**5. Present seismic hazard in Calabria**

*Earthquakes - From Tectonics to Buildings*

*lithosphere before a seismic slip in the Southern Dinarides.*

this figure, one can note that in the period considered all the shocks with M ≥ 6.0 in the Southern Apennines have been preceded within few years (less than 5) by one or more earthquakes with M ≥ 6 in the Southern Dinarides. The above correspondence does not worsen significantly even if a lower threshold (M = 5.5) is considered, given that only one of the 15 Southern Apennine events failed to be preceded by comparable events in the Southern Dinarides. The above evidence may indicate that a fault in the Southern Apennines cannot easily activate without the contribution of a post-seismic perturbation triggered by one or more major shocks in the Southern Dinarides. Since the probability that such a correspondence merely occurs by chance is very small [57, 58], it is plausible to suppose that a close tectonic connection exists between the two zones (**Figure 6C**). The occurrence of a major seismic slip at a thrust fault beneath the Southern Dinarides, such as the one that developed with the 1979 Montenegro event (estimated to be 1–2 metres, e.g. [63]), implies a comparable displacement of the adjacent Adria domain, which causes a reduction of vertical flexure in the southern Adriatic domain, as sketched in the section of **Figure 6**. Such process is expected to induce extensional strain in the Southern Apennines, which may favour the activation of the belt-parallel normal faults recognised in that zone, as for instance the one that generated the 1980 strong earthquake in the Irpinia zone e.g., [35, 64]. This hypothesis is confirmed by the results of numerical modelling of the strain perturbation that was presumably induced in the Irpinia zone by the 1979 Montenegro event [57–60]. Moreover, the strain rate induced by the Montenegro earthquake is expected to reach its maximum amplitude in the Southern Apennines about 1–2 years after the triggering event, a delay fairly consistent with the time interval that elapsed

*A) Geometry of the zones implied in the presumed interrelation between Southern Dinarides-Albanides and Southern Apennines and location of the earthquakes given in the table. The stars indicate the locations of the 1979 and 1980 earthquakes in Montenegro and Irpinia. B) List of the major seismic events occurred since 1810. The events with M* ≥ *6.0 are in red. Seismicity data as in Figure 1. C) Structural sketch, through a transversal section in the southern Adriatic area (S-S′), evidencing the vertical flexure of the Adriatic lithosphere overthrusted by the Dinaric belt, on one side, and plunged under the Apennine belt, on the other side (e.g., [62]). The vertical scale is exaggerated in order to make more evident the possible effect of a seismic slip (red arrow) along the subduction fault beneath the Dinaric belt. The dashed lines indicate the presumed profile of the Adriatic*

**24**

**Figure 6.**

The analysis of the seismic histories of Calabria and the Hellenides sector lying between the Ionian islands and Albania, along with the geodynamic context in the central Mediterranean area, suggests a possible connection between these two zones [58, 59, 61]. This interpretation is consistent with the structural/tectonic setting sketched in the section of **Figure 7**, which implies that a seismic slip at the Hellenic thrust zone reduces the upward vertical flexure of the Adriatic lithosphere, so attenuating the resistance that the Calabrian wedge encounters in overthrusting such lithosphere. Since, this last process underlies the main genetic mechanism of Calabrian shocks, one can realise why an earthquake in the Hellenides may cause an increase of seismic hazard in Calabria.

The above interpretation and its implications on the interaction of the Calabrian and Hellenic seismic sources is consistent with the quantification of the effects of post-seismic relaxation induced by strong earthquakes in the Hellenides [58, 59, 61], which provides insights into the most probable delay between the presumed precursor and the induced event.

The possibility that the above phenomenon was systematic is supported by the comparison of the seismic histories of the two zones involved (**Table 1**), which indicates that all Calabrian seismic crises with M ≥ 6.0 have been preceded, within

#### **Figure 7.**

*Geometry of the presumably interrelated Calabrian and Hellenic seismic zones and trace of the section (S-S*′*) are shown in the map. Red circles indicate the epicentres of the earthquakes that have occurred in the two zones since 1600 a. D (Table 1). The section illustrates a tentative reconstruction (vertically exaggerated) of the reduction of vertical flexure of the Adriatic plate (dashed line) that may occur in response to a strong decoupling earthquake in the Hellenic thrust zone. This effect may favour the outward escape of the uplifted Calabrian wedge towards the Ionian domain. Seismicity data as in Figure 1.*

#### **Table 1.**

*List of major Hellenic and Calabrian events, occurred since 1600 a.D. in the zones depicted in Figure 7 (the shocks with M* ≥ *6.0 in Calabria and M* ≥ *6.5 in Hellenides are in bold). Seismicity data as in Figure 1.*

10 years, by at least one event with M ≥ 6.5 in the Hellenides. Even if lower magnitudes (M ≥ 5.5) are considered, the correspondence remains fairly significant, since only 3 (out of 29) Calabrian events have not been preceded by equivalent shocks in the Hellenides. The above evidence could imply that a major earthquake can hardly occur in Calabria without being preceded by significant seismic activity in the Hellenides [58, 59].

**27**

**Figure 8.**

*Tectonics and Seismicity in the periAdriatic Zones: Implications for Seismic Hazard in Italy*

On the other hand, considering the opposite aspect of the presumed interrelation, one can note that only 11, out of 22, Hellenic seismic crises with M **≥** 6.5 were followed by a Calabrian earthquake with M **≥** 6.0. This indicates that the role of the Hellenic events as precursors of Calabrian shocks is affected by significant uncertainty. This problem mainly concerns the most recent time, given that since 1948 no Hellenic events with M **≥** 6.5 have been followed by an event in Calabria with M ≥ 5.5 (**Table 1**). Such long quiescence (73 years) is rather anomalous with respect to the previous behaviour, in particular with the fact that from 1626 to 1947 the average inter-event time between M ≥ 5.5 Calabrian shocks was about 16 years and was never longer than 41 years.

In order to find a possible explanation of the present long quiescence and of the fact that since the middle of the XX century the correspondence between Hellenic and Calabrian earthquakes has undergone a considerable worsening, we advance the hypothesis that such anomalous behaviour is an effect of the considerable increase of E-W compressional stress that developed in the Hellenic and Ionian zones in response to the large westward displacement of the Anatolian-Aegean system since 1939, when a very strong earthquake in Eastern Anatolia (M = 8) triggered the progressive activation of the entire North Anatolian fault

The peculiarity of the above seismic sequence in the NAF is the fact that it also

involved the activation of the central NAF, which had been almost silent for a long time e.g., [72]. This rare event favoured a significant westward displacement (some metres) of the whole Anatolian wedge, causing a considerable increase of E-W compression in the zones stressed by the convergence of this block with the Africa-Adriatic domain (**Figure 8**). The least action principle suggests that the fast shortening required by such dynamics was mainly accommodated by the outward extrusion of the Peloponnesus and the central Aegean zones, i.e. the orogenic

*Proposed plate/microplate configuration and kinematic pattern in the Central Mediterranean and Aegean-Anatolian region [14]. White arrows indicate the presumed velocity field with respect to Eurasia. Land and seafloor morphological features from Le Pichon and Biju-Duval [70]. Thick red lines delimitate for reference the inner part of the Alpine metamorphic belt. Al = Albanides; CA, NA, SA = Central, Northern and Southern Apennines, Cal = Calabrian Arc, Ce = Cephalonia fault system, ESA = Eastern Southern Alps, Ma = Marmara, NAF = North Anatolian fault system, ND = Northern Dinarides, NH = Northern Hellenides,*

*Pe = Peloponnesus, SD = Southern Dinarides, Si = Sicily. Symbols as in Figure 2.*

*DOI: http://dx.doi.org/10.5772/intechopen.94924*

system (NAF in **Figure 8**, e.g., [71]).

### *Tectonics and Seismicity in the periAdriatic Zones: Implications for Seismic Hazard in Italy DOI: http://dx.doi.org/10.5772/intechopen.94924*

On the other hand, considering the opposite aspect of the presumed interrelation, one can note that only 11, out of 22, Hellenic seismic crises with M **≥** 6.5 were followed by a Calabrian earthquake with M **≥** 6.0. This indicates that the role of the Hellenic events as precursors of Calabrian shocks is affected by significant uncertainty. This problem mainly concerns the most recent time, given that since 1948 no Hellenic events with M **≥** 6.5 have been followed by an event in Calabria with M ≥ 5.5 (**Table 1**). Such long quiescence (73 years) is rather anomalous with respect to the previous behaviour, in particular with the fact that from 1626 to 1947 the average inter-event time between M ≥ 5.5 Calabrian shocks was about 16 years and was never longer than 41 years.

In order to find a possible explanation of the present long quiescence and of the fact that since the middle of the XX century the correspondence between Hellenic and Calabrian earthquakes has undergone a considerable worsening, we advance the hypothesis that such anomalous behaviour is an effect of the considerable increase of E-W compressional stress that developed in the Hellenic and Ionian zones in response to the large westward displacement of the Anatolian-Aegean system since 1939, when a very strong earthquake in Eastern Anatolia (M = 8) triggered the progressive activation of the entire North Anatolian fault system (NAF in **Figure 8**, e.g., [71]).

The peculiarity of the above seismic sequence in the NAF is the fact that it also involved the activation of the central NAF, which had been almost silent for a long time e.g., [72]. This rare event favoured a significant westward displacement (some metres) of the whole Anatolian wedge, causing a considerable increase of E-W compression in the zones stressed by the convergence of this block with the Africa-Adriatic domain (**Figure 8**). The least action principle suggests that the fast shortening required by such dynamics was mainly accommodated by the outward extrusion of the Peloponnesus and the central Aegean zones, i.e. the orogenic

#### **Figure 8.**

*Earthquakes - From Tectonics to Buildings*

1601 (6.3)

1612 (6.3), 1613 (6.3)

1666 (6.2), 1674 (6.3)

1709 (6.2), 1714 (6.3)

**1769 (6.8)**, 1772 (6.1)

1872 (6.0)

1930 (6.2)

**1948 (6.5, 6.5) 1953** (6.0, **6.6, 7.0**, 6.2) **1983** (**6.7**, 6.0) 2003 (6.2)

2014 (6.1, 6.1), **2015 (6.5)**

1722 (6.3), 1723 (6.1, 6.3), **1732 (6.6)**

1809 (6.1), 1815 (6.3), **1820 (6.6)**

1914 (6.0), 1915 (6.1, 6.3, 6.0),

1854 (6.0), 1858 (6.0, 6.2, 6.4), 1859 (6.0,6.2) 1860 (6.4), 1862 (6.4, 6.2), 1865 (6.3), **1866 (6.6**, 6.2, 6.1. 6.4), **1867 (7.2), 1869** (6.0, **6.7**)

**Hellenides (M** ≥ **6.0) Calabria (M** ≥ **5.5)**

**1625 (6.5) 1626 (6.1) 1630 (6.5), 1636 (7.2) 1638 (6.8, 7.1)** 1638 (6.3) 1640 (5.8) 1650 (6.2), **1658 (6.7) 1659 (6.6)**

**1701 (6.6)**, 1704 (6.4) 1708 (5.6)

1759 (6.3), **1766 (6.6), 1767 (6.7)** 1767 (5.9)

**1773 (6.5) 1783 (7.1, 6.7, 7.0) 1783 (6.5, 6.6), 1786 (6.5) 1791 (6.1)**

1823 (6.3), **1825 (6.7) 1832 (6.7) 1833 (6.5)** 1835 (5.9), **1836 (6.2) 1851 (6.8) 1854 (6.3)**

1885 (6.0) 1886 (5.6), 1887 (5.6) **1893 (6.6) 1894 (6.1)**

**1920** (6.0, **6.5**) 1928 (5.9)

**1895** (6.2, **6.5**, 6.2, 6.2), **1897 (6.6)**, 1912 (6.1) **1905 (7.0), 1907 (6.0), 1908 (7.1)**, 1909 (5.5), 1913 (5.6)

**1870 (6.2)**

1947 (5.7)

1736 (6.0), 1741 (6.3), **1743 (6.9)** 1743 (5.9), 1744 (5.7), 1749 (5.8)

**26**

**Table 1.**

Hellenides [58, 59].

10 years, by at least one event with M ≥ 6.5 in the Hellenides. Even if lower magnitudes (M ≥ 5.5) are considered, the correspondence remains fairly significant, since only 3 (out of 29) Calabrian events have not been preceded by equivalent shocks in the Hellenides. The above evidence could imply that a major earthquake can hardly occur in Calabria without being preceded by significant seismic activity in the

*List of major Hellenic and Calabrian events, occurred since 1600 a.D. in the zones depicted in Figure 7 (the shocks with M* ≥ *6.0 in Calabria and M* ≥ *6.5 in Hellenides are in bold). Seismicity data as in Figure 1.*

*Proposed plate/microplate configuration and kinematic pattern in the Central Mediterranean and Aegean-Anatolian region [14]. White arrows indicate the presumed velocity field with respect to Eurasia. Land and seafloor morphological features from Le Pichon and Biju-Duval [70]. Thick red lines delimitate for reference the inner part of the Alpine metamorphic belt. Al = Albanides; CA, NA, SA = Central, Northern and Southern Apennines, Cal = Calabrian Arc, Ce = Cephalonia fault system, ESA = Eastern Southern Alps, Ma = Marmara, NAF = North Anatolian fault system, ND = Northern Dinarides, NH = Northern Hellenides, Pe = Peloponnesus, SD = Southern Dinarides, Si = Sicily. Symbols as in Figure 2.*

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

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

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

#### **Figure 9.**

*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.*

**29**

*Tectonics and Seismicity in the periAdriatic Zones: Implications for Seismic Hazard in Italy*

occurrence of major shocks in the Hellenides thrust zone. The fact that three earthquakes with M > 6 recently occurred in the Cephalonia zone (2014 and 2015) cannot easily be taken as a possible precursor of Calabrian shocks, since in the tectonic context created by the Anatolian westward displacement other very strong events (1953, M = 7.0, 6.6; 1983 M = 6.7) affected the Cephalonia fault without

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

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)

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

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

*DOI: http://dx.doi.org/10.5772/intechopen.94924*

inducing significant seismic activity in Calabria.

**6. Conclusions**

Apennine belt.

with M > 5.5.

be excluded.

*Tectonics and Seismicity in the periAdriatic Zones: Implications for Seismic Hazard in Italy DOI: http://dx.doi.org/10.5772/intechopen.94924*

occurrence of major shocks in the Hellenides thrust zone. The fact that three earthquakes with M > 6 recently occurred in the Cephalonia zone (2014 and 2015) cannot easily be taken as a possible precursor of Calabrian shocks, since in the tectonic context created by the Anatolian westward displacement other very strong events (1953, M = 7.0, 6.6; 1983 M = 6.7) affected the Cephalonia fault without inducing significant seismic activity in Calabria.