Waves in an elastic plate with an irregular boundary

Summary Green's function corresponding to the displacements for anSH-line source inside a wedge shaped medium finite or infinite in which the density and the elastic parameter are not constant throughout but some class of function ofr, the radial distance (i.e. and satisfy some differential equation) has been obtained. The paths of propagation of the cylindrical waves, their reflections on different boundaries and their continuous refractions within the medium has been clearly established. The scattered field from the different parts of the body have been pointed out with prescribed distribution of and in this inhomogeneous wedge. It is shown that for some distribution of and the rays are curved and the reflections at the two plane boundaries occur in such a way that no ray will go to infinity by reflection, rather they are coming back towards the apex after suitable number of reflections with variable intensity. Also there are some distributions of and in the same class as mentioned earlier, such that the behaviour of the waves is similar to that in a homogeneous wedge, i.e. these waves will go to infinity by a suitable number of reflections and ultimately die out. The first case is quite unlike the usual homogeneous medium with non parallel boundaries and so care must be taken in computing the field within the nonhomogeneous medium within the non-parallel boundaries.     

Quake Catalogs from an Optical Monitoring of an Interfacial Crack Propagation

Using an experimental setup which allows to follow optically the propagation of an interfacial crack front in a heterogeneous medium, we show that the fracture front dynamics is governed by local and irregular avalanches with large velocity fluctuations. Events defined as high velocity bursts are ranked in catalogs with analogous characteristics to seismicity catalogs: time of occurence, epicenter location and energy parameter (moment). Despite differences in the fracturing mode (opening for the experiments and shear rupture for earthquakes), in the acquisition mode and in the range of time scales, the distributions of moment and epicenter jumps in the experimental catalogs obey the same scaling laws with exponents similar to the corresponding distributions for earthquakes. The record-breaking event analysis also shows very strong similarities between experimental and real seismicity catalogs. The results suggest that the dynamics of crack propagation is controlled by the elastic interactions between microstructures within the material.     

Transient flexural vibrations of an elastic column supported by an elastic half-space

An analysis is presented of the transient flexural vibrations of an elastic column supported by an elastic half-space under the condition that an arbitrarily shaped free-field lateral acceleration and displacement are given as inputs. Applying Laplace transformations with respect to time and numerical inverse Laplace transformations, the time histories of the column acceleration at the interface and free end, and the column and half-space displacement distributions are obtained. After the input free-field acceleration terminates, slightly damped and almost harmonically variable acceleration is observed. The acceleration frequency after the disappearance of the input acceleration nearly coincides with the resonant frequency of the system. The slight damping with the first resonant frequency, even if the half-space is soft compared with the column, is characteristic of the transient flexural vibrations of a column supported by a half-space. Such a phenomenon is not typical of the transient longitudinal vibration problem. Therefore, it may be concluded: when buildings and structures are subjected to an earthquake or an explosive force, their flexural vibrations will continue with their first resonant frequencies, even if their foundations are soft.     

Repeating earthquakes from rupture of an asperity under an Antarctic outlet glacier

The inner regions of the Antarctic continent are generally regarded as nearly aseismic, although microseismicity is known to occur beneath some outlet ice streams, related to the interaction between the fast flowing ice and the bedrock. Here we show the occurrence of unusual earthquakes beneath an Antarctic outlet glacier that share almost the same magnitude, pointing to the repeated rupture of a single asperity. These seismic events produce waveforms with very high similarity and uncommon spectrum and are tightly clustered in space but, unlike other reported instances of repeating earthquakes on a patch of the San Andreas Fault, they occur in frequent irregular swarms. Evidence locates these events at the rock–ice interface under the glacier, and shows the existence of stick–slip motion on a smaller scale than the large slow slip events detected by global seismographs. Seismic behaviour of large glaciers can presumably be connected to surges in ice motion. This study determines a little known environment for fracture dynamics studies, while also contributing to the understanding of the coupling processes between fast flowing glaciers and bedrock that influence ice stream evolution and stability.     

She is an Astronomer

Helen Walker, Chair of the IYA2009 Cornerstone Project task group, describes the problems facing female astronomers and the steps being taken towards a solution.     

Jahreszeitliche untersuchung an bachhydracarinen

Ohne Zusammenfassung     

Resistivity and IP modelling of an anisotropic body located in an isotropic environment

A solution based on Tabarovskii's coupled pair of surface integral equations is given for the potential of a direct current flowing in an electrically anisotropic body and within the enclosing isotropic surroundings. The sources of the secondary potential exterior and interior to the body are fictitious surface charge distributions. The equations are solved numerically using point matching with pulse functions as subsectional basis functions. The model used in the applications is a long prism, excited by long line current electrodes aligned parallel to the strike. The strike length is set at a length sufficient to guarantee 2D behaviour of the model.   Comparisons of computation results indicate that for the models, electrode arrays and numerical procedures applied, the solutions based on fictitious surface sources converge faster and behave more regularly than those based on real surface charges. When compared with previously published integral equation solutions, the present solution seems to be relatively efficient, even in the case of purely isotropic models. The model experiments also showed that at moderate resistivity contrasts, the anomaly shapes are strongly dependent on the directions of the principal axes of the body resistivity. However, when the external resistivity is more than 100 times that of the geometric mean of the principal resistivities in the body, with the principal resistivities differing from each other by at most one order of magnitude, the contribution of the anisotropy to the anomaly diminishes as a result of electrical saturation.     

Thermal stresses in an elastic layer due to an instantaneous plane heat source

Summary The solution of the title problem has been obtained for stress free boundaries of the layer.     

Changes in an urban water resource,an example from Sydney,Australia

This paper examines present and future resource utilization in the tidal section of the Georges River in the southern part of Sydney. Over the last 30 years, the physical characteristics of the river have changed as a result of urbanization, changed hydrologic regime and sand dredging. Urbanization has resulted in an increase in population from 190,000 in 1947 to 550,000. This has resulted in an increase in runoff, sediment input to the river and waste disposal. The change in hydrologic regime has resulted in a substantial increase in the magnitude and frequency of flooding which has increased erosion. Sand dredging has increased the channel capacity, enlarged the tidal prism and increased water turbidity. It has also resulted in extensive bank erosion in the upper part of the river and extensive deposition lower down as tailings are deposited. Although most of these changes have resulted in the deterioration of environmental quality, use of the river for recreation and as an urban amenity has increased.     

Detection of an Impermeable Boundary in an Anisotropic Formation: A Case Study

Direct mirror-well image theory to locate an impermeable boundary does not apply in anisotropic formations because the relationship between flow lines and equipotential lines is distorted due to directional transmissivity. A simple approach enables an impermeable boundary to be located in anisotropic formations, and it can be verified with a field example.     

Deployment of an unmanned aerial system to assist in mapping an intermittent stream

Recent growth in the capabilities of unmanned aerial vehicles and systems (UASs) as airborne platforms for collecting environmental data has been very rapid. There are now ample examples in the literature of UASs being deployed to map fine‐scale vegetation, glacial, soil and atmospheric conditions. The purported advantages of UASs are their ability to collect spatial data at lower cost, lower risk, higher resolution and higher frequency than ground surveys or satellite platforms. In this specific study, whether or not obtaining high‐resolution UAS imagery was advantageous for identifying an intermittent stream network was determined by comparing it with coarse‐scale satellite imagery collected for the same purpose. It was also determined if the UAS imagery could be an improvement to Global Positioning System acquired ground‐truth points for classifying an intermittent stream network across the same large‐scale satellite image. The UAS‐acquired and satellite‐acquired imageries were derived from a visible spectrum camera capable of 2‐cm resolution and multispectral SPOT‐5 with 10‐m resolution, respectively. The SPOT‐5 imagery with its relatively coarse resolution could not always detect the narrow intermittent stream, which was well resolved in the UAS imagery. When a classified UAS image was applied as a training area for the SPOT‐5 image, the identification of the stream network and accuracy of the satellite imagery classification did not necessarily improve. UASs have the potential to revolutionize hydrological research the same way that geographic information systems did three decades ago. A final goal of the paper is to provide insight into the advantages and disadvantages of deploying a UAS for this kind of research. © 2015 Her Majesty the Queen in Right of Canada. Hydrological Processes. © 2015 John Wiley & Sons, Ltd.     

Responses of an intertidal gastropod to field exposure of an oil and a dispersant

Field observations have been made of short-term acute and long-term sublethal responses of Nerita (Melanerita) atramentosa melanotragus, to experimental applications of Maui Condensate, and a relatively new oil dispersing agent, Shell SD LTX. Short-term responses suggested condensate/dispersant mixtures to be highly toxic, whereas weathered condensate and LTX, when applied separately, produced no significant mortality. In the longer term, however, sublethal effects such as changes in wet weight, gonad weight, and gonad fertility indices were especially noticeable on LTX treatments. Unexpectedly large reductions in weight, and aberrations in gonad tissue structure were recorded. Ecological implications of these results are discussed.     

Reflection of an internal wave at an interface representing a rapid increase in viscosity

Abstract

Internal waves at high altitudes are greatly damped by the drastic increase in molecular viscosity and thermal diffusivity, resulting in important heating and other effects at those altitudes. Here we consider the case where this increase in viscosity is very rapid, idealized as an interface with inviscid flow in the lower layer and constant viscosity in the upper layer. The results show that waves are partially reflected by this interface, with a reflection coefficient that increases monotonically with an increase in the viscosity of the upper layer. This mechanism would have a significant impact on the vertical distribution of thermal energy at high altitudes.     

Living with an active star

Observations have shown that the Sun is far from being a stable, unchanging object. Rather, we live within the outer atmosphere of a variable star. Andy Breen presents a summary of a meeting that drew together new results from observations and computational modelling of the Sun, interplanetary space and the upper atmosphere of the Earth.     

Tidal observations near an amphidrome

The signal to noise ratio in tidal data in the diurnal and semidiurnal frequency bands is ordinarily so large that the noise contribution to the tidal harmonic constants is unimportant. However, as the observational locations are selected progressively closer to an amphidrome (point of no tide), the signal to noise ratio decreases, making the tidal harmonic constants less dependable. Standard deviations in amplitude of M₂ and S₂ obtained from 12 29-day analyses of a year of tide data obtained at a standard tide station, estimated to be 280 and 550 km away from the amphidromes for these constituents in the eastern Caribbean, are roughly one-third of the mean amplitudes for these constituents; the standard deviations in epoch are 38° and 30° respectively. Therefore, a program to locate an amphidrome precisely is self-defeating and the location can only be approximated by a grid of tide observations spanning the geographic position and/or by longer series of observations, using higher resolution to increase the signal to noise ratio. Amplitudes of 0.64 cm and 1.24 cm were calculated for M₂ and S₂ from a one-month series of pelagic observations obtained very close to an inferred position of the M₂ amphidrome in the northeast Caribbean Sea.Abbreviations C&GS Coast and Geodetic Survey - CICAR Cooperative Investigation of the Caribbean and Adjacent Regions - IAPSO International Association for the Physical Sciences of the Ocean - ICOT Institute of Coastal Oceanography and Tides - IDOE International Decade of Ocean Exploration - NOAA National Oceanic and Atmospheric Administration - NOS National Ocean Survey - NSF National Science Foundation - SCOR Scientific Committee of Oceanic Research - UNESCO United Nations Educational, Scientific, and Cultural Organization