GNGTS 2024 - Atti del 42° Convegno Nazionale

Session 1.1 GNGTS 2024 corresponding to the deep part of the MVFS (Fig. 2). This suggests that, when the MVFS is actvated, the statc Coulomb stress is increased only in the most superfcial porton of the crust crossed by the NFS, while when the NFS is actvated, it is the statc stress in the deep porton crossed by the MVFS that increases. Considering that the 1) vector slips observed on the feld along the coseismic breaks indicate pure dip-slip kinematcs, while the slickenlines measured on the border fault planes indicate lef oblique kinematcs (Galderisi and Galli, 2020), and that 2) any signifcant seismicity has been detected at depth along the NFS (Improta et al., 2019), it is likely that the 30 October 2016 surface slip of the NFS cannot be atributed to deep deformaton dynamics induced by Apennine tectonic stress release. Thus, excluding shaking-induced phenomena, as diferental compacton or slope movements, both impossible in the splays of the plain, we hypothesize that the sudden increase in Coulomb stress induced by the actvaton of MVFS have triggered a slight passive slip along the pre-existng fault planes of the NFS, which accommodated also the abrupt, 12 cm uplif of the Norcia block (GSI, 2016; Bignami et al., 2019). Probably, due to the lack of sufcient lithostatc weight, fricton was low and slip occurred seismically. Considering the long paleoseismic history of the two fault systems (Galli et al., 2018; 2019; 2023), it is possible that the MVFS and the NFS have been interactng for thousands of years, mutually inducing variatons in the Coulomb stress in the hanging wall of the other (Fig. 3). But while the actvaton of the MVFS seems to favour only the formaton of superfcial passive ruptures along the NFS, the actvaton of the NFS, which seems to precede that of the MVFS by 100-500 years, could act as a trigger for the complete rupture of the later. The same analysis methodology was applied for the GSFS and the UAFS. The results obtained from the simulatons for the calculaton of the variaton of the statc Coulomb stress show what was observed in the MVFS and NFS simulatons (S3 and S4 in Fig. 2). This evidence suggests that the GSFS and the UAFS also interact by mutually inducing variatons in the Coulomb stress in each other's hanging walls. The analysis of the afershocks of the 2009 seismic sequence (Valoroso et al, 2013) unequivocally shows that the GSFS recorded seismicity, where the Coulomb stress growth lobe is created (Fig. 2). In fact, according to King et al, (2004) the increase of even just 1 bar would be sufcient to trigger slip on a fault system. In our case, the transfer of Coulomb stress from MVFS to NFS may reach 2–3 bars only in the upper lobe (Fig. 2). What is shown by the arrangement of the afershocks of the 2009 seismic sequence, linked to the evidence of the 2016 coseismic ruptures observed along the NFS, shows that there is indeed an interacton between the parallel fault systems of the MVFS-NFS and the GSFS-UAFS, with possible Coulomb stress transfer.