Crustal anisotropy in the Bohemian Massif, Czech Republic: Observations based on Central European Lithospheric Experiment Based on Refraction (CELEBRATION) 2000

We study the azimuthal velocity variation of Pg waves in the Bohemian Massif using data collected during Central European Lithospheric Experiment Based on Refraction (CELEBRATION) 2000. We analyze travel times of waves generated by 28 shots and recorded by 256 portable and 19 permanent seismic stations deployed on the territory of the Czech Republic and in adjacent areas. We use recording offset ranging from 30 to 190 km with azimuths covering the whole interval of angles. The observed travel times are inverted for parameters of a velocity model formed by an isotropic low-velocity subsurface layer with a varying depth lying on a homogeneous transversely isotropic half-space with a horizontal axis of symmetry. The recovered velocity displays a systematic azimuthal variation indicating a regional-scale intrinsic or effective anisotropy in the Bohemian Massif. The mean, minimum and maximum values of the velocity are v_mean = 6.03 km/s, v_min = 5.98 km/s, v_max = 6.10 km/s, respectively, indicating an anisotropy of 1.5-2.5%. The direction of the maximum propagation velocity is ∼N35°E being approximately perpendicular to the present maximum compression in the Earth crust in central Europe. The observed anisotropy cannot be induced by stress-aligned cracks in the crust, because the crack models predict azimuthal velocity variations completely inconsistent with the observed one. Therefore we suggest the crustal anisotropy to be induced by a preferred orientation of rock-forming minerals and large-scale intrusion fabrics developed during a tectonic evolution of the Bohemian Massif.