Coseismic ground deformations related to the 2009 L’Aquila seismic sequence (Italy)

On April 6, 2009 an earthquake of Mw=6.3 occurred in the central Apennines (Abruzzi region), rupturing a SW-dipping normal fault for a total length of about 18 km. It severely hit an area of ~2000 km2 around L’Aquila town causing 300 fatalities and thousands injured. The mainshock was followed by two major aftershocks on April 7 (Mw=5.6) and April 9 (Mw=5.4). The seismic sequence is confined in the upper 10-12 km, with the exception of April 7 earthquake with a depth of about 15 km. The focal mechanisms of the main events clearly show NW-oriented normal faulting consistent with the NE-SW trending extensional regime of the central Apennines. In this work we present the data collected during the post-earthquake field campaigns performed by the Emergeo working group (INGV prompt geological survey team). The seismic sequence produced a widespread and diversified set of geological surface effects consisting of rock falls, soil compactions and mobilization of loose deposits as well as open tectonic fractures with or without throw along pre-existing faults. Particularly, as a result of the mainshock, surface faulting occurred along the SW-dipping Paganica normal fault with a continuous extent of ~2.5 km. Ruptures consist of extensional cracks (i.e. fissures) and flexural or normal fault scarps with an orientation of N130°-N140°. Small and sporadic tectonic slip took place along the NW and SE prolongation of the Paganica fault and along its antithetic normal faults. The continuity of the Paganica ruptures, the agreement between their average trend and the focal plane solutions, the consistency with the slip distribution at depth, their location and similarity in the geometrical organization with respect to the long term evidence of the Paganica fault, lead us to interpret them as the primary surface expression of the seismogenic fault at depth. The limited extent and the small throw of the surface ruptures in comparison to the size of the Paganica compound fault scarp and to many individual throws observed along the long-term Paganica fault, raise questions on the maximum expected Magnitude and the seismic hazard estimate of this area. Thus, although the 2009 L’Aquila earthquake and associated faulting caused major damage and loss of life, it does not fully characterize the seismic hazard associated to this area. Emergeo working group: Giuliana Alessio, Laura Alfonsi, Carlo Alberto Brunori, Francesca R. Cinti, Riccardo Civico, Luigi Cucci, Paolo Marco De Martini, Sofia Mariano, Maria Teresa Mariucci, Paola Montone, Rosa Nappi, Daniela Pantosti, Antonio Patera, Simona Pierdominici and Stefano Pucci.