Seismic sequences in the Italian Apennines influenced by fault network geometry

Stress interactions between neighbouring faults plays a key role in controlling earthquake recurrence and size, and therefore in the seismic hazard posed by individual faults within a fault network. In this study, we investigate how differences in the predominant arrangement of faults, specifically, whether it is along-strike or across-strike, affect earthquake recurrence rates and magnitude of earthquakes. To address this topic, we use the boundary-element code QDYN to simulate earthquake cycles of two fault systems within the actively extending region of the Italian Apennines: one to the south where faults are predominantly arranged along-strike, and another in the central Apennines where faults are predominantly arranged across-strike.  The different styles of fault network between the Central and Southern Apennines, and high seismic hazard of the region, make this the ideal area to investigate the role of fault geometry on earthquake behaviour across multiple seismic cycles in this region.

The models account for variable fault slip rates between faults and network geometry to determine their impact on seismic cycles and earthquake statistics. These simulations produce spontaneous ruptures, with slip modes encompassing full and partial ruptures as well as slow-slip events. We found a good fit between the modelled magnitudes and the ones derived from historical ruptures and empirical relationships. Fault networks with multiple across-strike faults produce more complex seismic sequences, including greater variability in recurrence times and higher proportion of partial ruptures, compared to fault networks with faults arranged predominantly along-strike. Lastly, we assessed the seismic hazard in the studied regions based on the modelled earthquake rates and magnitudes. Our findings show that the spatial distribution of peak ground acceleration corresponding to a 50-year exceedance probability has a greater heterogeneity compared to classical seismic hazard assessment approaches. Hazard levels are elevated in areas where multiple faults overlap, highlighting the influence of fault interactions on regional hazard patterns. These findings show the influence of fault system geometry on how stresses redistribute across multiple earthquake cycles and associated seismic hazard.