Day-night cycle of seismic noise HVSR and comparison with body waves and T waves

We analysed long (months) continuous recordings of ground motion at more than 30 sites in Calabria (Italy) in order to investigate the relationships among background signal amplitude, noise composition, day-night cycle and horizontal to vertical spectral ratio (HVSR). We computed the root mean square (rms), polarization and HVSR of the seismic signal. For many sites, the HVSR contains at least one well-defined peak of amplitude greater than 2 that is representative of site effects produced by the shallow geological structure and/or topography. At six of the investigated sites, we observe an important variation of the HVSR peak amplitude that is well correlated with the day-night cycle, the peak amplitude being greater during day hours, when the background signal amplitude is higher. The rectilinearity of particle motion computed from the polarization analysis is higher during day hours, thus showing a positive correlation with both signal rms and HVSR peak amplitude. For these sites we analysed also body waves of local earthquakes and T waves produced by regional earthquakes in order to compute the HVSR of signals composed predominantly by body waves. Results of body waves and T waves are more similar to the HVSR of day hour seismic noise than the HVSR of night hour seismic noise, thus suggesting that the stronger seismic noise recorded during day hour may contain a greater amount of body waves with respect to the night hour noise.