Geological and seismologic data review of the 2009 L’Aquila seismic sequence (central Apennines, Italy): deep-seated seismogenic structures and seismic hazard

Abstract

This study identified and characterized an important and complex Quaternary structure, the Celano-Cittareale Fault System (CCFS), crossing NNW/SSE-NW/SE through the entire area of the L’Aquila 2009 seismic sequence. The CCFS, which develops from the Fucino Plain and further north to Cittareale, is locally segmented by NE-SW/NNE-SSW transversal/oblique faults. It is a regional flower structure, deeply rooted in the crust, mainly NE-dipping and overall high-angle, characterized during the Quaternary by left transtensional kinematics. Based on the data acquired from the literature and on our reconstructions, the CCFS was the main seismogenic structure responsible for the 2009 L’Aquila seismic sequence and for other important seismic sequences, in particular those of 1703 and 915. According to our view, the SW-dipping Paganica Fault (PGF, a splay of the main Barisciano Fault System BFS) and the Mt. Gorzano Fault (GF), considered by previous authors to be the only ones responsible for the 2009 L’Aquila seismic sequence, were instead local and more superficial faults, antithetical to the CCFS, activated at different times by accommodation. During the 2009 seismic sequence, three CCFS segments, bounded by four transversal/oblique fault-systems, were active stepwise from the south to the north, producing three clusters of earthquakes that differed in energy and number of events. The activation of the CCFS southern segment caused the largest number of earthquakes, including the foreshocks, the main shock (MW=6.1) and earthquakes deeper than 20 km. The earthquakes of this cluster were characterized by left transtensional and extensional kinematics. The activation of the other two CCFS segments and of the BFS and GF produced earthquakes of lower magnitude that also were shallower and characterized by predominantly extensional kinematics. The main transversal faults played different roles: the RMF (Rocca di Mezzo Fault), MF (Marana Fault) and TFS (Torrita Fault System) passively confined the three clusters of the 2009 L’Aquila seismic sequence to the south and to the north, respectively; instead, the intermediate SFS (San Vittorino Fault System) first confined the southern cluster to the north at the onset seismic of the sequence and subsequently, by activating as a right transfer fault, it became itself seismogenic.