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Description
The background seismic vibrations of either natural or anthropogenic origin are generally dominated by surface Rayleigh waves, whose amplitude rapidly decrease with depth. As a consequence, underground sites offer the most favourable conditions for a low-noise environment. In this study, we provide an assessment of the ground vibrations at the dismissed Sos Enattos mines (Sardinia, Italy), one of the candidate sites for the construction of the ET gravitational wave detector. Earthquake rate at the site is extremely low, as evidenced from inspection of both historical and instrumental catalogues. Spectral amplitude and temporal evolution of the background seismic noise are assessed using continuous recordings collected at different sites above and within the mine. Most of the noise energy is concentrated within the 0.1-1 Hz frequency band, and it is markedly correlated with wave climate in the Tyrrhenian sea. Horizontal-to-Vertical spectral ratios indicate the lack of significant site amplification effects related to shallow impedance contrasts. We finally use recordings from an underground mine blast to evaluate (1) the average P-wave velocity at the site, (2) an empirical attenuation relationship describing the distance decay of the peak ground velocity, and (3) a preliminary estimate of the shallow (~40m) shear-wave velocity profile from inversion of the Rayleigh-wave dispersion function.