From long-term active tectonic model to seismic coupling: impacts of seismic hazard in the central Apennines

Fault slip rate is a key input for long-term seismic hazard models. However, fault slip behavior can vary significantly, ranging from aseismic creep to sudden rupture events during the seismic cycle. Accurately quantifying how these different slip modes partition deformation and release seismically remains a critical challenge for improving seismic hazard assessments. Thus, a rigorous probabilistic framework is required to explore uncertainties in the active fault model, including fault geometry (e.g., length, dip, seismogenic thickness), and seismotectonic potential (e.g., long-term slip rate and tectonic moment rate). This approach must also account for uncertainties in the regional seismic model, such as a tapered Gutenberg-Richter distribution.

In this context, we focus on the central Apennines, one of the most seismically active and extensively studied regions in Italy, where the largest and most frequent earthquakes occur mainly along the axis of the mountain chain. This setting has favored neotectonic studies, synthesized into a new high-quality active tectonic model (Di Naccio et al., 2025a), while the rich cultural heritage and long historical records of the region support a consistent earthquake catalog spanning several centuries.

Our findings (Di Naccio et al., 2025b) indicate that a non-marginal component of permanent deformation contributes to the long-term tectonic moment rate, with a significant impact on seismic hazard estimates. These results underscore the importance of practitioners utilizing fault-based models to explicitly account for seismic coupling when forecasting long-term seismicity.

Hazard calculations closely align with the official national hazard model, with our most probable coupling scenario reproducing the reference values. Thus, the assumption of full coupling (c≈1) is unrealistic and may critically bias hazard estimates, reinforcing the importance of robust seismic coupling assessments.

Di Naccio, D., Di Lorenzo, C., Falcone, G. , Kastelic, V., Sparacino, F., Del Sole, L., Carafa, M.M.C. (2025a)a. Active tectonic model in the central Apennines. Zenodo https://doi.org/10.5281/zenodo.15970471 (2025);

Di Naccio, D., Di Lorenzo, C., Falcone, G. , Kastelic, V., Sparacino, F., Del Sole, L., Carafa, M.M.C. (2025b). The impact of long-term seismic coupling on fault-based seismic hazard models: insights from the central Apennines (Italy). npj Nat. Hazards 2, 97. https://doi.org/10.1038/s44304-025-00150-y.