azimuth reveals a SW-NE preferential directionality of the noise sources within the secondary microseism band (6–8 s), consistent with previous studies. By nonlinear inversion of noise data we find the attenuation coefficient in the area of interest to range from ∼ 1 × 10−5 m−1 at 0.3 Hz to ∼ 4.5 × 10−7 m−1 at 0.065 Hz, and confirm the statistical robustness of this estimate by means of a bootstrap analysis. The result is compatible with previous observations based on both earthquake-generated and ambient Rayleigh waves. In this regard, the method proves to be promising in accurately quantifying surface-wave attenuation at relatively high frequencies
T-cell receptor (TCR) signal transduction: Biochemical mechanisms and possible clinical implications
No full textThe T-cell receptor (TCR) is a multiprotein complex composed of the Ti subunit responsible for antigen binding, the CD3 complex and the ζ family dimers involved in the TCR signal transduction. This function is mainly due to the presence of a 17 aminoacid motif, known as the ARAM (Antigen Recognition Activation Motifs) sequence, present in the cytoplasmic domains of the γ, δ, and ε chains of the CD3 complex and the ζ dimers. One of the earliest biochemical events after TCR cross-linking is the enhanced phosphorylation of several cellular substrates on tyrosine residues. Since the TCR/CD3 complex lacks intrinsic tyrosine kinase activity, five cytoplasmic protein tyrosine kinases (PTKs) belonging to three different families have been shown to be sequentially activated after TCR engagement: p56(lck) and p59(fyn) of the Src family, p70(zap) and p72(syk) of the Syk family and p50(csk) of the Csk family. p56(lck) interacts noncovalently with the cytoplasmic domains of the co-receptors CD4 and CD8, in helper and suppressor T cells, respectively; p59(fyn) has been shown to weakly associate directly with the TCR complex. The catalytic activity of the Src family kinases is regulated by the CD45 transmembrane phosphatase and the Csk cytosolic tyrosine-kinase. Both these enzymes act on the same C-terminal regulatory tyrosine residue of the Src-like PTKs. TCR stimulation rapidly induces tyrosine phosphorylation of p56(lck) and p59(fyn), whose most important cellular substrates are the ARAM sequences of the CD3 complex and the ζ chains. The phosphorylated ARAMs create the binding sites for the two SH2 domains of p70(zap), leading to p70(zap) activation. Even though p72(syk) is known to be activated after TCR stimulation, its exact temporal localization in the signaling cascade is not completely understood. Recruitment of downstream signal transducers, such as phospholipase C γ1 (PLCγ1) and Ras, follows p70(zap) activation. All these events lead to changes in gene expression, production of lymphokines and cell proliferation. This review focuses on the biochemical pathways activated by TCR engagement and on the posible clinic-therapeutic implications of the altered activity of the involved tyrosine-kinases
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
Surface-wave tomography using SeisLib: A Python package for multiscale seismic imaging
No full textTo improve our understanding of the Earth's interior, seismologists often have to deal with enormous amounts of data, requiring automatic tools for their analyses. It is the purpose of this study to present SeisLib, an open-source Python package for multiscale seismic imaging. At present, SeisLib includes routines for carrying out surface-wave tomography tasks based on seismic ambient noise and teleseismic earthquakes. We illustrate here these functionalities, both from the theoretical and algorithmic point of view and by application of our library to seismic data from North America. We first show how SeisLib retrieves surface-wave phase velocities from the ambient noise recorded at pairs of receivers, based on the zero crossings of their normalized cross-spectrum. We then present our implementation of the two-station method, to measure phase velocities from pairs of receivers approximately lying on the same great-circle path as the epicentre of distant earthquakes. We apply these methods to calculate dispersion curves across the conterminous United States, using continuous seismograms from the transportable component of USArray and earthquake recordings from the permanent networks. Overall, we measure 144 272 ambient-noise and 2055 earthquake-based dispersion curves, that we invert for Rayleigh-wave phase-velocity maps. To map the lateral variations in surface-wave velocity, SeisLib exploits a least-squares inversion algorithm based on ray theory. Our implementation supports both equal-area and adaptive parametrizations, with the latter allowing for a finer resolution in the areas characterized by high density of measurements. In the broad period range 4-100 s, the retrieved velocity maps of North America are highly correlated (on average, 96 per cent) and present very small average differences (0.14 +/- 0.1 per cent) with those reported in the literature. This points to the robustness of our algorithms. We also produce a global phase-velocity map at the period of 40 s, combining our dispersion measurements with those collected at global scale in previous studies. This allows us to demonstrate the reliability and optimized computational speed of SeisLib, even in presence of very large seismic inverse problems and strong variability in the data coverage. The last part of the manuscript deals with the attenuation of Rayleigh waves, which can be estimated through SeisLib based on the seismic ambient noise recorded at dense arrays of receivers. We apply our algorithm to produce an attenuation map of the United States at the period of 4 s, which we find consistent with the relevant literature
Rayleigh-wave attenuation across the conterminous United States in the microseism frequency band
Full text linkMapping variations in the attenuation of seismic energy is important for understanding dissipative mechanisms in the lithosphere, and for modeling ground shaking associated with earthquakes. We cross-correlate ambient seismic signal recorded across the EarthScope Transportable Array in the 3–15 s period range. We apply to the resulting cross correlations a new method to estimate lateral variations in Rayleigh-wave attenuation, as a function of period, beneath North America. Between 3 and 6 s, our maps are dominated by a strong eastward decrease in attenuation. This pattern vanishes at longer periods, confirming early observations based on regional earthquakes. Attenuation maps and phase-velocity maps are anti-correlated at periods between 3 and 6 s, but the anti-correlation is also largely lost at longer periods. This corresponds to the attenuation coefficient decreasing with period more rapidly in the west than in the east, while the change in phase velocity with period is more uniform across the continent. Our results point to a transition in the properties of upper-crustal materials with depth, probably related to the closure of fluid-filled cracks and pores, and imply that measures of attenuation from seismic noise carry significant information on crustal rheology
Aquileia and its Territory in the Fourth Century AD
No full textIn spite of the main opinion, new archaeological and epigrahical evidences show that during the fourth century Aquileia didn’t decline. Its political and economic importance grew in consequence of the administrative reform carried out by Diocletian and of its importance in the struggles between the rulers as well as of its location on the route of invasions. Actually the story of Aquileia in Late Antiquity is above all a story of great urban transformation: a new renovated forum, city walls, circus, new baths, market places and monumental storehouses and christian church buildings mark the new urban look of late antique Aquileia. Imported late roman wares (amphoras, ceramics an so on) from Easterm Mediterranean and from Africa witness the importance of the adriatic harbour. On the territory there is evidence of continuity in the distribution of rural settlements, even if the archaeological researches show structural transformations of the former villas and there are scanty evidences of late antique factories on the territory. So that the case of Carlino remains at the moment almost isolated
Arrival-angle effects on two-receiver measurements of phase velocity
Full text linkWe compile a data set of Rayleigh-wave phase velocities between pairs of stations, based on teleseismic events located on the same great circle as the two stations. We validate our observations against dispersion estimates based on ambient-noise cross correlations at the same station pairs. Discrepancies between the results of the two methods can in principle be explained by deviations in the wave propagation path between earthquake and receivers, due to lateral heterogeneity in the Earth's structure, but the latter effect has, so far, not been precisely quantified nor corrected for. We implement an algorithm to measure the arrival angle of earthquake-generated surface waves and correct the dispersion measurements accordingly. Application to a data set from the Central-Western Mediterranean shows that the arrival-angle correction almost entirely accounts for the discrepancy in question, decreasing significantly the velocity bias for a wide range of periods
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