Palaeoseismological evidence of the 1570 Ferrara earthquake

The May 20, 2012 Emilia earthquake (Mw = 6.1) produced secondary ground deformation effects, mainly associated with liquefaction phenomena, in several parts of the epicentral region, but particularly concentrated in the western sector of the Ferrara Province. South of San Carlo village, the eastern levee of the palaeo-Reno River was crossed by a system of parallel ground ruptures forming a complex system of overstepping and partly overlapping fractures. Single features were several decameters long, locally showing an extensional kinematics (i.e. opening, up to 35 cm) and/or vertical displacements (up to 30 cm). Locally, 1-10 m-wide grabens formed. On the top of the levee, we excavated a 50 m-long and locally 8 m-deep palaeoseismological trench mainly across some fractures where no sand ejection occurred during the 2012 event. In the trench walls, load structures and large slided blocks have been observed along the slope of the palaeo-levee, while in the deeper part of the trench, fluidification structures were detected showing clear mixing phenomena of sandy-silty layers with finer ones. The logs show that the natural levee body is clearly cross-cut by several sand filled dikes almost perpendicular to the trench's walls and at high-to-normal angle with respect to the depositional layering. Dikes were mainly formed as extensional joints characterised by opening up to 10-12 cm. Some dike was reactivated during the 2012 event, with a normal sense of shear and a maximum throw of ca. 15 cm, whereas others, showing a similar kinematics and throw (ca. 20 cm), were not visibly affected by the last earthquake. It is worth to note the lack of any ejection from the trenched dikes notewithstanding their opening and the presence of infilling sand at few tens of cm-depth. Moreover, two of the infilled dikes 5-10 cm-thick are clearly truncated upwards at 50-60 cm from the present-day surface. All these evidences document the occurrence of liquefaction phenomena (and the consequent diking) well predating the May 2012 earthquake. Based on a detailed reconstruction of the historical hydrographic network of the area, the affected levee sediments were mainly deposited between the XV and the XVI centuries, therefore constraining the possible time window for the causative event(s). Taking into account the historical catalogue of the period, the proposed magnitudes and the epicentral distances, only the 1570 Ferrara earthquake was able to sufficiently shake the trench site in order to induce the observed liquefaction phenomena.