Landslide inventory map of north-eastern Calabria ...

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Landslide inventory map of northeastern Calabria (South Italy) a

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Massimo Conforti , Francesco Muto , Valeria Rago & Salvatore Critelli

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CNR – Istituto per Sistemi Agricoli e Forestali del Mediterraneo (ISAFOM), Rende, Italy b

Dipartimento di Ingegneria per l'Ambiente e il Territorio e Ingegneria Chimica (DIATIC), Università della Calabria, Arcavacata di Rende, Italy c

Dipartimento di Biologia, Ecologia e Scienze della Terra (DiBEST), Università della Calabria, Arcavacata di Rende, Italy Published online: 17 Oct 2013.

To cite this article: Massimo Conforti, Francesco Muto, Valeria Rago & Salvatore Critelli (2014) Landslide inventory map of north-eastern Calabria (South Italy), Journal of Maps, 10:1, 90-102, DOI: 10.1080/17445647.2013.852142 To link to this article: http://dx.doi.org/10.1080/17445647.2013.852142

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Journal of Maps, 2014 Vol. 10, No. 1, 90 –102, http://dx.doi.org/10.1080/17445647.2013.852142

SCIENCE Landslide inventory map of north-eastern Calabria (South Italy) ∗

Massimo Confortia , Francesco Mutob, Valeria Ragoc and Salvatore Critellic a

CNR – Istituto per Sistemi Agricoli e Forestali del Mediterraneo (ISAFOM), Rende, Italy; Dipartimento di Ingegneria per l’Ambiente e il Territorio e Ingegneria Chimica (DIATIC), Universita` della Calabria, Arcavacata di Rende, Italy; cDipartimento di Biologia, Ecologia e Scienze della Terra (DiBEST), Universita` della Calabria, Arcavacata di Rende, Italy


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(Received 26 August 2013; Resubmitted 27 September 2013; Accepted 2 October 2013) Landslides are one of the most widespread natural hazards in many areas of Calabria region (Southern Italy), due to the combination of its peculiar geological, morphological, and climatic characteristics and very often to unsustainable land management. This study reports the reconnaissance and the characterization of landslides of north-eastern Calabria (south Italy). The landslide inventory map was obtained by combining field surveys with the analysis of topographic maps and multi-temporal air photos, taken in the period ranging from 1954 to 2006. This analysis has provided the spatial and temporal evolution of mass movements. The integration and elaboration of the data obtained in a GIS environment provided the inventory map of landslides on a scale 1:50,000. Landslides are widespread in the study area and play an important role in the present-day landscape evolution. A total of 1003 landslides were recognized, occupying a surface of 230.4 km2, about 30.5% of the whole study area. The landslides were mapped on the basis of the movement type, as follows: slides, flows, falls and complex landslides. Slide and complex type massmovements are very common, and represent more than 87% of the landslides mapped. The pelitic lithologies show the highest density of landslides, mainly complex type. Multitemporal air photo interpretation and field surveys provided data for distinguishing the state of activity of the landslides; therefore, 29% of the landslides mapped has been assessed active while the remaining 71% has been considered inactive. This kind of map is an useful tool for land planning policy. As all the data are digitized and stored in GIS database, this will provide the basic input needed to generate the landslide susceptibility assessments besides evaluate the landslide hazard and risk. Keywords: geomorphology; landslides inventory map; Calabria region; southern Italy

1. Introduction Landslides are responsible for rapid landscape evolution and represent a serious hazard in many areas of the World (Cendrero & Dramis, 1996; Glade, Anderson, & Crozier, 2005). Landslides are widespread in large areas of the Italian territory (Catenacci, 1992; Guzzetti, 2000) and, in particular, in Calabria region (south Italy), many areas are very prone to landsliding, due to the combination of their peculiar geological, morphological, climatic characteristics and, very often, to the ∗

Corresponding author. Email: [email protected]

# 2013 Massimo Conforti


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destabilizing effects of by human activity (Calcaterra & Parise, 2010; Conforti, Robustelli, Muto, & Critelli, 2012; Gulla`, Antronico, Iaquinta, & Terranova, 2008; Iovine & Merenda, 1996; Luca, Robustelli, Conforti, & Fabbricatore, 2011; Sorriso-Valvo, Greco, & Catalano, 2009 and many others). Landslide inventory maps are very important to investigate the distribution, types, activity, frequency of occurrence of the mass movements in a territory and to study the spatio-temporal evolution of landscapes dominated by landslides (Guzzetti, Cardinali, Reichenbach, & Carrara, 2000; Guzzetti et al., 2012). In addition, an inventory of landslides constitutes an essential basis for assessing of landslide susceptibility, hazard and risk (Fell et al., 2008). The reliability of predictions on the future distribution of landslides and the effectiveness of mitigation measures depend largely on the completeness and accuracy of available landslide databases (Van Den Eeckhaut & Herva´s, 2012). The aim of this study was the recognition of the landslides in north-eastern Calabria region (South Italy). Most of the study area is largely affected by landsliding because of its tectonics and lithological features (Carrara et al., 1977; Iovine & Merenda, 1996; Merenda, 1983). Many landslides are active and periodically cause serious damage (Crescenzi et al., 1996; Iovine & Merenda, 1996; Iovine & Petrucci, 1998; Merenda, 1983). In this work a geomorphological analysis was carried out, followed by the processing and management of collected data through a Geographic Information System (GIS) and the landslide inventory map was achieved. 2. Material and methods The inventory of landslides for the study area was produced through a geomorphological analysis, integrating field surveys, air photo interpretation and the analysis of topographic maps. In addition, a review of previous geological and geomorphological data about specific sites of the study area was carried out (e.g. Carrara et al., 1977; Iovine & Merenda, 1996; Iovine & Petrucci, 1998; Iovine & Tansi, 1998; Merenda, 1983; Monaco, Tortorici, Morten, Critelli, & Tansi, 1995; Sorriso-Valvo & Tansi, 1996). The spatial and temporal distribution of landslides was investigated through the stereoscopic analysis of two series of air photos, dating to 1954 (1:36,000 scale), 1991 (1:33,000 scale) and the analysis of orthophotos dating to 2006 (1:10,000 scale). The multi-temporal air photo interpretation was supported by field surveys carried out from May 2012 to May 2013. Type and state of activity of landslides were attributed following the classification proposed by Cruden & Varnes (1996). The surveyed landslides were reported on topographical map at 1:25,000 scale (published by the Istituto Geografico Militare Italiano). In order to accomplish the landslide inventory map, the data collected were scanned, geo-referenced and digitized as polygons or lines using a GIS software (Geographic Information System). Moreover, landslide area, type of movement and state of activity were recorded into a geo-database linked to the landslide inventory. The map legend mainly follows the schemes of Brancaccio et al. (1994) and Conforti, Pascale, Pepe, Sdao, and Sole (2013a). Different colors and symbols were assigned to each morphogenetic landform. In particular, brown was used for structural elements; red for active landslides and yellow for inactive ones; green for landforms due to fluvial landforms; blue for the drainage network; black and gray for anthropogenic features. Plain solid-filled polygons were used to represent the lithology. The cartographic representation is performed on a smaller scale (1:50,000) compared to the survey scale. The simplified topographic base, with a 50-m contour interval and elevations of the main summits, derives from a digital elevation model (DEM) obtained by digitizing of contour lines and points of the 1:25,000 scale topographic maps.

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M. Conforti et al. Geographical, geological and geomorphological setting

The study area is located in the north-eastern side of Calabria (South Italy) and covers an area of more than 756 km2 (Figure 1); this sector of the region is known as one of the most landslide prone areas of Calabria because of its geological and climatic characters (Carrara et al., 1977; Iovine & Merenda, 1996; Iovine & Petrucci, 1998; Merenda, 1983; Rago, Conforti, Muto, & Critelli, 2013). The topographic elevation has an average value of 472 m a.s.l., with a maximum value of 1707 m a.s.l. (Sparviere Mt.). Slope gradients, computed from DEM, range from 0 to 76 degrees, while its average is about 15 degrees (Figure 2). Climate in the study area is of Mediterranean type, with hot and dry summers and mild and wet winters (Buttafuoco, Caloiero, & Coscarelli, 2011). The precipitation varies with elevation and distance from the coast ranging between 500 and over 1100 mm/year in mountainous areas. It is concentrated in the period autumn-winter, with rainfall peaks in December and January; conversely, the summers are dry hot, with a minimum of rainfalls in July and August. The mean annual rainfall is approximately 800 mm. (Caloiero, Mercuri, & Reali, 1990; Iovine & Merenda, 1996). North-eastern Calabria is drained, mainly, by gravel-bed river, named Fiumare, characterized by an ephemeral and torrential regime and the middle and lower streams are characterized by braided pattern (Sabato & Tropeano, 2004). These rivers have a great transport and erosion capacity because of their flow regime, dominated by episodic flash floods alternated with long periods of inactivity during which their beds become completely dry. From a geological point of view, the study area is located on the border between the Southern Apennine and Calabrian Arc (Bonardi et al., 1988; Monaco et al., 1995; Mostardini & Merlini, 1986). The area comprises, in the southwestern sector, carbonate and terrigenous deposits, while is confined in the NE sector by the Bradanic Foredeep, which borders the Apulian Foreland (Casero et al., 1988; Catalano, Monaco, Tortorici, & Tansi, 1993; Cello & Mazzoli, 1999; Corbi et al., 2009; Critelli, 1999; Critelli, Muto, Tripodi, & Perri, 2013; Ferranti, Santoro, Mazzella,

Figure 1. Location of the study area.


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Figure 2. Slope map of the study area.

Monaco, & Morelli, 2009; Mostardini & Merlini, 1986; Rebesco et al., 2009; Tripodi, Muto, & Critelli, 2013). The southern Apennines are a NE verging fold and thrust belt derived from the deformation of the African paleomargin (Cello & Mazzoli, 1999). In this area, the geological units are formed by Mesozoic – Cenozoic sedimentary rocks of the Apenninic platform (Pollino Unit Autc.) and of the Sicilide Unit and Liguride Complex (sensu Bonardi et al., 1988) that crop out at the top of the thrust belt. Also, in this part of the Apennine chain outcrop sedimentary units derived from the Neogene—Pleistocene foredeep deposits (Pescatore, Renda, Schiattarella, & Tramutoli, 1999). The geological units outcropping in the studied area are mainly formed by flyschoid terrains of Cenozoic age: Saraceno Formation (Upper Oligocene – Aquitanian age) and Albidona Formation (Upper Burdigalian – Langhian). The first is a carbonatic turbiditic succession, formed by calcarenites and calcilutites with chert, clays and sandstones; it constitutes the uppermost portion of the North – Calabrian Unit of Bonardi et al. (1988). However, the Albidona


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Formation is a turbiditic succession, with alternance of sandstones and marls and clayey marls, where intercalations of thick calcareous marls and conglomerate beds are present. Varicolored Clays (Cretaceous – Middle Eocene) outcrop in chaotic setting in the study area; it is a complex formation constituted by different rock types (calcarenites, sandstones, marls, limestones) in silty – clayey matrix. Further, Middle to Upper Miocene deposits outcrop in the northern sector of area. Unconformably on all the described sedimentary units, there are Pliocene to Pleistocene marine sediments (clay, sand and conglomerate) of the Bradanic cycle and Quaternary terraced deposits, found up to several hundreds of meters above the present coastline (Sabato & Tropeano, 2004). In the south-westernmost sector of the study area, limestone of Mesozoic-tertiary age also crops out (Bonardi et al., 1988), representing the western extension of the Apulian platform rocks exposed to the east (Ferranti et al., 2009). These successions belong to the Unit Pollino and are homocline structures generally dipping toward NE extruded from allochthonous northCalabrian terrains in consequences of Pleistocene strike-slip tectonics (Ferranti et al., 2009). The geological units are affected by structures related to the complex tectonic history of the area (Iovine, Parise, & Tansi, 1996). The main tectonic structures are grouped into three tectonic phases (Iovine & Merenda, 1996; Monaco et al., 1995): (1) NE – verging thrusts and folds, with mainly NW-SE trends and related to the building of the Apenninic Chain (Miocene); (2) left lateral strike – slip faults belonging to the WNW – ESE system identified by Catalano et al. (1993) and associated thrust (Plio – Pleistocene). These tectonic structures upfaulted the Mesozoic carbonate sequences into Ligurian units; and (3) high-angle normal faults, trending N – S to NE – SW (Quaternary). They are responsible of uplift and displacement of Pleistocene claysandy-conglomeratic sediments (Guerricchio & Melidoro, 1986). Taking into consideration the geological map of Calabria (1:25,000 scale sheet map, CASMEZ, 1971) and the geological map of Italy (1:50,000 scale sheet map, ISPRA, 2010) the lithologies outcropping in the study area were grouped on the basis of their geotechnical behavior as follows: alluvial deposits, slope deposits, conglomeratic deposits, sand deposits, clay deposits, sandstones, calcareous – clayey complex, clayey – calcareous complex, turbiditic complex, black shales, carbonatic complex. The geomorphological setting of the study area is strongly controlled by geological and structural features. The western and middle sector are typically characterized by a hilly to mountainous landscape built on rocks of different composition and erodibility, where selective erosion has given alternatively way to steep slopes cut on hard rocks in contrast with typically rounded and gentler slopes carved on mainly pelitic, and more erodible rocks. Also, the variety of outcropping lithologies and the tectonic influence determined the development of structural landforms, such as morpho-structural ridges bounded by fault scarps (Figure 3) and minor morpho-tectonic alignments (e.g. straight channels, saddles, and straight ridges). The south-western area, where, carbonate rocks outcrop, are dominated by high relief landscapes characterized by steep slopes and narrow V-shaped valleys, often controlled by fault systems. The eastern sector, where rocks erodibility is higher, is characterized by low relief with undulating topography, gentle slopes and wide, slightly incised valleys. Nevertheless the presence of more competent rocks (calcareous and/or sandstone) locally determines high-gradient slopes or sub-vertical rocky cliffs. These latter landforms notably emerge from the surrounding terrains, given their greater resistance to erosion. Also, processes of selective erosion has produces structurally controlled mesa landforms on sub-horizontal resistant bedrock layers overlying soft rocks while homoclinal ridges develop on dipping strata. The coastal area is characterized by the presence of marine terraces, dissected by narrow Vshape valleys, developed in response to regional uplift and sea level changes (Cucci & Cinti, 1998; Ferranti et al., 2009).


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Figure 3. Morpho-structural ridge of Timpa San Lorenzo.

Great alluvial fans were observed along the coastal plain; in many cases, these fans were intensely dissected by longitudinal rivers. Minor alluvial fans were mapped also along the confluence of several tributary valleys of the main water courses. The landscape of the study area is mainly dominated by gravitational landforms even if, in many places, landforms related to running water were observed (Iovine & Merenda, 1996; Merenda, 1983). Hillslopes carved into pelitic deposits, are locally affected by slope wash processes (sheet, rill, and gully erosion), particularly in non-vegetated areas. Their evolution is conditioned by rainfall regimes as well as by human activities (Buttafuoco, Conforti, Aucelli, Robustelli, & Scarciglia, 2012; Conforti et al., 2013b; Luca et al., 2011). Slope wash processes are responsible for the development of calanchi landforms on clayey slopes, producing a typical stream dissected morphology, with very steep gullies separated by narrow ridges (Figure 4). Calanchi landforms are restricted to relatively steep valley side slopes and fluvial scarps.

4. Landslide inventory map A landslide inventory map was produced through a detailed geomorphological survey that allowed to define that the mass wasting are widespread in the study area and represent the main geomorphic processes that control the present-day morphoevolution of slopes (Carrara et al., 1977; Iovine & Merenda, 1996; Iovine & Petrucci, 1998; Merenda, 1983; Rago et al., 2013); many landslides involve settlements and man-made infrastructures (e.g. roads and/or houses) (Figure 5). Our geomorphological analysis has identified that the spatial distribution, size and typology of landslides are largely controlled by the geological and geomorphological context, including structural and stratigraphic conditions, local relief and fluvial downcutting (Crescenzi et al., 1996; Iovine & Merenda, 1996; Iovine & Tansi, 1998; Iovine, Parise, & Tansi, 1997; Merenda, 1983; Sorriso-Valvo & Tansi, 1996). A total number of 1003 landslides was detected and mapped, which covered an area of about 230.4 km2 (30.5% of the total area); the landslide frequency is of about 1.3 landslide/km2. The minimum, mean and maximum landslide areas are 2905, 227,742 and 10,904,946 m2, respectively.


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Figure 4. Calanchi landforms on clayey slope near Oriolo settlement, characterize by very steep gullies separated by narrow ridges.

The landslides mapped were classified on the basis of the prevalent type of movement and state of activity, according to the Cruden & Varnes (1996) classification. In particular, the types of mass movement observed in the study were classified as follows: falls (12 bodies, 1.2%), slides (542 bodies, 54.1%), flows (113 bodies, 11.3%) and complex landslides (334 bodies, 33.4%). In addition, three deep landslides interpreted as Deep-Seated Gravitational Slope Deformations (DSGSD, sensu Agliardi, Crosta, & Zanchi, 2001; Dramis & SorrisoValvo, 1994; Hutchinson, 1988; Mahr, 1977; Zaruba & Mencl, 1969), involving wide slope areas, were mapped. These DSGSDs are characterized by kinemetic mechanisms strongly influenced by structural setting and show relatively small displacements and evident morphological features such as doubled ridges, scarps, counterscarps, trenches, and toe bulging (Crescenzi et al., 1996; Iovine & Tansi, 1998; Sorriso-Valvo & Tansi, 1996).

Figure 5. Examples of landslides that caused the cut of roads.


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The spatial distribution of the different types of mass movement are reported in Table 1, which shows that the complex landslides, characterized by the combination of two or more types of movement (Figure 6a), with a spatial predominance of one of them, are very common, and account for 53.7% of the unstable area. In the study area rotational slides that evolve over time and space into earth or mud flows are the most frequent (Carrara et al., 1977; Iovine & Merenda, 1996; Rago et al., 2013). Slide-type mass movements, mainly rotational slides (Figure 6b), which essentially affect pelitic rocks, represent more than 31% of the whole landslide area. In many cases, high contents of clays promote the down-slope evolution of rotational earth slides into flows. Translational slides are common along dip and near dip slopes, e.g. along the road near Cerchiara di Calabria where outcrop carbonatic rocks. Flow-type landslides, constitute about 7.4% of the of unstable areas, are particularly diffused where clayey lithotypes crop out (Figure 6c). Characteristic features of these mass movements are gently hummocky topography and ridges of accumulated material in the toe area, often causing the deviation of river channels. Many flow-type landslides show elongated accumulation zones, ending with fan-shaped toes (Figure 6d). Falls, mainly rock falls, representing only 1.5% of the mapped landslides, are predominantly located in the zones where steep slopes and more resistant rocks crop out (e.g. carbonatic complex

Figure 6. (a) Complex landslide (rotational slide-earth flows); (b) rotational slide that involve the clay deposits; (c) earth flow that caused the deviation of the river channel; and (d) overview of large flow type landslide that has created a fan in the toe area.

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Table 1.

Spatial distribution of the type and state of activity of the landslides mapped in the study area. Landslide type


Fall

Slide

Flow

Complex

DSGSD

Total

Landslide activity

km2

%

km2

%

km2

%

km2

%

km2

%

km2

%

Active Inactive Total

2.0 1.3 3.2

0.9 0.6 1.5

19.5 52.8 72.3

8.5 22.9 31.4

7.4 10.1 17.5

3.2 4.4 7.6

25.4 98.5 123.9

11.0 42.7 53.7

– – 13.4

– – 5.8

54.3 162.6 230.4

23.6 70.6 100.0

and/or sandstones). Rock fall source areas are often connected with fault scarps, joints, or multiple cleavage systems. Rock falls may be represented by single boulder falls or events that move large volumes of rock. The falling material is deposited at the base of the cliffs and often produces typical debris cones. Also, small earth falls were observed along the outer edge of fluvial valleys, caused by bank erosion; these processes have not been mapped because their size, too small compared to the scale of the map. The state of activity of the landslides was deduced by field observation and the analysis of multi-temporal air photos. In particular the landslides that showed signs of activity after 2006 were considered active (e.g. bright light colors on the orthophotos, unvegetated landslide area, evident cracks in the source area and by a distinct bulge at the toe). Therefore, 29% of the landslides recognized has been assessed active while the remaining 71% has been considered inactive. Many active landslides represent a reactivation of pre-existing dormant landslides. In addition, the diachronic analysis showed that the dominant evolutionary trend of landslides are retrogressive and/or enlarging (sensu Cruden & Varnes, 1996). Interplay between landslides and lithology showed that mass movements mainly occurred on slopes carved in the slope deposits, clay deposits, black shales and clayey-calcareous complex (Figure 7a). In addition, high values of landslide index, expressed as the ratio in percentage between landslide area and total area in each lithology class, were obtained for these lithologies, respectively, of 50.1%, 41.1%, 48.9 and 42.9%; in contrast, low values of landslide index were observed for conglomeratic, sand and alluvial deposits (Figure 7a). Moreover, many landslides are closely related to tectonic setting because the faults have caused an intense fracturing and deformation of the rocks, steep slopes and fluvial undercutting of the slope; in some cases, the main landslide scarps are aligned with the major fault lineaments (Iovine et al., 1997). In many cases the mass movements are related to stratigraphic/tectonic contact between more and less competent sequences and a high degree of fracturation of rocks. Slope angle was considered one the main parameter influencing slope stability (Conforti et al., 2012; Lee & Min, 2001); therefore, the comparison between landslides occurrence and the slope gradient classes was examined as shown in Figure 7b. Landslide occurrence increases with the slope gradient, until this latter reaches the values of 208– 308; above these values of slope gradients, landslide occurrence generally decreases with increasing steepness. The highest values of landslide index (ratio in percentage between landslide area and total area in each slope class) are reached for 208– 308 and 15– 208 classes of slope gradient; on the contrary, landslide index decreases for slope gradients higher than 308 (see Figure 7b). Finally, the primary triggering factors are meteorological events, such as heavy and/or prolonged rainfall and, subordinately, by human activity (e.g. excavation, overloading, concentrated water infiltration), and earthquakes. Most landslides occur during or immediately after intense and prolonged precipitations (Ferrari, Iovine, & Petrucci, 2000). An example of this was the extreme

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Figure 7. (a) Total surface, landslide area and landslide index (expressed as the ratio in percentage between landslide area and total area in each lithologic class) for the lithologic classes. Lithology: (1) alluvial deposits; (2) slope deposits; (3) conglomeratic deposits; (4) sand deposits; (5) clay deposits; (6) sandstone; (7) calcareous – clayey complex; (8) clayey – calcareous complex; (9) turbiditic complex; (10) black shales; (11) carbonatic complex. & (b) Total surface, landslide area and landslide index (expressed as the ratio in percentage between landslide area and total area in each slope class) for the slope gradient classes.

meteorological event of winter 1972– 1973, characterized by prolonged rainfalls, which triggered numerous landslides in the study area (Iovine & Merenda, 1996; Iovine & Petrucci, 1998; Merenda, 1983).

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M. Conforti et al. Conclusions

In this study, geomorphological analyses coupled with data processing in GIS environment allowed to characterize the spatial distribution, the type and the activity of landslides in the north-eastern sector of the Calabria. The analysis showed a widespread diffusion of landslides, that have caused a rapid evolution of slopes and valley bottoms. Also, the inventory maps show that many settlements and manmade infrastructures are damaged and/or destroyed by landslides. A total of 1003 landslides were mapped and they occupy a surface of 230.4 km2, corresponding to about 30.5% of the whole study area. Multi-temporal air photo interpretation and field surveys provided data for distinguishing between active (29%) and inactive (71%) landslides. Some active landslides represent a reactivation of pre-existing dormant landslides. The activation and/or reactivation of the landslides are, mainly, linked to rainfall events. The spatial distribution of landslides reflects the complex interplay between lithology and structural conditions. The gravitational phenomena mainly affect the slopes carved in pelitic rocks. Among the landslide types, the rotational earthflows (complex type) are widespread, and account for 53.7% of the unstable area. Finally, the landslide inventory map compiled in this study provides a baseline information for further assessment of landslide hazards and related risks as well as a tool for environmental management and planning of the north-eastern Calabria. Acknowledgements The authors thank, Prof. Francesco Dramis, Dr. Stefania Pascale and Dr. John Abraham for their critical comments and suggestions, which greatly improved the quality of our manuscript and map.

Software The topographic base, the landslide inventory map and related layout were drafted using the ESRI ArcGIS 9.3. Data collected, using previous geological and geomorphological information, air photo interpretation, topographic maps and field survey, were geo-referenced and digitized and a database containing attributes of the main features observed was created for each mapped landform.

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