Synthetic spectral structure of the seismic ambient vibrations generated by a distribution of superficial random sources with a finite extension

A model of seismic ambient vibrations is presented in the form of a random wavefield generated by a uniform continuous distribution of correlated aleatory forces, located at the surface of a flat layered weakly-dissipative Earth. The frequency-dependent spatial correlation among these sources is assumed to be representative of their spatial extension and has been parametrized by considering the role played by sea-wave dynamics in generating ambient vibrations. This enables a realistic description of the average power spectral density function of ambient vibrations observed worldwide at reference soil conditions. Effectiveness of the model is tested by reproducing observed average Horizontal-to-Vertical Spectral Ratios obtained from ambient vibration measurements at a well documented site in Northern Italy.