Site effects in the urban area of L'Aquila damaged by the April 6, 2009 earthquake

This paper presents the results of numerical analyses carried out to assess the different seismic response of two sites in the urban area of L'Aquila (Italy), selected as representative of typical subsoil conditions in the old city centre and in the recently developed suburban Pettino district. Both areas were severely damaged by the April 6, 2009 earthquake. The geotechnical model of the subsoil at each of the two sites and the related parameters, defined based on accurate site investigations, are described. The comparison of results of seismic response analyses at the two sites, in agreement with strong motion recordings of the April 6, 2009 main shock, confirms that site effects due to different subsoil conditions played an important role in the observed non-uniform damage distribution. Particularly in the city centre, characterized by an inversion of the shear wave velocity Vs with depth, the simplified approach based on elastic response spectra defined according to ground type (Vs,30) of the Italian building code tends to underestimate the seismic action and should be used with caution.