Applying the Multiple Inverse Method to the analysis of earthquake focal mechanism data: New insights into the active stress field of Italy and surrounding regions

In order to obtain new insights into the active tectonic setting of the Italian territory and surrounding regions, the Multiple Inverse Method (MIM) has been applied to the analysis of fault plane solutions from more than 700 earthquakes with Mw ≥ 4. The active stress field in the top 40 km of the lithosphere has been defined for four 10 km-thick layers, each including 810 square cells of 1.5° side. The obtained stress field maps point out that most of the upper crustal seismicity of the Western and Central Alps is controlled by a strike-slip regime, which is dominant also in part of the Dinarides, Albanides and Hellenides and in a large sector encompassing eastern Sicily and the Malta area to the eastern Tunisia offshore. On the other hand, the well-known extensional belt occurring in the interior of the Apennines appears to extend well beyond the backbone of Italy, potentially reaching the outer foothills of the northern Marche region, while the adjacent Adria block (extending to the eastern Po Plain and the outer Dinarides) sticks out as a major area characterized by dominant thrust faulting in the upper crust. A similar regime characterizes also a large sector of the western Tyrrhenian Sea, from NE Tunisia through western Sicily and the west coast of Sardinia, to the Provence coast. Besides lateral variations, our analysis also points out a significant vertical heterogeneity of the stress field, the deeper levels (20 to 40 km) investigated in this study being characterized by dominant horizontal maximum compression even in areas of upper crustal extension. The application of the MIM to a large seismological dataset, providing basic information for the compilation of active stress maps, contributes to a better understanding of active tectonic processes and may be used for improving seismotectonic zoning and reservoir management