GNGTS 2017 - 36° Convegno Nazionale

GNGTS 2017 S essione 2.2 383 Combining passive-geophysical and stratigraphic data for a preliminary mapping of the Po Plain seismic bedrock (Northern Italy) C. Mascandola 1 , M. Massa 1 , D. Albarello 2 , L. Martelli 3 , S. Lovati 1 , V. Poggi, S. Barani 4 , F. Pergalani 5 , M. Compagnoni 5 1 Istituto Nazionale di Geofisica e Vulcanologia (INGV), Milano, Italy 2 Dip. di Scienze Fisiche, della Terra e dell’Ambiente, Università degli Studi di Siena, Italy 3 Regione Emilia-Romagna, Servizio Geologico, Sismico e dei Suoli, Bologna, Italy 4 Dip. di Scienze della Terra dell’Ambiente e della Vita, Università degli Studi di Genova, Italy 5 Dip. di Ingegneria Civile e Ambientale, Politecnico di Milano, Italy According to the actual Italian and European seismic codes (NTC, 2008; CEN, 2003), the transition between soft and hard soil is ascribable where the shear-wave velocity exceeds 800 m/s. This interface represents the seismic bedrock and it is of relevant importance for the seismic site effects evaluation. This issue is particularly significant in the Po alluvial basin, which is the deepest and most extended sedimentary basins in Italy. The recent case of the 2012, M w 6.1, Emilia (Northern Italy) seismic sequence highlighted the importance of the site effects estimation in the Po Plain, where despite the low-to-moderate seismic hazard, the high density of both population and infrastructure makes this area a high seismic-risk zone. The aim of this work is to preliminary map the Po Plain seismic bedrock through the association of the available passive geophysical analysis and the known regional stratigraphic discontinuities. This study is based on an extensive collection of geophysical and geological data in the entire area. In particular: - all permanent and temporary (i.e. seismic emergency or project ) seismic stations installed in the area in the previous years and now available, for a total of 73 measure points; - 56 new single station measures of ambient noise have been performed throughout the basin in order to obtain a regular grid of equidistant measures; - 8 microtremor array measurements recently executed in the framework of several projects; - borehole data if available, in particular the cross-hole tests inMedolla, Mirandola (Paolucci et al. , 2015; Laurenzano et al. , 2017) and Casaglia (Laurenzano et al. , 2013), besides the down-hole test in Mirabello (Minarelli et al. , 2016); - a huge dataset collected by Regione Emilia-Romagna, with 60 shear-wave velocity profiles and 2000 H/V executed in the Po Plain area, has been reviewed and selected obtaining a representative subdataset composed of 47 single station measure and 15 microtremor array measurements; - regarding the stratigraphic data, different regional unconformities corresponding to different aquifers limits have been extensively mapped throughout the basin in two subsequent publications (i.e. Regione Emilia-Romagna, ENI – AGIP, 1998; Regione Lombardia, Eni Divisione Agip, 2002). These works have been the primary source of geological and stratigraphic information. However, other different studies in the Po Plain area have been considered (e.g. Pantaloni 2007; Scardia et al. , 2012; Fantoni and Franciosi 2010; Fontana et al. , 2014; GeoMol Team 2015; Martelli et al. , 2017). All available data of ambient noise (Fig. 1) were analyzed with the Nakamura technique (Nakamura, 1989) to determine the H/V amplification function and the collected microtremor array measurements were homogeneously reprocessed to obtain the relative shear-wave velocity profile, through a joint inversion of the Rayleigh wave dispersion curve (fundamental mode only) and the ellipticity curve estimated from the H/V results. Moreover, 71 H/V curves were inverted with the Sanchez-Sesma et al. (2011) method in order to obtain the correspondent velocity profile for 71 scattered sites in the basin (Fig. 2). Based on these analyses, the H/V peaks corresponding to the seismic bedrock interface have been selected and a frequency-depth