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  • 1
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    Preserved history of global mean spreading rate: 83 Ma to present (2017)
    Oxford University Press
    Details
    Publication Date: 2017-01-07
    Description: Using an up-to-date global plate rotation model, applied to the endpoints of preserved major spreading ridge isochrons, we have calculated the explicitly reconstructable length-weighted mean global half-spreading rate (HSR), ridge length and area production as a function of time since the end of the Cretaceous Normal Superchron at 83.0 Ma. Our calculations integrate uncertainties in rotation parameters and chron boundary ages with the partial sampling uncertainties arising from progressive subduction of older oceanic lithosphere and its preserved spreading record. This record of directly reconstructable oceanic ridge production provides a well-constrained baseline that can be compared to reconstructions that include the largely unconstrained extrapolated histories of entirely subducted oceanic plates. The directly reconstructable global mean HSR has not varied by more than ±15 per cent about an average rate of 28.4 ± 4.6 mm a –1 since 83 Ma. No long-term secular trend is evident: a maximum global mean half-rate of 32 ± 6 mm a –1 occurred from 33.1 Ma to about 25.8 Ma, with minima of 26 ± 5 mm a –1 between about 56 and 40.2 Ma, and 24 ± 1 mm a –1 since 3.2 Ma. Only this most recent interval has a rate that differs significantly (at ±2) from the long-term mean. The global, reconstructable ridge length at 56 Ma decreases by less than 15 per cent relative to the modern ridge system; by 83 Ma it has decreased by 38 per cent. These relatively high preserved ridge fractions mean that the estimated uncertainty due to partial sampling stays roughly equivalent to the estimated rotation model uncertainties, allowing long-term spreading rate variations of 〉20 per cent since the Late Cretaceous to be ruled out. In contrast, prior to 83 Ma too little oceanic lithosphere is preserved to reliably reconstruct global spreading rates.
    Keywords: Geodynamics and Tectonics
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 2
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    Socioeconomic and Demographic Determinants of the Nutritional Content of National School Lunch Program Entree Selections (2017)
    Oxford University Press
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    Publication Date: 2017-01-05
    Description: New National School Lunch Program (NSLP) guidelines aim to reduce sodium and saturated fats, limit calories, and eliminate trans-fat and whole milk. This paper provides a novel approach to understanding how the healthfulness of NSLP participants’ entrée selections varies across socioeconomic and demographic groups. Unlike previous studies that rely on dietary recalls, we use a mixed logit model to examine students’ entrée choices in a school cafeteria. We estimate the likelihood that an entrée is selected from the available lunch choices as a function of the entrée’s nutrients (fat, carbohydrate, protein, and sodium) and entrée’s taste profile characteristics (e.g., Mexican, Pizza-like), as well as the student’s socio-economic and demographic characteristics. Using these estimates, we examine how changing the nutritional content of an offering impacts the probability of selecting each of the offerings. Free lunch recipients are more likely to choose entrées higher in fat but lower in sodium than other students. Full-price lunch recipients are the most responsive to changes in nutritional content of the offerings and are most likely to respond to changes in the nutritional content of the offered entrées by substituting a lunch brought from home for the school-purchased lunch. Replacing less healthy menu items with popular but healthier items reduces the selection of total calories, calories from fat, and sodium by approximately 4%, 18%, and 8%, respectively, over the study period. The new guidelines should be effective at improving the nutrition of school-age children, and potentially reducing childhood obesity, provided NSLP participation does not decline appreciably.
    Keywords: D12 - Consumer Economics: Empirical Analysis, Q18 - Agricultural Policy ; Food Policy
    Print ISSN: 0002-9092
    Electronic ISSN: 1467-8276
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition , Economics
    Published by Oxford University Press
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  • 3
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    Crop Choice, School Participation, and Child Labor in Developing Countries: Cotton Expansion in Burkina Faso (2017)
    Oxford University Press
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    Publication Date: 2017-01-05
    Description: We estimate the effects of changes in cotton adoption on children’s schooling and child labor in rural Burkina Faso. Using time and spatial variations, we find evidence that expansion of cotton farming has led to an increase in enrollment and to a reduction of participation in child labor for girls. There are, however, no detectable effects on boys. In theory, cotton adoption could increase household income, leading to increased demand for schooling and reduced child labor. On the other hand, because children are productive on cotton farms, adoption of cotton could increase the opportunity cost of child time and the demand for child labor. We provide suggestive evidence showing that boys are more productive than girls on cotton farms. Taken together, the results suggest that the income effect from cotton adoption might have been larger than the wage effect for girls, hence the overall positive impacts on school enrollment for girls.
    Keywords: O12 - Microeconomic Analyses of Economic Development, O13 - Agriculture ; Natural Resources ; Energy ; Environment ; Other Primary Products, O15 - Human Resources ; Human Development ; Income Distribution ; Migration, Q18 - Agricultural Policy ; Food Policy
    Print ISSN: 0002-9092
    Electronic ISSN: 1467-8276
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition , Economics
    Published by Oxford University Press
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  • 4
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    Present-day velocity field and block kinematics of Tibetan Plateau from GPS measurements (2017)
    Oxford University Press
    Details
    Publication Date: 2017-01-01
    Description: In this study, we present a new synthesis of GPS velocities for tectonic deformation within the Tibetan Plateau and its surrounding areas, a combined data set of ~1854 GPS-derived horizontal velocity vectors. Assuming that crustal deformation is localized along major faults, a block modelling approach is employed to interpret the GPS velocity field. We construct a 30-element block model to describe present-day deformation in western China, with half of them located within the Tibetan Plateau, and the remainder located in its surrounding areas. We model the GPS velocities simultaneously for the effects of block rotations and elastic strain induced by the bounding faults. Our model yields a good fit to the GPS data with a mean residual of 1.08 mm a –1 compared to the mean uncertainty of 1.36 mm a –1 for each velocity component, indicating a good agreement between the predicted and observed velocities. The major strike-slip faults such as the Altyn Tagh, Xianshuihe, Kunlun and Haiyuan faults have relatively uniform slip rates in a range of 5–12 mm a –1 along most of their segments, and the estimated fault slip rates agree well with previous geologic and geodetic results. Blocks having significant residuals are located at the southern and southeastern Tibetan Plateau, suggesting complex tectonic settings and further refinement of accurate definition of block geometry in these regions.
    Keywords: Geodynamics and Tectonics
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 5
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    Current deformation in Central Afar and triple junction kinematics deduced from GPS and InSAR measurements (2016)
    Oxford University Press
    Details
    Publication Date: 2016-12-22
    Description: Kinematics of divergent boundaries and Rift-Rift-Rift junctions are classically studied using long-term geodetic observations. Since significant magma-related displacements are expected, short-term deformation provides important constraints on the crustal mechanisms involved both in active rifting and in transfer of extensional deformation between spreading axes. Using InSAR and GPS data, we analyse the surface deformation in the whole Central Afar region in detail, focusing on both the extensional deformation across the Quaternary magmato-tectonic rift segments, and on the zones of deformation transfer between active segments and spreading axes. The largest deformation occurs across the two recently activated Asal-Ghoubbet (AG) and Manda Hararo-Dabbahu (MH-D) magmato-tectonic segments with very high strain rates, whereas the other Quaternary active segments do not concentrate any large strain, suggesting that these rifts are either sealed during interdyking periods or not mature enough to remain a plate boundary. Outside of these segments, the GPS horizontal velocity field shows a regular gradient following a clockwise rotation of the displacements from the Southeast to the East of Afar, with respect to Nubia. Very few shallow creeping structures can be identified as well in the InSAR data. However, using these data together with the strain rate tensor and the rotations rates deduced from GPS baselines, the present-day strain field over Central Afar is consistent with the main tectonic structures, and therefore with the long-term deformation. We investigate the current kinematics of the triple junction included in our GPS data set by building simple block models. The deformation in Central Afar can be described by adding a central microblock evolving separately from the three surrounding plates. In this model, the northern block boundary corresponds to a deep EW-trending trans-tensional dislocation, locked from the surface to 10–13 km and joining at depth the active spreading axes of the Red Sea and the Aden Ridge, from AG to MH-D rift segments. Over the long-term, this plate configuration could explain the presence of the en-échelon magmatic basins and subrifts. However, the transient behaviour of the spreading axes implies that the deformation in Central Afar evolves depending on the availability of magma supply within the well-established segments.
    Keywords: Geodynamics and Tectonics
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 6
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    Compound dislocation models (CDMs) for volcano deformation analyses (2016)
    Oxford University Press
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    Publication Date: 2016-12-22
    Description: Volcanic crises are often preceded and accompanied by volcano deformation caused by magmatic and hydrothermal processes. Fast and efficient model identification and parameter estimation techniques for various sources of deformation are crucial for process understanding, volcano hazard assessment and early warning purposes. As a simple model that can be a basis for rapid inversion techniques, we present a compound dislocation model (CDM) that is composed of three mutually orthogonal rectangular dislocations (RDs). We present new RD solutions, which are free of artefact singularities and that also possess full rotational degrees of freedom. The CDM can represent both planar intrusions in the near field and volumetric sources of inflation and deflation in the far field. Therefore, this source model can be applied to shallow dikes and sills, as well as to deep planar and equidimensional sources of any geometry, including oblate, prolate and other triaxial ellipsoidal shapes. In either case the sources may possess any arbitrary orientation in space. After systematically evaluating the CDM, we apply it to the co-eruptive displacements of the 2015 Calbuco eruption observed by the Sentinel-1A satellite in both ascending and descending orbits. The results show that the deformation source is a deflating vertical lens-shaped source at an approximate depth of 8 km centred beneath Calbuco volcano. The parameters of the optimal source model clearly show that it is significantly different from an isotropic point source or a single dislocation model. The Calbuco example reflects the convenience of using the CDM for a rapid interpretation of deformation data.
    Keywords: Geodynamics and Tectonics
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 7
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    Insights into pulverized rock formation from dynamic rupture models of earthquakes (2016)
    Oxford University Press
    Details
    Publication Date: 2016-12-18
    Description: Pulverized rocks (PR) are extremely incohesive and highly fractured rocks found within the damage zones of several large strike-slip faults around the world. They maintain their crystal structure, show little evidence of shearing or chemical alteration, and are believed to be produced by strong tensile forces. Several mechanisms for pulverization have been proposed based on simple qualitative analyses or laboratory experiments under simplified loading conditions. Numerical modelling, however, can offer new insights into what is needed to produce PR and likely conditions of formation. We perform dynamic rupture simulations of different earthquakes, varying the magnitude, the slip distribution, and the rupture speed (supershear and subshear), while measuring the stresses produced away from the fault. To contextualize our results, a basic threshold of 10 MPa is set as the tensile strength of the rock mass and recordings are made of where, when, and by how much this threshold is exceeded for each earthquake type. Guided by field observations, we discern that a large (〉 M w 7.1) subshear earthquake along a bimaterial fault produces a pulverized rock distribution most consistent with observations. The damage is asymmetric with the majority on the stiffer side of the fault extending out for several hundred metres. Within this zone there is a large and sudden volumetric expansion in all directions as the rupture passes. We propose that such an extreme tensile stress state, repeated for every earthquake, eventually produces the PR seen in the field.
    Keywords: Geodynamics and Tectonics
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 8
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    Explaining the Impact of USDA's Healthy Incentives Pilot on Different Spending Outcomes (2016)
    Oxford University Press
    Details
    Publication Date: 2016-12-07
    Description: This article reports spending results for the USDA's Healthy Incentives Pilot (HIP), which tested a 30% incentive on fruit and vegetable purchases with Supplemental Nutrition Assistance Program (SNAP) benefits. Self-reported mean usual monthly spending for all fruits and vegetables was $6.15 higher for randomly assigned HIP participant households than for a control group. Much of the additional spending appears to have taken place in ways that did not earn the incentive—spending with non-SNAP resources or in retailers that did not participate in HIP. This article investigates mechanisms that might explain the HIP impact on fruit and vegetable purchases that did not earn the incentive.
    Keywords: Q18 - Agricultural Policy ; Food Policy
    Print ISSN: 2040-5790
    Electronic ISSN: 2040-5804
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition , Economics
    Published by Oxford University Press
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  • 9
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    Did Revisions to the WIC Program Affect Household Expenditures on Whole Grains? (2016)
    Oxford University Press
    Details
    Publication Date: 2016-12-07
    Description: The food packages provided by the Supplemental Nutrition Program for Women, Infants, and Children (WIC) program changed in 2009. This article examines purchases of whole grain products before and after the change. Nielsen Homescan panel data from 2008 to 2010 provide information on households’ food purchases, demographics, and self-reported WIC participation status. We estimate the effect of WIC participation and the 2009 package change on whole grains purchases using a difference-in-difference method, and find that participation in the WIC program was associated with more whole grain purchases during the observed period; the package change in 2009 roughly doubled the associated effect of WIC participation on the purchases of whole grain products. These results are consistent with recommendations in the Dietary Guidelines for Americans and suggest that moderate innovations in the design of food assistance programs can lead to beneficial dietary choices.
    Keywords: D10 - General, I18 - Government Policy ; Regulation ; Public Health, Q18 - Agricultural Policy ; Food Policy
    Print ISSN: 2040-5790
    Electronic ISSN: 2040-5804
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition , Economics
    Published by Oxford University Press
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  • 10
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    How Meat Demand Elasticities Vary with Price, Income, and Product Category (2016)
    Oxford University Press
    Details
    Publication Date: 2016-12-07
    Description: As US beef and pork prices approached record high levels in 2014, industry analysts expressed surprised at consumer response. Because the relative price swings have occurred only recently, traditional approaches to demand analysis that rely on historical data series may be less useful than is typically the case. Employing one of the largest and longest-running choice experiments, we analyze data on 110,295 choices made by 12,255 consumers observed over a year-long time period coinciding with historically high meat prices. Our findings reveal nonlinear demands for meat products, with demand being more inelastic at higher prices. Ground beef, steak, and pork chop demands are more sensitive to changes in chicken breast price than the reverse. Moreover, cross-price elasticities between disaggregate meat products shrink as prices rise. Consumers' incomes significantly affect demand inter-relationships. Higher income consumers are more likely to choose steak and chicken breasts and are less likely to choose ground beef, chicken wings, and deli ham than are lower income consumers. High-income consumers tend to be less responsive to own-price changes and more responsive to cross-price changes than lower income consumers. This analysis provides estimates of structural demand parameters that help explain current meat expenditure patterns, and the results have implications for the assumption of linearity often invoked in policy analyses. 
    Keywords: C83 - Survey Methods ; Sampling Methods, D12 - Consumer Economics: Empirical Analysis, Q11 - Aggregate Supply and Demand Analysis ; Prices, Q18 - Agricultural Policy ; Food Policy
    Print ISSN: 2040-5790
    Electronic ISSN: 2040-5804
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition , Economics
    Published by Oxford University Press
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