Interseismic deformation and dynamic rupture along the Alto Tiberina normal fault: insights from 2D and 3D numerical models.

The mechanical behavior of low-angle normal faults (dip <30°) is a debated issue. I tackle this issue by considering the Alto Tiberina fault (ATF), a very low angle normal fault dipping toward NE with an average dip of 17° and cutting the upper crust (from 2 to 15 km of depth) in the Northern Apennines. Here, geological data suggest a stable sliding motion (creeping), whereas historical seismic events of medium magnitude can not exclude a stick-slip behavior (earthquakes).
In order to facilitate the understanding of this dilemma, I perform two-and three-dimensional quasi-static mechanical models simulating the interseismic phase of the ATF. I define the effects of locking depth, synthetic and antithetic fault activity and lithology on the velocity gradient measured by continuous GPS stations along the ATF system. The innovation of the proposed models is that a three-dimensional subsurface “topography” of the ATF is available on the basis on seismic reflection profiles. In this way, the along‐strike stress lateral variations due to the geometrical irregularities are considered. Successively, I investigate how these geometrical irregularities can influence the dynamic propagation of a rupture simulated on the ATF plane. The results show that the ATF system is characterized by a transition of prevalent stick-slip deformation mode above approximately 5 km, to a mixed behaviour characterized by creeping and stick-slip from 5 to 15 km of depth.
Finally, if on the one hand the fault roughness produces local stress accumulations during the interseismic phase, on the other hand it reduces the maximum magnitude expected during the dynamic rupture simulations.

contact:luca.formaggia@polimi.it