The 2022 Seismic Sequence in the Northern Adriatic Sea and Its Long-Term Simulation

We studied the long-term features of earthquakes caused by a fault system in the northern Adriatic sea that experienced a series of quakes beginning with two main shocks of magnitude 5.5 and 5.2 on 9 November 2022 at 06:07 and 06:08 UTC, respectively. This offshore fault system, identified through seismic reflection profiles, has a low slip rate of 0.2-0.5 mm/yr. As the historical record spanning a millennium does not extend beyond the inter-event time for the largest expected earthquakes (M?6.5), we used an earthquake simulator to generate a 100,000-year catalogue with 121 events of M-w=5.5. The simulation results showed a recurrence time (T-r) increasing from 800 yrs to 1700 yrs as the magnitude threshold increased from 5.5 to 6.5. However, the standard deviation s of inter-event times remained at a stable value of 700 yrs regardless of the magnitude threshold. This means that the coefficient of variation (C-v=s/T-r) decreased from 0.9 to 0.4 as the threshold magnitude increased from 5.5 to 6.5, making earthquakes more predictable over time for larger magnitudes. Our study supports the use of a renewal model for seismic hazard assessment in regions of moderate seismicity, especially when historical catalogues are not available.