Three-dimensional segmentation and different rupture behavior during the 2012 Emilia seismic sequence (Northern Italy)

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 from a 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 then modeled the Coulomb stress changes produced by two mainshocks to analyze on- and off-fault seismicity. Discrepancies between the magnitude 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.