Seismic vs aseismic deformation in the Northern Apennines constrained from dense GNSS velocities

The Upper Tiber Valley, in the Northern Apennines of Italy, is a key natural laboratory for investigating how continental extension is partitioned between seismic and aseismic deformation. Extension in this sector of the Apennines has been primarily accommodated by the Altotiberina Fault (ATF), a low-angle (~15°) normal fault that is mechanically unfavorable for elastic shear failure, and by a network of high-angle synthetic and antithetic faults in its hanging wall. While the ATF is characterized by persistent background micro-seismicity, the high-angle faults host larger historical earthquakes and frequent seismic swarms, likely induced by fluid circulation and elevated pore pressure. Since the study of Anderlini et al. (2016), the local GNSS network has been significantly densified within the framework of the Alto Tiberina Near Fault Observatory (TABOO-NFO). The updated dataset now better resolves a sharp ~3 mm/yr chain-normal interseismic velocity gradient across the Upper Tiber Valley, providing unprecedented constraints on how ongoing extension is distributed across the fault system. We use the new GNSS velocity field to reassess the relative contribution of low-angle versus high-angle faults to crustal deformation and to quantify the partitioning between seismic and aseismic slip. We apply a block-modeling approach that jointly estimates rigid block rotations and spatially variable interseismic coupling through a newly developed iterative inversion strategy. The model includes 3D geometries, discretized in triangular dislocation elements, of both the ATF and its antithetic structures, permitting assessment of distributed slip rates across the fault system. Preliminary results show that shallow locking on high-angle syn- and antithetic faults plays a first-order role in explaining the observed velocity gradient, whereas the ATF accommodates a significant fraction of extension through aseismic creep. These findings refine earlier interpretations and provide new insight into how low-angle normal faults can interact with steeper faults during the earthquake cycle.