**4. Present seismic hazard in the Southern Apennines: further evidence from a seismotectonic correlation**

Further information on the present seismic hazard in the Southern Apennines could be inferred from a correlation that has been recognised between the major earthquakes in that zone and the ones in the Southern Dinarides [56–61].

The possibility that intense seismic activity in the Southern Apennines may be influenced by the occurrence of major shocks in the Southern Dinarides has been first suggested by the fact that the strong April 1979 Montenegro event (M = 6.9) was followed by the strong November 1980 Irpinia earthquake (M = 6.8) in the Southern Apennines (**Figure 6**). The idea that the above correspondence may be a systematic phenomenon was then suggested by the fact that in the last two centuries similar correspondences occurred other times (**Figure 6B**). From the list of events given in

*Earthquakes - From Tectonics to Buildings*

northern RMU wedge.

shown in **Figure 5**.

periods of about one-two decades.

(Alta Valtiberina trough) and outer (Rimini-Ancona thrust) boundaries of the

• Major earthquakes at the northern Adria front mostly occurred some years after the main seismic decouplings around the northern RMU wedge (**Figure 4**). This tendency is consistent with the hypothesis that the release of the RMU wedge

The spatial distribution of the shocks in the 8 sequences evidenced in **Figure 4** is

In most of the proposed sequences seismic activity took place in all periAdriatic zones. Moreover, one could note that when a zone is characterised by low seismicity, in the following sequence such zone is often characterised by intense earthquakes. For example, in the sequence **c** the Central Apennines did not experience any event with M ≥ 5.0 while strong earthquakes (1646 M = 5.9, 1654 M = 6.3) hit that zone in the subsequent sequence. In the Southern Apennines, after a period of low activity from 1562 to 1687 (only one earthquake with M ≥ 5.0 in the sequences **c** and **d**), a phase of intense seismicity took place in the following sequence **e** (1688 M = 7.1, 1692 M = 5.9,

*Spatial distribution of major (M ≥ 5.0) earthquakes in the seismic sequences tentatively evidenced in Figure 4.* 

• Then, seismic activity mostly involves the main fault systems in the Emilia Apennines, the Eastern Southern Alps and the Northern Dinarides, over

favours the acceleration of the northern Adria domain [11, 42].

**22**

**Figure 5.**

*Numbers as in Figure 3.*

#### **Figure 6.**

*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 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

**25**

**Figure 7.**

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

between the April 1979 Montenegro and November 1980 Irpinia shocks. The possible relationship between stress/strain rate increase and triggering of seismic activity has

The fact that the above significant correlation can be recognised for the most recent**,** complete and reliable part of the seismic catalogue may imply that this phenomenon can represent a tool for recognising the periods when the probability of strong shocks in Southern Apennines is undergoing a significant increase. In this view, the fact that since 1979 no earthquakes with M ≥ 6.5 have occurred in the Southern Dinarides (**Figure 6**) could imply that at present the probability of major shocks in the Southern Apennines is relatively low. Some doubts about this prediction may be raised by the fact that a significant shock recently occurred in the Southern Dinarides (2019, M = 6.2). The previous seismic histories would suggest that such event is slightly weak for triggering significant seismicity in the Southern Apennines. However, possible uncertainties in the estimated

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

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

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

*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.*

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

**5. Present seismic hazard in Calabria**

increase of seismic hazard in Calabria.

and the induced event.

been pointed out in several works e.g., [52–54, 60, 65–69].

magnitude could reflect on the reliability of the above prediction.

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

between the April 1979 Montenegro and November 1980 Irpinia shocks. The possible relationship between stress/strain rate increase and triggering of seismic activity has been pointed out in several works e.g., [52–54, 60, 65–69].

The fact that the above significant correlation can be recognised for the most recent**,** complete and reliable part of the seismic catalogue may imply that this phenomenon can represent a tool for recognising the periods when the probability of strong shocks in Southern Apennines is undergoing a significant increase. In this view, the fact that since 1979 no earthquakes with M ≥ 6.5 have occurred in the Southern Dinarides (**Figure 6**) could imply that at present the probability of major shocks in the Southern Apennines is relatively low. Some doubts about this prediction may be raised by the fact that a significant shock recently occurred in the Southern Dinarides (2019, M = 6.2). The previous seismic histories would suggest that such event is slightly weak for triggering significant seismicity in the Southern Apennines. However, possible uncertainties in the estimated magnitude could reflect on the reliability of the above prediction.
