Azimuthal anisotropy in the wider Vienna basin region: a proxy for the present-day stress field and deformation

Abstract

SUMMARY We 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.

Azimuthal anisotropy from eikonal tomography: example from ambient-noise measurements in the AlpArray network

“…S1) and thus the peak sensitivity may already lie below the 5 km estimate for the stress dominated anisotropic fast-axis alignment. A stress-related fast axis orientation at short periods is also proposed by Schippkus et al (2018Schippkus et al ( , 2019 who study the anisotropy in the Vienna basin region (Fig. S12).…”

“…S12). A direct comparison between the studies is not possible, because Schippkus et al (2019) use group velocities that have a different depth sensitivity; the strong velocity contrast to the adjacent low-velocity basin and the position at the edge of our model area limits the interpretability of the results in our study. The observed anisotropic fast axis curve around the low-velocity Vienna basin and are aligned in SW-NE direction within the basin correspond very well to the bias modeled in Fig.…”

“…Figure S12: Comparison of the work of Schippkus et al (2018, top), Schippkus et al (2019, left) and this work (right) for the Vienna basin region. The studies of Schippkus et al (2018Schippkus et al ( , 2019 are based on group velocity measurements as opposed to phase velocities in this work.…”

Bayesian analysis of azimuthal anisotropy in the Alpine lithosphere from beamforming of ambient noise cross-correlations

“…Stehly et al, 2008;Lu et al, 2018bLu et al, , 2020Soergel et al, 2020;El-Sharkawy et al, 2020;Alder et al, 2021;Nouibat et al, 2022), and a single study focusing on azimuthal anisotropy (Kästle et al, 2021). At the local scale, azimuthal anisotropy of surface waves from noise correlations with AlpArray data has been studied in the Vienna basin by Schippkus et al (2019).…”

“…First, the surface-wave measurements need to overcome the same challenges as those based on teleseismic surface waves: the presence of 3D heterogeneities can create an apparent anisotropy that does not correspond to the intrinsic anisotropy of the structure at depth (Bodin & Maupin, 2008;Ritzwoller et al, 2011;Schippkus et al, 2019). Another challenge specific to ambient noise measurements is that the uneven distribution of noise sources can create biases in the recovered Green's function, and hence on measured velocities (e.g.…”