GNGTS 2016 - Atti del 35° Convegno Nazionale

GNGTS 2016 S essione 3.1 517 3D geological model of L’Aquila historical downtown: preliminary results of high-resolution seismic reflection profiles M. Tallini 1 , M. Porreca 2 , M. Ercoli 2 , P. Mancinelli 2 , M. Barchi 2 , M. Nocentini 3 , D. Cosentino 3 , V. Di Fiore 4 , G. Cavuoto 4 1 Dipartimento di Ingegneria Civile, Edile-Architettura e Ambientale, Università dell’Aquila, Italy 2 Dipartimento di Fisica e Geologia, Università di Perugia, Italy 3 Dipartimento di Scienze, Università di Roma Tre, Italy 4 IAMC, CNR, Napoli, Italy Introduction. The Mw 6.1 earthquake struck on April 6, 2009 a wide area of the Abruzzi region in central Italy, causing strong damages in L’Aquila town and surroundings. L’Aquila historical downtown, encircled inside the medieval walls, was strongly affected and some important site effects were recognized (Martelli et al. , 2012). Previous works have suggested that different amplification effects, recorded into the historical downtown, may be related to the complex subsurface geologic architecture, given by the variability of the thickness and the lithology of the Quaternary deposits, on which the city was built (Martelli et al. , 2012). Thus, after April 6, 2009 earthquake, to improve the subsoil model for the seismic site response estimation and the Seismic Hazard evaluation of L’Aquila historical downtown, a multitask project has been carried out. It consists of the integration of subsurface dataset, including geological and geophysical surveys. The data have been interpreted with the aim to reconstruct a detailed 3D geological model of the Quaternary filling part of the continental basin on which L’Aquila historical downtown is located and the geometries of the Meso-Cenozoic bedrock located below the Quaternary cover. More precisely, we report the preliminary results concerning the interpretation of three high-resolution seismic reflection profiles provided in SEG-Y format by the IAMC-CNR research group. Geological setting. L’Aquila historical downtown is founded on terrace that gently slopes down toward SW direction and strongly dissected by the Aterno River. The terrace is formed by slope-derived Middle Pleistocene calcareous breccia (the so-called L’Aquila breccia pertaining to Colle Macchione-L’Aquila Synthem by Nocentini et al. , 2016), with a variable thickness in the range of 20 to 100 m. In the southern sector of L’Aquila downtown, a thin unit of shallow red soils dated back to the Eemian (Nocentini et al., 2016) causes a further shallow seismic impedance contrast that may have influenced the damage distribution during the 2009 earthquake (Del Monaco et al. , 2013; Tallini et al. , 2016a). The L’Aquila breccia and the red soils overlie a thick alluvial sequence consisting of Calabrian silty and sandy layers (Madonna della Strada Synthem, Nocentini et al. , 2016), with a thickness of 200 m as reported by recent drilling data (Nocentini et al. , 2016b). The Meso-Cenozoic limestone bedrock was intercepted in one single borehole (Amoroso et al. , 2010), whereas its geometry, which is poorly known, was studied in detail through microtremor array measurements (Di Giulio et al. , 2014). The high-resolution seismic reflection profiles: results and discussion. This work shows preliminary results obtained by the integration of subsurface dataset, including geological (above all several hundreds of borehole logs) and geophysical surveys, such as high-resolution seismic reflection profiles, microtremor data, down-hole and deep cross-hole investigations, seismic tomography and electrical tomography. The data have been interpreted with the aim to reconstruct a detailed 3D geological model of the filling part of the continental basin on which L’Aquila historical downtown is located. More precisely, we present the preliminary results of three high-resolution seismic reflection profiles provided in SEG-Y format. Such three lines were collected in order to shed light on a restricted subsurface area of L’Aquila historical downtown. The equipment used encompassed by 24 channels seismographs (GEODE, Geometrics), 10 Hz geophones (Geospace), and a “MINIVIB” source, able to generate sweep (P-wave in this case) in a frequency range between 5-150 Hz. We analyzed the seismic lines in Fig. 1, named as “Corso” (NNE-SSW direction), “Duomo” and “Sallustio” (both in WNW-ESE direction). The