Active oblique extension in the central Apennines (Italy): evidence from the Fucino region

The Fucino Basin is a flat Quaternary depression within the central Apennines. It is surrounded by active normal faults with small oblique-slip components. Surface faulting was observed along the east side of the basin during the M-s = 7.0 Avezzano earthquake of 1915. In order to understand the kinematics of recent strain better and to assess the seismic hazard of this region, we carried out a detailed geomorphic and structural study of the faults around the basin. Fault scarp heights were accurately measured with total station profiles, and vertical slip rates were estimated assuming slope offsets to postdate the end of periglacial abrasion (14 +/- 4 kyr BP). To the north, the most prominent fault, the Magnola fault, appears to have an average postglacial throw rate of 0.7 +/- 0.3 mm yr(-1) and, together with the Velino fault, to be capable of generating earthquakes of maximum magnitudes of 6.9-7.3 with recurrence intervals of 1000-3000 yr. East of the basin, the Serrone, Parasano and Ventrino faults form a right-stepping horsetail of the Giovenco fault. Current vertical slip rates on:the three former faults appear to be between 0.5 and 1.4, 0.5 and 1.0, and 0.3 and 1.1 mm yr(-1), respectively. Infrequent maximum-magnitude earthquakes on them may also exceed 7, particularly in the case of coupled rupture, with loosely constrained recurrence intervals (up to several thousand years). The right-lateral slip components implied by the most recent slickensides and by geomorphic offsets on the NW-SE-trending normal faults of the area suggest that the blocks they bound rotate counterclockwise, consistent with oblique left slip on the NNW-SSE-trending Giovenco and Ovindoli faults to the east. Sinistral shear parallel to the latter faults, the maximum relief across the Magnola fault, and the postglacial slope offsets measured suggest that the extension rate across the central Apennines might be of the order of 6 +/- 3 mm yr(-1) in a N20 degrees +/- 10 degrees E direction, more northerly than hitherto inferred.