Shear-wave velocity profiles across the Ferrara Arc: a contribution for assessing the recent activity of blind tectonic structures

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. The sequence has been characterized by two main shocks, with ML ranging from 5.9 and 5.8, respectively. The causative faults of this seismic sequence are two segments of the Ferrara Arc thrust system, which is one of the three major arcs of blind, north-verging thrusts and folds that represent the Apennines front. Both these seismogenic sources were associated with blind, mainly dip-slip reverse, faulting while the uppermost tip segment of the sliding planes has been estimated to reach a minimum depth of 3-4 km. The recurrence of similar ‘areal morphogenic earthquakes’ 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. 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 tectonic evolution of the region in the past, but also to help understanding where active faults are located, and which is their seismogenic potential. We planned a geophysical survey across some major tectonic structures affecting the subsoil of the eastern Po Plain. Our investigation is based on several passive seismic measurements carried out along the profiles with a mean distance of 1 km; in particular, we applied the ESAC and Re.Mi. methods which could provide information in the first 100-150 m. Accordingly, each measurement gives the vertical distribution of the shear waves velocity to a depth of ca. 150 m and allow to generate a pseudo-2D section. In this paper, we describe and comment the profile west of Ferrara, while measurements for the eastern profile are still in progress.