Ambient noise levels in north central Italy

The characteristics of background seismic noise in north central Italy have been investigated by means of velocity power spectral analysis within the frequency range 0.1 - 15 Hz. The method proposed by McNamara and Buland ( 2004) has been applied to estimate the probability density function ( PDF) of power spectra computed for ten different stations. Since the target region is the most industrialized area of Italy, a large variability among the power spectra for different sites is observed in the frequency range 1 15 Hz, with the noise levels at two stations exceeding the New High Noise Model ( NHNM) of Peterson ( 1993). The 95th percentile of the PDF varies from - 165 to - 125 dB ( relative to ( m/s) (2)/ Hz). This variability could significantly affect the detection capabilities of a network installed for recording the small to moderate size seismicity occurring in north central Italy. We also observed that the dispersion of the powers, estimated at each site as the difference between the 95th and the 5th percentiles, shows a positive trend with frequency that can be ascribed to the diurnal variation of the background noise levels. In the frequency range 0.1 - 1 Hz, the dominant feature is the double frequency ( DF) peak of microseisms generated by oceanic storms. At one of the considered stations, the seasonal variability of the maximum amplitude of the DF peak has been observed in the period from April 2004 to December 2005. Considering the barometric maps provided by the UK Meteorological Office, we observed that the strongest powers in the range 0.10 - 0.25 Hz occur when intense storms are present over the North Atlantic Ocean, whereas the measurements of the height, frequency, and azimuth of the sea waves at two buoys of the Rete Ondametrica Italiana deployed in the Adriatic and Tyrrhenian seas suggest that the DF microseisms in the frequency range 0.25 - 0.50 Hz are generated by storms over the Mediterranean Sea. Finally, the analyzed region is characterized by two large- scale geologic features, namely, the Po Plain and the Alps. The impedance contrast between these two units causes a noise reduction of about 10 dB in the frequency range 0.2 - 0.6 Hz.