# The 2022 Seismic Sequence in the Northern Adriatic Sea and Its Long-Term Simulation
## 4 Simulation of the Seismicity in the Adriatic Thrust Fault Systems
Smaller inter-event times are pertinent to clustering (with particular regard to small magnitude earthquakes), while the maximum at 1500-1750 yrs denotes quasi-periodicity (typical of larger magnitude earthquakes). As expected, the number of events decreases and the average inter-event time \(T_{r}\) increases when the magnitude threshold is increased (Table 2). However, the standard deviation \(\sigma\) maintains a value close to 700 yrs. In this way, the coefficient of variation (\(C_{\rm p}\)), which is defined by the ratio between \(\sigma\) and \(T_{r}\), decreases from 0.90 for \(M\geq 5.5\) to 0.40 for \(M\geq 6.5\). It is well known that \(C_{\rm p}\) is equal to 1.0 for a sequence of events following a memoryless Poisson distribution and is equal to 0 for a perfectly periodical sequence. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Magnitude** & **Number of** & **\(T_{r}\)(yrs)** & \(\sigma\)**(yrs)** & \(C_{\rm p}\) \\
**Threshold** & **Events** & **\(T_{r}\)(yrs)** & \(\sigma\)**(yrs)** & \(C_{\rm p}\) \\ \hline
5.5 & 121 & 814 & 733 & 0.90 \\
6.0 & 84 & 1170 & 797 & 0.68 \\
6.5 & 57 & 1713 & 690 & 0.40 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Results of the statistical analysis of the simulated 100,000 yrs earthquake catalogue for the DISS composite source labelled ITCS106 for three different magnitudes thresholds. \(T_{r}\) = recurrence time, \(\sigma\) = standard deviation, \(C_{\rm p}\) = coefficient of variation. Figure 3: Density distributions of the number of inter-event times in bins of 250 yrs. The first bin corresponds to the range of 0–250 yrs and so on. Thus, the earthquake sequence of our simulated catalogue behaves more and more as a quasi-periodical process when, increasing the threshold magnitude of the analysis, the ruptured areas of the considered earthquakes approach the size of whole seismic structure. Shortest inter-event times (smaller than 1000 yrs) are typical of small magnitude earthquakes (Table 2, and Figure 3), while the opposite situation happens for the inter-event times larger than 2000 years (Figure 3). An exceptionally long inter-event time of 4625 yrs is estimated between two earthquakes of magnitude larger than 6.5 (Figure 3). The simulations analysed in this study were obtained adopting the largest slip rates reported for the earthquake source in the DISS database. If we adopted smaller slip rates, the time scale of our inter-event times would become inversely longer. The synthetic 100,000-year catalogue highlights a spatiotemporal behaviour that can hardly be compared with analog features observed in reality, due to their brevity, but can be considered realistic in light of an earthquake cycle hypothesis. Long-term patterns preceding and following a strong earthquake are shown in Figure 4. This figure is obtained by stacking the number of \(M\geq 4.2\) earthquakes before and after an \(M\geq 5.2\) earthquake within a distance of 20 km between their respective epicenters in bins of 5 years. Here, we can observe an increase in seismic activity starting a couple of centuries before a strong earthquake, and a sharp increase in occurrence rate in the first 5 years after that event. The latter feature is clearly related to aftershock sequences following strong events. After this aftershock phase, the plot shows a long-lasting quiescence with some modest fluctuations. This long-term pattern highlights the existence of a preparatory phase before every strong earthquake. Presumably, during this preparatory phase, stress accumulation causes an increase in the rate of moderate-magnitude events. This hypothesis is supported by a detailed study of the time history of the average stress and its standard deviation during several seismic cycles in the Corinth (Greece) and Nankai (Japan) fault systems ([PERSON] and [PERSON] [24], [PERSON] et al. [25]).