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


Stefano Mazzolia,*, Stefano Santinib, Chiara Macchiavellic, Alessandra Ascionea

aDipartimento Scienze della Terra, Università di Napoli Federico II, Largo San Marcellino 10, 80138 Napoli, Italy
bDipartimento di Scienze di Base e Fondamenti, Sezione di Fisica, Università di Urbino “Carlo Bo”, Via Santa Chiara 27, 61029 Urbino, Italy
cScuola di Scienze e Tecnologie, Università di Camerino; Via Gentile III da Varano, 62032 Camerino (MC), Italy
Journal of Geodynamics, 84, 62–76, 2015
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 to the Po Plain area, where the 2012 northern Italy earthquakes confirmed active frontal thrusting, the new focal mechanisms obtained in this study for the 2013 Marche offshore earthquakes indicate that only minor thrust fault reactivation occurs in the Adriatic domain, even for a theoretically favourably oriented maximum horizontal compression. Recent seismicity in this domain appears to be mainly controlled by transcurrent crustal faults dissecting the Apennine thrust belt. The along-strike stress field variation from the Po Plain to the Adriatic area has been quantitatively investigated by applying the Multiple Inverse Method (MIM) to the analysis of the entire seismicity recordedfromJanuary 1976 to August 2014, from the top 12 km of the crust (fault plane solutions from 127 earthquakes with MW ≥ 4), allowing us to obtain a 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° x 1.5° surface area and 12 km depth. The obtained stress field maps point out that, although the entire outer northern Apennines belt is characterized by a sub-horizontal maximum compressive axis (σ1), the minimum compression (σ3) is sub-vertical only in the Po Plain area, becoming sub-horizontal in the Adriatic sector, thus confirming that the latter region is dominated by an active tectonic regime of strike-slip type.
Keywords: seismotectonics; earthquake focal mechanisms; stress inversion; thrust faulting; strike-slip faulting