Active tectonics of the outer northern Apennines:Adriatic vs. Po Plain seismicity and stress fields

The recent earthquake sequences of 2012 (northern Italy) and 2013 (Marche offshore) provided new,fundamental constraints to the active tectonic setting of the outer northern Apennines. In contrast tothe Po Plain area, where the 2012 northern Italy earthquakes confirmed active frontal thrusting, the newfocal mechanisms obtained in this study for the 2013 Marche offshore earthquakes indicate that onlyminor thrust fault reactivation occurs in the Adriatic domain, even for a theoretically favourably orientedmaximum horizontal compression. Recent seismicity in this domain appears to be mainly controlled bytranscurrent crustal faults dissecting the Apennine thrust belt. The along-strike stress field variation fromthe Po Plain to the Adriatic area has been quantitatively investigated by applying the multiple inversemethod (MIM) to the analysis of the entire seismicity recorded from January 1976 to August 2014, fromthe top 12 km of the crust (fault plane solutions from 127 earthquakes with MW≥ 4), allowing us to obtaina comprehensive picture of the state of stress over the outer zone of the fold and thrust belt. The present-day stress field has been defined for 39 cells of 1.5◦× 1.5◦surface area and 12 km depth. The obtainedstress field maps point out that, although the entire outer northern Apennines belt is characterized by asub-horizontal maximum compressive axis (_1), the minimum compression (_3) is sub-vertical only inthe Po Plain area, becoming sub-horizontal in the Adriatic sector, thus confirming that the latter regionis dominated by an active tectonic regime of strike-slip type.