Site Effects in the Pollino Region from the HVSR and Polarization of Seismic Noise and Earthquakes

Site effects have been studied at 15 sites in the area of Mt. Pollino (Italy)
through the analysis of seismic noise and earthquakes by horizontal-to-vertical spectral
ratios (HVSRs) and polarization methods. The HVSR method has been applied to seismic
noise and to 83 local and regional earthquakes with the goal to investigate site effects in
the 0.5–20 Hz frequency band. At least 20 hrs of seismic noise were selected at each site
by taking into account day and night hours, and week days during several months to
include any possible environmental condition. Results show stable HVSR curves characterized
by small standard deviation, without high peaks at most stations. The same
method was also applied to S waves and early coda waves of earthquakes, showing,
for most of the sites, results very similar to the HVSR curve obtained from the seismic
noise. At some sites, the HVSR is very flat, with amplitude levels between 0.7 and 2 in the
entire considered frequency band. Some other sites show well-defined peaks of amplitude
up to 4.5. The relationship of site effects seen in the HVSR curves with the local structure
has been further investigated through polarization analysis of seismic noise. Results indicate
that topography gives the main contribution to site effects in four cases, whereas the
effects possibly associated with fault zones near some stations are less obvious.