GNGTS 2015 - Atti del 34° Convegno Nazionale

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. Gemona del Friuli is located above an alluvial fan of exceptional dimension: 2.0 km of ray with frontal arc of 3.6 km and a total volume of sediments of about 3.0x10 9 m 3 . The thickness of quaternary deposits is variable form 100 to 150 m, the elevation is about 560 m at the top of the alluvial fan and close to 195 m at the edge of the alluvial fan. Gemona del Friuli, located in a high seismic area, is interested by the presence of an important regional thrust: the Barcis- Starasella tectonic line. In the past, several events of medium-high intensity occurred in the area. The most recent destructive events were the Friuli earthquakes in May and September 1976. GEMO, located near the borehole, lied at the alluvial fan border. The stratigraphy of the borehole, drilled in 1981 by Friuli Venezia Giulia Region, shows a sequence of gravel, sand and clay until the carbonate rock is reached at a depth of about 115 m. GESC seismic station is installed in the middle of the alluvial fan. This area presents strong amplification effects, as testified by the damages caused by 1976 earthquakes. Polcenigo is located at the border between Carnic Prealps and High Friulan Plain. At E-SE of Polcenigo there are some hills that are the result of the recent evolution of Carnic Prealps and are made of Pre-Quaternary rocks, as a result of the Quaternary activity of Aviano thrust (Carulli et al. , 1982). POLC seismic station is installed on one of these hills, inside San Floriano rural park. The area is characterized by an alternation of marl, sandstone and siltstone, and conglomerate as end-member of the molasse sequence. This area, as demonstrated by the analysis of seismic recording at POLC station, highlights significant amplification effects. Monte Pala, located at east from Pradis village, belongs to Carnic Prealps and reaches an altitude of 1231 m. The mountain is made by a narrow anticline tilted down toward the south, 184 GNGTS 2015 S essione 2.2