Seismotectonics of the Nubia plate compressive margin in the south-Tyrrhenian region, Italy: clues for subduction inception.

Seismological and structural data are used to constrain the active tectonics of the Nubia plate compressive margin in southern Italy. In this region, compressional displacements have resumed at the rear of the Maghrebian orogenic wedge since about 700-500 ka and are presently accommodated along a seismic belt in the south Tyrrhenian area, where a passive margin has developed during late Neogene-Quaternary times. Earthquake data from the south Tyrrhenian belt are analyzed with Bayloc, a probabilistic nonlinear location method. Results show that the seismic belt is segmented and involves a series of NE-SW and NW-SE elongated clusters. The NE-SW clusters are interpreted as high-angle reverse fault zones verging toward the southeast, whereas the perpendicular clusters are interpreted as possible strike-slip fault zones transferring the contractional displacements to advanced segments of the belt in the southeast. Brittle deformations observed in the Quaternary volcanic island of Ustica are consistent with the geometry and kinematics of the seismic belt deduced from the seismological data. The south Tyrrhenian active belt may constitute an early stage of subduction of the Tyrrhenian oceanic crust beneath Sicily. A similar scenario has been hypothesized also for the westward prolongation of the south Tyrrhenian belt off the Algerian coast. By integrating instrumental and historical seismic data with tectonic evidences, we infer that the south Tyrrhenian belt is probably capable of producing earthquakes with a maximum size close to magnitude 7. The analyzed data suggest a multifault system behavior for the studied compressional belt. This inference is important to estimate and mitigate the seismic and tsunamic hazards in such a densely populated region.