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Geological and seismological observations have been used to parameterize 2D numerical models to simulate the interseismic deformation of a complex extensional fault system located in the Northern Apennines (Italy). The geological system is dominated by the presence of the Altotiberina fault (ATF), a large (60 km along strike) low-angle normal fault 20° dipping in the brittle crust (0-15 km). The ATF is currently interested by a high and constant rate of microseismic activity and no moderate-to-large magnitude earthquakes have been associated to it for the past 1000 years. Modelling results have been compared with GPS data in order to understand the mechanical behaviour of this fault and a suite of minor syn- and antithetic normal fault segments located in the main fault hanging-wall.

The results of the simulations demonstrate the active role played by the Altotiberina fault in accommodating the on going tectonic extension in this sector of the chain. The GPS velocity profile constructed through the fault system cannot be explained without including the ATF's contribution to deformation, indicating that this fault although misoriented has to be considered tectonically active and with a creeping behaviour below 5 km of depth.

The low angle normal fault also shows a high degree of tectonic coupling with its main antithetic fault (the Gubbio fault) suggesting that creeping along the ATF may control the observed strain localization and the pattern of microseismic activity. [ABSTRACT FROM AUTHOR]