On the stationarity of the horizontal to vertical noise spectral ratio

The Horizontal to Vertical Noise Spectral Ratio (HVNSR) method is nowadays widely used to estimate the resonance frequencies of geological structures. In the HVNSR method, seismic noise is considered as a stationary stochastic process. However, especially in industrialized/urbanized area, this is a very strict assumption seldom occurred. Several sources of noise can generate non stationary and anisotropic microtremor fields. To investigate the stationarity of microtremor, we have carried out several long-term measures of seismic noise with broad-band seismic sensors, in areas where the main source of anthropogenic noise is well known. The signals acquired have been analyzed both in frequency and in time domain. Our analysis have showed as the shape of the HVNSR curves can be strongly conditioned by the presence of near anthropogenic sources of noise. Useful information can be derived by the analysis of the data spectral density and by checking the azimuthal dependence of the HVNSR. The first one allows to easy identify different sources of seismic noise while the second one, in particular its time variability, is a useful instrument to evaluate the reliability of the data to estimate resonance frequencies of geological structures.