Interseismic stress build-up and stress rotations between low and high-angle normal faults in the Northern Apennines (Italy): Insight from 2D and 3D numerical simulations

The mechanical behavior of low-angle normal faults is a debated issue. We tackle this issue by considering the Altotiberina fault (ATF), a very low-angle normal fault dipping toward NE with an average dip of 15° and cutting the upper crust (from 0 to 15-18 km of depth) in the Northern Apennines of Italy. Here, geodetic and geological data suggest a velocity strengthening behavior, whereas historical earthquakes of medium magnitude can not exclude a velocity weakening behavior. In order to facilitate the understanding of this dilemma, we perform two-and three-dimensional mechanical models simulating the interseismic phase of the Altotiberina low-angle extensional fault 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 also the along-strike stress field lateral variations due to the geometrical irregularities are considered. The faults are defined by a thick fault core characterized by a variation of the elastic parameters. Finally, the effects of a damage zone 1km thick, associated with the ATF, are also considered. The results of the 2D and 3D models can be summarized as follows: 1) the greatest build-up of the shear stress is localized mainly in the first kilometers of depth and at the ATF hangingwall; 2) a velocity strengthening behavior associated to the ATF could promote a local rotation of the principal stress axes; 3) this rotation is even more complex considering a thick damage zone around the ATF; 4) the rotation of the principal stress axes, also associated with along-strike geometrical irregularities, can generate anomalous accumulations of shear stress and therefore promote a velocity weakening behavior along certain patches of the fault. Those preliminary findings suggest that the Altotiberina low-angle extensional fault system is characterized by a gradual transition to prevalent velocity weakening behavior (above approximately 5 km), to changes to a mixed behavior (velocity weakening and velocity strengthening from 5 to 15 km of depth ), and finally to a prevalent velocity strengthening behavior.