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

Abstract

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 (Mw=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 pelite and sand; and (iv) the Meso-Cenozoic carbonate bedrock.The HVNSR analysis permitted the identification of two peaks (f0 and f1) nearly everywhere. f0 (0.4–0.7Hz) is due to the superposition of L'Aquila pelite and sand on the carbonate bedrock at 200–300m below ground level. f1 (3–15Hz) is caused by shallower impedance contrasts due to 3–20m 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 100m 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.