GNGTS 2018 - 37° Convegno Nazionale

GNGTS 2018 S essione 2.2 439 Rovida A., Locati M., Camassi R., Lolli B. e Gasperini P. (a cura di); 2015: CPTI15, la versione 2015 del Catalogo Parametrico dei Terremoti Italiani . doi: https://doi.org/10.6092/INGV.IT-CPTI15 Tertulliani A., Arcoraci L., Berardi M., Bernardini F., Brizuela B., Castellano C., Del Mese S., Ercolani E., Graziani L., Maramai A., Rossi A., Sbarra M. and Vecchi M.; 2012: The Emilia 2012 sequence: a macroseismic survey . Annals of Geophysics, supplement to v. 55 , n. 4., 679-687. doi: https://doi.org/10.4401/ag-6140 SHEAR WAVE SEISMIC REFLECTION SURVEY IN THE 2012 EMILIA EPICENTRAL AREA L. Petronio 1 , A. Affatato 1 , L. Baradello 1 , A. Barbagallo 1 , G. Cristofano 1 , D. Sorgo 1 , C.G. Lai 2 , V. Poggi 3 1 Istituto Nazionale di Oceanografia e di Geofisica Sperimentale - OGS, Trieste, Italy 2 University of Pavia, Italy 3 GEM - Seismic Hazard Italy, Pavia, Italy Introduction. We present the results obtained from a reflection seismic survey performed in the Cavezzo area. This acquisition was performed to evaluate if SH-wave reflection seismic method can be applied for delineating the subsurface in the study area and, possibly, to obtain new information in term of geometry and seismic velocity of the sedimentary sequences. The study area is located at Cavezzo (Emilia-Romagna, Northern Italy) in the Po plain, very close to the epicenter of the earthquake (M L 5.8) that occurred at 29 May 2012. Geology. The subsurface geology of the Cavezzo area is marked by the presence of compressional structures of the Apennines covered by marine and continental clastic deposits of the Po plain. The subsoil of the Cavezzo area is mainly composed of Plio-Quaternary sedimentary sequences with alternating sand, silts and clays. These alluvial deposits are referable to two major depositional cycles: the Emiliano-Romagnolo Inferiore (AEI) synthem (0.7 – 0.4 Ma) and the Emiliano-Romagnolo Superiore (AES) synthem (0.4 Ma – present). This continental succession overlays the marine and transitional deposits of the lower and middle Pleistocene (Quaternario Marino - QM), referable to the Yellow Sands or Imola sands of the Appennino-Po Valley margin (RER & ENI-Agip, 1998). The depth of the base of the alluvial succession varies according to the presence of the geometry of Ferrarese folds. In the study area, located close to the Mirandola anticline, the AEI synthem has a reduced thickness and the QM top expected depth is at about 220 m below sea level. Several Quaternary thrusts are pointed out by a dense network of seismic lines acquired for hydrocarbon exploration (e.g., Pieri and Groppi, 1981; Cassano et al. , 1986; Fantoni and Franciosi, 2009). Stratigraphic, morphologic, geodetic and seismic evidences indicate a still active convergence of northeast-verging Apennines and south- verging Southern Alps structures (e.g. Serva, 1990; Castiglioni et al. , 1997; Boccaletti et al. , 2004; Toscani et al. , 2009; Boccaletti et al. , 2011; Comerci et al. , 2012; Michetti et al. , 2012). Earthquake focal mechanisms (Frepoli and Amato, 1997) and other stress indicators (Montone et al. , 2004) confirmed this evidence. Methodology. At the beginning, a seismic transect along NE-SW direction to cross the buried Mirandola high structure in dip direction was proposed. The lack of the landowner authorization forced to change the orientation from dip to strike direction. During five days, including crew mob/demobilization, field layout and parameter testing, one SH-wave reflection seismic profile and one P-wave reflection seismic profile were acquired. The L1 seismic line was 478 m length and it was located along the Malaspina road. Cavezzo municipality allowed to close the road during the acquisition time so the absence of vehicular traffic was beneficial for the S/N ratio improvement. This line was first acquired by SH-wave seismic source and horizontal geophones and afterwards was re-acquired by P-wave seismic source and vertical