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Teper, L. (1998). Wpływ nieciągłości podłoża karbonu na sejsmotektonikę północnej części Górnośląskiego Zagłębia Węglowego, Wyd. Uniw. Śląskiego, Katowice, 107pp.

Seismotectonics

in the Northern Part of the Upper Silesian Coal Basin: Deep-seated Fractures-Controlled Pattern

Abstract

The Upper Silesian Coal Basin (in the paper called the GZW) is situated in southern Poland, next to Polish-Czech state border (see fig. 1). The Upper Carboniferous hard coals have extensively been mined there for last centuries. The GZW is the region where great seismic activity is observed. The seismicity is assumed to be induced by mining operations. Most of the seismic events occur in the northern part of the GZW that was selected to present study.

The paper is an effort to deal with a problem of interrelation between tremors triggered by mining and natural seismicity in a rock mass. The main point is that some of seismic events occur independently of works related to coal extraction. The study tends towards discovering causes of this group of tremors. The hypothesis is tested that the occurrence of these tremors is influenced by geological structure.

Investigations of the mine tremors focal mechanisms are accompanied by conventional structural analysis and modern techniques of structural geology just introduced into research work on the GZW. Study of geometrical attributes of fold arrays enables determining dynamics of principal tectonic structures in the area. Shear strain values and minimum displacements are calculated for the main discontinuities. Evidences of interlayer slip, widely represented within the sedimentary sequence, demonstrate spatial orientation of tectonic-transport direction consistent with kinematics and dynamics of primary structures recognised by means of conventional structural analysis of folds and faults. Structural and fractal methods applied enable to find out symptoms of the basement dislocations control on the deformation of the cover basin. Seismic moment tensor and regional tensor estimated on the basis of classic fault plane solution as well as strain ellipsoid and structural pattern obtained as a result of tectonic study have many features in common. The similarity is visible especially in case of the largest tremors and the largest faults. Investigation of the fault spatial distribution together with analysis of the fault size parameters reveal features that let regard the Upper Carboniferous rock mass as a non-linear system and point to fractal geometry of the fault network in the northern part of the GZW.

Location and orientation of active fundamental faults deduced from the focal mechanism solutions as well as structural and fractal analyses are determined in proposed seismotectonic model of the area. It is suggested that the primary faulting in the basement produces displacements on elements of the fault network in the Upper Carboniferous sedimentary complex. These displacements beside mining operations are regarded as a source of large tremors generation.

Subequatorial fracture zone is recognised as a second order boundary zone separating two segments of the GZW basement: the Bytom block and the centralny block. Two marginal dislocations of this zone are identified as main seismogenic structures: the southern (see figs. 5, 16 and 21 - discontinuity B, also fig. 28 - in the part I) and northern one (see fig. 21 - discontinuity b, also fig. 28 - in the part III). Both of them are marked by linear gravimetric anomalies (see fig. 21). Moreover, the seismogenic faults frame an area of anomaly of geothermal field (Karwasiecka, 1996a, 1996b).

Results point to long-continued seismogenic activity of the latitudinal deep-seated faults in the northern part of the GZW. Frequent reactivation of these faults is subsequent to position that the intra-plate border fracture (B - b) has kept in successive global structural patterns: within the Caledonian Transeuropean suture zone (see fig. 6), on the foreland of approaching orogenic front in the Variscan time (see fig. 7) and in front of the Carpathian arc since the Tertiary (see fig. 33). This position has favoured oblique rifting and faulting. The intra-plate border zone is thought to be at present a complex deep-seated left-lateral wrench belt. It is not unlikely that the considered seismogenic structure forms a segment of the "Fifty-north Fracture Zone" (see fig. 8) postulated by J. Kutina (1974). Seismotectonic framework of the northern part of the GZW may be conditioned by orientation of the "Fifty-north Fracture Zone" in relation to present-day compression vector in the Carpathians.

Seismogenic potentials are also attributed to deep-seated fundamental fault that makes the north-eastern border of the Bytom block (see figs. 5, 16 and 21 - discontinuity A, also fig. 28 - in the part IV). As against the active latitudinal zone, the NW-SE trending dislocation A is of lesser importance for recent seismotectonics of northern part of the basin. The fracture A, however, as positioned in the peripheral part of broad border zone of the GZW basement (see fig. 5 - belt between A and II-II), where maximum concentration and intensity of movement occurred in the Palaeozoic (cf. figs. 6, 7 and 31) and the Mesozoic (see fig. 32) tectogenetic phases, was probably much more important for generation of seismic events in the past.

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