New data on the late quaternary tectonics in the upper Sangro river valley (central Apennine, Italy): Seismotectonic implications

Abstract

The Quaternary activity in the upper Sangro valley (central Italy) is indicated by the displacement of Early Pleistocene continental deposits (Pescasseroli Formation). This activity is related to a fault system made of NW-SE left-lateral or oblique (with normal component) fault-branches. The most striking evidence of recent activity is, however, related to the E-W to N120 portion of the Colli Bassi fault (characterised by normal movements), which has been responsible for the formation of an impressive scarp (up to 120 m high) in the Early Pleistocene gravels. The lack of outcrops of more recent deposits along the faults of the Upper Sangro system did not permit the recognition of the Late Quaternary activity. Recently, during a specific study performed along the Colli Bassi fault, the displacements which affect a succession of debris and colluvial deposits, more recent than about 27,000 years B.P., have been analysed in the Pescasseroli village. Another outcrop along the same fault showed the deformation affecting a Late Pleistocene colluvial deposit. The deformation is due to minor shear planes related to the Colli Bassi fault. Another evidence of the Late-Quaternary activity along this fault is represented by the sharp and linear contact between the Miocene flysch and the recent alluvium in the Sangro valley bottom (to which the Colli Bassi fault is almost perpendicular). This contact demonstrates the activity of the Colli Bassi fault also after the most recent alluvial episodes of the Sangro river, with the relative uplift of the valley sector north of the fault and the lowering of the Pescasseroli basin. On the basis of the available data it is possible to hypothesise the activity of the Upper Sangro fault system after the last glacial maximum. Considering the length of this system (about 20 km), and the evidence of surficial displacements, it may be responsible for earthquakes of M=6.5. The upper Sangro valley was struck by the May 1984 earthquake sequence (main shock Ms=5.8). These events, however, do not seem related to the re-activation of the Upper Sangro fault system between Villetta Barrea and the Fucino Plain. Moreover, other earthquakes which affected the upper Sangro valley during the past centuries originated in adjacent areas (Fucino Plain, Liri valley). Therefore, large historical earthquakes which may be related to the studied fault system are unknown. Considering the length of the time-interval covered by the Italian seismic catalogues, the upper Sangro valley may represent a seismic gap in the Apennine chain.