Seismotectonics and landslides of the NE border of the Calabrian Arc (Southern Italy)

ABSTRACT

The north-eastern border of CA represents the accretionary system developed during its continuous collision with Apulian block. The tardive oblique tectonic component is highlighted by pervasive NW-SE crustal transpressive left-lateral strike slip faults developed from the Upper Miocene to Quaternary tectonic stages. The seismotectonic and landslides Main Map represents the update of the main faults, the style of their arrangement and kinematics, the seismotectonic features and landslide area distribution with risk implications. Seismotectonics allows us to recognise a vertical crustal zonation with the distribution of earthquakes and their focal mechanism solution. The prevalent modality of ruptures depicts a mainly transcurrent and compressive solution in mid-lower crust portions respectively. This data, together with the historical seismicity analysis of the area, lead to consider the NW-SE strike-slip faults improved on the Main Map, as recent and tectonically active. The areal distribution of landslide and susceptibility of the area is due to the interaction between seismotectonic peculiarity, lithological typologies and fault zone architectures. Landslides and flood phenomena of this sector involve many municipalities, major and minor infrastructures and economic and social activities. The landslides and the faults have been identified, mapped and classified, originally at detail scale and, then, represented at 1:50,000 scale in the Main Map, included as supplementary material. The geo-structural and geomorphological data were analysed in a geographic information system allowing data management and implementation. The work presents an updated knowledge framework of risk conditions of the study zone, available to plan and reduce the fundamental elements that determine the landslide and seismic risks in this region.

KEYWORDS:

• Calabria
• Seismotectonics
• Mapping
• GIS
• Landslide hazard

1. Introduction and geological setting

The study area is located on the NE border of the Calabrian Arc (CA), Southern Italy, along an important NW-SE trending left-lateral regional tectonic lineament in the central Mediterranean area (Figure 1).

Figure 1. Geological sketch-map of the Central Mediterranean area and geological section on the bottom (after Tansi et al., 2007, modified), with tectonic simplified sketch of the Map.

The CA is an arc-shaped accretionary wedge of the Mediterranean orogenic belt connecting the Maghrebian and Southern Apennine chains, trending E–W and NW–SE, respectively (Amodio-Morelli et al., 1976; Malinverno & Ryan, 1986) (Figure 1). It is the result of accretionary processes and the development of compressional structures on its eastern flank (Ionian Sea) and coeval rifting in the internal domain (Tyrrhenian Margin). These processes were produced by the subduction of the Ionian slab towards north-west and the roll back of the same towards south east, during the Neogene-Quaternary time span (Critelli, 1993, 2018; Critelli et al., 2013; Critelli & Martín-Martín, 2022, 2024; Zecchin et al., 2020). Collisional and post-collisional tectonics of this part of orogen is mainly influenced by oblique component of motion to which is referred the development of a composite transpressional and transtensional arrangement (Barone et al., 2008; Civile et al., 2022; Ferranti et al., 2009, 2014; Gori et al., 2017; Monaco & Tansi, 1992; Muto et al., 2015; Pirrotta et al., 2022; Tansi et al., 2007).

The chain consists of Alpine units made up of basement crystalline nappes and Mesozoic cover (Calabride complex; Ogniben, 1973) overthrusted since the Eocene on a series of ophiolite-bearing tectonic units of the Neo-Tethys domain (Liguride complex; Barbera et al., 2011; Critelli, 1993; Critelli & Martín-Martín, 2024; Ogniben, 1973). Pre-Mesozoic basement consists of different tectonic units affected by Variscan metamorphism and emplaced during the Alpine orogenesis (Langone et al., 2010; Messina et al., 1994). Overlying the basement of the Sila Unit the sedimentary cover was subdivided into the Longobucco and Caloveto Groups; these Mesozoic successions reflect the submarine depositional system and the tectonic/sedimentary evolution (Innamorati & Santantonio, 2018).

External and Sicilide Mesozoic-derived terranes constitute the thick accreted Apennine thrust wedge terrane. Slices of Sicilide-derived succession were emplaced through the internal domain during the Neogene back thrusting phase recorded along the entire western Calabria margin (Cavazza & Barone, 2010; Tripodi et al., 2018).

The Alpine units, during the Oligocene-Early Miocene, overthrusted on the Mesozoic carbonate terranes belonging to the Apennines-Maghrebide Chain (Amodio-Morelli et al., 1976). An Oligocene to Quaternary basin successions overlay the Paleozoic Alpine units and their Mesozoic cover (Bonardi et al., 2005; Zecchin et al., 2012).

On the basement rests an Oligocene to Holocene sedimentary succession that shows inside a series of back thrusts on various stratigraphic levels. In particular, the Upper Tortonian–Messinian sequences show an internal architecture that reflects the interplay between tectonics and relative sea-level changes.

The Miocene units are unconformably overlain by Pliocene to Upper Pleistocene succession made up of clay, sand and conglomerate, the latter forming a staircase of terraces, as observed north and south of the study area (Carobene, 2003; Corbi et al., 2009; Lucà et al., 2011; Robustelli et al., 2009a, 2009b). To better understand the tectonostratigraphic architecture of the area, in the Map we used the stratigraphic subdivision as reported in chapter three of this manuscript.

In the Middle Miocene-Middle Pleistocene time, the CA was affected by a regional NW–SE left-lateral transcurrent fault system which dissected the orogenic belt generating transverse and longitudinal structural highs and basins (Corradino et al., 2023; Dewey et al., 1989; Ferranti et al., 2009; Gullà et al., 2005; Muto & Perri, 2002; Tripodi et al., 2013; Van Dijk et al., 2000). Tansi et al. (2007) documented that in the central sector of the CA, the strike-slip tectonics produced, at the tips or in areas of change of direction of the fault segments, transtensional or transpressional structures. Moreover, in the sector immediately adjacent to the western side of the study area, the strike-slip tectonics produced the transtensional Crati Graben, structured since the Pliocene by N–S normal faults still active (Tansi et al., 2005, 2016).

In the study area, transcurrent tectonics – started in Upper Miocene and still active – produced a complex network of NW-SE left-lateral strike-slip faults and associated thrusts and back thrusts, (Brutto et al., 2016; ; Maffione et al., 2013; Mangano et al., 2023; Muto et al., 2014, 2017; Tripodi et al., 2022; Zecchin, Civile, et al., 2013). Since the early Messinian, Neogene basins have been dissected by tectonic processes into sub-basins.

The catalogue of historical earthquakes (Guidoboni et al., 2019) reports three important events that occurred in the study area in the years 951, 1824 and 1836. The latter was the strongest, with an estimated Imax = X MCS. The lack of reliable seismic information does not permit associating these earthquakes with seismogenic structures recognised in the area. Sorriso-Valvo and Tansi (1996) have highlighted the recent character of NW-SE trending normal faults, active up to the Middle Pleistocene in the study area.

The study area is affected by many landslides, large-scale landslides and deep-seated gravitational and tectono-gravitational slope deformations.

The aim of this study is the development of a multidisciplinary approach combining geological mapping, macro and meso-structural lineaments, geomorphological analysis, and the study of the main historical and instrumental earthquakes, in order to define the seismotectonic and landsliding characteristics of an area of high geo-structural risk. The map may represent so a useful tool for territorial engineering-geological planning and Civil Protection (Petrucci et al., 2017).

The results of this approach are summarised in the 1:50,000 scale Main Map of the study area.

2. Methodology

Through geo-structural and geomorphological analysis, performed from the macro to the mesoscale, we have depicted on the Main Map the main faults, distinguished in ancient and recent-active. Field geological surveys and analysis of aerial photos at various scales and interpretation of satellite images were carried out, as well as the processing and interpretation of digital terrain models. Morpho-tectonic indicators such as fault scarps, triangular and trapezoidal facets, rectified waterways and morphological steps and saddles aligned along adjacent ridges, were identified through the macro-structural analysis carried out by aerial and satellite photographic interpretation. At the macroscale, we performed meso-structural studies collected at 30 stations located along the main fault zones. For each striated fault plane we measured strike, dip and pitch, and so determined the direction of movement.

Our studies led us to detail the most recent faults, these are closely associated with the morphodynamics recognisable in the current landscape and with the seismotectonic setting of the area.

Moreover, we have been mapped the most representative elements of gravitational and tectono-gravitational origin primarily using geomorphological criteria (detection of scarps and trenches along the ridges and on the slopes, the analysis of the shape of the latter and interference with the drainage network, etc.) and multi-temporal analysis of aerial photographs and satellite images followed by field surveys.

We leveraged velocity vector data from GPS measurements (Palano et al., 2023) to deepen our understanding of the current tectonic regime. In order to depict the present-day crustal stress in the study area and the correlation between seismicity and tectonic structures, we analysed the recent instrumental seismicity. A very important information about the current stress field comes from the earthquake kinematics. Therefore we collected the focal mechanism of past earthquakes from published papers (D’Amico et al., 2011; Frepoli & Amato, 2000; Orecchio et al., 2014; Presti et al., 2013; Totaro et al., 2013, 2016) and from online databases (Pondrelli & Salimbeni, 2006; Scognamiglio et al., 2006). Furthermore, we computed the focal mechanism of 20 earthquakes that occurred during the last decades in the investigated area. We used the software CAP (Tan et al., 2006; Zhao & Helmberger, 1994; Zhu & Helmberger, 1996), based on the comparison between observed and synthetic seismograms, and the software FOCMEC (Snoke, 1989; Snoke et al., 1984), based on the polarity of direct waves. Seismic results were interpreted and correlated to the main tectonic structures by means of geological analysis.

The above data were also digitised and analysed in a geographic information system (GIS), the data were georeferenced in UTM (Universal Transverse Mercator) with WGS84 Datum.

3. Lithostratigraphy

In the study area, the following main tectono-stratigraphic and tectono-metamorphic units (Figure 1) crop out, on which Paleogene-Quaternary succession lies unconformably. The units are briefly described from bottom to top (Figure 2):

• Polia- Copanello – Gariglione Unit – High-grade metamorphic rocks (biotite-sillimanite garnet gneiss) intruded by plutonic bodies. Garnetiferous sillimanite biotite gneiss. There are interbedded marble lenses, granulite lenses and metabasite lenses felsic veins of microgranites, aplites and pegmatites (Cambrian?-Devonian) (Amodio-Morelli et al., 1976).

• Plutonic Sila Complex – Plutonic and metamorphic rocks of the Sila Batholith, constituted by intrusive bodies of regional extension with variable composition from gabbro to tonalite, granodiorite and monzogranite intruded by felsic dykes, micro granite and aplite-pegmatite (upper Carboniferous –lower Permian) (Messina et al., 1994).

• Mandatoriccio Complex – Paragneiss micaschists with schistose structure and heterogeneous grain, intercalations of ortho gneisses, layers of marbles and metabasites, intrusion of lamprophyre veins (Cambrian-Devonian) (Messina et al., 1994).

• Bocchigliero Unit – Phyllites composed of quartz, mica, albite, chlorite and calcite, of very low metamorphic grade. Porphyroids, metarenites, meta-andesites and metalimestones (Cambrian-Carboniferous) (Messina et al., 1994).

• Sedimentary cover of Longobucco and Caloveto Groups – The Longobucco group consists of conglomerates, quartz arenites, sublitharenite and mudstones of a purplish-red to white colour at the base. The cover is also characterised by dark grey to blackish limestones, hybrid sandstones and ooid limestones, marls, grey marly limestones with thin intercalations of sandstones and turbiditic succession (Lower-Middle Jurassic) (Innamorati & Santantonio, 2018; Young et al., 1986).

• Paludi Formation – Conglomerates and polygenic breccias with a sandy matrix; reddish and greenish marls containing layers of feldspathic quartz sandstones and conglomerates (Upper Oligocene – Lower Miocene, Bonardi et al., 2005).

• Pre-evaporitic deposits – Conglomerates, sandstones, silts and silty clay, tripolaceous marls and marly clays (Serravallian-Upper Tortonian) (Barone et al., 2008; Zecchin et al., 2020).

• Evaporitic deposits: Light brown or whitish limestone. White – yellowish evaporite limestone with local thin marls, siltites and gypsum intercalations. In the northern sector the white limestones are unconformably covered by conglomerate, breccias, and sandstones. Finely crystalline, well stratified gypsum, ranging from sandy gypsum or chalky sandstone to relatively pure gypsum (Messinian) (Barone et al., 2008; Gindre-Chanu et al., 2020; Massari et al., 2010; Zecchin, Caffau, et al., 2013; Zecchin, Civile, et al., 2013).

• Chaotic Complex – In the upper portion of Miocene succession is interbedded an exotic chaotic complex, characterised by varicoloured shales and clays with matrix ranging in colour from red to green, including olistoliths and blocks of limestones and marls, argillites, calcarenites (Cretaceous-Eocene) (Critelli et al., 2014a; 2014b; Ogniben, 1973).

• Post-evaporitic deposits – Siliciclastic argillaceous and arenaceous deposits during the late Messinian. These consist of greyish fossiliferous marl and clays alternating with thin turbidite sandstone beds and grades upward into shallower sandstones (Upper Messinian-Early Pliocene) (Barone et al., 2008).

• Conglomerates, sands, calcarenites and clays of alluvial fluvial, littoral, platform and bathyal environments (Middle Pliocene – Middle Pleistocene).

• Gravels, sands, silts and clays of alluvial, colluvial, lacustrine, aeolian, coastal and marine deposits (Middle/Upper Pleistocene-Holocene).

Figure 2. Tectono-stratigraphic scheme of the study area.

4. Tectonic structures

The examined area experienced a wrench tectonic setting starting at least from the Middle Miocene-Quaternary producing transpressional and transtensional fault zones displacing the older chain structures (Figure 1).

The main NW-SE-trending faults are characterised by strike-slip, reverse and normal kinematics. NW-SE-trending lineaments are arranged in an en-échelon pattern subparallel to the Rossano-San Nicola fault zone (Figure 3) (Mangano et al., 2023). Morphostructures between Cirò Marina and the Trionto River area are aligned parallel to the NW-SE regional lineaments and, in general, is characterised by an est dipping displaced growing monocline.

Figure 3. (A) Panoramic view of a flower structure bordered by left-lateral strike-slip faults with inverse component (Corigliano-Rossano town); B) Thrust ramps NNW-SSE trending, with left component, involving the Palaeozoic crystalline-metamorphic bedrock and soil horizons (Corigliano-Rossano town); C) Flower structure crossing the crystalline-metamorphic bedrock; D) Normal fault with left-lateral component affecting the Miocene pre-evaporitic deposits (Cariati town); E) Panoramic view of the fault scarp with triangular facets of a left normal-transcurrent structure WNW-ESE trending (Corigliano-Rossano town).

The faults with reverse kinematics, dipping both to the northeast and southwest (see Main Map and Figure 4), show pitch of nearly 90°, reverse kinematics appear on low angle-dipping thrust planes. These planes are displaced by high-angle dipping transcurrent and oblique-slip faults with the same strike. At the outcrop scale, the NW-SE fault planes commonly exhibit typical transpressive structures forming a braided pattern of anastomosing contemporaneous faults in the area between Cirò- Longobucco and Rossano (Figure 1(b) and Main Map). At the mesoscale, the fault system is characterised by transcurrent kinematics with variable values of pitch, between 0° and 25°, and fault planes dip towards NE and SW with variable angles (Figure 4). Left-lateral slip dominates, with subordinate right slip; on the same planes, the pitch may exceed 45° with a predominant reverse component of displacement. Tectonic structures belonging to the same trend, but with extensional kinematics of the planes, crosscut the deposits of the Lower-Late Pleistocene succession outcropping along the coast area, from the villages of Cirò towards the Cariati and Calopezzati countries (Main Map). The N-S oriented faults mainly outcrop along the western portion of the Main Map cutting the Sila Massif paleozoic rocks and the late quaternary cover deposits. This normal fault system is characterised by high-angle fault planes dipping top to the east and to the west, forming intramontane tectonic depression in correspondence with the Cecita Lake (Main Map). The system belongs to the regional extensional array faults that identify the Crati Graben (Tansi et al., 2007, 2016). Among others, the faults belonging to the Cecita Lake faults are part of this system; these are active faults which historically have produced high-magnitude earthquakes. These structures displace towards the eastern portion of the foreland system with respect to the metamorphic crystalline chain. The NE-SW faults have been identified and observed at the mesoscale and do not correspond with major mapped faults in the area. Ancient thrusts showing sub-horizontal planes E–W to NNW–SSE strikes, were also recognised. These thrusts do not show any morphological evidence and are documented only at the mesoscale, where they are commonly dislocated by the younger faults. These planes display reverse dip-slip slickensides, documenting – if tilting is ignored – an N–S-oriented sub-horizontal σ1: they can be related to the overthrusts responsible for the Oligocene-Early Miocene building of the chain.

Figure 4. Mesoscopic-scale structural data gathered at the stations shown on the Map. Diagrams (Schmidt’s net, lower hemisphere) show the attitude of fault planes, slickensides (arrows). Data elaboration was performed by means of the software Daisy 3.

5. Seismotectonics

The northern sector of the Ionian Calabria was affected in the past centuries by many earthquakes with Imax = VIII–X MCS. Three of them occurred right in the middle of our study area (Figure 5). The strongest occurred near Rossano village on April 25, 1836, with Imax = X MCS (Tertulliani et al., 2017), generated by the Rossano fault (Galli et al., 2010), a NW-SE striking tectonic structure characterised by left lateral transcurrent kinematics. Another notable earthquake occurred on December 11, 1824, of Imax = VIII MCS (Rovida et al., 2020, 2022). Much less information is available about another earthquake that occurred in the same area in 951, with presumed Imax = IX MCS (Guidoboni et al., 2019).

Figure 5. Historical earthquakes of Imax ≥ 8 MCS and epicenters of the earthquakes occurred since 1981.

During the last few decades, an increasing number of seismic stations managed by Università della Calabria (www.sismocal.org) and by Istituto Nazionale di Geofisica e Vulcanologia (www.ingv.it) allowed for reliable detection and location of low magnitude earthquakes. Figure 5 shows the epicentres of earthquakes with magnitude M ≥ 2 located in the area during the last 43 years. The official catalogue (www.ingv.it) reports more than 4000 earthquakes located in the study area, occurred from 1981 to 2023. Among them, about 1250 have M ≥ 2, 149 have M ≥ 3, and 11 earthquakes have magnitude M ≥ 4. The strongest recorded events in the last 40 years were two earthquakes of magnitude M4.7 that occurred in 1988 and in 2002, both of them located offshore Mirto Crosia. Regarding the depth distribution, 65% of this seismicity is located in the upper crust, from the surface to 20 km depth. The epicentre distribution of the recent seismicity is characterised by some places where earthquakes have been more numerous than other areas. Earthquake swarms have been recorded offshore Mirto Crosia, at Punta Alice, at Umbriatico, near Cropalati, and others. Although the recent earthquakes are of low to moderate magnitude, the analysis of their kinematics is essential to infer the present stress field active in the area. The collection of focal mechanisms from published papers, from open databases and those computed in this work consists of 37 earthquakes of magnitude M ≥ 2.8. The kinematics of these earthquakes indicate a quite complex picture, particularly in the upper crust. Figure 6 shows the focal mechanism solutions in four depth ranges. All kinds of mechanisms are observed in the area, but with a predominance of strike slip motions, while reverse mechanisms are very few. The direction of the P axis of strike slip and reverse motions is mostly oriented in the EW direction. This feature is more evident for earthquakes deeper than 20 km, while for shallow earthquakes normal kinematics is more frequent. This result indicates that in the study area the active stress field has an important compressive contribution in the EW direction in the middle and lower crust, while at shallow depth the gravity contribution is likely the most important at present.

Figure 6. Focal mechanism of 37 earthquakes of magnitude M ≥ 2.8 is shown in four depth ranges as indicated in each plot.

The occurrence of earthquakes at depths greater than 30 km (nearly 40 km in some cases) in the study area indicates a complex crustal structure that is not surprising considering the subduction of the Ionian slab and the interaction with the Apulian crust in this sector of the Calabrian Arc. However, establishing if such earthquakes occur in the lower crust of the Calabrian Arc or in the crust of the subducting slab is not easy.

6. Gravitational and tectono-gravitational phenomena

The study area is marked by a large-scale instability of slopes. Many of these landslides are reactivated when particularly intense rainfall occurs: during the years 2008–2012 and in 2015, a strong rainy seasons, marked by exceptional weather conditions, have caused disasters that have damaged large areas of the territory, particularly affecting the road network and private construction, as well as economic activity in the study area and northern Calabria. In particular, the surroundings of Corigliano-Rossano town were damaged by a short-lasting and localised rainstorm on 12 August 2015, when more than 230 mm of rain in 16 h caused damage exceeding 100 million euros (Figure 7).

Figure 7. Landslides that occurred in Corigliano-Rossano town during the flood event of 12 August 2015. Historic centre of Rossano: (A) Vittorio Emanuele road, (B) Via Minnicelli road.

This instability is highlighted by widespread gravitational and tectonic-gravitational phenomena – of various types and dimensions – connected with the particular geodynamic context that determines a high relief energy along the slopes (produced by active tectonic uplift occurring in the Calabrian Arc), the degree of fracturing of the rock types involved in the tectonic deformations (Tansi et al., 2000) and the particular structural styles predisposing the slopes to instability. The tectonic pattern is also responsible for an underground water circulation strongly conditioned by the brittle tectonic structures (faults), locally causing strong increases in the neutral pressures and slope instability. Tectonic and hydrogeological conditions triggered the development and the kinematics of large landslides, deep gravitational slope (Conforti et al., 2014; Iovine et al., 1996; Tansi et al., 2005) and tectonic gravitational (Iovine & Tansi, 1998) deformations on land and in submarine environments (e.g. Mangano et al., 2022; Zecchin et al., 2018). All of which results in a close correlation between the distribution of faults, of every type, and the distribution of mass movements, of every order and degree. The Map summarises the efforts to integrate available databases with original surveying, trying to respect the original classifications. We have distinguished the following groups of gravitational and tectono-gravitational phenomena: Fall (FL), Slide (SL), Intense Erosion Area (IEA), Shallow Landslide Area (SLA), Deep Landslide Area (DLA), Large Slide (LS), Deep-seated Gravitational Slope Deformation (DGSD) and Sackung (SK). In addition, landforms were distinguished based on the period of activation as follows:

• - Landslides before 2001 affecting residential areas and roads, collected by the Basin Authority of the Region of Calabria within the definition of the Hydrogeological Plan For Soil Protection (PAI) – of the Region of Calabria, Italy (ABR-Calabria, 2001). They are represented by SL, SLA, DLA, DGSD.

• - Landslides activated in the period 2001–2016. These data derive from both an update of 2016 year of the PAI – Basin Authority of Region of Calabria (ABR-Calabria, 2016) and the CNR-IRPI which collected data (landslides activated during the period 2008–2012) in the light of an agreement stipulated with the province of Cosenza for mapping the geo-hydrology of risk areas and the updating of the risk forecasting and prevention programme (Provincia di Cosenza, 2014). They are represented by FL, SL, IEA, SLA.

Landslides are differentiated as dormant or active, and distinguished in the same typologies of the above class according to the Cruden and Varnes (1996) classification.

Finally, the large-scale gravitational and tectonic-gravitational phenomena (LS, SK) – identified by Sorriso-Valvo and Tansi (1996), Iovine and Tansi (1998) and Conforti et al. (2014).

Tables 1–3 summarise the results of statistical analysis on the different types of the analysed mass movements.

Table 1. Statistical data on the landslides activated before 2001.

Type of movement	n. landslides	% landslides	Area (km^2)	% area
SL	226	57.36	6.10	20.37
SLA	73	18.53	2.77	9.24
DLA	93	23.6	18.72	62.48
DGSD	2	0.51	2.37	7.91
Total	394	100.00	29.76	100.00

Note: Total area of the Map: 1541 km².

Table 2. Statistical data on the landslides activated in the period 2001–2016.

Type of movement	n. landslides	% landslides	Area (km^2)	% area
FL	2	2.74	0.03	1.06
SL	3	4.11	0.03	1.08
IEA	60	82.19	1.56	63.17
SLA	8	10.96	0.86	34.69
Total	73	100.00	2.48	100.00

Note: Total area of the Map: 1541 km².

Table 3. Statistical data on the LS and SK phenomena.

Type of phenomena	n. phenomena	% phenomena	Area (km^2)	% area
LS	25	71.43	26.15	58.21
SK	10	28.57	18.78	41.79
Total	35	100.00	44,93	100.00

Note: Total area of the Map: 1541 km².

7. Conclusions

The eastern area of the Sila Massif is mainly controlled by several tectonic structures documenting the transition between fold and thrust belt deformation to wrench-dominated tectonic modality. The NW-SE transpressive left lateral transcurrent faults offset the entire studied area determining up-thrust blocks along the main faults and associated N-S transpressional upthrust. Major faults of this margin of Calabria conditioned the development of wedge-top basins of the southern Apennines-Calabrian Arc system in consequence of oblique collision between Calabria and Apulia blocks. Relationships between the NW-SE main faults and the seismicity of the area are testified by the distribution of earthquake epicentres.

The examined area is affected by recent and active faults and by historical and present earthquakes. The mechanisms measured in the area show mainly strike-slip left-lateral movement. These results are more evident for the earthquakes deeper than 20 km, indicating that in the study area compressive stress is present and active in the middle and lower crust. The deeper compressive mechanism could also testify the persistence of active subduction with geodynamic implications on this oblique collisional segment.

The main seismogenic structures were compared with historical and instrumental seismicity, in order to identify numerous severe crustal earthquakes (IX–X MCS) that occurred in the 17th and 19^th centuries in onshore and offshore study areas.

Tectonic deformation strongly conditioned the degree of faulting and fracturing and as well as the weathering grade of a crystalline and metamorphic complex of the Sila basement rocks. These factors, together with widely outcrops of clay and silty neogenic and quaternary lithologies in the entire piedmont ionian area, predispose the slopes to large and deep landslides, to shallow and flood events.

The large-scale seismotectonic assessment and landslides map may represent a useful tool for territorial engineering-geological planning and Civil Protection.

Software

The basemap and related layout were drafted using the software open source QGIS. Daisy 3 software (Salvini, 2002) was used for the stereoplot representation of faults. Adobe Illustrator was used to produce the final layout of the Map.

Acknowledgments

The Authors warmly thank the Editor Mike Smith and the three Reviewers Steven Bernard, Mohsen Ehteshami-Moinabadi and Ray Y. Chuang for the significant improvements to the map and manuscript. We are indebted to Federica Lucà for precious suggestions and support during the cartographic revision.

Disclosure statement

No potential conflict of interest was reported by the author(s).