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Abstract. Geological and seismological observations have been used to parameterize 2-D numerical elastic 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 Alto Tiberina fault (ATF), a large (60 km along strike) lowangle normal fault dipping 20 • in the brittle crust (0-15 km). The ATF is currently characterized by a high and constant rate of microseismic activity, and no moderate-to-large magnitude earthquakes have been associated with this fault in 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 Alto Tiberina fault in accommodating the ongoing 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 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.

[1] Two-and three-dimensional mechanical models simulating the seismic cycle along thrust faults are devoted to the active front of the Northern Apennines buried under the Plio-Pleistocene sediments of the eastern Po Plain. The models aim at understanding two peculiar characters of the stress field in the region: (1) analyses of borehole breakouts and other well data, integrated with seismological information and field evidence, show along-depth variations (from extensional to strike slip to compressional for increasing depth) of the stress style along the active Mirandola anticline and (2) seismic and borehole breakout data suggest strong lateral rotations of the stress axes along the frontal and lateral ramp portions of the Mirandola thrust fault. Modeling results allowed us to understand that the mechanical decoupling along faults strongly affects the stress distribution in their vicinity, under the form of strong rotations of the principal stress axes. As a consequence the slip (either fast or slow) along active blind thrust faults could drive to local stress fields very different from that associated with regional tectonics. This could explain, together with other processes (such as differential compaction), the along-depth variation of the stress field observed in the Mirandola region and the strong lateral rotations of the principal stress axes observed along the lateral ramp of the Mirandola thrust fault. Finally, the seismic activity along one of the thrust fault ramps along the Apennines front should not interfere with the state of stress along adjacent ramps.Citation: Carminati, E., and L. Vadacca (2010), Two-and three-dimensional numerical simulations of the stress field at the thrust front of the Northern Apennines, Italy,

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