International audienceWe derive the 3-D crustal structure (S wave velocity) underneath Italy and the Alpine region, expanding and exploiting the database of ambient noise Rayleigh-wave phase- and group-velocity of Verbeke et al. (2012). We first complement the database of Verbeke et al. (2012) with a dense set of new ambient-noise-based phase-velocity observations. We next conduct a suite of linear least squares inversion of both phase- and group-velocity data, resulting in 2-D maps of Rayleigh-wave phase and group velocity at periods between 5 and 37 s. At relatively short periods, these maps clearly reflect the surface geology of the region, e.g., low velocity zones at the Po Plain; at longer periods, deeper structures such as Moho topography under Alps and Apennines, and lower-crust anomalies are revealed. Our phase- and group-velocity models are next inverted via the Neighbourhood Algorithm to determine a set of one-dimensional shear-velocity models (one per phase/group-velocity pixel), resulting in a new three-dimensional model of shear velocity (vS) parameterized in the same way as the European reference crustal model EPcrust. We also show how well vS is constrained by phase and group dispersion curves. The model shows the low velocity area beneath the Po Plain and the Molasse basin; the contrast between the low-velocity crust of the Adriatic domain and the high-velocity crust of the Tyrrhenian domain is clearly seen, as well as an almost uniform crystalline crust beneath the Alpine belt. Our results are discussed from the geological/geodynamical standpoint, and compared to those of other, interdisciplinary studies
[1] Ambient-noise seismology is of great relevance to high-resolution crustal imaging, thanks to the unprecedented dense data coverage it affords in regions of little seismicity. Under the assumption of uniformly distributed noise sources, it has been used to extract the Green's function between two receivers. We determine the imprint of this assumption by means of wave propagation and adjoint methods in realistic 3-D Earth models. In this context, we quantify the sensitivity of ambient-noise cross-correlations from central Europe with respect to noise-source locations and shear wave-speed structure. We use ambient noise recorded over 1 year at 196 stations, resulting in a database of 864 cross-correlations. Our mesh is built upon a combined crustal and 3-D tomographic model. We simulate synthetic ambient-noise cross-correlations in different frequency bands using a 3-D spectral-element method. Traveltime cross-correlation measurements in these different frequency bands define the misfit between synthetics and observations as a basis to compute sensitivity kernels using the adjoint method. We perform a comprehensive analysis varying geographic station and noise-source distributions around the European seas. The deterministic sensitivity analysis allows for estimating where the starting crustal model shows better accordance with our data set, and gain insight into the distribution of noise sources in the European region. This highlights the potential importance of considering localized noise distributions for tomographic imaging, and forms the basis of a tomographic inversion in which the distribution of noise sources may be treated as a free parameter similar to earthquake tomography.
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