Present-day stress in the area of L'Aquila April 6 2009 earthquake (Italy)

Abstract

On April 6 2009 a strong earthquake (Mw= 6.3) occurred in Central Italy destroying the old town of L’Aquila and causing the death of hundreds of people. The mainshock was followed by two major aftershocks on 7 April (Mw= 5.6) and 9 April (Mw= 5.4) and more than 30000 minor events, ~170 with Ml> 3.0, during the following two months. The seismic sequence is confined in the upper 10 km, with exception of 7 April earthquake located at 15 km depth. The focal mechanisms of the main events show NW-normal faulting, consistent with the NE-SW trending extensional regime of the central Apennines. The earthquake occurred along the SW-dipping Paganica fault, and produced a surface rupture ~3 km long and maximum 10 cm high. Nevertheless, the cumulated size of the Paganica fault scarp and those of other active faults in the area, suggest that the Paganica fault could rupture with larger magnitude earthquakes than the 2009 event. Thus, although this earthquake caused loss of lives and major damage it does not fully reflect the seismic hazard of the area. We have analyzed in detail the only two deep boreholes of the area to infer the present-day horizontal stress orientation and discriminate regional and local sources of stress. We have compared stress orientation from borehole breakout data with those deduced from focal mechanism solutions and active faults to fully depict the stress pattern from surface to focal depths. Notwithstanding the results are comparable with the well-known regional present-day stress trend, they reveal a more complex pattern, that is probably due to the presence of active structures differently oriented with respect to the average regional trend.