Azimuthal Anisotropy in the Eastern Alpine Crust from Ambient Noise Tomography

Making use of the dense AlpArray and SwathD networks in the eastern Alps, a large dataset of Rayleigh phase-velocity measurements is extracted. This dataset is the basis for a 3D azimuthally anisotropic shear-velocity model of the Alpine crust. A 2-step inversion approach is followed: First, phase-velocity maps are created which are inverted for the shear velocity structure at depth in a second step. In both steps, a Bayesian (rjMcMC) approach is used to find the posterior distribution of anisotropic models. The model uncertainties are propagated from the phase-velocity maps to the depth inversion to make sure that the data is not overfitted. The final model shows a 2 layer anisotropy in the Alpine crust, the upper crustal layer is mostly orogen parallel and follows the major fault structures. The lower crustal to uppermost mantle layer shows orogen-perpendicular fast axis in the Alps and an anisotropy following the curvature of the Alps in the northern foreland. The importance of microfabric such as microcracks and oriented mineral grains is difficult to estimate from the presented model on the effective regional-scale anisotropy. But the results suggest that the azimuthal anisotropy may be largely controlled by macro-scale structures. The transition from upper to lower crustal anistotropy takes place at approx. 20 km depth which is unlikely to be due to the brittle-ductile transition. But it could indicate that upper and lower crust are only weakly coupled underneath the Alps.