Azimuthal variation of Pg velocity in the Moldanubian, Czech Republic: observations based on a multi-azimuthal common-shot experiment

We study the azimuthal velocity variation of Pg waves in the Moldanubian, which is a crystalline segment within the Bohemian Massif in the Czech Republic. We use the data from a multi-azimuthal common-shot experiment performed as part of the ALP 2002 refraction experiment, complemented by profile refraction data from the CELEBRATION 2000 experiment. We analyze the travel times of waves recorded by 72 portable seismic stations deployed along two circles with radii of 35 and 45 km around a shot. The observed travel times display an azimuthal variation indicating anisotropy of 2%. The minimum and maximum velocity values are 5.83 and 5.95 km/s, respectively. The direction of the maximum velocity is N50°E. These values characterize horizontal anisotropy of the uppermost crust down to 3 km. The strength and orientation of uppermost crustal anisotropy in the Moldanubian is consistent with the overall upper crustal anisotropy in the entire Bohemian Massif. The high-velocity direction is roughly perpendicular to the present-day maximum compressive stress in the Bohemian Massif and Central Europe and coincides with the orientation of structures formed by the main Variscan tectonic events in the area. This indicates that the anisotropy is caused predominantly by alignment of textural elements and minerals in the rocks, which developed in early geological stages rather than by a preferred orientation of cracks or microcracks due to present-day stress. If the crack-induced anisotropy is present in the medium, then its strength should not exceed 1% and the cracks should be water saturated.