Bedform effect on the reorganization of surface and subsurface grain size distribution in gravel bedded channels

Quantification of river bedform variability and complexity is important for sediment transport modeling as well as for characterization of river morphology. Alluvial bedforms are shown to exhibit highly nonlinear dynamics across a range of scales, affect local bed roughness, and vary with local hydraulic, hydrologic, and geomorphic properties. This paper examines sediment sorting on the crest and trough of gravel bedforms and relates it to bed elevation statistics. The data analysed here are the spatial and temporal series of bed elevation, grain size distribution of surface and subsurface bed materials, and sediment transport rates from flume experiments. We describe surface topography through bedform variability in height and wavelength and multiscale analysis of bed elevations as a function of discharge. We further relate bedform migration to preferential distribution of coarse and fine sediments on the troughs and crests, respectively, measuring directly surface and subsurface grain size distributions, and indirectly the small scale roughness variations as estimated from high resolution topographic scans.     

Seismic response of masonry-infilled steel frames via multi-scale finite-element analyses

Effects of masonry infills on the seismic vulnerability of steel frames is studied through multi-scale numerical modelling. First, a micro-modelling approach is utilized to define a homogenized masonry material, calibrated on experimental tests, which is used for modelling the nonlinear response of a one-story, single span, masonry-infilled portal under horizontal loads. Based on results of the micro-model, the constitutive behavior of a diagonal strut macro-element equivalent to the infill panel is calibrated. Then, the diagonal strut is used to model infill panels in the macro-scale analysis of a multi-span multi-story infilled moment-resisting (MR) steel frame. The seismic vulnerability of the MR frame is evaluated through a nonlinear static procedure. Numerical analyses highlight that infills may radically modify the seismic response and the failure mechanism of the frame, hence the importance of the infill correct modelling.     

Displacement capacity of masonry piers: parametric numerical analyses versus international building codes

The nonlinear behaviour of masonry piers loaded in their plane is investigated by parametric numerical simulations. Each pier has a cantilever scheme, is loaded by a constant axial load and is subjected to an increasing horizontal displacement at the top. The macro-modelling approach is used to perform numerical analyses, adopting two different constitutive laws: a total strain crack model and a plastic model. The numerical model is calibrated on a block-masonry type for which experimental tests are available in literature. Parametric numerical simulations are performed by varying the aspect-ratio and the compression level, in order to assess the influence of such parameters on both shear strength and displacement capacity. By comparing numerical results with formulas of international codes, a good agreement for the shear strength is obtained, while significant differences are observed for the displacement capacity, which is influenced by both parameters. The authors propose a simple empirical formula for the displacement capacity, obtained by fitting the numerical results. The expression can be useful in the practical design for considering the influence of aspect-ratio and compression level, currently neglected by building codes.     

Damping reduction factors and code‐based design equation for structures using semi‐active viscous dampers

This study uses a semi‐active viscous damper with three different control laws to reshape the structural hysteresis loop and mitigate structural response, referred to as 1–4, 1–3 and 2–4 devices, respectively. The 1–4 control law provides damping in all four quadrants of the force‐displacement graph (it behaves like a standard viscous damper), the 1–3 control law provides resisting forces only in the first and third quadrants, and the 2–4 control law provides damping in the second and fourth quadrants. This paper first outlines the linear single degree of freedom structural performance when the three types of semi‐active viscous dampers are applied. The results show that simultaneous reduction in both displacement and base‐shear demand is only available with the semi‐active 2–4 device. To enable guidelines for adding a 2–4 device into the design procedure, damping reduction factors (RF_ξs) are developed, as they play an important role and provide a means of linking devices to design procedures. Three methods are presented to obtain RF_ξ and equivalent viscous damping of a structure with a 2–4 semi‐active viscous damper. In the first method, the relationship between RF_ξ and the damping of a semi‐active structure can be obtained by calculating the area under the force‐deformation diagram. The second and third method modified the Eurocode8 formula of RF_ξ and smoothed results from analysis, respectively. Finally, a simple method is proposed to incorporate the design or retrofit of structures with simple, robust and reliable 2–4 semi‐active viscous dampers using standard design approaches. Copyright © 2016 John Wiley & Sons, Ltd.     

Caldera subsidence in extensional tectonics

A 45-km-long regional dike was emplaced over a period of 2 weeks in August 2014 at the boundary between the East and North Volcanic Zones in Iceland. This is the first regional dike emplacement in Iceland monitored with modern geophysical networks, the importance of which relates to regional dikes feeding most of the large fissure (e.g., Eldgja 934 and Laki 1783) and lava shield (e.g. early Holocene Skjaldbreidur and Trölladyngja) eruptions. During this time, the dike generated some 17,000 earthquakes, more than produced in Iceland as a whole over a normal year. The dike initiated close to the Bardarbunga Volcano but gradually extended to the northeast until it crossed the boundary between the East Volcanic Zone (EVZ) and the North Volcanic Zone (NVZ). We infer that the strike of the dike changes abruptly at a point, from about N45°E (coinciding with the trend of the EVZ) to N15°E (coinciding with the trend of the NVZ). This change in strike occurs at latitude 64.7°, exactly the same latitude at which about 10 Ma dikes in East Iceland change strike in a similar way. This suggests that the change in the regional stress field from the southern to the northern part of Iceland has been maintained at this latitude for 10 million years. Analytical and numerical models indicate that the dike-induced stress field results in stress concentration around faults and particularly shallow magma chambers and calderas in its vicinity, such as Tungnafellsjökull, Kverkfjöll, and Askja. In particular, the dike has induced high compressive, shear, and tensile stresses at the location of the Bardarbunga shallow chamber and (caldera) ring-fault where numerous earthquakes occurred during the dike emplacement, many of which have exceeded M5 (the largest M5.7). The first segment of the dike induced high tensile stresses in the nearby part of the Bardarbunga magma chamber/ring-fault resulting in radially outward injection of a dike from the chamber at a high angle to the strike of the regional dike. The location of maximum stress at Bardarbunga fluctuates along the chamber/ring-fault boundary in harmony with dike size and/or pressure changes and encourages ring-dike formation and associated magma flow within the chamber. Caldera collapse and/or eruption in some of these volcanoes is possible, most likely in Bardarbunga, but depends largely on the future development of the regional dike.     

Assessing the rock glacier kinematics on three different timescales: a case study from the southern Swiss Alps

Surface temperature increases since the 1990s have often been associated with an increase in the speed of rock glaciers. Evidence of similar links on the centennial to millennial scale are, however, still lacking due to less focus to date on the medium‐ and long‐term kinematics of these landforms. In order to assess (palaeo)climatic variations in rock glacier kinematics, we analysed the movements of the Stabbio di Largario rock glacier in the southern Swiss Alps using three different timescales. The Schmidt hammer exposure‐age dating (SHD) was applied to study long‐term kinematics in order to extrapolate the minimal age of the formation of the rock glacier, which may have started its development after the Mid‐Holocene climate optimum, and to detect possible accelerations of the horizontal surface velocity during the Medieval Warm Period. Georeferentiation and orthorectification of six historical photographs of the rock glacier taken between ad 1910 and today were analysed using monoplotting to detect the rock glacier displacement on the decennial scale from the end of the Little Ice Age. Finally, differential global positioning system (dGPS) monitoring data available since ad 2009 were used to assess annual and seasonal creep rates of the rock glacier at present. Our results show a link between the periods of increase in mean air temperature on different timescales and variations in rock glacier kinematics and provide important new insights into rock glacier development and evolution on the long‐term scale. Copyright © 2014 John Wiley & Sons, Ltd.     

The sodium exosphere of Mercury: Comparison between observations during Mercury's transit and model results

In this study we compare the sodium exosphere observations made by Schleicher et al. [Schleicher, H., and 4 colleagues, 2004. Astron. Astrophys. 425, 1119-1124] with the result of a detailed numerical simulation. The observations, made during the transit of Mercury across the solar disk on 7 May 2003, show a maximum of sodium emission near the polar regions, with north prevalence, and the presence of a dawn-dusk asymmetry. We interpret this distribution as the resulting effect of two combined processes: the solar wind proton precipitation causing chemical alteration of the surface, freeing the sodium atoms from their bounds in the crystalline structure on the surface, and the subsequent photon-stimulated and thermal desorption of the sodium atoms. While we find that the velocity distribution of photon desorbed sodium can explain the observed exosphere population, thermal desorption seems to play a minor role only causing a smearing at the locations where Na atoms are released on the dayside. The observed and simulated distributions agree very well with this hypothesis and indicate that the combination of the proposed processes is able to explain the observed features.     

Soil carbon and nitrogen dynamics in southern African savannas: the effect of vegetation-induced patch-scale heterogeneities and large scale rainfall gradients

Savanna ecosystems are mixed plant communities in which trees and grasses co-exist thereby providing a heterogeneous landscape with a mosaic of tree-dominated and grass-dominated soil patches. Despite the important role that nutrient availability plays in these systems, detailed knowledge of differences in carbon and nitrogen cycling in soil patches predominantly covered by tree canopies or by grasses is still lacking. In this study, a process-based model was used to investigate the carbon and nitrogen dynamics in soil plots located in grass-dominated and tree/shrub-dominated soil patches along the Kalahari Transect (KT). The KT in southern Africa traverses a dramatic aridity gradient, across relatively homogenous soils, providing an ideal setting for global change studies. Here we show that there are distinctly different dynamics for soil moisture, decomposition and nitrogen mineralization between soil plots located under tree canopies and in open canopy areas, especially at the dryer end of the KT. Such differences diminished when approaching the wetter end of this transect. This study shows that in savanna ecosystems, water availability determines the patterns and rates of nutrient cycling at large scales, while at the local scales, vegetation patchiness plays an important role in nutrient cycling. Savannas are relatively stable ecosystems, resilient to small rainfall modifications, although irreversible changes may occur with stronger shifts in climate conditions.     

Large wood transport modelling by a coupled Eulerian–Lagrangian approach

The fractal characteristics of the ultra-low-frequency (ULF) magnetic field variations recorded prior to the Tohoku earthquake (EQ) with M _W = 9 which happened on 11 March 2011 are studied in this article with the use of detrended fluctuation analysis and Higuchi fractal dimension algorithm. In the specific study, we use for our calculations only nighttime (LT = 3 a.m. ± 2 h) data because of their lowest contamination by industrial noise. A key aspect of our analysis is the investigation about any possible correlation of the ULF magnetic field variations or their calculated fractal characteristics with geomagnetic indices. Different preprocessing approaches are examined aiming at the minimization of any possible influences from global phenomena in the fractal analysis results, while in the same time retaining the scale-invariant character of ULF magnetic field variations after preprocessing. The obtained fractal analysis results imply locally driven change in the fractal characteristics of the ULF data prior to the Tohoku EQ, which is compatible with the change that has been reported prior to other large EQs.     

Stratigraphic features of the Maltese Archipelago: a synthesis

The present study gathers a large amount of both existing and unpublished biostratigraphic data, which allows a detailed and complete definition of the stratigraphic features of the late Oligocene–late Miocene Maltese Archipelago sedimentary succession, recording in turn the tectonic and eustatic history of the Central Mediterranean region. We selected five sections in the Malta Island and three in Gozo, representative of the entire sedimentary succession, affected by well-known erosional surfaces, correlated to low-stands of the sea level, often associated with phoshatic layers, linked to the subsequent high-stands. The sedimentary interval, and thus the associated hiatuses, was constrained both by the bio-chronostratigraphic attribution and by the comparison with the third-order succession of the New Jersey passive margin, which shows strict analogy with the geodynamic context in which the Maltese succession deposited. The diachroneity at the base of the formations in the different sections, and the presence of intraformational unconformity/hiatuses, highlighted the role of the tectonic, which depicted a complex sedimentary basin, characterized by more distal versus more marginal sectors. Furthermore, the possibility to compare the sedimentary succession with the oxygen isotope curve connects the sedimentation interruptions, recorded within the Maltese Archipelago deposits, to global cooling events.