Normal faults and thrusts reactivated by deep fluids: The 6 April 2009 Mw 6.3 L'Aquila earthquake, central Italy

On 6 April 2009, a M-w = 6.3 earthquake occurred in the central Apennines (Italy) damaging the city of L'Aquila and the surrounding country. We relocate the October 2008 to 6 April 2009 foreshocks and about 2000 aftershocks occurred between 6 and 30 April 2009 by applying a double-difference technique and determine the stress field from focal mechanisms. The events concentrate in the upper 15 km of the crust. Three main NW-SE to NNW-SSE striking, 30 degrees-45 degrees and 80 degrees-90 degrees dipping faults were activated during the seismic sequence. Among these, a normal fault and a thrust were reactivated with dip-slip movements in response to NE-SW extension. The structural maturity of the seismogenic fault system is lower than that displayed by other systems in southern Apennines because of the lower strain rate of the central sector of the chain with respect to the southern one. V-P/V-S increases progressively from October 2008 to the 6 April 2009 main shock occurrence along a NW-SE strike because of an increment in pore fluid pressure along the fault planes. Pore pressure diffusion controls the space-time evolution of aftershocks. A hydraulic diffusivity of 80 m(2) s(-1) and a seismogenic permeability of about 10(-12) m(2) suggest the involvement of gas-rich (CO2) fluids within a highly fractured medium. Suprahydrostatic, high fluid pressure (about 200 MPa at 10 km of depth) within overpressurized traps, bounded by preexisting structural and/or lithological discontinuities at the lower upper crust boundary, are required to activate the April 2009 sequence. Traps are the storage zone of CO2-rich fluids uprising from the underlying, about 20 km deep, metasomatized mantle wedge. These traps easily occur in extensional regimes like in the axial sector of Apennines but are difficult to form in strike-slip regimes, where subvertical faults may cross the entire crust. In the Apennines, fluids may activate faults responsible for earthquakes up to M-w = 5-6. Deep fluids more than tectonic stress may control the seismotectogenesis of accretionary wedges.