HVNSR survey in historical downtown L'Aquila (central Italy): Site resonance properties vs. subsoil model Francesco Del Monaco a , Marco Tallini a, ⁎, Carmelina De Rose b , Federica Durante a a Dipartimento di Ingegneria Civile, Edile-Architettura e Ambientale - Centro di Ricerca e Formazione in Ingegneria Sismica, Università dell'Aquila, Via Giovanni Gronchi, 18, 67100, L'Aquila, Italy b Dipartimento di Medicina clinica, Sanità pubblica, Scienze della Vita e dell'Ambiente, Università dell'Aquila, Piazzale Salvatore Tommasi 1, 67100 Coppito (L'Aquila), Italy abstract article info Article history: Received 10 September 2012 Received in revised form 11 February 2013 Accepted 2 March 2013 Available online 20 March 2013 Keywords: Seismic noise Resonance frequency Site effects Subsoil model Downtown L'Aquila Central Italy This paper presents and discusses the results of a detailed seismic noise survey carried out in historical down- town 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 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 (0.4–0.7 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 var- iation 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 pres- ence 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. © 2013 Elsevier B.V. All rights reserved. 1. Introduction After the L'Aquila (central Italy) earthquake of 6 Apr. 2009 (M w = 6.3, Ameri et al., 2009; Amoruso and Crescentini, 2009; Chiarabba et al., 2009), activities of rebuilding and new urban planning for mitigating the seismic risk were started in L'Aquila's medieval historical centre. Therefore, pro- jects funded by several institutions, including the L'Aquila University, were implemented with a view to more thoroughly characterising local seismic effects. The paper presents and discusses the results of a detailed seismic noise survey conducted in downtown L'Aquila. Seismic noise monitoring, carried out before (De Luca et al., 2005) and after (Çelebi et al., 2010; Bergamaschi et al., 2011; Milana et al., 2011; Martelli et al., 2012) the earthquake, showed a typical reso- nance frequency f0 of 0.5–0.6 Hz in the entire historical centre. As reported in previous studies (De Luca et al., 2005; Bordoni et al., 2011), the f0 was caused by the sharp seismic impedance due to the superposition of the Quaternary 300 m-thick detrital succession of the L'Aquila basin on the seismic Meso-Cenozoic carbonate bedrock. As higher frequencies in ranges of engineering interest (i.e. 1–20 Hz), have not been studied in detail so far, a high density seismic noise monitoring survey was conducted in the L'Aquila centre. Seismic noise data were also integrated with geological field mapping, borehole data and in situ seismic investigations in order to improve the accuracy of the subsoil model. The results permitted the correlation of high reso- nance frequencies with building damage distribution. 2. Geological and geophysical setting The centre of L'Aquila rests on a nearly triangular hill with a 700 m-high (above sea level, asl) flat terraced top. It is located in the Aterno River basin, which was struck by the M w = 6.3 earthquake of 6 April 2009 (Galli et al., 2010). The Aterno River basin is a typical WNW–ESE-trending Plio-Quaternary intramontane basin of central Italy which is located in the lower Pliocene–upper Miocene thrust and fold belt of central Apennines (Doglioni, 1991; Cosentino et al., 2010). Afterwards, from the upper Pliocene through the Quaternary, the area experienced extensional tectonics, with mainly S-dipping and NW–SE- or W–E-trending normal faults, many of which are of a seismogenic Engineering Geology 158 (2013) 34–47 ⁎ Corresponding author. Tel.: +39 862434553; fax: +39 862434548. E-mail addresses: francesco.delmonaco@libero.it (F. Del Monaco), marco.tallini@univaq.it (M. Tallini), carmelina.derose@gmail.com (C. De Rose), durante_federica@libero.it (F. Durante). 0013-7952/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.enggeo.2013.03.008 Contents lists available at SciVerse ScienceDirect Engineering Geology journal homepage: www.elsevier.com/locate/enggeo