**2. Short-term kinematics of Adria and spatio-temporal distribution of seismicity in the surrounding belts**

In the short-term the northward displacement of Adria does not develop continuously over time. Each seismic decoupling along the Adria lateral boundaries (Dinaric and Apennine belts) triggers the acceleration of the involved Adriatic sector e.g., [52–54]. These local accelerations induce an increase of stress at the other still blocked Adria boundaries, where consequently the probability of earthquake occurrence gets higher. When such stressed zones are then affected by major shocks, the acceleration involves more northern zones of Adria up to reach the thrust front of Adria in the Eastern Southern Alps.

In order to check the above seismotectonic interpretation, we have divided the periAdriatic boundary zones in a number of sectors (**Figure 3**). The eastern lateral boundary of Adria includes the Central-Southern Dinarides (CSD in **Figure 4**), and the Northern Dinarides (ND). The western lateral boundary (Apennine belt) is divided in more sectors, being characterised by a more complex tectonic setting, as discussed in the previous section. The Southern Apennines (SA) are mainly characterised by extensional faulting. In the Central Apennines (CA) transtensional decoupling fault systems (L'Aquila and Fucino) prevail. The Northern Apennines are divided in various sectors, due to their complex tectonic setting, with particular regard to the Romagna-Marche-Umbria wedge (RMU). The southern part of the western RMU boundary (Norcia-Colfiorito-Gubbio fault system, RMUWB), is mainly characterised by extensional faults. Considering the peculiar seismotectonic role of the Northern RMU wedge, its boundaries, i.e. the Rimini-Ancona thrust front (RA), the Romagna fault (Rom) and the Alta Valtiberina trough (AVT), are taken as three independent zones. The Emilia Apennines (EM) is the belt sector that lies just north of the RMU wedge. The last sector in **Figure 4** (ESA-ND) is the zone where the Adria plate underthrusts the Eastern Southern Alps.

#### **Figure 3.**

*Geometries of the periAdriatic boundary zones adopted for determining the seismicity patterns shown in Figure 4. 1) Central-Southern Dinarides (CSD in Figure 4), 2) Southern Apennines (SA), 3) Central Apennines (CA), 4) Southern part of the western boundary of the RMU wedge (RMUWB), 5) Rimini-Ancona thrust front (RA), 6) Romagna fault system (Rom), 7) Alta Valtiberina trough (AVT), 8) Emilia Apennines (EM), 9) Eastern Southern Alps and Northern Dinarides (ESA-ND).*

**21**

following peculiar features:

**Figure 4.**

ally lasts some tens of years.

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

In the seismicity time patterns shown in **Figure 4**, we tentatively recognise the

*CA = Central Apennines, CSD=Central-Southern Dinarides, ESA-ND = Eastern Southern Alps and Northern Dinarides, EM = Emilia Apennines, RA = Rimini-Ancona thrust front, RMUWB=Southern western boundary of the RMU wedge, Rom = Romagna fault system, SA = Southern Apennines. Earthquakes are indicated by bars with colours related to magnitude (scale in* **A** *and* **B***). Sources of seismicity data in the caption of Figure 1. The grey and white bands tentatively include the events that are supposed to belong to the presumed migrating sequences, identified by letters (***a-h***). The geometries of the zones considered and the spatial distribution of* 

• In the zones considered, most intense seismicity tends to concentrate in short

• The crises tend to occur later and later as the zones involved are located more and more to the north, delineating a sort of migrating pattern (seismic sequence).

• A number of sequences may be recognised in the period considered, as tentatively evidenced by grey and white bands and letters (from **a** to **h)** in **Figure 4**.

• The time development of the proposed sequences tends to occur in two phases. During the first phase, seismicity mainly affects the southern and central Dinarides, the central and southern Apennines and the western extensional boundary of the RMU wedge (the Norcia-Colfiorito-Gubbio fault system). This phase generally involves several shocks of M ≥ 5.0 in each zone and gener-

• In most cases, the second phase starts with a crisis in the Romagna decoupling fault system, followed (within 10–20 years) by the activation of the inner

periods (crises), separated by longer phases of lower activity.

*major shocks in the various sequences are shown in the Figures 3 and* **5** *respectively.*

**(A, B)** *Time patterns of seismic activity in the periAdriatic zones. AVT = Alta Valtiberina trough,* 

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

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

#### **Figure 4.**

*Earthquakes - From Tectonics to Buildings*

**of seismicity in the surrounding belts**

thrust front of Adria in the Eastern Southern Alps.

where the Adria plate underthrusts the Eastern Southern Alps.

*Geometries of the periAdriatic boundary zones adopted for determining the seismicity patterns shown in Figure 4. 1) Central-Southern Dinarides (CSD in Figure 4), 2) Southern Apennines (SA), 3) Central Apennines (CA), 4) Southern part of the western boundary of the RMU wedge (RMUWB), 5) Rimini-Ancona thrust front (RA), 6) Romagna fault system (Rom), 7) Alta Valtiberina trough (AVT), 8) Emilia* 

*Apennines (EM), 9) Eastern Southern Alps and Northern Dinarides (ESA-ND).*

**2. Short-term kinematics of Adria and spatio-temporal distribution** 

In the short-term the northward displacement of Adria does not develop continuously over time. Each seismic decoupling along the Adria lateral boundaries (Dinaric and Apennine belts) triggers the acceleration of the involved Adriatic sector e.g., [52–54]. These local accelerations induce an increase of stress at the other still blocked Adria boundaries, where consequently the probability of earthquake occurrence gets higher. When such stressed zones are then affected by major shocks, the acceleration involves more northern zones of Adria up to reach the

In order to check the above seismotectonic interpretation, we have divided the periAdriatic boundary zones in a number of sectors (**Figure 3**). The eastern lateral boundary of Adria includes the Central-Southern Dinarides (CSD in **Figure 4**), and the Northern Dinarides (ND). The western lateral boundary (Apennine belt) is divided in more sectors, being characterised by a more complex tectonic setting, as discussed in the previous section. The Southern Apennines (SA) are mainly characterised by extensional faulting. In the Central Apennines (CA) transtensional decoupling fault systems (L'Aquila and Fucino) prevail. The Northern Apennines are divided in various sectors, due to their complex tectonic setting, with particular regard to the Romagna-Marche-Umbria wedge (RMU). The southern part of the western RMU boundary (Norcia-Colfiorito-Gubbio fault system, RMUWB), is mainly characterised by extensional faults. Considering the peculiar seismotectonic role of the Northern RMU wedge, its boundaries, i.e. the Rimini-Ancona thrust front (RA), the Romagna fault (Rom) and the Alta Valtiberina trough (AVT), are taken as three independent zones. The Emilia Apennines (EM) is the belt sector that lies just north of the RMU wedge. The last sector in **Figure 4** (ESA-ND) is the zone

**20**

**Figure 3.**

**(A, B)** *Time patterns of seismic activity in the periAdriatic zones. AVT = Alta Valtiberina trough, CA = Central Apennines, CSD=Central-Southern Dinarides, ESA-ND = Eastern Southern Alps and Northern Dinarides, EM = Emilia Apennines, RA = Rimini-Ancona thrust front, RMUWB=Southern western boundary of the RMU wedge, Rom = Romagna fault system, SA = Southern Apennines. Earthquakes are indicated by bars with colours related to magnitude (scale in* **A** *and* **B***). Sources of seismicity data in the caption of Figure 1. The grey and white bands tentatively include the events that are supposed to belong to the presumed migrating sequences, identified by letters (***a-h***). The geometries of the zones considered and the spatial distribution of major shocks in the various sequences are shown in the Figures 3 and* **5** *respectively.*

In the seismicity time patterns shown in **Figure 4**, we tentatively recognise the following peculiar features:


(Alta Valtiberina trough) and outer (Rimini-Ancona thrust) boundaries of the northern RMU wedge.


The spatial distribution of the shocks in the 8 sequences evidenced in **Figure 4** is shown in **Figure 5**.

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,

#### **Figure 5.**

*Spatial distribution of major (M ≥ 5.0) earthquakes in the seismic sequences tentatively evidenced in Figure 4. Numbers as in Figure 3.*

**23**

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

1694 M = 6.7). The strong 1915 Fucino earthquake (M = 7.1, sequence **g**) took place after a sequence (**f**) characterised by relatively low seismicity (few events with

**3. Some remarks on the Apennine zones most prone to the next strong** 

Taking into account the regularity patterns that we tentatively recognise in the seismic sequences so far developed (**a-g** in **Figure 4**), we try to gain insights into how the last, still ongoing, sequence (**h** in **Figure 4**) might develop in the next future. In this regard, it must be considered that the first phase of that sequence has so far involved several earthquakes in the Southern and Central Dinarides and the Southern and Central Apennines. The acceleration of southern Adria triggered by such seismic decouplings has presumably stressed and deformed the RMU wedge, increasing its tendency to separate from the inner belt. This hypothesis may explain why a number of major extensional shocks (1979 M = 5.8, 1984 M = 5.6, 1997 M = 6.0, 5.7, 5.6, 5.5, 2016 M = 6.2, 6.1, 6.6, [7]) occurred along the western border of the RMU wedge (Norcia-Colfiorito-Gubbio fault system). The NE ward acceleration of the southern RMU wedge may have emphasised stresses (and thus seismic hazard) at the northern boundaries of that wedge (Romagna fault, Alta Valtiberina trough and Rimini-Ancona thrust front). Thus, one could expect that the present seismic hazard in such zones is higher than in the other Apennine fault systems. This hypothesis is also suggested by the fact that the last significant earthquakes (M ≥ 5.3) in the above zones occurred about 100 years ago, i.e. a quiescence longer than the previous ones (**Figure 4**).

Another zone where tectonic load may currently be high is the Emilia Apennines and the related buried folds (**Figure 2**), since such structures, including the Mugello trough, have been stressed by the push of the RMU wedge during the last tens of years. The above hypothesis is consistent with the fact that intense earthquakes (2012, M = 6.1, 5.9) recently occurred in the Ferrara buried folds (lying outside the Emilian Apennines) and that moderate seismicity (M = 4.5) affected the Mugello

The kinematic field delineated by geodetic data [11, 50, 51] suggests that the separation between the inner and outer Apennine belts is developing at rates of about 3–4 mm/y, which implies that a displacement of about 30–40 cm has been accumulated since the last activations of the fault systems surrounding the northern RMU wedge (about 100 years). This displacement is comparable to the fault slip

**4. Present seismic hazard in the Southern Apennines: further evidence** 

Further information on the present seismic hazard in the Southern Apennines could be inferred from a correlation that has been recognised between the major

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 in that zone and the ones in the Southern Dinarides [56–61].

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

M ≤ 5.5).

**earthquakes**

trough on December 2019.

associated with a M = 5–6 earthquake e.g., [55].

**from a seismotectonic correlation**

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

1694 M = 6.7). The strong 1915 Fucino earthquake (M = 7.1, sequence **g**) took place after a sequence (**f**) characterised by relatively low seismicity (few events with M ≤ 5.5).
