The results of a systematic study of shear-wave splitting, observed in the three-component digital seismograms of about 400 local microearthquakes that occurred during the 1985/1986 West-Bohemian earthquake swarm are presented. Two shear phases, polarized nearly perpendicularly in horizontal projection, were observed at all five stations of the local seismic network for most of the events. The polarization of the faster shear wave is aligned in the WNW-ESE direction, regardless of the mechanism and hypocenter location of the individual events, and coincides with the direction of the maximum horizontal compressive tectonic stress in the region. These effects are interpreted in terms of effective anisotropy of the upper crust in the region. The maximum delay time between the split shear waves is 0.15 sec and corresponds to 6% anisotropy.

Directional variation of the delay time was observed at the nearest station (epicentral distance ≈ 5 km). This variation was compared with functions predicted by theoretical models of cracked media to resolve which model is the most appropriate for crustal anisotropy. Surprisingly, no agreement could be found for Hudson's model of dry or water-filled parallel cracks that is widely used by many authors. A successful fit was obtained for Schoenberg — Douma's model of a medium with parallel fractures. The fracture normal pointed N31°E, and the optimum fracture parameters were EN = 0.4 and ET = 0.02. The theoretically calculated fractures are aligned parallel to the tectonic stress direction and do not correspond to any real tectonic lines in the region.

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