Seismic anisotropy — the state of the art: II

Summary. The theory, causes, observations, and possible applications of seismic anisotropy in the Earth have developed considerably since the previous state of the art paper was published in 1977. The behaviour of waves in layered anisotropic media is now much better understood and the evidence for seismic anisotropy indicates that anisotropy is likely to be present throughout much of the crust and upper mantle. The top few hundred kilometres of the mantle appears to be anisotropic with the orientations aligned by the present or palaeo stress-field. The upper part of the crust is frequently anisotropic, probably due to cracks differentially aligned by the non-lithostatic stresses. The possibility of being able to monitor crack geometry by seismic techniques opens a wide range of applications in currently important activities.     

Dew water collector for potable water in Ajaccio (Corsica Island, France)

We report on the development of an inexpensive radiative condenser for collecting atmospheric vapor. Based on the experience gained using a small working model in Grenoble (France), a prototype of 10×3 m² was established in Ajaccio (Corsica, France). The condensing surface is a rectangular foil made of TiO₂ and BaSO₄ microspheres embedded in polyethylene and has an angle of 30° with respect to horizontal. The hollow part of the device, thermally isolated, faces the direction of the dominant nocturnal wind. Dew measurements were correlated with meteorological data and compared to dew condensed on a horizontal polymethylmethacrylate (PMMA, Plexiglas) reference plate. The plate served as a reference standard unit and was located nearby. Between July 22, 2000 and November 11, 2001 (478 days), there were 145 dew days for the reference plate (30%), but 214 dew days for the condenser (45%). This yield corresponds to 767 l (3.6 l, on average, per dew day). The maximum yield in the period was 11.4 l/day. Dew mass can be fitted to a simple model that predicts dew production from simple meteorological data (temperature, humidity, wind velocity, cloud cover). Chemical analyses of the water collected from the plate were performed from October 16, 1999 to July 16, 2000 and from the condenser, from July 17, 2000 to March 17, 2001. The following parameters were investigated: suspended solids, pH, concentration of SO₄²⁻, Cl⁻, K⁺, Ca²⁺ ions. Only Cl⁻ and SO₄²⁻ ions were sometimes found significant. Wind direction analyses revealed that Cl⁻ is due to the sea spray and SO₄²⁻ to the combustion of fuel by an electrical plant located in the Ajaccio Gulf. Except for a weak acidity (average pH≈6) and high concentration of suspended solids, dew water fits the requirements for potable water in France with reference to the above ions.     

Geometrical structure of shear wave surfaces near singularity directions in anisotropic media

There are three types of surfaces which are used for studying wave propagation in anisotropic media: normal surfaces, slowness surfaces and wave surfaces. Normal surfaces and slowness surfaces have been researched in detail. Wave surfaces are the most complicated and comparatively poorly known compared with the other two. Areas of complicated geometrical structure of the wave surfaces are located in the vicinity of conical acoustic axes. There is an elliptical hole on the quick shear wave surface and complicated folds and cusps on the slow shear wave surface. Decomposition of the slow shear wave surface into smooth sheets is used for the study of its geometrical structure. Complexity of shear wave surfaces can be expressed by the number of waves corresponding to a fixed ray. An original approach to the calculation of wave normals depending on ray direction is presented.     

Environment and paleoecology of a 12 ka mid-North American Younger Dryas forest chronicled in tree rings

Until now, availability of wood from the Younger Dryas abrupt cooling event (YDE) in N. America ca. 12.9 to 11.6 ka has been insufficient to develop high-resolution chronologies for refining our understanding of YDE conditions. Here we present a multi-proxy tree-ring chronology (ring widths, “events” evidenced by microanatomy and macro features, stable isotopes) from a buried black spruce forest in the Great Lakes area (Liverpool East site), spanning 116 yr at ca. 12,000 cal yr BP. During this largely cold and wet period, the proxies convey a coherent and precise forest history including frost events, tilting, drowning and burial in estuarine sands as the Laurentide Ice Sheet deteriorated. In the middle of the period, a short mild interval appears to have launched the final and largest episode of tree recruitment. Ultimately the tops of the trees were sheared off after death, perhaps by wind-driven ice floes, culminating an interval of rising water and sediment deposition around the base of the trees. Although relative influences of the continental ice sheet and local effects from ancestral Lake Michigan are indeterminate, the tree-ring proxies provide important insight into environment and ecology of a N. American YDE boreal forest stand.     

An empirical evaluation of three elevation change symbolization methods along routes in bicycle maps

ABSTRACT

Elevation change is critical for choosing a route when bicycling. We detail an empirical study in which we comparatively evaluated three linear symbolization types (color hue, color-coded arrows, and elevation profiles) to depict elevation change in bicycle maps for two common bicycle route planning tasks: relative height detection and slope identification. Participants performed most accurately with the color-coded arrows for relative height detection tasks, whereas symbolization did not significantly influence map-use performance for slope identification. Participants preferred the elevation profile, in spite of their lowest performance with this method overall. Our rare empirical findings offer much needed new insights into the function and appropriateness of common elevation symbolization methods, specifically to identify elevation change in bicycle route planning tasks in urban areas where map display real estate is already very limited.