Analiza prędkości sejsmicznych w Karpatach Zachodnich

Abstract

Analysis of seismic velocities in the Western Carpathians The analysis of seismic velocities carried out for the area of the Western Carpathians and their Foreland (Southern Poland), is based upon measurements of average velocities in 24 bore-holes. Most of these bore-holes are situated in the marginal zone of the Carpathians within the Silesian and the Sub-Silesian nappe. No measurements of seismic velocities were carried out in the inner Carpathians, with the exception of the Podhale Flysch, where one bore-hole provided data. Bore-holes situated in the immediate foreland of the Carpathians were also used for the present analysis, since they were indispensable for the determination of seismic velocities in beds forming the basement of the Carpathians. Same preliminary data on seismic velocities in the Western Carpathians were mentioned earlier by A. Kisłow (1971) and J. Łaski (1967). The present analysis comprise average velocities, layer velocities, velocities in lithological complexes and velocities in depth intervals. Layer velocities and velocities in lithologic complexes change within the Flysch beds of the Silesian and the Sub-Silesian nappe at contacts of various rock types. Generally no changes are observed at stratigraphic and tectonic contacts of petrographically similar rocks. Changes of seismic velocities are associated with petrographic changes e.g. at boundaries of sandstone and shale lithosomes. This situation is observed in sandy-shaly deposits of the Carpathian Flysch, in the Miocene rocks, as well as in the older basement, especially in the Upper Carboniferous beds (Figs. 1a , b, c). A marked discontinuity of velocity is associated with the contact of the high-velocity carbonate rocks of various age forming part of the basement, and the sandy-shaly complex (Figs. 1d, e). Functions of layer velocities v(w) (H) were established for the Sub- -Silesian nappe, for the Miocene beds underlying the Carpathians and forming their immediate foreland (Fig. 2 e), and for the Upper Carboniferous (Fig. 2g). No such functions can be established for other systems and rock series in the present state of investigations (Figs 2 a, b, d, f, h, i). From the tabulation of bed velocities (Fig. 3) in the Western Carpathians it follows that two contrasting velocity media can be distinguished, one comprising the Flysch and Miocene beds, the other comprising the underlying Mesozoic and Palaeozoic rocks. The velocity function Vfc (H) was established in the first medium comprising the Flys ch/Miocene lithological complex, while no such function could be determined in the second medium. The horizontal velocity gradient is shown in maps (Fig. 5 and Fig. 6). The values of layer velocities and velocities in lithological complexes are tabulated in Table I and Table II.