Faults like to hide: subsurface evidence of poorly known and possibly active normal faults at the border between central and southern Apennines (Italy)

The Italian Apennines are among the Mediterranean areas with the highest seismic hazard. Geodetic data show that the belt is experiencing slow deformation rates (3-4 mm/yr, D’Agostino, 2014; Carafa et al., 2020), with a prevalent SW-NE extension. Tectonic activity is expressed by well-exposed normal-fault planes dissecting the carbonate ridges. To infer the activity of these faults, several investigations using morphotectonic, paleoseismological, geophysical, and field survey techniques have been applied, leading to a robust literature in which the active structures are characterized and parametrized. Also, in recent times strong earthquakes with extensional kinematics struck the belt (e.g., 2009 L’Aquila, Mw 6.1; 2016 Norcia, Mw 6.5).

This work focuses on the sector between the central and southern Apennines, the Abruzzo-Molise region boundary (AMB), bordered to NW and SE by well-known active normal fault systems with opposite dip (SW-dipping and NE-dipping, respectively). AMB is characterized by a seismic gap and a complex lithological arrangement composed of prevalent flysch-like and clayey-marls outcrops, whose thickness reaches 2.5 km, which hamper the recognition of active faults at the surface. In a recent study, taking advantage of morphotectonic and remote sensing analysis, Battistelli et al. (2025) highlighted the presence of an organized strip of slope instabilities that could represent the surface expression of unknown normal faults, possibly active from the Late Quaternary to present. The structures align with the fault systems outcropping at the AMB border and define a 10 km wide corridor marked by subtle evidence of recent tectonic activity, such as linear scarps and crest offsets (Castel di Sangro-Rionero Sannitico corridor, CaS-RS).

With this contribution, we made a step forward to constrain the aforementioned lineaments also in the subsurface by interpreting two commercial seismic reflection profiles (that cross-cut the CaS-RS) calibrated by two deep well (ViDEPI Project). Three geological cross sections were also drawn to cross-check the subsurface with the available geological and structural maps.

Seismic line interpretation and time-to-depth conversion pointed out normal faults that align well with the lineaments highlighted by Battistelli et al. (2025), and thus also the presence of minor extensional structures that do not seem to directly affect the topography. The estimated fault offsets range between 100 and 400 m, and increase moving from NW to SE. Tentatively assuming an age of 120-750 kyr for these offsets, the resulting fault slip rates range from 0.1 to 0.9 mm/yr.

In this peculiar geo-lithological context, we propose that faulting can be strongly influenced by the mechanical stratigraphy, producing, at the shallower structural levels, a wide area marked out by diffuse and partly off-fault deformation (sensu Ferrill et al., 2017). A complementary interpretation envisages the possibility that the CaS-RS corridor could represent a linkage zone, between fault systems with opposite dip, whose evolutionary stage has not yet led to well-developed normal fault structures and related basins.

 

Battistelli et al., 2025. https://doi.org/10.3390/rs17142491

Carafa et al., 2020. https://doi.org/10.1029/2019JB018956

D'Agostino, 2014. https://doi.org/10.1002/2014GL059230  

Ferrill et al., 2017. https://doi.org/10.1016/j.jsg.2016.11.010