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.