Site response estimation in the Vittorio Veneto area (N.E. Italy) Part 1: geophysical measurements and in situ soil characterization

In 1936, the Vittorio Veneto (Treviso, NE Italy) area was heavily hit by a M=5.8 earthquake that occurred about 20 km from the city. The damage distribution was heterogeneous, suggesting the influence of site effects. In order to assess the seismic response of this area, surveys were conducted consisting of seismic noise measurements (Nakamura’s approach), reference site spectral ratios (RSSR) of weak earthquakes and shallow seismic refraction profiling. The horizontal-to-vertical spectral ratios (HVSR) from the ambient noise measurements reveal clear resonant peaks with variable HVSR amplitudes. The large amplitude of most peaks suggests an impedance contrast at shallow depths. The RSSR of about 40 weak events provide quantitative estimates of the seismic amplification in the northern part of Vittorio Veneto. Two sites, located on the left bank of the river, show high values of amplification, about 5 times larger than the signal amplitude at the reference sites, with a narrow-band peak at a frequency of about 5 Hz. The results generally agree with the HVSR resonant frequencies. In addition, a shear wave velocity profiling based on the inversion of Rayleigh waves has been performed at some sites. The technique consists of a surface wave analysis on both single and multichannel seismic records to retrieve the phase and group velocity of the Rayleigh wave fundamental mode and an inversion of the dispersion curves to obtain S-wave velocity models. The resulting equivalent S-wave velocity profiles are then constrained to match the observed HVSR fundamental resonant frequencies.