**4. Results**

#### *4.1. Density Maps (Heatmaps) of Landslide Types over Abruzzo Hilly Piedmont Area*

Heatmaps of various slope instability processes over the Abruzzo hilly piedmont area (Figure 3) were produced using GIS technology. These maps allowed us to outline the spatial distribution of landslide phenomena. For this kind of analysis, landslides data were labelled according to the type of movement (rotational and translational slides, complex landslides, earth flows, and rockfalls). Colored areas represent the sites with a higher density of slope instability processes in each category. In the current study, a heterogeneous spatial distribution of landslide types was identified, reflecting the physiographic, geological–structural, and geomorphologic setting of the hilly piedmont area.

The analysis allowed us to identify that (i) rotational and translational slides are most widespread in central and southern sectors (Figure 3a) with high density in correspondence of the mesa-plateau landscape on clayey–sandy and conglomeratic deposits and the incision of the main rivers; (ii) complex landslides are heterogeneously widespread in the study area, with the highest density in the southern sectors following the complex rough topography developed on allochthonous pelagic deposits (Figure 3b); (iii) earth flows mainly characterize the northernmost sectors of the study area reflecting the physical landscape on sandy-pelitic turbidites (Figure 3c). Rockfall density map (Figure 3d) shows a moderate to low spatial distribution as the result of episodic and localized slope instability processes related to the morphostructural setting in the inner sectors [76] and cliff recession processes combined with wavecut and gravity-induced slope processes in coastal areas [77]. Regarding this latter case, no landslide events have been identified as clearly representative of this mass movement type in the study area. In detail, we selected the following case studies intended to be representative of the most characterizing and frequent slope instability processes:


The georeferenced location of selected case studies is graphically shown in Figure 3 with capital letters in white circles.

**Figure 3.** Density maps (heatmaps) of various slope instability processes over the Abruzzo hilly piedmont area: (**a**) rotational and translational slides; (**b**) complex landslides; (**c**) earth flows; (**d**) rockfalls. Colored areas represent the sites with a higher density of slope instability processes in each category (black dots). Capital letters in white circles locate the selected case studies. The black line represents the study area.

#### *4.2. Relationship between Lithology and Spatial Distribution of Landslide Types*

A detailed landslide analysis allowed us to differentiate landslide types in order to define the role played by lithological units on landscape development and build up a statistical relationship with the spatial distribution of landslide type.

Preliminary GIS-based analysis of the data derived from available databases (i.e., georeferenced location and detailed landslide information) allowed us to recognize the presence of a large number of landslide phenomena in the study area, reaching 5605 recorded events. In order to promote a relationship between mass movements and lithological units outcropping in the area, recorded landslides were classified according to their typology of movement (e.g., rotational and translational slides, complex landslides, earth flows, and rockfalls). Then, a spatial overlapping between the landslide distribution layer and the vector lithology layer was performed, and a new table of attributes was built (Figure 4).

**Figure 4.** Lithological sketch map of the Abruzzo hilly piedmont area (modified from [56,57]) and spatial landslides distribution [60,61,64,65]. Legend: (1) eluvial–colluvial deposits; (2) sandy shore deposits; (3) recent fluvio-lacustrine deposits; (4) travertine deposits; (5) morainic deposits; (6) old fluvio-lacustrine deposits; (7) conglomeratic deposits; (8) clayey–sandy deposits; (9) sandy turbidites; (10) pelitic turbidites; (11) carbonate deposits in conglomeratic and calcarenitic facies; (12) allochthonous pelagic deposits; (13) carbonate ramp limestones; (14) basin limestones and marls; (15) slope limestone; (16) open carbonate shelf-edge limestones; (17) carbonate shelf limestones and dolomites. Capital letters in white circles locate the selected case studies. The black line represents the study area.

The area of each landslide was obtained from this estimation so that the area ratio of the distribution of landslides in each lithology was derived.

The spatial overlapping allowed us to quantitatively estimate the extension of each lithological unit in the study area in terms of area (km2) and percentage (Table 1). This GISbased technique was useful to define the major lithological abundance (both in percentage and area) of clayey–sandy deposits and pelitic turbidites over the study area. Then, the analysis of spatial distribution compared to the outcropping lithologies was carried by comparing the percentage and number of landslides (rotational and translational slides, complex landslides, earth flows, and rockfalls) on each lithological unit as graphically shown by the pie charts and tables in Figure 5.

This overlapping process shows a heterogeneous relationship between lithological units and the distribution of different types of landslides in the Abruzzo hilly piedmont area. Landslides on Quaternary continental deposits were mostly small flows and slides located along the scarp edge of fluvial terraces. Landslides affecting the cuesta and mesa reliefs on the sands and conglomerates on high gradient slopes or else along structural scarps are represented by rapid earth flows affecting surface colluvial cover; falls and topples affecting the edge of structural scarps on sandstones and conglomerates; rotational and translational sliding, which was less frequent but developed for a long time after the event due to deep water infiltration in the permeable conglomerates and sandstones laying on impermeable clays. Landslides on the hilly slopes and cuesta and mesa slopes affecting clayey–sandy deposits were mostly earth flows, from the small to the very wide. Landslides on the arenaceous-pelitic and marly rocks of the turbiditic succession consisted of mostly rapid surface flows and sliding, affecting the eluvial and colluvial cover, particularly where it is more clay-rich. Landslides on the slopes and isolated reliefs on allochthonous pelagic deposits outcropping in the southernmost sectors were mostly flows and complex landslides occurring on all the slopes with a low gradient due to its complex geological– structural setting.


**Table 1.** Extension of each lithological unit in the study area.

The study area is characterized by 2694 rotational and translational slides, 851 complex landslides, 2003 earth flows, and 57 rockfalls. In detail, rotational and translational slides mostly develop on pelitic turbidites (31.1%), clayey–sandy deposits (29.8%), and conglomeratic deposits (23.2%), with a higher number of events recorded (839) on pelitic turbidites. Complex landslides mostly develop on pelitic turbidites (47.0%), clayey–sandy deposits (16.5%), carbonate ramp limestones (10.8%), conglomeratic deposits (10.2%), with the higher number of events recorded (400) on pelitic turbidites. Earth flows develop on pelitic turbidites (47.9%) and clayey–sandy deposits (35.9%), with a higher number of events recorded (959) on pelitic turbidites. Rockfalls develop on conglomeratic deposits (31.6%), pelitic turbidites (17.5%), carbonate ramp limestones (15.8%), and clayey–sandy deposits (12.3%) with 18 recorded events on conglomeratic deposits. This latter relationship shows a moderate to low distribution as the result of episodic and localized processes related to morphostructural setting in the inner sectors and cliff recession processes combined with wavecut and gravity-induced slope processes in coastal areas.

**Figure 5.** Relationships between lithological units and the distribution of different types of landslides in the Abruzzo hilly piedmont area. (**a**) Pie chart and table showing the percentage and number of rotational and translational slides on each lithological unit; (**b**) pie chart and table showing the percentage and number of complex landslides on each lithological unit; (**c**) pie chart and table showing the percentage and number of earth flows on each lithological unit; (**d**) pie chart and table showing the percentage and number of rockfalls on each lithological unit. Numbers and colors refer to legend in Table 1 and Figure 4.

#### *4.3. Selected Landslide Case Studies*

#### 4.3.1. San Martino Sulla Marruccina Landslide

The case study area is located in the central-eastern hilly area of the Abruzzo Region with heights ranging from 200 to 450 m.a.s.l.; this landscape is interrupted by the S–Noriented Dendalo River valley, where lower altitudes (up to 200 m.a.s.l.) are reached. The study area shows a homogeneous slope distribution (about 5◦–15◦), with some peaks (>20◦) especially in correspondence with the main steep scarps and along the secondary slopes.

From a lithological standpoint, bedrock lithology is composed of a thick marine succession, composed of arenaceous-pelitic and pelitic-arenaceous deposits, known in the literature as the *Mutignano Formation* [78,79]. This succession is composed of clays and silty clays alternated with gray to yellow sands in the lower part, and by gray to yellow sands in medium layers with frequent intercalations of fine-grained sandstone, in the upper part. Quaternary continental deposits include landslide, alluvial, and eluvial– colluvial deposits mainly observed along fluvial incisions and slopes. Strength features of the outcropping rocks are considerably complex, being linked not only to the lithological and structural setting (sub-vertical fracture-sets NNW–SSE to E–W-oriented) but also to the alteration, rearrangement, and loosening processes during complex gravitational phenomena [80]. The landslide phenomenon covers an area of about 2.5 km<sup>2</sup> extending between 400 and 300 m.a.s.l.; it presents a medium length of about 750 m and a significant width of surface rupture area. It is characterized by the main crown of about 2.5 km long, which is locally more than 20 m high. Multitemporal analysis of air-photos, technical cartography, and dendrochronological analysis reveals the first signs of activity in the second half of the 1960s, causing the definition of the first slopes and causing huge damage to roads, buildings, and crops [79,81]. These geomorphological effects, definable in the timespan 1968–1981, are represented by complex landslide bodies with related scarps in the northernmost areas and rotational–translational landslide bodies in the central sector. Nowadays, the movements recorded by the monitoring network are due to a residual activity, but the central sectors are currently affected by a significant local instability due to retrogressive evolution (Figure 6a). Currently, landslides mainly show a rotational and translational sliding surface, as highlighted by counterslopes, counterscarps, and formation of ponds and peatbogs recognized in landslide bodies; smaller instability phenomena are represented by complex landslides and earth flows. Landslide scarps (Figure 7) have different morphological and geomorphological characteristics: where the pelitic deposits outcrop, they are highly degraded, while where sandy deposits are present, they are fresh and evident. The geometrical development of the main and the subordinate crowns are influenced by the spatial disposition of the structural landforms. The planimetric development of the scarps, corresponding in part to the disposition of the families of faults, shows how the geomorphologic processes have been conditioned by the structural setting. The area that surrounds the currently active landslide also presents an old and generalized familiarity with the slope instability processes. Relict shapes and quiescent minor instability phenomena have been observed owing to detailed field surveys and stereoscopic observations [80].

The geomorphological cross-section (Figure 6b) shows how the landslides are in close connection with each other, often presenting several coalescent bodies, also involving landslides activated in the previous time frame. These landslides are characterized by deep failure surfaces, often in the range of several tens of meters. The geometry of the sliding surfaces shows a strong structural control, mainly connected to fault zones and bedding planes; in fact, most of the main landslide scarps and flanks coincide with inferred faults, while the geometry of the sliding surfaces, especially in the middle and lower part of the landslide body, is conditioned by the bedding of the pelitic sequences.

**Figure 6.** San Martino sulla Marruccina: (**a**) multitemporal geomorphological map (derived from unpublished data and modified and updated from [79–81]); (**b**) geomorphological cross-section.

**Figure 7.** Photo documentation of geomorphological features of San Martino sulla Marruccina landslide. (**a**) Aerial view of the landslide area; (**b**) panoramic view of the landslide scarp of Casa dell'Arciprete. Red lines show the planimetric development of main landslide scarps.

The complex landslide system could be divided into fairly regular "blocks", dislocated from each other and generally prismatic in form, originally created by the intersection of tectonic fracturing and faulting systems. The main direction of the landslide mass movement is SW–NE, that is, obliquely to the slope.
