Present-day stress in the surroundings of 2009 L'Aquila seismic sequence (Italy)

The axial zone of the Apenninic belt in central Italy is a tectonically active region affected by post-orogenic Quaternary extension. The present-day stress field is characterized by a minimum horizontal stress (Sh(min)) similar to NE-SW oriented, derived mainly from earthquake focal mechanisms and secondarily from borehole breakouts and fault data. The paper describes the computation of the Sh(min) orientation along two deep boreholes located in the vicinity of the area hit by the 2009 April 6, Mw 6.3 L'Aquila earthquake. The analysed wells show breakout zones at a depth range between 1.4 and 4.6 km, giving precious information on a depth interval usually not investigated by any other data. The results show an Sh(min) N81 +/- 22 degrees and N74 +/- 10 degrees oriented for Varoni 1 and Campotosto 1 wells, respectively. The comparison among the breakouts, the 2009 seismic sequence, the past seismicity and the Quaternary faults indicates a small rotation of Sh(min) orientation from similar to NE, in the southern, to similar to ENE in the northern sector of the study area, where the wells are located. These differences are linked both to the natural variations of data and to the orientation of the main tectonic structures varying from NW-SE in the Abruzzi region to similar to N-S moving toward the Umbro-Marchean Apennines. The identification of constant Sh(min) orientations with depth derived from all the examined active stress data, confirms the breakouts as reliable stress indicators also for aseismic areas.