Information Diffusion, Facebook Clusters, and the Simplicial Model of Social Aggregation: A Computational Simulation of Simplicial Diffusers for Community Health Interventions

By integrating the simplicial model of social aggregation with existing research on opinion leadership and diffusion networks, this article introduces the constructs of simplicial diffusers (mathematically defined as nodes embedded in simplexes; a simplex is a socially bonded cluster) and simplicial diffusing sets (mathematically defined as minimal covers of a simplicial complex; a simplicial complex is a social aggregation in which socially bonded clusters are embedded) to propose a strategic approach for information diffusion of cancer screenings as a health intervention on Facebook for community cancer prevention and control. This approach is novel in its incorporation of interpersonally bonded clusters, culturally distinct subgroups, and different united social entities that co-exist within a larger community into a computational simulation to select sets of simplicial diffusers with the highest degree of information diffusion for health intervention dissemination. The unique contributions of the article also include seven propositions and five algorithmic steps for computationally modeling the simplicial model with Facebook data

GRB 050223: A dark GRB in a dusty starburst galaxy

Aims: We aim at detecting and determining the properties of the host galaxy of the dark GRB 050223. Methods: We use VLT optical/NIR images coupled to Swift X-ray positioning, and optical spectra of the host galaxy to measure its properties. Results: We find a single galaxy within the Swift error box of GRB 050223. It is located at z = 0.584 and its luminosity is L ~ 0.4 L*. Emission lines in the galaxy spectrum imply an intrinsic SFR > 7 Msun/yr, and a large extinction A_V > 2 mag within it. We also detect absorption lines, which reveal an underlying stellar population with an age between 40 Myr and 1.5 Gyr. Conclusions: The identification of a host galaxy with atypical properties using only the X-ray transient suggests that a bias may be present in the former sample of host galaxies. Dust obscuration together with intrinsic faintness are the most probable causes for the darkness of this burst.Comment: 4 pages, 2 figures, accepted for publication in Astronomy & Astrophysic

The Role of Viral Introductions in Sustaining Community-Based HIV Epidemics in Rural Uganda: Evidence from Spatial Clustering, Phylogenetics, and Egocentric Transmission Models

Background:It is often assumed that local sexual networks play a dominant role in HIV spread in sub-Saharan Africa. The aim of this study was to determine the extent to which continued HIV transmission in rural communities-home to two-thirds of the African population-is driven by intra-community sexual networks versus viral introductions from outside of communities.Methods and Findings:We analyzed the spatial dynamics of HIV transmission in rural Rakai District, Uganda, using data from a cohort of 14,594 individuals within 46 communities. We applied spatial clustering statistics, viral phylogenetics, and probabilistic transmission models to quantify the relative contribution of viral introductions into communities versus community- and household-based transmission to HIV incidence. Individuals living in households with HIV-incident (n = 189) or HIV-prevalent (n = 1,597) persons were 3.2 (95% CI: 2.7-3.7) times more likely to be HIV infected themselves compared to the population in general, but spatial clustering outside of households was relatively weak and was confined to distances <500 m. Phylogenetic analyses of gag and env genes suggest that chains of transmission frequently cross community boundaries. A total of 95 phylogenetic clusters were identified, of which 44% (42/95) were two individuals sharing a household. Among the remaining clusters, 72% (38/53) crossed community boundaries. Using the locations of self-reported sexual partners, we estimate that 39% (95% CI: 34%-42%) of new viral transmissions occur within stable household partnerships, and that among those infected by extra-household sexual partners, 62% (95% CI: 55%-70%) are infected by sexual partners from outside their community. These results rely on the representativeness of the sample and the quality of self-reported partnership data and may not reflect HIV transmission patterns outside of Rakai.Conclusions:Our findings suggest that HIV introductions into communities are common and account for a significant proportion of new HIV infections acquired outside of households in rural Uganda, though the extent to which this is true elsewhere in Africa remains unknown. Our results also suggest that HIV prevention efforts should be implemented at spatial scales broader than the community and should target key populations likely responsible for introductions into communities.Please see later in the article for the Editors' Summary

The 30 December 2002 landslide-induced tsunamis in Stromboli: sequence of the events reconstructed from the eyewitness accounts

International audienceOn 30 December 2002 the coast of the volcanic island of Stromboli, in the Tyrrhenian sea, Italy, was attacked by two tsunamis generated by landslides that took place on the north-west flank of the volcano. The landslides and the tsunamis represented the most impressive and threatening episodes of a strong effusive eruption, started on 28 December from a new vent which opened close to the north-east crater of the volcano. In spite of the intensified monitoring carried out in response to the eruption, the landslides and the ensuing tsunamis were not foreseen, and the available instrumental data are insufficient to allow a precise reconstruction of the sequence of the events. The seismic network recorded two main landslides along the steep slope of Sciara del Fuoco, with onset around 13:15 and 13:23 local time (GMT+1). The tsunamis were the direct consequence of the mass movements. Three main post-event surveys helped make assessment on the wave impact on the coast. In this paper the attention is focussed on the accounts of the eye-witnesses, that help us clarify and understand what happened. People in the source area (Sciara del Fuoco) reported a small-volume subaerial slide taking place first, then a sharp cut forming in the sea water down to the sea floor (about 10?20 m deep) and propagating almost parallel to the coastline, be concomitantly associated with a sea retreat and a subsequent sea advance. It is suggested here that the cut was the effect of a large submarine landslide that detached from very close to the coast and produced the 13:15 signal in the recorded seismograms. The second, mostly subaerial, slump was observed to slide down 7?8 min later and to excite a train of waves some distance offshore. Not all the witnesses realised that two distinct tsunamis occurred. The tsunami period was probably in the order of 100 s, but shorter period crests were seen to travel on the top of the long-period waves by several persons. The duration of each tsunami was appreciated to be around 5?7 min. It is difficult to ascertain which tsunami was the largest, since there is no full agreement among the observers, but certainly by accounts both were characterised by large destructive waves

A new computational method based on the minimum lithostatic deviation (MLD) principle to analyse slope stability in the frame of the 2D limit-equilibrium theory

The stability of a slope is studied by applying the principle of the minimum lithostatic deviation (MLD) to the limit-equilibrium method, that was introduced in a previous paper (Tinti and Manucci, 2006; hereafter quoted as TM2006). The principle states that the factor of safety &lt;i&gt;F&lt;/i&gt; of a slope is the value that minimises the lithostatic deviation, that is defined as the ratio of the average inter-slice force to the average weight of the slice. In this paper we continue the work of TM2006 and propose a new computational method to solve the problem. The basic equations of equilibrium for a 2-D vertical cross section of the mass are deduced and then discretised, which results in cutting the cross section into vertical slices. The unknowns of the problem are functions (or vectors in the discrete system) associated with the internal forces acting on the slice, namely the horizontal force &lt;i&gt;E&lt;/i&gt; and the vertical force &lt;i&gt;X&lt;/i&gt;, with the internal torque &lt;i&gt;A&lt;/i&gt; and with the pressure on the bottom surface of the slide &lt;i&gt;P&lt;/i&gt;. All traditional limit-equilibrium methods make very constraining assumptions on the shape of &lt;i&gt;X&lt;/i&gt; with the goal to find only one solution. In the light of the MLD, the strategy is wrong since it can be said that they find only one point in the searching space, which could provide a bad approximation to the MLD. The computational method we propose in the paper transforms the problem into a set of linear algebraic equations, that are in the form of a block matrix acting on a block vector, a form that is quite suitable to introduce constraints on the shape of &lt;i&gt;X&lt;/i&gt;, but also alternatively on the shape of &lt;i&gt;E&lt;/i&gt; or on the shape of &lt;i&gt;X&lt;/i&gt;. We test the new formulation by applying it to the same cases treated in TM2006 where &lt;i&gt;X&lt;/i&gt; was expanded in a three-term sine series. Further, we make different assumptions by taking a three-term cosine expansion corrected by the local weight for &lt;i&gt;X&lt;/i&gt;, or for &lt;i&gt;E&lt;/i&gt; or for &lt;i&gt;A&lt;/i&gt;, and find the corresponding MLDs. In the illustrative applications given in this paper, we find that the safety factors associated with the MLD resulting from our computations may differ by some percent from the ones computed with the traditional limit-equilibrium methods

Self-induced deformation on the fault plane during an earthquake part I: Continuous normal displacements

The problem of the coseismic deformation induced by an earthquake on its own fault plane is investigated here. The analysis concentrates on the on-fault displacement field accompanying the occurrence of an earthquake in response to a prescribed uniform shear slip and it is carried out on the basis of the classical analytical model by OKADA (1992) for a rectangular fault buried in an elastic homogeneous half-space delimited by a planar free surface. The analysis is subdivided into two separate papers: the first dealing with normal and the second with tangential on-fault displacements. In this first paper, concerning the study of the normal displacement component, the contributions of the source and of the correction introduced by the free surface are investigated separately and their dependence on the fault characteristics is thoroughly discussed. Particular attention is also devoted to the effects of the normal displacement on the fault surface geometry. It will be shown that the main effect is that of deforming the fault itself, with deformation consisting chiefly in a rotation of the plane and in a bending of the fault edges. The rotation angle is negligibly small (on the order of 1-10 μrad) for a single earthquake, although repeated seismic events occuring on the same fault might result in rotations of several degrees over geological time scales