GNGTS 2014 - Atti del 33° Convegno Nazionale

Shear-wave velocity profiles across the Ferrara Arc: a contribution for assessing the recent activity of blind tectonic structures N. Abu-Zeid 1 , S. Bignardi 1 , R. Caputo 1,2 , A. Mantovani 1 , G. Tarabusi 1,3 , G. Santarato 1 1 Department of Physics and Earth Sciences, University of Ferrara, Italy 2 Research & Teaching Centre for Earthquake Geology, Tyrnavos, Greece 3 Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma 1, Roma, Italy Introduction. In the late May 2012, a noticeable seismic sequence affected a wide area of the eastern sector of the Po Plain, causing 27 casualties, thousands of injuries, severe damages to historical centers and industrial areas, and extensive liquefaction phenomena along abandoned river channels (Caputo and Papathanassiou, 2012; Galli et al. , 2012; GdL Liquefazione RER, 2012; Emergeo Woking Group, 2013; Papathanassiou et al. , 2012). The sequence has been characterized by two main shocks, with M L ranging from 5.9 and 5.8, respectively. The first occurred on May 20 at 02:03 UTC, with epicenter between Finale Emilia and San Felice sul Panaro (44°51’50”N, 11°14’31”E, h = 6.3 km), while the second occurred on May 29 at 07:00 UTC, 15 km SW of the first shock, near Mirandola (Massa et al. , 2012). The causative faults of this seismic sequence are two segments of the Ferrara Arc thrust system, which is one of the threemajor arcs of blind, north-verging thrusts and folds that represent the Apennines front. The Ferrara Arc, with a total length of more than 100 km, is characterized by several second order structures showing a complex internal geometry and, in particular, the activated structures have a left-stepping largely overlapping geometry (Pieri and Groppi, 1981; Bigi et al. , 1982; Boccaletti et al. , 2004; Bonini et al. , 2014). Both these seismogenic sources were associated with blind, mainly dip-slip reverse, faulting ( e.g. Scognamiglio et al. , 2012; Pondrelli et al. , 2012), while the uppermost tip segment of the sliding planes has been estimated to reach a minimum depth of 3-4 km (Bignami et al. , 2012). One of the coseismic effects due to the reactivation of reverse blind faults, as in the case of the Po Plain, is the bending of the topographic surface and the consequent uplift of the broader epicentral area. In fact, as a consequence of the fault geometry and kinematics, the rock volume above the co-seismic rupture tip is characterised by a typical fault-propagation folding process (Okada, 1985; Bignami et al. , 2012; Salvi et al. , 2012). Depending on the seismotectonic parameters of the underlying seismogenic source, the uplifted area has an elliptical shape that is characterized, in correspondence of the epicentral area, by a maximum vertical displacement of some tens of centimeters. The application of satellite interferometry (DinSAR technique) to the Emilia seismic sequence clearly show that the main shocks of the 20th and 29th of May produced two uplifted areas, characterized by a maximum vertical displacement of 25 cm, partly overlapping and with a cumulative length of about 50 km in E-W direction (Bignami et al. , 2012; Salvi et al. , 2012). The recurrence of similar ‘areal morphogenic earthquakes’ (Caputo, 2005) and the competition with the high subsidence and depositional rates that characterize the Po Plain, have progressively modified the geomorphology and stratigraphy of the region. In this conditions, the hydrographic network has proven to be particularly sensitive to vertical deformations, so even small altimetric and gradient changes could induce river avulsions and diversions, highlighted by the presence of several drainage anomalies (Burrato et al. , 2003, 2012). Consequently, the alluvial plain is actually crossed by numerous abandoned river channels, some of which are still well preserved. Obviously, the presence of active tectonic structures responsible for the local uplifts and even for the complex interactions with the hydrographic network has influenced not only the distribution of the sediments on the surface, but also in the subsoil (down to some tens of metres) producing important stratigraphic variations and therefore also changes in the geophysical properties of the materials. Determining the space distribution of the tectonically induced uplift rate together with the location of active faults is therefore of the utmost concern not only for reconstructing the GNGTS 2014 S essione 1.2 117