Contemporaneous extension and compression in the Northern Apennines from earthquake fault-plane solutions

The Northern Apenninic are belongs to the deformation zones surrounding the Adriatic plate, which behaves as a rigid block. it is a NE-verging fold-and-thrust belt, which developed since the Miocene with the migration of an extensional-compressional pair. Previous seismological data are roughly in agreement with this deformation picture, although the outer compressional front was not clearly defined by the few available solutions, In this work we confirm the existence of two adjacent zones of contemporaneous extension and compression in the Northern Apennines, defining the extent of these two zones and their internal deformation better than was previously done. This is accomplished through the analysis of 125 new focal mechanisms of earthquakes (2.6 <Md<4.8) recorded by the national network of the Istituto Nazionale di Geofisica in the period 1988-1995. The two deformation zones are clearly separated and lie very close to each other with a partial overlapping in the Emilian-Tuscan apennines. Strike-slip events are scattered in most of the study area. Thrust-faulting earthquakes are located in the internal part of the most recently active thrust front (middle-upper Pleistocene). There is a general trend of the compressional P-axes of thrust-fault and strike-slip solutions to be perpendicular to the Apenninic direction in the external zone, while the tensional axes of the normal-fault and strike-slip solutions in the belt do nor show a uniform orientation. However, clearly evident is the similar to ENE direction of extension in the peri-Tyrrhenian region, consistent with the direction of S-hmin in the Quaternary volcanoes of this region inferred from borehole breakouts and micro-earthquake fault-plane solutions, We determined the orientation of the stress tensor in the two main deformation zones by inversion of the 125 fault-plane solutions determined in this study. We found that the external belt is characterized by NE-SW compression (sigma(1) horizontal, oriented N45 degrees E), while a normal-faulting regime with sigma(3) similar to E-W oriented is characteristic of the Umbria-Tuscany (back-arc) region.