A Study on Seismic Noise Variations at Colfiorito, Central Italy: Implications for the Use of H/V Spectral Ratios

Seismic noise recorded by broad‐band stations in the middle of and around the Colfiorito plain is analyzed in the frequency band 0.1 to 10 Hz. Small daily variations in noise amplitude are found, on the order of 2 for f > 1 Hz. In contrast, long‐term amplitude variations due to weather conditions are significant throughout the analyzed frequency band; for f < 1 Hz, the amplitude increase can be as large as a factor of 50. In the low‐frequency band, horizontal components vary much more than the vertical at both firm and soft sites. However, these noise variations at low frequencies do not contaminate significantly the 0.9‐Hz peak of the H/V spectral ratio that fits the fundamental eigenfrequency of the sedimentary fill of the basin, resonating during earthquakes. Correlating the long‐term variations of noise with different meteorological parameters, we find that wind speed best matches the low‐frequency noise disturbances.