Surface-wave attenuation and phase-velocity maps using the AlpArray seismic network: implications for crustal heterogeneity

The Apennines-Alps-Carpathians-Dinarides orogenic belt results from complex geodynamic processes, manifesting as pronounced crustal heterogeneities across much of Europe. This geotectonic setting has been extensively studied in order to unveil the processes underlying its formation and evolution. Several geophysical methods such as deep reflection and refraction seismic surveys, receiver function analysis, gravimetry studies, local earthquake tomography, and more recently ambient-noise tomography have been applied to this region. The dense and homogeneous coverage of recently deployed seismic stations in this area, such as the AlpArray Seismic Network, offers unprecedented seismic coverage enabling high-resolution tomographic imaging. However, one of the main challenges when studying the Earths crust is to interpret unambiguously the role of fluids, composition, and temperature. Seismic velocities are not sufficient alone for resolving these properties. On the other hand, seismic wave attenuation is more sensitive to the physical conditions of the crust and mapping its variations is indeed important for a better understanding of the dissipative mechanisms which act in the lithosphere. Therefore, we decided to apply to our study region a novel method, that is capable of sampling the crust at high resolution compared to other earthquake-based methods, to estimate attenuation from the seismic ambient noise. We also performed new phase-velocity measurements with unprecedented resolution, to complement our attenuation measurements, providing a more robust interpretation of the area. Two years of continuous data from 749 broadband seismic stations, densely deployed throughout the Alps-Apennines-Carpathians-Dinarides orogenic system, were used to compute Rayleigh-wave phase velocities and attenuation coefficients from seismic ambient noise. The excellent seismic coverage allows us to measure phase velocities at shorter surface-wave periods compared to previous studies (down to 3s). Preliminary results indicate that the spatial variations in Rayleigh-wave velocities correlate with known geological features, such as the relatively low-velocities of Cenozoic basins (Po plain, Molasse basin, Rhine graben) and the relatively high-velocity crust (Apennines, Alps, Bohemian massif, Dinarides). Attenuation maps between 3 and 20 seconds were computed and are the first of their kind for the study region. Preliminary results show a clear anomaly pattern of seismic attenuation related to the Po plain and the Apennines. The correlation between attenuation coefficients and phase velocities presents an intriguing pattern, still under debate, that is consistent with what has been observed in previous studies using the same methodology in the United States. Combining the new constraint on seismic attenuation to phase velocity results enables us to improve interpretation on temperature and composition of the crust, including the role of fluids. These results also provide an indirect constraint on the current rheological properties of the crust.