Integrated passive and active geophysical prospections for seismic sites characterization

The seismic monitoring of the territory is extremely important in a seismic area as Italy, in order to determine the strong ground motion parameters, as peak ground acceleration and velocity, and develop risk management strategies for earthquake hazards. For a correct definition of strong ground motion parameter, the knowledge of site effects is determinant. In fact, the seismic motion is strongly affected by the materials through which seismic waves travel especially within the shallowest part of the subsurface. Local geological (lithology) and geomorphological conditions may produce strong modifications in the amplitude level, spectral content and time duration of earthquake ground motion. Furthermore, the knowledge of local site conditions (geology, topography, shear wave velocity, soil deposit thickness and resonant frequency) is fundamental for the definition of the EC8 site class (CEN, 2003) for each seismic station. Geological surveys are not enough for a detailed seismic site characterization and must be integrated by other investigation such as dedicated geophysical surveys. Among these, the most commons are the passive and active seismic investigations: seismic refraction and reflection, multichannel analysis of surface waves, H/V spectral ratio, spatially averaged coherency spectrum and frequency-wavenumber methods. This contribution shows the results obtained from the integration of some geophysical methods for the characterization of some RAF sites in Friuli Venezia Giulia (Maselli, 2007): two are seismic noise techniques (H/V spectral ratio and frequency–wavenumber (F-K) methods) while the other is an active seismic method (seismic refraction) including both P and S waves analysis. H/V spectral ratio (Nakamura, 1989) is a single-station method used to evaluate the resonance frequency of soft sediment over hard bedrock. This method calculates the ratio between the horizontal and the vertical Fourier spectra of ambient noise recorded at a single site by a three-component sensor. F-K method (Lacoss et al. 1969; Horike 1985) is an array-based method used to evaluate Vs profiles of soft, low velocity, sedimentary layers overlying hard bedrock. It extracts surface wave dispersion curves from ambient noise recordings. Vs profiles are estimated inverting the dispersion curves. Seismic refraction method measures the travel time of the seismic wave between the seismic source and the receivers to measure the velocity and the depth of the refracting surface. Four sites were investigated through these methods, one in correspondence with a borehole in Gemona del Friuli (GEMO) and the other three in correspondence with three RAF seismic stations: Gemona del Friuli (GESC), Polcenigo (POLC) and Monte Pala (PALA). Due to the presence pf a 120 meters long borehole reaching the basement, GEMO was selected as a reference site to compare the results obtained with the other methods and also to have a calibration site for the analysis.