Abstract
SUMMARYWe infer seismic azimuthal anisotropy from ambient-noise-derived Rayleigh waves in the wider Vienna Basin region. Cross-correlations of the ambient seismic field are computed for 1953 station pairs and periods from 5 to 25 s to measure the directional dependence of interstation Rayleigh-wave group velocities. We perform the analysis for each period on the whole data set, as well as in overlapping 2°-cells to regionalize the measurements, to study expected effects from isotropic structure, and isotropic–anisotropic trade-offs. To extract azimuthal anisotropy that relates to the anisotropic structure of the Earth, we analyse the group velocity residuals after isotropic inversion. The periods discussed in this study (5–20 s) are sensitive to crustal structure, and they allow us to gain insight into two distinct mechanisms that result in fast orientations. At shallow crustal depths, fast orientations in the Eastern Alps are S/N to SSW/NNE, roughly normal to the Alps. This effect is most likely due to the formation of cracks aligned with the present-day stress-field. At greater depths, fast orientations rotate towards NE, almost parallel to the major fault systems that accommodated the lateral extrusion of blocks in the Miocene. This is coherent with the alignment of crystal grains during crustal deformation occurring along the fault systems and the lateral extrusion of the central part of the Eastern Alps.
Highlights
Tectonics and seismic activity are consequences of the acting stress field
We investigate the azimuthal anisotropy of Rayleigh waves in the region to gain insight into the orientation of the stressfield and historical deformation
We present our findings on the directional dependence of measured group velocities (4.1), of modelled group velocities from the isotropic velocity model (4.2), and of group velocity residuals (4.3)
Summary
Tectonics and seismic activity are consequences of the acting stress field. The wider Vienna Basin region is one of the seismically most active regions in Austria. Due to the limited earthquake record in the area and the sparse knowledge of the regional stress field, there are major uncertainties in seismic hazard assessments in the region. Various techniques such as palaeoseismology have been used to better constrain past and current seismic hazard (Gribovszki et al 2017; Hintersberger et al 2018), they remain mostly pointwise measurements that lack the coverage over the whole Vienna Basin region. Understanding the stress field better, and which faults may be reactivated to produce significant earthquakes in the near future—as dictated by the stress field—is of great importance for accurate assessment of seismic hazard
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