GNGTS 2024 - Atti del 42° Convegno Nazionale

Session 3.2 GNGTS 2024 Imaging the buried Mirandola and Casaglia antclines in the Po Plain, northern Italy, based on HVSR frequencies and amplitudes analysis G. Tarabusi 1 , G. Sgatoni 1 , R. Caputo 2,3 1 Isttuto Nazionale di Geofsica e Vulcanologia, Italy 2 Dept. of Physics and Earth Sciences, Ferrara University, Italy 3 CRUST, UR-UniFE, Italy The Mirandola and Casaglia antclines are two buried fault-propagaton folds that started forming during Quaternary due to the seismogenic actvity of blind segments belonging to the broader Ferrara Arc (Po Plain, Italy, Fig. 1). The last reactvaton of segments of this Arc was during the May 2012 Emilia sequence (20 May, Mw 6.1 and 29 May, Mw 5.9 earthquakes, Pondrelli et al. 2012). On top of these structures the thickness of the marine and contnental deposits of the Po Plain foredeep is partcularly reduced. The results of a previous study carried out in correspondence of the Mirandola antcline area (Tarabusi and Caputo, 2017) represent the startng point of this research, in which we largely increased the dataset and extended the method to the area of the Casaglia antcline, where the stratgraphy and the relatonships between the subsoil units are similar though partly diferent (Fig. 1). Indeed, in Casaglia, as in Mirandola, in correspondence with the structural culminatons of the fault-propagaton antclines, the thickness of the contnental Quaternary deposits is generally reduced and they directly overlay the Miocene units. As a consequence, a high impedance contrast occurs due to the abrupt increase of material density and hence of the seismic waves velocity. For the purpose of the present research, numerous passive seismic measurements were carried out for obtaining the horizontal-to-vertcal spectral ratos (HVSR) to identfy resonance frequencies (Fig. 1). This approach has a twofold target: to supplement the existng data in the Mirandola region in order to enhance and extend the existng subsoil model, and to develop an analogous detailed subsoil model of the Casaglia area, which had not been explored previously. A detailed image of the antcline structures was successfully obtained (Fig. 1), confrming the strong correlaton between the HVSR outcomes, such as peak frequencies and amplitudes, and the available stratgraphic data primarily obtained from boreholes. We obtained high resoluton resonance frequency and HVSR peak amplitude maps and a 3D reconstructon of both antclines (Fig. 2).