Abstract
On 6 April 2009, a Mw = 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, 30-45 deg and 80-90
deg 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.
VP/VS 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 m2
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 Mw = 5-6. Deep fluids more
than tectonic stress may control the seismotectogenesis of accretionary
wedges.
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