This paper concerns the space-time evolution of the late Miocene to present contractional and extensional deformations within east-central Italy, with an analysis of the pattern and orientation of the related stress field, an insight into the cover-basement relations, and a discussion of the seismotectonic implications.
The pattern and orientation of the deformation and stress fields have remained unchanged since late Miocene time and have been always characterized by the coexistence and parallelism in contiguous areas in a given time of shortening to the east and elongation to the west. With time, during a multistage deformation history, both the contraction and the extension fields have moved eastward. Basement-involved tectonics with a west-dipping multiple detachment system would have allowed the progressive eastward shift of crustal slices, corresponding at the surface to well-defined tectonic domains, with consequent crustal thinning in the Tuscan zone and crustal thickening beneath the Apennine mountain chain.
By comparing macroseismic epicenters, hypocenters, and focal mechanisms with crustal structure and independently known active faults, it appears evident that the pattern, frequency, and distribution of the crustal seismicity are tectonically controlled and that a contraction-extension pair is active also at the present time. In fact, along the Adriatic coastline, contractional deformation prevails with involvement of Pleistocene layers, whereas beneath the Apennine Range, extensional deformation prevails. Both T-axes in the inner domain and P-axes in the outer domain are oriented southwest-northeast or west-south-west east-northeast, and this coaxiality reflects the situation depicted by the geologic stress field. On this basis, we have produced a seismotectonic zoning of east-central Italy with three distinct domains: Intramountain Seismic Zone (ISZ), Foothills Seismic Zone (FSZ), and Coastal Seismic Zone (CSZ).
A plausible explanation for the genesis of the "active" seismic deformations of east-central Italy is that the locally intense extensional upper-crustal seismicity of the Intramountain Seismic Zone is confined to the hanging wall of a major, west-dipping, active extensional discontinuity, whereas the less frequent contractional seismic activity of the Coastal Seismic Zone is within the hanging wall of an outer major, active, west-dipping contractional discontinuity. An area of overlap between these two zones is represented by the Foothills Seismic Zone, where superposed epicentral patterns from different surfaces of weakness at different depths are the predictable result of the asymmetry of the deformation.