575 research outputs found

    Logarithmic rate dependence in deforming granular materials

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    Rate-independence for stresses within a granular material is a basic tenet of many models for slow dense granular flows. By contrast, logarithmic rate dependence of stresses is found in solid-on-solid friction, in geological settings, and elsewhere. In this work, we show that logarithmic rate-dependence occurs in granular materials for plastic (irreversible) deformations that occur during shearing but not for elastic (reversible) deformations, such as those that occur under moderate repetitive compression. Increasing the shearing rate, \Omega, leads to an increase in the stress and the stress fluctuations that at least qualitatively resemble what occurs due to an increase in the density. Increases in \Omega also lead to qualitative changes in the distributions of stress build-up and relaxation events. If shearing is stopped at t=0, stress relaxations occur with \sigma(t)/ \sigma(t=0) \simeq A \log(t/t_0). This collective relaxation of the stress network over logarithmically long times provides a mechanism for rate-dependent strengthening.Comment: 4 pages, 5 figures. RevTeX

    Fault healing inferred from time dependent variations in source properties of repeating earthquakes

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    We analyze two sets of repeating earthquakes on the Calaveras fault to estimate in-situ rates of fault strengthening (healing). Earthquake recurrence intervals t, range from 3 to 803 days. Variations in relative moment and duration are combined to study changes in stress drop, rupture dimension, rupture velocity, and particle velocity as a function of tr. Healing rates and source variations are compared with predictions of laboratory derived friction laws. Two interpretations of event duration τ are used: one in which τ: is given by the ratio of slip to particle velocity and one in which it scales as rupture dimension divided by rupture velocity. Our data indicate that faults strengthen during the interseismic period. We infer that source dimension decreases with tr due to aseismic creep within the region surrounding the repeatinge vents. Stress drop increases 1-3MPa per decade increase in tr, which represents an increase of a factor of 2-3 relative to events with tr between 10 and 100 days. This rate of fault healing is consistent with extrapolations of laboratory measurements of healing rates if fault strength is high, on order of 60MPa, ands tress drop is roughly 10% of this value

    The barium isotopic mixture for the metal-poor subgiant star HD140283

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    Current theory regarding heavy element nucleosynthesis in metal-poor environments states that the r-process would be dominant. The star HD140283 has been the subject of debate after it appeared in some studies to be dominated by the s-process. We provide an independent measure of the Ba isotope mixture in HD140283 using an extremely high quality spectrum and an extensive chi^2 analysis. We exploit hyperfine splitting of the BaII 4554 \AA\ and 4934 \AA\ resonance lines in an effort to constrain the isotope ratio in 1D LTE. Using the code ATLAS in conjunction with KURUCZ06 model atmospheres we analyse 93 Fe lines to determine the star's macroturbulence. With this information we construct a grid of Ba synthetic spectra and, using a \chi^2 code, fit these to our observed data to determine the isotopic ratio, fodd, which represents the ratio of odd to even isotopes. We also analyse the Eu lines. We set a new upper limit of the rotation of HD140283 at vsin{i}\leq3.9\kms, a new upper limit on [Eu/H] < -2.80 and abundances [Fe/H] = -2.59\pm0.09, [Ba/H] = -3.46\pm0.11. This leads to a new lower limit on [Ba/Eu] > -0.66. We find that, in the framework of a 1D LTE analysis, the isotopic ratios of Ba in HD140283 indicate fodd=0.02\pm0.06, a purely s-process signature. This implies that observations and analysis do not validate currently accepted theory. We speculate that a 1D code, due to simplifying assumptions, is not adequate when dealing with observations with high levels of resolution and S/N because of the turbulent motions associated with a 3D stellar atmosphere. New approaches to analysing isotopic ratios, in particular 3D hydrodynamics, need to be considered when dealing with the levels of detail required to properly determine them. However published 3D results exacerbate the disagreement between theory and observation.Comment: 16 pages, 10 figures, 7 tables, 1 online appendix Accepted by A&

    Mechanisms for slow strengthening in granular materials

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    Several mechanisms cause a granular material to strengthen over time at low applied stress. The strength is determined from the maximum frictional force F_max experienced by a shearing plate in contact with wet or dry granular material after the layer has been at rest for a waiting time \tau. The layer strength increases roughly logarithmically with \tau -only- if a shear stress is applied during the waiting time. The mechanisms of strengthening are investigated by sensitive displacement measurements and by imaging of particle motion in the shear zone. Granular matter can strengthen due to a slow shift in the particle arrangement under shear stress. Humidity also leads to strengthening, but is found not to be its sole cause. In addition to these time dependent effects, the static friction coefficient can also be increased by compaction of the granular material under some circumstances, and by cycling of the applied shear stress.Comment: 21 pages, 11 figures, submitted to Phys. Rev.

    Food security monitoring via mobile data collection and remote sensing: results from the Central African Republic

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    The Central African Republic is one of the world's most vulnerable countries, suffering from chronic poverty, violent conflicts and weak disaster resilience. In collaboration with Doctors without Borders/Midecins Sans Frontieres (MSF), this study presents a novel approach to collect information about socio-economic vulnerabilities related to malnutrition, access to resources and coping capacities. The first technical test was carried out in the North of the country (sub-prefecture Kabo) in May 2015. All activities were aimed at the investigation of technical feasibility, not at operational data collection, which requires a random sampling strategy. At the core of the study is an open-source Android application named SATIDA COLLECT that facilitates rapid and simple data collection. All assessments were carried out by local MSF staff after they had been trained for one day. Once a mobile network is available, all assessments can easily be uploaded to a database for further processing and trend analysis via MSF in-house software. On one hand, regularly updated food security assessments can complement traditional large-scale surveys, whose completion can take up to eight months. Ideally, this leads to a gain in time for disaster logistics. On the other hand, recording the location of every assessment via the smart phones. GPS receiver helps to analyze and display the coupling between drought risk and impacts over many years. Although the current situation in the Central African Republic is mostly related to violent conflict it is necessary to consider information about drought risk, because climatic shocks can further disrupt the already vulnerable system. SATIDA COLLECT can easily be adapted to local conditions or other applications, such as the evaluation of vaccination campaigns. Most importantly, it facilitates the standardized collection of information without pen and paper, as well as straightforward sharing of collected data with the MSF headquarters or other aid organizations

    (New) Bulgarian Enlighteners and Ambassadors? The Reinvention of National Identity in Times of Crisis

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    Drawing on empirical data from 37 Bulgarian students and young professionals in the UK, this article explores the intersection of the discourses produced by the European crises and migrants' national identity. In Bulgaria, the crisis narrative is embedded in the arguably never‐ending democratic transition, manifested in socio‐economic instability and political volatility. Simultaneously, “Brexit Britain” is enveloped in strong Eurosceptic sentiments, propelled by a combination of austerity measures and intensified Eastern European migratory flows. Both contexts subject Bulgarian migrants to stigmatizing representations. Looking at migrants' everyday practices, the data reveals that young Bulgarians draw on the related ideas of the “new” Enlightener and Ambassador to counterbalance negative discourses. Thus, the article explores the meanings and significance attributed to the Enlighteners and the Ambassadors, arguing that the participants engage in “social creativity” and “individual mobility” strategies that lead to reinvention of national identity
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