Describing the slip behavior of an active fault system is central to understanding the seismic potential of
seismogenic areas. Different elements control the nature and the extent of the coseismic and post-seismic
ruptures, including the geometry of faults, the nature of faulted rocks, and the stress changes caused by the
mainshocks. In May–June 2012 a severe seismic sequence struck a portion of the Po Plain (Northern Italy),
where a thick blanket of Plio-Quaternary sediments hides a number of seismogenic sources corresponding to
the external thrust systems of the Northern Apennines. We used deep seismic reflection data to reconstruct
the geometry of the faults responsible for the sequence. These faults exhibit significant non-planarity due to
tectono-stratigraphic heterogeneities inherited froma complex pre-thrusting extensional tectonic phase. A comparison
of the fault parameters derived from our geological analysis and the evidence supplied by seismological
(aftershock distributions) and geodetic data (InSAR) allowed to identify the causative fault segments of the two
mainshocks.We thenmodeled the Coulomb stress changes produced by two mainshocks to analyze on- and offfault
seismicity. Discrepancies between themagnitude of the earthquakes and the extent of active faults suggest
that the mainshocks did not rupture the entire thrust planes.We contend that seismogenic ruptures were confined
in the Mesozoic carbonates and were stopped by lithological changes and/or mechanical complexities of
the fault planes, both along dip and along strike. Our findings highlight that along the active structures of the
Po Plain slip tends to be seismogenic where faults are located in Mesozoic carbonate rocks.
