Seismic response analysis in the southern part of the historic centre of the City of L’Aquila(Italy)

This paper deals in the 2011 extensive site investigation by means of boreholes up to a depth of 80 m., Down-Hole (D-H) tests, Seismic Dilatometer Marchetti Tests (SDMT), Multichannel Analysis of Surface Waves (MASW) tests in the area of via XX Settembre of the city centre of L’Aquila (Italy), to obtain a detailed geotechnical model. Results of site investigations showed a marked variability of shear wave velocity Vs profiles, especially in the upper 5-10 m, where different soil types are commonly encountered, all characterised by low values of Vs. In this paper the main features of the April 6, 2009 L’Aquila Earthquake (ML=5.8; MW=6.3) are also discussed. The earthquake caused 308 casualties and heavy damage in the city of L’Aquila and in the surroundings villages. Some accelerometric stations were located across the Aterno River Valley, while only one station (namely AQK) was located in the city centre of L’Aquila. The peak acceleration values ranged from 0.35g recorded in the city centre to 0.65g recorded in the middle Aterno valley. The recorded time histories were characterised by short durations and high peak accelerations both in the horizontal and in the vertical directions. The area of via XX Settembre (southern part of the historic centre of the city of L’Aquila) was severely damaged by the earthquake. This area, located at a very short distance from the city centre, includes also some reinforced concrete frame buildings, mostly 5-7 storey high, built between 1950 and 1965. Old masonry buildings and some of these r.c. buildings collapsed or suffered severe damage due to the main shock, causing several tens of victims. The peculiar subsoil conditions locally detected down to about 40 m depths in this area include fine-grained soils interposed within, or placed above, “Brecce dell’Aquila” (typical of L'Aquila) and man-made fills. Low and variable shear wave velocity VS values in the upper portion of the subsoil have locally originated major amplification of the ground motion during the main shock. Therefore the paper deals also in specific one-dimensional numerical seismic response analyses performed. Significant amplification effects related to local subsoil conditions, bigger than the amplification factors given by the Italian Building Code NTC 2008, have been discovered by the seismic response analyses carried out at the site. The results of soil response analyses were compared with the occurred damage in the area. Keywords: 2009 L’Aquila Earthquake, Geotechnical Characterisation, Historic Centre, Site Effects, via XX Settembre.