HVNSR survey in historical downtown L'Aquila (central Italy): Site resonance properties vs. subsoil model

This paper presents and discusses the results of a detailed seismic noise survey carried out in historical downtown L'Aquila after the earthquake of 6 Apr. 2009 (M-w = 6.3). Seismic noise data were interpreted through numerical simulations, with the support of a subsoil model, characterised by four main units, from the top to the bottom: (i) soft soils (anthropogenic filling material and terra rossa); (ii) L'Aquila breccia; (iii) L'Aquila petite and sand; and (iv) the Meso-Cenozoic carbonate bedrock. The HVNSR analysis permitted the identification of two peaks (f0 and f1) nearly everywhere. f0 (0.4-0.7 Hz) is due to the superposition of L'Aquila pelite and sand on the carbonate bedrock at 200-300 m below ground level. f1 (3-15 Hz) is caused by shallower impedance contrasts due to 3-20 m thick soft sediments overlying L'Aquila breccia and by the weathering profile of L'Aquila breccia. A numerical simulation was carried out for three different models characterised by: (i) lithology lateral variation in the first 100 m of thickness due to the substitution, from north to south, of L'Aquila breccia with L'Aquila pelite and sand; (ii) the thickness variation of soft top soil onto L'Aquila breccia; and (iii) the presence of a weathered layer in the upper part of the L'Aquila breccia. The numerical simulation results fit the experimental seismic noise data. A spatial correlation between areal distributions of severe damages with that of f1 resonance frequency is found. So it follows that the severe damage during the L'Aquila earthquake of 6 Apr. 2009 could have been caused by the seismic local effect of the soft top soil and the weathering profile of L'Aquila breccia. (c) 2013 Elsevier B.V. All rights reserved.