Contemporary stress state in Italy: updated map

Abstract

Borehole breakouts are considered one of the most reliable indicators of the present-day stress field in the shallow crust, providing information about the orientation of the main axes of the stress tensor. Here we present an updated stress map of Italy in terms of horizontal stress orientations in particular focusing on three areas where recently new data from oil wells have been available: Abruzzi region, central-southern Apennines and Sicily. We have performed borehole breakout analysis in 57 new wells to increase the Italian database of present-day stress and have inferred 41 new reliable horizontal stress orientations analyzing SHDT logs up to a maximum depth of 5541 m. In order to resolve also the stress regime we have considered the focal mechanisms of earthquakes computed for these zones. In the Abruzzi region (central Italy) we have analyzed in detail two deep boreholes close to the April 6, 2009 earthquake (Mw=6.3) which destroyed the old town of L’Aquila and caused the death of more than 300 people. The focal mechanisms of the main events show NW-normal faulting, consistent with the NE-SW trending extensional regime of the central Apennines. We have compared stress orientations from borehole breakout data with those deduced from earthquake focal mechanisms and active faults in the surroundings 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, in the NW sector the stress indicators reveal a rotation linked to the changing trend of the main structures of Apennines. In the wide area belonging to the central-southern Apennines, new horizontal stress orientations confirm the well-known NE-extension along the belt and the foredeep, although evidence some local variations. The latter are probably due to the different tectonic regime acting in the foredeep, but also to the influence of active faults differently oriented with respect to the main trend. Stress data along the Tyrrhenian coast are relevant as very few data existed before. Shmin orientations in this area are quite variable and interpreted as due to an extensional tectonic regime with a sub-vertical σ1 and without a prevailing horizontal stress component. Breakout results from southeastern Sicily (Hyblean foreland) and offshore are in agreement with the Africa-Europe convergence ~NNW-SSE oriented. Whereas, in central Sicily the presence of a foredeep running with different orientations, from ~EW to ~NS, produced a complex tectonic setting originating local stress sources. Unfortunately, few earthquake data do not properly define the stress state of this part of Sicily. In fact most of the seismicity is located along a narrow band in the Tyrrhenian offshore, north of Sicily, and around the Etna volcano area. The new data included in the Italian stress map have increased the previous present-day stress compilation of about 20%.