Vladimír Nehybka

P-Wave anisotropy of rocks from the seismic area in Western Bohemia

Abstract P-Wave velocity of metamorphic and magmatic rocks from the West Bohemian Saxothuringian (e.g. phyllites, quartzites, metabasites, mica schists, orthogneisses, granites and young volcanics) was measured using an acoustic method. The specimens were saturated with water in order to avoid the influence of open microcracks and porous space. Anisotropy coefficients were calculated from measurements in three perpendicular directions. Phycodes phyllitic slates as well as phyllites of the Frauenbach group from the region near the towns Luby and Kraslice exhibit relatively high V P up to 6800 m/s and extremely high anisotropy in the interval 25–49%. The lowest V P , under 4744 m/s, was found in light and phyllitic quartzites exhibiting, their anisotropy up to 29%. The high anisotropy of phyllites and quartzites is caused by an arrangement of phyllosilicates defining the metamorphic schistosity S 2 and controlling directional changes of V P . In the case of mica-schists of the Svatava crystalline complex, significant roles may be played also by other foliations. The relatively lower anisotropy of mica-schists from the vicinity of the town of Olovi compared to that of phyllites is partly due to this feature, partly due to lesser amount of mafic minerals. The anisotropy found by laboratory measurements has helped to find the relation between the geological structure and elastic wave propagation in the seismic area under study.

EARTHQUAKES IN THE CZECH REPUBLIC AND SURROUNDING REGIONS IN 1995-1999

In the time span from January 1995 to December 1999 the Czech National Seismological Network (CNSN), consisting of ten permanent digital broadband stations, several local networks and two data centers, detected and recorded 9530 regional natural seismic events, 27 greater than magnitude 2. Most of these events were located by the Czech Seismological Service (CSS), and the most prominent of them were analyzed in detail. A large number of quarry blasts were recorded as well but were not included in the analysis. We provide basic information on the configuration of the CNSN and on the way of routine data processing employed by the CSS in this paper. The over-all regional seismicity monitored by the CNSN in 1995–1999 is briefly reviewed. The main results of observations and evaluation of the local (NW-Bohemia/Vogtland, South Bohemia, Sudeten) and induced (Kladno, Příbram, Upper Silesia, Lubin/Poland) seismic activity within this period are presented in a condensed form. Finally, a summary on macroseismic observations on the territory of the Czech Republic in 1995–1999 is also presented.

The Moho Structure in The Western Eger Rift: A Receiver Function Experiment

We used teleseismic recordings of a temporary deployment of seismic stations and of permanent short period stations in the western Eger Rift system to study the lithosphere with the help of Receiver Functions. The crust-mantle boundary (Moho) is observed at almost all stations by strong P-to-S converted phases. The Moho is basically flat between about 26 – 30 km depth in the entire region. At one station in the Eger Rift (BOH-1, Loket castle) no Moho is observed. We interpret this with the existence of a broad gradient zone there, instead of a sharp discontinuity. This observation, however, needs to be confirmed by more data.