The transition zone between the Eastern Alps and the Pannonian basin imaged by ambient noise tomography

Abstract The aim of this study was to establish a 3D shear-wave velocity model of the transition zone between the Eastern Alps and the Pannonian basin by means of ambient noise tomography. Datasets from permanent networks as well as past and recently deployed temporary networks of broadband seismic stations have been processed. Empirical Green's functions were estimated via cross-correlation of vertical component ambient seismic noise and further processed to obtain Rayleigh-wave group-velocity dispersion curves. They were directly inverted for shear velocities through a wavelet-based sparsity-constrained tomography method avoiding the intermediate step of constructing 2D group velocity maps. A horizontal resolution of 0.2° in the upper crust and 0.3°–0.5° in the lower crust was achieved. Our upper crustal shear velocity model revealed low velocities in the sedimentary depocenters and high velocities below the surrounding mountains. However, at greater depths (24–29 km), high shear velocities beneath the basins suggest crustal thinning accompanied by mantle updoming. An extremely deep low velocity anomaly was mapped beneath the Vienna basin, which we argue is caused by either sediment transfer to the lower crust; ductile deformation suggested by seismic anisotropy; or the presence of fluids.