Coseismic and other short-term strain changes recorded with Sacks-Evertson strainmeters in a deep mine, South Africa

Summary. During 1977 March and April, three Sacks-Evertson borehole dilatometers were installed at the ends of boreholes drilled into the sidewall of an experimental tunnel at a depth of 3.1 km in the ERPM gold mine near Johannesburg. In the following year coseismic strain changes ranging from 5 ± 10⁻¹⁰ to values exceeding 5 ± 10⁻⁶ were recorded for hundreds of mine tremors in the magnitude range -1 to 3.7 and at hypocentral distances of 50 m to about 2 km. Hypocentral coordinates and magnitudes were determined from seismograms recorded from an underground array of geophones. Amplitudes and polarities of the coseismic strain steps are generally in excellent agreement with theoretical expectations based on point-source dislocation theory; specifically, the strain steps are proportional to the seismic moment divided by the cube of hypocentral distance. At a strain level of 5 ± 10⁻⁹ or greater the tremors do not appear to be preceded by any short-term indications of instability even for tremors producing coseismic steps greater than 5 ± 10⁻⁶ and for which the strainmeters were within a source radius of the hypocentre. Continuous strain changes observed at the times when the mine excavation, at a distance of about 100 m, is extended are in good agreement with calculated changes based on the theory of elasticity. A similar calculation is consistent with post-seismic strain changes observed to follow some of the closer tremors. These post-seismic strains show a logarithmic dependence on time following the tremor and appear to be due to the interaction of a tremor with the adjacent mine excavation rather than to deformation within the actual seismic source region.     

Can gravitational effects damp Alfvén waves?

We show that Alfvén-gravity waves propagating in a gravitationally stratified atmosphere do not suffer damping as a result of the rate of working of the gravity drift current on the electric field of the waves. A self-consistent treatment involving conservation of total energy, Poynting's theorem, and the rate of working of the various drift currents on the electric field demonstrates that dissipation only arises from real dissipative processes such as Ohmic heating or viscous effects, otherwise the system is adiabatic.     

Solar rotation in the subphotospheric layers

The solar rotation rate at latitudes 0°, 15° and 30° has been inferred by averaging the results of 120 regions of 15°×15°, which have been studied over a total area of about 75° in latitude and 360° in longitude. A local helioseismology technique, the ring diagram analysis method, has been used to analyse the horizontal velocity vectors from about 0.95 R up to the surface. Our results are in very good agreement with those of other authors over most of the depth range. However, near the surface we find sharp local features which are not reported in other studies. The independent measurements of the rotation rate in the north and south hemispheres show asymmetries below 0.975 R . The data used are full-disc dopplergrams taken by Solar Oscillation Investigation/Michelson Doppler Imager (SOI/MDI) on board of the SOlar and Heliospheric Observatory (SOHO) during its first Dynamic Program, between 1996 May and June.     

Considerations about antenna pattern measurements of 2-D aperture synthesis radiometers

Accurate measurement of the antenna voltage patterns of large-aperture synthesis radiometers is critical in order to achieve good radiometric accuracy, and a very time consuming and expensive task. Measurement requirements and a tradeoff study relating radiometric accuracy degradation and number of elements to be measured are presented.     

Using data of snow survey monitoring in the Samara region for ecological mapping

Some results of use of snow survey monitoring data within the system of the Privolzhie Administration for Hydrometeorological Service for studying the degree of snow cover pollution and mapping pollution distribution in several urban and rural areas of the Samara region are considered.     

Influence of hydrological regime on carbon dioxide fluxes over oligotrophic bog massif in northwestern Russia

It is found that the surface bog temperature and bog water level position are major factors that determine CO₂ emission from the bog massif. The emission intensity increases with increasing temperature and with a bog water level decrease. The dependence of CO₂ emission is shown to be different for each bog microlandscape. Monthly mean CO₂ emissions are given for the central bog massif in the years with different water content. Based on modeling, it is shown that in the case of a low bog water level the CO₂ flux is directed to the atmosphere (the emission value exceeds the gas amount consumed for photosynthesis), while in the case of a high level, it is taken up by the growing plants.