**6. Conclusions**

*Earthquakes - From Tectonics to Buildings*

structures which were facing the thin and dense (low buoyancy) Ionian oceanic lithosphere e.g., [73]. The extrusion of the northern Hellenides (facing the thicker and more buoyant Adriatic continental domain) would have instead encountered much higher resistance. This hypothesis may explain why since about 1947 (when the effects of such strong perturbation might have reached the western Hellenic zone) most seismic activity has occurred in the Aegean zones lying south of the Cephalonia fault system and the North Aegean trough, while minor activity has

Since the activation of that Hellenic thrust zone is supposed to be a necessary condition for the occurrence of Calabrian earthquakes (**Figure 7** and **Table 1**), the above evidence could explain why since 1947 no major events have occurred in Calabria. The same interpretation may help to understand why in the 1850–1908 time interval, characterised by very high seismic activity in the Hellenides sector,

The evidence and arguments described above suggest that the probability of strong earthquakes in Calabria will not undergo a significant increase until the

*Distribution of major earthquakes occurred in two time intervals (***A** *and* **B***) which respectively preceded and followed the presumed arrival in the Aegean area of the effects of the large westward displacement of the Anatolian wedge, triggered by the strong 1939 earthquake (M = 8) in the easternmost north Anatolian fault system [74, 75]. 1) Africa-Adriatic domain 2) oceanic Ionian domain 3) Alpine metamorphic belt 4) orogenic belts. Circles and triangles respectively indicate focal depths lower and greater than 60 km. Seismicity data as in Figure 1.*

instead occurred in the Northern Hellenides (**Figure 9**).

very strong earthquakes occurred in Calabria (**Table 1**).

**28**

**Figure 9.**

It is advanced the hypothesis that the spatio-temporal distribution of major earthquakes in the periAdriatic zones (**Figure 4**) is closely connected with the progressive roughly northward displacement of the Adria plate. This motion is allowed by the seismic activations of the decoupling fault systems located along the lateral boundaries of Adria (Dinarides and Apennines) and the Eastern Southern Alps. This migrating pattern of earthquakes may tentatively be recognised in the period considered (1300–2020), delineating 7 already developed sequences and one partially developed migration. Taking into account the regularities that we tentatively recognise in the first 7 seismic sequences and the main features of the last ongoing one (which has already involved intense seismic crises in the Southern and Central Apennines and in the western boundary of the RMU wedge in the Northern Apennines) we suppose that the boundaries of the northern RMU wedge (Rimini-Ancona thrust, Romagna fault and Alta Valtiberina trough), along with the Emilia Apennines (stressed by the RMU wedge, **Figure 2**) are the zones most prone to the next strong earthquakes in the Apennine belt.

Further insights into the present seismic hazard in two major Italian seismic zones (Southern Apennines and Calabria) are tentatively inferred from the presumed tectonic connection of such regions with other periAdriatic zones. The first tectonic connection (suggested by seismic histories of about two centuries, **Figure 6**) provides that a strong earthquake (M **≥** 6.0) in the Southern Apennines cannot easily occur if not preceded (within 5 years) by a shock with M **≥** 6.5 or by more than one shock with M **≥** 6.0 in the Southern Dinarides. Even if weaker shocks are taken into account, the correlation remain significant, since almost all Southern Apennines shocks with M **≥** 5.5 (14 out of 15) have been preceded by seismic phases in the Southern Dinarides involving more than one shock (2–5) with M > 5.5.

Assuming that the presumed implications of the above correspondence can be taken as realistic for the next years, one could try to estimate the present seismic hazard in the Southern Apennines. To this purpose, one have to take into account the recent seismic activity in the Southern Dinarides, which only includes an event with M = 6.2 in 2019 (Albania). The fact that the magnitude of such shock was lower than 6.5 would imply a low probability for the occurrence of a Southern Apennine shock with M ≥ 6, while the occurrence of a weaker shock cannot easily be excluded.

The possible tectonic connection between Calabrian and Hellenic earthquakes (**Figure 7** and **Table 1**) is suggested by the seismic histories of these two zones for the period 1600–1947. However, in the subsequent time, this correspondence cannot be recognised, mainly due to the fact that no more earthquakes with M ≥ 5.5 have occurred in Calabria. We suggest that such quiescence is an effect of the considerable westward displacement that the whole Anatolian wedge has undergone due to the activation of the full NAF fault system. That event has caused a noticeable increase of E-W compression in the Ionian and Calabrian zones, so enhancing the

### *Earthquakes - From Tectonics to Buildings*

resistance against the outward escape of the Calabria wedge. Since this process is the main genetic mechanism of seismicity in Calabria, the above effect can help to explain the recent seismic quiescence in that zone (**Table 1**). If this interpretation is realistic, one could suppose that in the present context seismic hazard in Calabria is not high, since in recent times no major earthquakes have occurred in the Hellenides sector lying north of the Cephalonia zone, i.e. the area that generated the main precursors of the strongest Calabria earthquakes. Recent seismicity only affected the Cephalonia fault system (2014, M = 6.1, 6.1 and 2015, M = 6.5), that in the ongoing stress regime have already involved other major shocks with no effects in Calabria.

The above considerations about the present seismic hazard in the Southern Apennines and Calabria reinforce our conviction that the Northern Apennine zones cited above should be taken as priority zones in a prevention plan in Italy.
