Caratterizzazione sismica di tre siti della rete sismometrica dell'Italia Nord Orientale mediante l'analisi di terremoti e di rumore sismico ambientale

After the strong earthquake of magnitude M=6.4 occurred in 1976 in the Italian Friuli-Venezia Giulia region, the Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS, Italian National Institute for Oceanography and Experimental Geophysics) started to operate the Northeastern Italy (NI) Seismic Network. Since that time, OGS has installed stations in NE Italy, from Garda Lake to the border with Slovenia (Figura 1). In 1994 the network was substantially improved with the installation of digital good-quality acquisition system, and nowadays it is composed by more than 40 digital stations. Data from the network are available on the OASIS database (http://oasis.crs.inogs.it), the OGS archive system, where waveforms and information concerning the seismological sites are stored. The quality of information at each seismic station is mandatory to take correctly into account the local seismic response in the site-specific hazard analysis and to provide useful results to be used in the validation testing of hazard models. Since hazard seismic map is referred to a flat, rigid soil with no-amplification, while the real data are affected by local site condition, it is necessary to understand and quantify the site response due to topographic and soft soils effects. Topographic effects are due to lens effect operated by crest and the apex relief, while soft soils entrap the seismic input in superficial and little cohesive layers. Moreover, in the layers with low rigidity, shear waves slow down and, for the conservation of the energy, their amplitude grows. This master thesis' work wants to characterize the site condition at three station of the NI Seismic Network. The three sites are: CGRP, Cima Grappa in the Southern Alps, TEOL, Teolo - Monte Grande, Euganei hills and ADRI, Adria in the Po Plain. For all stations the site effects are reached with geophysical techniques, that are based on the spectral ratio of horizontal-to-vertical component (HVSR) of the ground motion. The analyzed data are both earthquake and microtremors (ambient noise recordings). For two sites (TEOL and ADRI), a specific geophysical survey has been acquired, and for ADRI the shear waves profile has been defined by the ambient noise inversion, in order to determinate the Vs30, as expected by Eurocode (EC8). This master thesis' results are: . good agreement between the HVSR technique for earthquake and ambient noise; . all stations are affected by site effects, so it is mandatory to correct them for hazard maps; . ADRI station has a Vs30 of 164 m/s, the soil class is "D", according to EC8; . there are evidences that CGRP station is affected by site effects due to topography and tectonic framework; . TEOL station is affected by the hill fundamental frequency.