Possible Multiple Sources of the Strong 1117 Po Plain Earthquake, Inferred from the Plio-Quaternary Evolution of the Northern Adriatic Area

The strongest documented seismic disaster ever occurred in the Po Plain area (January 3, 1117, M = 6.5) involved significant damage over a large zone. The genetic mechanism of such an event, most probably caused by more than one earthquake, is still an object of debate. Above all, the sources so far proposed cannot account for significant features of the observed macroseismic field. In this work, we suggest that the damage in the Verona zone was caused by the activation of a fault in the Lessini tectonic district, while damage in the central Po Plain may be related to a thrust fault located beneath the Giudicarie belt. The effects felt in northern Tuscany might derive from the seismic activation of the presumed SW-ward buried prolongation of the Giudicarie fault. The presence of such transpressional lithospheric discontinuity in the Adriatic domain since the upper Miocene and its reactivation (Pliocene-Pleistocene) as a thrust zone is mainly suggested by an accurate analysis of the observed deformation pattern in the central Mediterranean region. The proposed Giudicarie source may also help to explain the damage observed in the central Po Plain on December 25, 1222, which is not compatible with the seismic sources so far proposed.