Seismic Array Measurements in the Italian Candidate Site for the Einstein Telescope, the Third-Generation Gravitational Wave Detector

Geophysical investigations and studies on the local seismic noise are of paramount importance for evaluating the candidate locations for hosting Einstein Telescope (ET), the third-generation gravitational wave detector. In the Italian candidate site, several active and passive geophysical acquisitions have been carried out, mainly focusing on the imaging of the shallow subsurface, while a full characterization of the noise wavefield was lacking. In this work, we present the results from three temporary seismic arrays installed at the Italian candidate site, each with different geometrical layouts, recording durations, and total numbers of stations. Here, we provide an overview on the seismic noise characteristics and its azimuthal distribution, obtained through beamforming. Moreover, we leverage the noise recordings to extract (i) Rayleigh wave dispersion curves using fk analysis, which are then inverted to obtain a one-dimensional, shear-wave velocity model of the subsurface and (ii) HVSR spectra across all arrays' stations. The recordings confirm the exceptionally low level of seismic noise approaching Peterson's New Low Noise Model for frequencies > 1 Hz. The arrays allowed to reliably reconstruct the seismic wavefield in the 10–20 Hz range, showing an almost azimuthally homogeneous noise source distribution, with slowness values between 0.4 and 0.5 s/km. The inversion of dispersion curves in the same frequency range highlighted a rather homogeneous, high-velocity terrain (VS=2–3 km/s) in the first 100 m. The flat HVSR spectra across all arrays excludes the presence of a resonant, low-velocity layer at shallow depth.