No abstract

The ambient seismic field is now routinely used for imaging and monitoring purposes. Most commonly, applications aim at resolving crustal-scale features and utilize ocean-generated surface waves. At smaller scales and at frequencies above the microseismic peaks, local sources of seismic energy, often anthropogenic, are dominant, and understanding of their contributions to the ambient seismic field becomes important to apply ambient noise techniques. This study uses data of an industrial-scale seismic deployment covering ∼500  km2 with 10,532 stations, each equipped with several collocated 10 Hz geophones, to provide unique insight into anthropogenic sources of seismic energy in a suburban-to-rural area. We compute amplitude levels, their distance dependency, power spectral densities, and spectrograms to describe the source characteristics. The sources we observe in great detail include windmills, a railway track and trains, cars, oil pumpjacks, power lines, gas pipelines, and airplanes. These sources exhibit time-dependent behavior that is illustrated strikingly by videos of amplitude levels in certain frequency bands that we provide as supplemental material. The data described in this study are a potential resource for future studies, such as automatic signal classification, as well as underground imaging using microseismic noise or the sources presented here.

Summary To constrain seismic anisotropy under and around the Alps in Europe, we study SKS shear-wave splitting from the region densely covered by the AlpArray seismic network. We apply a technique based on measuring the splitting intensity, constraining well both the fast orientation and the splitting delay. 4 years of teleseismic earthquake data were processed, from 723 temporary and permanent broadband stations of the AlpArray deployment including ocean-bottom seismometers, providing a spatial coverage that is unprecedented. The technique is applied automatically (without human intervention), and it thus provides a reproducible image of anisotropic structure in and around the Alpine region. As in earlier studies, we observe a coherent rotation of fast axes in the western part of the Alpine chain, and a region of homogeneous fast orientation in the Central Alps. The spatial variation of splitting delay times is particularly interesting though. On one hand, there is a clear positive correlation with Alpine topography, suggesting that part of the seismic anisotropy (deformation) is caused by the Alpine orogeny. On the other hand, anisotropic strength around the mountain chain shows a distinct contrast between the Western and Eastern Alps. This difference is best explained by the more active mantle flow around the Western Alps. The new observational constraints, especially the splitting delay, provide new information on Alpine geodynamics.

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.