Multidisciplinary geophysical approach for investigating the recent activity of blind tectonic structures in the eastern sector of the Po Plain (North Italy).

The bending of the topographic surface and the consequent uplift of the broader epicentral area are among the major coseismic effects due to the reactivation of reverse blind faults as, for example, in the case of the Northern Apennines underlying the Po Plain (Okada, 1985; Bignami et al., 2012). Determining the uplift spatial distribution is crucial for reconstructing the recent tectonic evolution of the region as well as for understanding where active faults are located, and which is their probable seismogenic potential. With this premise, we carried out a set of geophysical surveys across some major tectonic structures of the eastern Po Plain. The investigations included several passive seismic measurements carried out along two profiles, ca. 27 km-long and oriented SSW-NNE, i.e. almost perpendicular to the regional trend of the buried structures of the Ferrara Arc. The western one runs between Cento and Bondeno, while the eastern one between Traghetto (near Molinella) and Formignana. We used the ESAC (Aki, 1957; 1964) and Re.Mi. (Louie, 2001) strategies to reconstruct pseudo-2D shear wave velocity sections, providing information about the shallow subsurface (100-150 m depth), and the HVSR technique (Nakamura, 1989) to infer the depth of the major impedance contrasts. In addition, in order to deduce the subsurface density distribution along the eastern profile and to increase the investigation depth of the former seismic methods (down to some km), we carried out a gravimetric survey. Assuming that the overall Vs pattern is determined by lithological variations, the pseudo-2D sections allow to emphasize the occurrence of buried anticlinal structures in correspondence with the higher velocity gradients due to a 'condensed' stratigraphy. Conversely, where gradients are lower, the thickness of the coeval sedimentary units is greater (i.e. synclinal structures). The reconstructed shear wave velocity profiles document the possibility to detect the recent tectonic activity of buried structures underlying the Po Plain by means of low-cost geophysical surveys. The comparison with the location of the major tectonic structures at greater depths as inferred using the support of the gravimetric survey and some available seismic reflection profiles suggests that the shallow stratigraphic variations highlighted in this research may be directly associated with a recent activity of the buried folds.