Tectonically asymmetric Earth: From net rotation to polarized westward drift of the lithosphere

AbstractThe possibility of a net rotation of the lithosphere with respect to the mantle is generally overlooked since it depends on the adopted mantle reference frames, which are arbitrary. We review the geological and geophysical signatures of plate boundaries, and show that they are markedly asymmetric worldwide. Then we compare available reference frames of plate motions relative to the mantle and discuss which is at best able to fit global tectonic data. Different assumptions about the depths of hotspot sources (below or within the asthenosphere, which decouples the lithosphere from the deep mantle) predict different rates of net rotation of the lithosphere relative to the mantle. The widely used no-net-rotation (NNR) reference frame, and low (1°/Ma) net rotation (shallow hotspots source), all plates, albeit at different velocity, move westerly along a curved trajectory, with a tectonic equator tilted about 30° relative to the geographic equator. This is consistent with the observed global tectonic asymmetries

Simulation of viscoelastic Cosserat rods based on the geometrically exact dynamics of special Euclidean strands

We propose a method for the description and simulation of the nonlinear dynamics of slender structures modeled as Cosserat rods. It is based on interpreting the strains and the generalized velocities of the cross sections as basic variables and elements of the special Euclidean algebra. This perspective emerges naturally from the evolution equations for strands, that are one-dimensional submanifolds, of the special Euclidean group. The discretization of the corresponding equations for the three-dimensional motion of a Cosserat rod is performed, in space, by using a staggered grid. The time evolution is then approximated with a semi-implicit method. Within this approach, we can easily include dissipative effects due to both the action of external forces and the presence of internal mechanical dissipation. The comparison with results obtained with different schemes shows the effectiveness of the proposed method, which is able to provide very good predictions of nonlinear dynamical effects and shows competitive computation times also as an energy-minimizing method to treat static problems

Mantle thermal structure at northern Mid–Atlantic ridge from improved numerical methods and boundary conditions

We computed mantle flow and thermal structure beneath a segment of the northern Mid-Atlantic Ridge using numerical simulations adopting asymmetric spreading and ridge migration as boundary conditions. The objective is to obtain new insights on mantle processes acting at this ridge segment. We explored different lateral boundary conditions based on velocity, stress and stress–velocity constraints highlighting differences in the depth of the thermal base of the lithosphere versus domain width. Here, we propose a new formulation of lateral and bottom boundary conditions based on the choice of a proper tangential stress at the bottom and on lateral boundaries of the domain accounting for ridge migration. Moreover, dimensional analysis of governing equations suggests that heat generation due to work of the viscous forces cannot be neglected in the computations. Therefore, we included this thermal contribution into the numerical experiments providing an application to the northern Mid-Atlantic Ridge at the reference latitude of 43°N. Results are compared with available geophysical data in the area, including also mantle tomography models. Asymmetric spreading and ridge migration in numerical modelling account for an asymmetric accretion of the oceanic lithosphere, supporting the evidence of the asymmetries described by geophysical data across the northern Mid-Atlantic Ridge segments

HPC simulations of brownout: A noninteracting particles dynamic model

Helicopters can experience brownout when flying close to a dusty surface. The uplifting of dust in the air can remarkably restrict the pilot’s visibility area. Consequently, a brownout can disorient the pilot and lead to the helicopter collision against the ground. Given its risks, brownout has become a high-priority problem for civil and military operations. Proper helicopter design is thus critical, as it has a strong influence over the shape and density of the cloud of dust that forms when brownout occurs. A way forward to improve aircraft design against brownout is the use of particle simulations. For simulations to be accurate and comparable to the real phenomenon, billions of particles are required. However, using a large number of particles, serial simulations can be slow and too computationally expensive to be performed. In this work, we investigate an message passing interface (MPI) + graphics processing unit (multi-GPU) approach to simulate brownout. In specific, we use a semi-implicit Euler method to consider the particle dynamics in a Lagrangian way, and we adopt a precomputed aerodynamic field. Here, we do not include particle–particle collisions in the model; this allows for independent trajectories and effective model parallelization. To support our methodology, we provide a speedup analysis of the parallelization concerning the serial and pure-MPI simulations. The results show (i) very high speedups of the MPI + multi-GPU implementation with respect to the serial and pure-MPI ones, (ii) excellent weak and strong scalability properties of the implemented time-integration algorithm, and (iii) the possibility to run realistic simulations of brownout with billions of particles at a relatively small computational cost. This work paves the way toward more realistic brownout simulations, and it highlights the potential of high-performance computing for aiding and advancing aircraft design for brownout mitigation

Tectonically asymmetric Earth: From net rotation to polarized westward drift of the lithosphere

The possibility of a net rotation of the lithosphere with respect to the mantle is generally overlooked since it depends on the adopted mantle reference frames, which are arbitrary. We review the geological and geophysical signatures of plate boundaries, and show that they are markedly asymmetric worldwide. Then we compare available reference frames of plate motions relative to the mantle and discuss which is at best able to fit global tectonic data. Different assumptions about the depths of hotspot sources (below or within the asthenosphere, which decouples the lithosphere from the deep mantle) predict different rates of net rotation of the lithosphere relative to the mantle. The widely used no-net-rotation (NNR) reference frame, and low (1°/Ma) net rotation (shallow hotspots source), all plates, albeit at different velocity, move westerly along a curved trajectory, with a tectonic equator tilted about 30° relative to the geographic equator. This is consistent with the observed global tectonic asymmetries. © 2014 China University of Geosciences (Beijing) and Peking University.The possibility of a net rotation of the lithosphere with respect to the mantle is generally overlooked since it depends on the adopted mantle reference frames, which are arbitrary. We review the geological and geophysical signatures of plate boundaries, and show that they are markedly asymmetric worldwide. Then we compare available reference frames of plate motions relative to the mantle and discuss which is at best able to fit global tectonic data. Different assumptions about the depths of hotspot sources (below or within the asthenosphere, which decouples the lithosphere from the deep mantle) predict different rates of net rotation of the lithosphere relative to the mantle. The widely used no-net-rotation (NNR) reference frame, and low (1°/Ma) net rotation (shallow hotspots source), all plates, albeit at differ

A novel OpenSees element for single curved surface sliding isolators

The increasing use of curved surface sliding bearings as seismic isolators benefits from the improvement of analytical models that can accurately capture their experimental performance and enhance the predictive capability of nonlinear response history analyses. The mathematical formulation proposed in this paper aims at addressing the variability of the coefficient of friction based on experimental data that can be retrieved from prototype tests on Curved Surface Sliders. The formulation accounts for variation in the coefficient of friction with the instantaneous change of axial load and sliding velocity at the contact interface, and the accumulated heat due to cyclic motion; furthermore, it incorporates new features such as the static friction developed in the transition from the pre-sliding phase to the dynamic sliding condition. The proposed model has been coded in the object-oriented finite element software OpenSees by modifying the standard SingleFPSimple3d element that describes the force – displacement relationship of a bearing comprising one concave sliding surface and a spherical articulation. The main novelties of the new CSSBearing_BVNC element are inclusion of the static friction before the breakaway and degradation of kinetic friction induced by the heat developed during the motion of the articulated slider. The primary assumptions in the development of the friction model and the verification of the newly developed element are validated by agreement with available data. A case study helps to demonstrate the improved prediction capability of the new bearing element over its standard counterpart when applied to real situations, such as estimating a +50% increase in isolator displacement, superstructure drift and base shear demand under high intensity earthquakes, and possible non-activation of the sliding isolators under weak or medium intensity earthquakes

N-3 fatty acids in patients with multiple cardiovascular risk factors

BACKGROUND: Trials have shown a beneficial effect of n-3 polyunsaturated fatty acids in patients with a previous myocardial infarction or heart failure. We evaluated the potential benefit of such therapy in patients with multiple cardiovascular risk factors or atherosclerotic vascular disease who had not had a myocardial infarction. METHODS: In this double-blind, placebo-controlled clinical trial, we enrolled a cohort of patients who were followed by a network of 860 general practitioners in Italy. Eligible patients were men and women with multiple cardiovascular risk factors or atherosclerotic vascular disease but not myocardial infarction. Patients were randomly assigned to n-3 fatty acids (1 g daily) or placebo (olive oil). The initially specified primary end point was the cumulative rate of death, nonfatal myocardial infarction, and nonfatal stroke. At 1 year, after the event rate was found to be lower than anticipated, the primary end point was revised as time to death from cardiovascular causes or admission to the hospital for cardiovascular causes. RESULTS: Of the 12,513 patients enrolled, 6244 were randomly assigned to n-3 fatty acids and 6269 to placebo. With a median of 5 years of follow-up, the primary end point occurred in 1478 of 12,505 patients included in the analysis (11.8%), of whom 733 of 6239 (11.7%) had received n-3 fatty acids and 745 of 6266 (11.9%) had received placebo (adjusted hazard ratio with n-3 fatty acids, 0.97; 95% confidence interval, 0.88 to 1.08; P=0.58). The same null results were observed for all the secondary end points. CONCLUSIONS: In a large general-practice cohort of patients with multiple cardiovascular risk factors, daily treatment with n-3 fatty acids did not reduce cardiovascular mortality and morbidity. Copyright © 2013 Massachusetts Medical Society