Geomorphology 208 (2014) 50–73

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Geomorphology

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Geology, geomorphology and dynamics of the 15 February 2010Maierato landslide (Calabria, Italy)

Luigi Borrelli ⁎, Loredana Antronico, Giovanni Gullà, Giovanni Marino Sorriso-ValvoCNR-IRPI, Via Cavour 4/6, 87036 Rende, CS, Italy

⁎ Corresponding author. Tel.: +39 0984841451.E-mail address: borrelli@irpi.cnr.it (L. Borrelli).

0169-555X/$ – see front matter © 2013 Elsevier B.V. All rhttp://dx.doi.org/10.1016/j.geomorph.2013.11.015

a b s t r a c t

a r t i c l e i n f o

Article history:Received 26 July 2013Received in revised form 18 November 2013Accepted 20 November 2013Available online 1 December 2013

Keywords:LandslideComplex landslideLandslide borehole stratigraphyLandslide dynamicsGeological slope modelCalabria (Italy)

On 15 February 2010, as a result of intense and long-lasting rainfalls, a large landslide affected a wide areanear the town of Maierato (Calabria, Italy). The studies conducted – including (i) aerial photo interpretation,(ii) geological and geomorphological field investigations, (iii) interpretations of lithology and stratigraphyfrom borehole data, and (iv) observation of videos filmed during the main diastrophic phases of the landslideand of antecedent Google Street View® images – allowed researchers to reconstruct the geological and tectonicsetting of the slope and the internal structure of the landslide with the estimation of the depth of the sliding sur-face, the triggering mechanisms and its evolution.The analysis of the prelandslide event setting demonstrates that this mass movement is the reactivation of apreexisting landslide of alleged seismic origin, remaining at an incipient stage.TheMaierato landslide occurred on a gentle slopemade of late Miocene to Plio-Pleistocene clastic and evaporiticsedimentary rocks. Themainbasal failure surface that developedon thehemipelagicmarls has amaximumdepthof 50 m. The volume of the landslide is ~5 million cubic meters.The type of landslide movement is a complex one, consisting of a very rapid slide of rock and earth and of flow ofdebris and earth. The landslide clearly shows three major types of failuremechanisms: the first type is describedas a rapidlymoving rotational slidewhere back-tilted blocks of sediment are preserved; the second type includesa very rapidly moving translational slide of large rock blocks; the third type includes sudden, extremely rapidflow-slides where the slide material is disaggregated while flowing downward along a gentle slope.The slide is a compound one,with a retrogressive evolution and transformation into earth and debris flow duringthe failure. After the triggering of the landslide, and as a result of the relevant displacement, an important portionof the lower evaporitic unit (Calcare di Base Formation), close to the failure surface, collapsed, therebyundergoinga quick change of its mechanical behavior that became similar to that of a viscous fluid. During the landslide evo-lution, large rocky blocks consisting ofMiocene evaporitic limestones, Pliocene silty clays, and sands were rafted,without severe disturbance, on the destructurated and fluidized limestone. The intense destructuration and thepresence of water transformed the limestone (in the lower parts of the unit) into a viscous material that wassqueezed out of the landslide mass through the jags between the several rafted rocky blocks and along thenatural levees of flow tongue.This event is a rather frequent combination of mass movements made complicated and spectacular by the fluid-ization of theweak limestone that imparted great dynamics to themovements. Such fluidization is an infrequentphenomenon especially in this geological context.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

On 15 February 2010 a large landslide affected the left bank of theScotrapiti Torrent in the municipality of Maierato (Calabria, southernItaly; Fig. 1). The town's residential area has been developed upstreamof the landslide, while the old town center is located on its easternedge. Themass movement caused the destruction of an ~800-m stretchof an important access road to the inhabited area, moved a building at110 m, generated a small lake along the Scotrapiti Torrent, and finally

ights reserved.

caused the loss of about 18 ha of farmland (mainly olive orchards).Moreover, the landslide event determined a warning situation becauseof the presence of dwellings and auxiliary/service buildings (waterand methane supply systems) that were located near the top of themain scarp.

The Maierato landslide occurred at the end of 20 days of precipita-tion (from 25 January to 15 February 2010), characterized by cumula-tive rainfalls of 269.8 mm and a maximum daily rainfall of 35.6 mmrecorded by the rain gauge located in Vibo Valentia, a few kilometersfrom the landslide site. Precipitation depth values are normal for thearea and have return periods of only a few years (Antronico et al.,2010); however, some relevant elements that characterize the rainfall

Fig. 1. (A) Geological setting of theMesima graben (from Tortorici et al., 2002,modified) and (B) digital terrainmodel of the study areawith location of the 2010Maierato landslide. Key tothe symbols: 1) Holocene alluvial and coastal deposits; 2) middle–upper Pleistocene fluvial and marine terraces; 3) lower Pliocene–lower Pleistocene marine succession; 4) upperMiocene terrigenous and evaporitic sediments; 5) crystalline basement rocks (Polia-Copanello unit); 6) Quaternary normal fault; 7) tilting direction; 8) epicenter of M N 6 earthquakes;9) area surveyed at 1:10,000 scale.

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conditions that preceded the landslide can be detected: (i) severe rain-falls have been recorded in the previous 2008–2009 winter period,(ii) the value of the cumulative rain in the 120 days preceding the land-slide onset has been exceeded only a few times in the rain time series,and (iii) the value of the cumulative rain (obtained by summing thetwo single rainfall events that occurred from 25 January to 15 February

2010) ranks sixth out of 4470 events (Coscarelli, 2012). Further criticalfeatures for rainfalls in this time interval had been identified also byFilice et al. (2012), Guerricchio et al. (2012), and byDoglioni et al. (2013).

The particular hazard of the case study derives from the very rapiddisplacement of the sliding mass and its following evolution into flow;more importantly, the possibility to infer important elements on its

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dynamics from the images of the movement makes this case study ofparticular interest.

This work focuses on (i) the reconstruction of the geological, geo-morphological, and tectonic features of the site where the landslideoccurred and its surroundings; (ii) the reconstruction of the geologicalmodel of the slope where the instability occurred (before and after theevent); (iii) the analysis of the dynamics of the landsliding performedthrough videos filmed during the paroxysmal phase of the event.

2. Regional geological setting

The study area (Fig. 1) is located in the northwestern sector of theMesima graben, which constitutes the central part of the Calabro–Sicularift zone (Tortorici et al., 2003). The graben is a tectonic depressionactive since Pliocene to Holocene, filled by marine and continental clas-tic deposits and structured by NE–SW trending normal faults. On itswestern and eastern sides, the graben is bounded by Palaeozoic crystal-line metamorphic rocks of the Capo Vaticano peninsula and the SerreMountain horsts, covered by discontinuous remnants of Miocene andPliocene carbonate and terrigenous deposits, respectively (Rao et al.,2007; Gramigna et al., 2008).

The normal faults delimiting the two sides of the graben (Fig. 1A) areconnected to a tectonic extensional phase, with NW–SE trending maxi-mum extension axes, lasting since the upper Pliocene to the presenttime (Tortorici et al., 1995, 2003; Cucci and Tertulliani, 2006).

The activity of the faults is suggested by the seismicity of the area,which has been affected by several historical earthquakes with epi-centers close to the study area, including those of 1659, 1783, 1905with intensities between 9 and 11 of the MCS scale and magnitudeM = 6 ÷ 7.4 (Boschi et al., 1995; Galli and Bosi, 2002; Cucci andTertulliani, 2006; Fig. 1A). Several environmental effects includingcracks, landslides, and liquefaction phenomena were caused by suchearthquakes. In particular, Cotecchia et al. (1986) indicated that thelong seismic sequence of February–March 1783 produced relevantground effects mainly represented by the activation of large landslides.Several of these phenomena are still visible todaymainly through aerialphotos, and some of them are in an incipient development stage, asindicated in the geomorphologic map annexed to the authors' work.The conditions outlined so far are, therefore, a reference element toidentify contexts prone to the reactivation and evolution of landslideswith features similar to the Maierato landslide of 15 February 2010.

3. Material and methods

Interdisciplinary research on the Maierato landslide extended overseveral months after the event and included the following main steps.

In the first step, geological and geomorphological investigationshave been carried out in an area of about 25 km2 (Fig. 1B), where theMaierato landslide developed, through the interpretation of aerialphotograms (black and white IGM aerial photos of 1954 and 1991 at a1:33,000 scale) and field surveys at a 1:10,000 scale. This study wasconducted to (i) identify geological and tectonic structures that couldhave influenced the context where the event occurred and/or made itprone to such phenomena, and (ii) detect and typify existing massmovements. In particular, the study has been extended over a largerarea in order to verify the presence of a deep-seated gravitationalslope deformation (DSGSD) indicated by some authors (Guerricchioet al., 2010, 2012; Gattinoni et al., 2012; Gattinoni and Scesi, 2013)and its possible effect on the development of the Maierato landslide.

In the second step, the study focused on the slopewhere the instabil-ity occurred (an area of ~2.5 km2), considering the pre- and post-2010event setting. The study of the geomorphological features of the slopepreceding the occurrence of the 15 February 2010 Maierato landslide(pre-2010 event setting) has been conducted through aerial photo in-terpretation and the analysis of Google Street View® pictures of 2009.The photo interpretation has been carried out by observing high quality

color aerial photographs (photo scale 1:15,000); the photo interpretedelements were mapped at a 1:5000 scale. The preexisting activity levelof mapped landslides has been assessed through the comparison ofmultitemporal aerial photos (photos of 2001 compared with photos of1991 and 1955), the analysis of official landslide maps (PAI, Pianostralcio per l'Assetto Idrogeologico della Calabria, 2001), as well as of thedocuments filed at the municipal and provincial records and interviewswith residents. Immediately after the event, geological and geomorpho-logical detailed surveys (1:5000 scale) have been carried out to character-ize the landslide event and the processes that had governed itsdevelopment and evolution (post-2010 event setting). The aim was toobtain the maximum amount of information on the genesis and innerstructure of the slope deformation. The detailed study made it possibleto define themechanism of themotion and the features of the phenome-non. Emphasiswas put on the documentation of outcrops in the landslidebody that provide useful elements for the kinematics of involved rockmasses and for the basal rupture surface localization. Fragments of lime-stone also have been investigated by a scanning electron microscope(SEM). Moreover, data collected through 15 geotechnical boreholes,integrated with the surface data, allowed us to define the geological andgeomorphological model of the slope affected by the landslide.

Finally, in the third step, the dynamics of the landslide –with refer-ence to its reactivation on 15 February 2010 – has been investigated bymeans of (i) information collected through municipal authorities andthrough interviews with residents; (ii) a selection of pictures taken afew hours before the slope collapse; and (iii) two videos taken fromtwo different shooting points during the collapse. In particular, thefilmed evidence of the climax of the paroxysmal phase of the massfailure, besides being a rare document in the literature of landslidephenomena (e.g., Fujisawa et al., 2006; Nomura and Fujisawa, 2006;Fujisawa andOhara, 2008), is extremely useful in understanding the dy-namics of the Maierato landslide.

The collected data has been stored in a GIS database, specificallyimplemented for the Maierato area, and utilized to build and updatethe geological and geomorphological maps of the area.

4. The study area

4.1. Geology and tectonics

The sedimentary succession of the study area mainly consists ofsilicoclastic–carbonate sediments unconformably lying on the Erciniancrystalline basement (Rao et al., 2007; Gramigna et al., 2008). ThePalaeozoic crystalline basement, belonging to the Polia-Copanello unit(Amodio-Morelli et al., 1976), is made up of garnet–biotite–sillimanitegneiss and by various biotite paragneisses (Ioppolo et al., 1978), withmarble and metabasite intercalations. In the north-central sector ofthe study area (Fig. 2), gneisses have taken a band-like foliated structure,characterized by the alternance of light quartzo/feldspatic layers anddark garnetiferous biotite–sillimanite layers. At elevations higher than350 m asl, gneisses are characterized by deeply weathered conditions.

The Miocene transgressive sequence starts with the Tortonianconglomerate (basal conglomerate) consisting of crystalline pebbleschaotically mixed with coarse sandy matrix. Most commonly conglom-erate occurs in an up to 2-m-thick layer. Components are directly de-rived from the underlying basement rocks.

In several areas (Fig. 2), such conglomerates are totally absent andtheMiocene succession directly startswith gray fossiliferous sandstonesof the Tortonian age, gradually evolving upward to yellowish-brown orgrayish, poorly cemented sandstones (Selli, 1957; Papazzoni and Sirotti,1999). The visible thickness of the sandstone unit is about 50 m.

Along the Scotrapiti Torrent (in the area surroundingMaierato; Fig. 2),the sandstone is overlain with an abrupt contact by bluish-grayhemipelagic marls of the upper Tortonian/early Messinian age (Raoet al., 2007). The total estimated outcropping thickness ranges from 5 mto 15 m.

Fig. 2. Geostructural and landslides map of the study area (1:10,000 scale) andmesostructural data. Legend: 1) alluvial fan (Holocene); 2) landslide debris (Holocene); 3) alluvial deposits (Holocene); 4) colluvial soils (Holocene); 5) conglomeratesand sands (middle Pleistocene); 6) silty sands (middle–upper Pliocene); 7) silty clays (lower Pliocene); 8) evaporitic limestones (Messinian); 9) hemipelagic marls (upper Tortonian/early Messinian); 10) fossiliferous sandstones (Tortonian); 11)gneiss (Paleozoic); 12) major normal fault; 13) secondary normal fault; 14) transcurrent fault; 15) thrust; 16) structural measurement station; 17) 2010 Maierato landslide; 18) Draga active block-slide; 19) dormant trench; 20) ancient landslidescarp; 21) dormant slide; 22) dormant block-slide. 53

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Fig. 3. Examples of tectonic structures detected in the study area belonging to the NE–SW fault system (most recent one): (A) fault scarp in the area between Cresta Basilica and CeramidaTorrent; (B) detail of the fault plane; (C) fault scarp to the NW of Cresta Basilica; (D) detail of the fault plane.

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These hemipelagic marls pass upward (Fig. 2) into the Messinianevaporitic limestones (i.e., Calcare di Base Formation) (Selli, 1957;Grasso et al., 1996). The evaporitic limestones are constituted by highlyporous, weak, white to yellow, fine-grained, calcareous banks interbed-ded with a decimetric layer of brownish-red and green laminated marls.The evaporitic limestones show a layering with a generally subhorizontalattitude. The visible thickness of the unit is about 50 m. The top of the unitshows evidence of superficial and underground paleokarst erosion.

Silty clays of the lower Pliocene (Nicotera, 1959) lay in a transgressiveand discordant arrangement on the Messinian evaporitic limestones (in

Fig. 4.Mesoscale fault systems

the south-central part of the area; Fig. 2) and on the Tortonian sand-stones (in the north-central part). Silty clays vary in color from gray togray–blue; they are stratified with layers whose thickness may rangefrom a few decimeters to 1 m, and they are generally subhorizontallystratified. The apparent thickness of this unit is about 15 m on the leftbank of the Scotrapiti Torrent and about 40 m on its right bank.

Silty clays gradually evolve toward the top (Fig. 2) into fossiliferous,poorly consolidated silty sands of themiddle–upper Pliocene, character-ized by high energy depositional structures (plane lamination, hum-mocky and swaley cross-stratification) arranged in decimeter-thick

detected in the study area.

Fig. 5. Evolution scheme of the drainage network of the study area, from the beginning ofthe uplift of theMt. Poro (A) to the present (E), with capture by the Scotrapiti Torrent andinversion of drainage direction of the Ceramida Torrent upper trait.

Table 1Maierato historical earthquake records (Locati et al., 2011, modified).

Siteintensity(MCS)

Date andtime

Epicentral area Epicentralintensity(MCS)

Magnitude

VIII–IX 1638 03 27 15:05 Calabria XI 7.03 ± 0.12VIII 1659 11 05 22:15 Central Calabria X 6.55 ± 0.13VII 1783 02 05 12:00 Calabria XI 7.02 ± 0.08VII 1783 02 07 13:10 Calabria X–XI 6.62 ± 0.11VI–VII 1783 03 01 01:40 Central CalabriaIX 1783 03 28 18:55 Calabria XI 6.98 ± 0.08VI–VII 1832 03 08 18:30 Crotonese X 6.59 ± 0.16VI–VII 1894 11 16 17:52 Southern Calabria IX 6.07 ± 0.10VIII 1905 09 08 01:43 Southern Calabria 7.04 ± 0.16VII 1908 12 28 04:20 Southern Calabria–Messina XI 7.10 ± 0.15VII 1947 05 11 06:32 Central Calabria VIII 5.70 ± 0.13IV–V 1997 06 09 14:10 Vibonese VI 4.47 ± 0.14III–IV 1997 09 03 23:15 Southern Calabria V–VI 4.55 ± 0.13

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sets. Fossiliferous silty sands vary in color from grayish-brown toyellowish-white and present local intercalation of weak sandstones.The visible thickness of this unit is about 10 m on the left bank of theScotrapiti Torrent and about 35 m on its right bank.

Consistent thickness variations in the Pliocene succession coincidingwith the gorge of the Scotrapiti Torrent suggest thepresence of a normalsynsedimentary fault to which the valley adapted.

Finally, Quaternary deposits unconformably close the sedimentarymarine sequence of the Scotrapiti basin (Fig. 2). In particular, they arerepresented by middle Pleistocene continental deposits (organized interraces) with a predominance of light reddish-brown conglomerates(Tortorici et al., 2002), made up of rounded and subangular pebbles ofcrystalline rocks with a coarse sand matrix. Their thickness is variablein the outcrops, and it is mainly lower than 10 m.

Holocene unconsolidated deposits represented by colluvial soils andlandslide debris conclude the stratigraphic sequence (Fig. 2); they havebeen mapped where the estimated outcropping thickness is over 5 m(sufficient to alter the ground-surface topography or conceal the under-lying deposits).

From a neotectonic point of view, the study area is crossed by severalnormal fault segments, mainly striking NE–SW and WNW–ESE andbelonging to the Siculo–Calabrian rift zone (Tortorici et al., 2003). Thefault systems recognized in the study area are represented in Fig. 2. Thekinematics of the fault system has been defined bymesostructural analy-sis performed in 16 measure stations, whose location is shown in Fig. 2.

The NE–SW normal faults are arranged into a southeastward step-wise system whose master faults are represented by two 8-km-long,SE-dipping fault segments (Cresta Basilica-La Rocca Diruta and Scotrapitifaults), which control the morphology of the study area (Fig. 2). Thesefault segments are clearly recognizable on a morphological basis. Theyare characterized by well-developed escarpments, with triangular and/or trapezoidal facets, and control the drainage network; this fault systemjuxtaposes the Neogene–Quaternary sedimentary units with the under-lying Paleozoic metamorphic basement (Figs. 2 and 3). At the mesoscale,fault planes strike from N. 35° E. to N. 80° E. and dip 60°–80° mostly to-ward the SE (Fig. 4). Planes are characterized by normal subvertical slick-ensideswith a pitch angle ranging from75° to 90°, that testify to amainlynormal kinematics (with a minor oblique right-lateral component),induced by the last Quaternary deformational events (Fig. 4).

The N–S system, less evident from a geomorphological viewpointand more ancient than the previously described one, is composed ofmainly normal faults (Fig. 2). Faults belonging to this system, mainlydipping toward the ESE, near the residential area of Maierato, causedthe outcrop of the late Miocene succession (Fig. 2). At the mesoscale,the fault planes in some cases showdiversified and overlapping kinemat-ics. The chronological analysis of such elements points out that this sys-tem presents right-lateral motion, followed by dip-slip displacements,caused by passive reactivation of the still ongoing tectonics (Fig. 4).

The third fault system (the most ancient one) shows WNW–ESEtrending and is identified, from a morphological point of view, by thepattern of streams flowing toward the Scotrapiti Torrent (Fig. 2). Atthe mesoscale, faults display a mainly left-lateral motion. The overlap-ping of dip-slip slickensides on some fault planes indicates a normalreactivation of these structures in late extensional stages (Fig. 4).

4.2. Geomorphology

The elevation of the study area ranges between 100 and 450 m asl(Fig. 1B). The form of the land surface is rather complex and largelycontrolled by the lithological features of the outcropping rocks and bythe structural setting (Fig. 2). The several normal faults have generatedalmost rectilinear slopes and cliffs, the altimetric and plano-altimetricdiscontinuity of the ridge, triangular and trapezoidal facets, saddles,and diverted streams (with elbow-like bends and/or double elbowbends). The main slopes are dissected by NW–SE trending, deep andnarrow valleys (e.g., Ceramida Torrent; Fig. 1B). The drainage network

is strongly asymmetrical with more numerous and longer left channels(Fig. 1B). The upper part of the Scotrapiti Torrent and the wholeCeramida Torrent (Fig. 1B) have dissected their valleys so as to simulatethe trench features to be found in the uppermost part of the body and

Fig. 6. Pre-2010 event landslidesmap (1:5000 scale) carried out through photo interpretation. The state of landslides activity is not determined in this analysis phase. Legend: 1) landslidescarp; 2) landslide body; 3) slide; 4) block-slide; 5) block-slide with uncertain boundary; 6) graben-like trench; 7) uncertain scarp.

Fig. 7. Pre-2010 event landslides map with state of activity assessed through a comparison of multitemporal aerial photos (photos of 2001 compared with photos of 1991 and 1955),official landslidemaps (PAI, Piano stralcio per l'Assetto Idrogeologico della Calabria, 2001), aswell as consulting documents filed at themunicipal and provincial records. Legend: 1) activeslide; 2) dormant slide; 3) dormant block-slide and relative trenches; 4) slope failure (900 m long) made evident by an old, long, discontinuous scarp in incipient development stage.

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the flanks of a DSGSD (Radbruch-Hall et al., 1976; Radbruch-Hall, 1978;Varnes et al., 1989). Other authors (Gattinoni et al., 2012; Guerricchioet al., 2012), in fact, interpreted such morphological features as a conse-quence of a DSGSD and considered the Maierato landslide as being asso-ciated with such phenomenon. Also, the writers of this paper, based onaerial photo interpretation, at first sight considered such phenomenonas a landslide located along the flank of a DSGSD, as observed in othercase studies (Sorriso-Valvo and Tansi, 1996; Sorriso-Valvo et al., 1999;Borrelli et al., 2007). Presently, our detailed investigations have shownthat the drainage orientation in the upper part of the slope (NE of CrestaBasilica) is controlled by faults that are visible at the macroscale and atthemesoscale survey (Fig. 3C andD).Moreover, other structural and geo-morphological indicators of a DSGSD (such as graben-like trenches alongthe upper part of the slope, horizontal displacements of the supposeddeforming body with right-lateral component of movements along theright flank, and left-lateral component along the left flank, as well asthe presence of decompression features and subhorizontal fractures inthe lower part of the slope) have not been found (Dramis and Sorriso-Valvo, 1994; Sorriso-Valvo, 1995; Bisci et al., 1996).

Fig. 5 shows the possible evolution of the drainage network, be-tween the beginning of the uplift of the Mt. Poro and the present time,simulating the typical setting of an area affected byDSGSD,with captureby the Scotrapiti Torrent and inversion of drainage direction of theupper trait of the Ceramida Torrent. The Scotrapiti Torrent and its trib-utaries (e.g., Corvolì and Ceramida Torrents) are deeply incised, formingconvex slopes on the left-hand side of the valley (Fig. 1B). Thismorphol-ogy, combined with the general dip-slope attitude of rock layering, isstrongly favorable to mass movement.

Landslides are largely present also in the medium–low parts of thevalley of the Scotrapiti Torrent, where they prevalently affect Plioceneclays (Fig. 2). In these areas, the slides (generally rotational) are themost widespread landslide type, in some cases evolving into earthand/or debris flows. Such phenomena, currently dormant, show an esti-mated thickness up to 20 m.

Fig. 8. Google Street View® 2009 images taken along the bypass road and S.P. 55 road: (A) sudifferent portions of the limestone; (B) subvertical shear fracture observed on the road scarp ssmall and still visible fall occurred in winter 2009; (C) and (D) lower part of the slope, in the hacracks with horizontal displacement, corresponding to the left and right flank of the 2010 land

Moreover, near the town of Maierato, features related to old slidesand/or block-slides connected with the geological structure of the areahave been observed. In particular, block-slides develop through low-angle sliding surfaces located at the contact between the evaporiticlimestones and the hemipelagic marls (Fig. 2). Such phenomena(some of which probably are of seismic origin) will be described morethoroughly in Section 5.2.

The analysis of the Italianmacroseismic database (Locati et al., 2011)allows us to clarify that from1638 to 1997,Maierato has been affected by13 seismic events. Table 1 shows the most destructive events took placeon 28March 1783 and in 1905. In particular, concerning the 1905 earth-quake, the historical news reports, besides the almost total destruction ofthe town's buildings, were that ‘.....in several places, large fracturesopened in the soil…, the water flowing from fountains was milk-like incolor and it was hot....’ (Rizzo, 1907). As for the ground effects of the1905 earthquake, however, locating them was not possible.

5. The Maierato landslide

5.1. Pre-2010 event setting

The aerial photo interpretation allowed us to identify somepreexisting landslide phenomena and morphological features relatedto them in the portion of the slope affected by the landslide of February2010 (Fig. 6). In this area it was possible to distinguish two side-by-sidelandslide units crossed by the Corvolì Torrent: the NE unit (the Giardino-Mosto landslide), formed by a unique landslide group made of severalslides, confined upstream by an old, long, discontinuous scarp inthe incipient development stage; the second one toward the SE(Draga landslide), where aerial photos allowed the detection of somemorphological features interpreted as two graben-like trenches(upper trench and lower trench), NNE–SSW trending, because of thepresence of a translational block-slide phenomenon; the latter is in a

bvertical shear fracture at the beginning of the hairpin turn, which brings in contact twohowing a displacement of the marl layer interbedded in the evaporitic limestone, with amlet of Giardino, where we observe the presence on the S.P. 55 roadbed of two transverseslide, respectively.

Fig. 9. Photos shot to the day after the landslide triggering (16 February 2010): (A) panoramic view of the phenomenon (view from SW), with indication of the building shifted by about110 m; (B) secondary debris and earth slide flow developed along the left flank of the landslide (view from SE); (C) detail of the landslidemain scarp (view fromNE); (D) and (E) bulgesand lateral levees, respectively, formed along the left flank.

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more advanced stage of evolution but its complete development ishampered by the facing, steep right-side slope of the Scotrapiti Torrent.

The activity state of mapped landslides between the hamlets ofMosto, Giardino, and Draga is shown in Fig. 7.

In the lower part of the slope, interviews to residents and technicaldocuments provided by theMaierato community pointed out that the ac-tivity of the landslide, in the hamlet of Giardino, had been known for along time. In fact, the S.P. 55 road involved in the landslide of February2010 had often been subject to maintenance work caused by the defor-mations and damages that occurred to the road carpet. This active land-slide displayed amain scarp about 1 m high and almost rectilinear flanks.

In the medium part of the slope, on the left and right side of theCorvolì Torrent, some dormant slides have been mapped; these phe-nomena involved the evaporitic limestones, disarranging them intoblocks. In particular, the rock slides mapped on the left side of theCorvolì Torrent can be considered as a unique landslide group made ofmultiple slides presenting a single envelope scarp located at 270 m asland whose height varies from 5 to about 10 m, from now on referredto as scarp at 270-m elevation (Fig. 7).

At about 140 m upstream from the scarp at 270-m elevation, a900-m-long slope failure extends SW–NE at an elevation of 300 m asl,from the hairpin turn of the bypass road to the area of Draga, crossing

Fig. 10. Post-2010 event geostructural and landslidesmap carried out throughfield surveys at 1:5000 scale. Legend: 1) landslide debris; 2) colluvial soils (Holocene); 3) conglomerates andsands (middle Pleistocene); 4) silty sands (middle–upper Pliocene); 5) silty clays (lower Pliocene); 6) evaporitic limestones (Messinian); 7) hemipelagic marls (upper Tortonian/earlyMessinian); 8) fossiliferous sandstones (Tortonian); 9) basal conglomerate (Tortonian); 10) gneiss (Paleozoic); 11) major normal fault; 12) secondary normal fault; 13) transcurrentfault; 14) attitude of strata; 15) 20 February 2010 Maierato landslide scarp; 16) 20 February 2010 Maierato landslide body; 17) trace of 15 February 2010 main scarp; 18) toe of the 15February 2010 sliding surface; 19) active secondary scarps; 20) Draga active block-slide delimited by active graben-like trench; 21) dormant graben-like trenches; 22) dormant sliding;23) ancient landslide scarp; 24) ancient block-slide with uncertain boundary; 25) borehole; 26) track of geological cross section.

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the Corvolì Torrent (Fig. 7). Such failure, classified as dormant, in thehamlet of Mosto was made evident by a degraded and discontinuousscarp whose height was 5 m at most. Immediately downstream of thisscarp, the top of the outcropping succession appears slightly back-tilted.From now on, this scarp shall be referred to as scarp at 300-m elevation.

Subsequently, Google Street View® images of 2009, taken along theS.P. 55 road and along the bypass road between the S.P. 5 and the S.P. 55,have been analyzed. These images show further elements indicatingthat the scarp at 300-m elevation was evidence of a preexisting failuresurface. In particular, close to the hairpin turn of the bypass road, twosubvertical shear fractures affecting the evaporitic limestone were ob-served. Fig. 8A shows the presence of a first subvertical shear fractureat the beginning of the hairpin turn that brings two different levels ofthe limestone into contact (a lighter part on the left and a darker oneon the right). Two subhorizontal discontinuities are also present, andthey clearly stop against the shear fracture. Going some 150 m downthe road, a second subvertical shear fracture can be observed on theroad scarp showing a displacement of about 1 m of themarl layer inter-bedded in the evaporitic limestone (Fig. 8B). In correspondence of thisshear fracture, a small and still visible fall occurred in winter 2009;based on the information collected at the Municipal Technical Office ofMaierato, such a fall occurred in concomitance with two other landslide

events triggered on the valley side of the roadbed of the bypass road(Fig. 7).

Notably, the shear fractures in Fig. 8A and B coincide with the leftflank of the landslide of 15 February 2010 (Fig. 6), thus confirming theobservations drawn from aerial photos.

In the lower part of the slope, in the hamlet of Giardino, GoogleStreet View® 2009 images show the presence of two transverse crackson the roadbed (Fig. 8C, D), which correspond to the left and the rightflank of the 2010 landslide, respectively.

5.2. Post-2010 event setting

Immediately after the 2010 paroxysmal phase, the landslide pre-sented a subvertical main scarp, ~50 m high, where large slickensideswere visible (Fig. 9A–C). Along the left flank of the landslide, in thehamlet of Giardino, some bulgings formed on the slope surface sidingof the landslide (Fig. 9D). Along the right flank, in the hamlet of Draga,ground cracks, conjugate scarps, bulgings, and a small dammed lakeformed along the Scotrapiti Torrent. Large lateral levees were evidentalong the flanks of the flow portion of the landslide consisting of soiland evaporitic limestone, which was destructurated and evidently ex-truded during the movement (Fig. 9E). Small lakes formed inside the

Fig. 11.Rock types outcropping in theMaierato landslide site: (A) fossiliferous sandstones (Tortonian); (B)hemipelagicmarls (upper Tortonian/earlyMessinian), degraded andweatheredin outcrop; (C) evaporitic limestones (Messinian) with a decimetric interbedded laminated marl layer; (D) Pliocene succession, overlying the evaporitic limestones, outcropping on themain scarp of the Maierato landslide.

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landslide body at the base of the main scarp, as well as along the flanksof the flow.

On 20 February, a partial further retreat of the main scarp (of about80 m) occurred in its central sector (Fig. 9A).

Fig. 12. Microphotographs of the evaporitic limestone taken by scanning electron microscopepresent microgeodes. The rock structure shows a brecciated facies, due to autobrecciation proclution diagenesis. Later alteration has removed most of the halite, leaving cubic pores and poro

The 15 February 2010Maierato landslide can be classified as a com-plex slide and flow phenomenon, with compound slide, involving rockand earth material (Varnes, 1978), displaying the following morpho-metric characteristics: maximum elevation of the crown 300 m asl;

(SEM). The rock structure is constituted by rhombohedral calcite crystals, some of whichesses induced by dissolution of halite and gypsum during subaerial weathering and disso-us layers.

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height of the main scarp ranging from 50 m in its central part to about20 m in the lateral sections; maximum length 1400 m; maximumwidth about 530 m; and area about 3 × 105 m2.

The retreat of the main scarp on 20 February affected a surface ofabout 2 × 104 m2 and a volume of about 4 × 105 m3.

5.2.1. Geology and geomorphology of the landslide siteIn the section of the area surveyed in detail (Fig. 10), the Paleozoic

metamorphic basement is covered by sedimentary rocks of theMioceneand Plio-Pleistocene age; the outcrops along the Scotrapiti valley andthe landslide main scarp enable a thorough reconstruction of the bed-rock stratigraphic sequence (Fig. 11).

The Paleozoic gneissic basement is evident only in a small outcrop inthe bed of the Scotrapiti Torrent, in the proximity of the tongue ofthe flow and in correspondence of the footwall of the NE–SW striking,SE-dipping Scotrapiti normal fault (Fig. 10). It is made up of garnet–biotite–sillimanite gneiss (Ioppolo et al., 1978)with a band-like foliatedstructure caused by the alternance of light quartzo/feldspatic layers anddark garnet-, biotite-, and sillimanite-rich layers. The small outcrops ofthe gneiss, detected at the base of the fault scarp, are characterized by ahigh weathering grade.

In the same area, the Tortonian conglomerate transgressive on thegneissic basement crops out, with a thickness of about 1 m; it is madeof poorly sorted conglomerate with clasts deriving from the underlyingsubstratum. This conglomerate passes upward, with a sharp strati-graphic contact, into gray Clypeaster sandstones of Tortonian age(Selli, 1957), characterized by layers with concentration of pebblesand oyster shell lags. The Clypeaster sandstones gradually evolve up-ward to yellowish-brown or grayish, poorly cemented sandstones,rich inHeterostegina papyracea (Papazzoni and Sirotti, 1999). The sand-stone unit (Fig. 11A) has a visible thickness of about 40 m and forms thefault scarp located at the base of the left slope of the Scotrapiti Torrent.

Along the edge of the fault scarps the overlying unit made of upperTortonian/earlyMessinian thin-bedded blue or bluish-gray hemipelagicmarls (Fig. 10) follows with a sharp contact. This unit shows a thicknessfrom a few meters to 20 m. In the most superficial layers, marls aredecolorized as a result of weathering (Fig. 11B).

In the surrounding areas ofMaierato, themarls pass upward througha discontinuity to theMessinian evaporitic limestone (Figs. 10 and 11C)made of highly porous, weak, yellowish-white, fine-grained, calcareous

Fig. 13. 2010 Google Earth image with positioning of the optical cones photos (red color) takeyellow optical cones are related to two videos taken during the paroxysmal phases of the pshows the three major homogeneous sectors identified (in terms of evolution phases in the fa

banks interbedded with decimetric brownish-red and green laminatedmarls. The evaporitic limestones show a layering with a generallysubhorizontal attitude. The visible thickness of the unit is about40 m. This unit shows a brecciated facies caused by autobrecciationprocesses induced by dissolution of halite and gypsum during subaerialweathering and dissolution diagenesis (Pedley and Grasso, 1993; Ryan,2009). Therefore, later alteration has removed most of the halite,leaving cubic pores, porous layers, and halite pseudomorphs filledwith calcite. Granules of evaporitic limestone that make up the struc-ture of the rock appear to have sharp or slightly subrounded edgesunder binocularmicroscope observation, and they consist of aggregatesof rhombohedral calcite microcrystals under the SEM observation(Fig. 12). Some crystals present microgeods. This evidence of pervasiveporosity makes the evaporitic limestones prone to collapse of the inter-nalmicrostructure (Hansen, 1965; Kastens and Spiess, 1984; Anson andHawkins, 2002; Ryan, 2009).

The landslidemain scarp shows the stratigraphic succession lying onthe evaporitic limestone and allows us tomeasure the thicknesses of thelithological units and the attitude of strata. In particular, on the evaporit-ic limestone outcropping at the base of the main scarp, a silty-clayeyunit of the lower Pliocene lies in slight angular discordance (Figs. 10and 11D); the unit shows a color variable from gray to grayish-greenand a thickness of about 15 m, being slightly stratified with strata atti-tude dipping a few degrees into the slope.

Toward the edge of the main scarp, the silty-clayey unit is graduallysubstituted by fossiliferous grayish-yellow sands of the medium-upperMiocene, with thin intercalations of soft sandstones (Figs. 10 and11D); their visible outcropping thickness is about 10 m.

Reddish sandy-gravelly deposits of the Pleistocene, about 5 m thickand visible only in the central part of the scarp, close the succession(Fig. 10).

Upslope of the landslide scarp, at about 60 m from its edge, a normalfault belonging to the NE–SW system and dipping toward the SE, dis-places the Pliocene silicoclastic succession by about 30 m (Fig. 10).

The 2010 Maierato landslide that represents the reactivation ofthe NE landslide unit (Fig. 6) has developed thus forming a complex,1.4-km-long landslide body. During this movement, the SW siding ofthe Draga landslide underwent only a slight reactivation.

The Maierato landslide can be divided into three sectors: upper,intermediate, and lower (Fig. 13).

n during field surveys in the landslide body; the asterisks refer to the close-up shots; thehenomenon (V1, amateur video; V2, video of a private broadcast company). The figureilure mechanisms) and described in the text.

Fig. 14. Photos of the study case: (A)–(D) upper sector (location in Fig. 13). (A)Main scarp (~600 m) that extends fromNE to SW, making a gentle curve; (B) valley of the Corvolì Torrentthat split themain scarp into two parts; (C) band of parallel joints (~15 mwide), NE–SW trending, related to the decompression that occurred during landslidemovement along themainscarp; (D) large rock blocks rotated backward.

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In the upper sector the main scarp, characterized by a length of~600 m and which develops itself from NE to SW, makes a gentlecurve (Fig. 14A); it is split into two parts by the valley of the Corvolì Tor-rent (Fig. 14B). A 15-m-wide band of parallel joints, NE–SW trending, ispresent on a section of the main scarp parallel to the mass movementdirection and located to the NE of the Corvolì Torrent (Fig. 14C). Jointsare subvertical, with an average spacing of about 1 m. Such featuresare related to the decompression that occurred during the 15 Februarylandslide movement along the main scarp. Immediately downstreamfrom the main scarp, in the whole upper portion of the landslide body,large blocks where the original stratigraphy is considerably preservedare rotated backwards up to 40° (Fig. 14D). Along the sides of theseblocks, pressure ridges made up of Pliocene silty clays and Messinianevaporitic limestone are present.

In the intermediate sector of the landslide (Fig. 13), some very largerock blocks (from2 to 6 ha) are present; they are slightly deformed, andtheir base is made up of evaporitic limestone (Fig. 15E). Such blockshave been displaced over a distance of up to 400 m and still bear nearlyunaltered vegetation cover with olive trees. Tongues of destructuratedevaporitic limestone and completely reworked hemipelagic marlshave been observed through the large blocks and along the flanks ofthe landslide.

The trenches dug to drain the landslide body have exposed brownish-red and green strata of laminated marls interbedded inside the blocks ofevaporitic limestone. Such marls are stretched and deformed withasymmetrical centimeter-size folds verging toward the direction ofthe landslide movement. Locally these layers are doubled and wedgedinto the cracks of the rafted blocks (Fig. 15F). Locally, where the top ofthe laminatedmarl strata is exposed to daylight, slightly striated sliding

surfaces are observed (Fig. 15G). The ductile deformation of the marllayers and the sliding of the limestone banks on such layers allowedkeeping the integrity of the blocks, without severe disturbance.

In the terminal part of the intermediate sector (Fig. 13), excavationscarried out to facilitate water drainage after the mobilization have ex-posed the toe of the failure surface of theMaierato landslide. The slidingsurface is clearly exposed for about 20 m inside the hemipelagic marls(Fig. 15H). Slickensides are to be observed on it; the marls take on areddish-brown color, visible for a thickness of about 50 cm, and havethe consistence of terra cotta.

In the lower sector of the landslide, a chaotic mixture is present: it ismade up of yellowish-white limestone fragments, ranging in size fromsand to small blocks, and reworked blue marls (Fig. 16I); it is the flowtongue overlapping the in situ Tortonian sandstones (Fig. 16J).

In the Draga landslide, the upper trench develops as a prolongationof the Maierato main scarp (Fig. 10), probably as a consequence of thestructural control exerted by joints and/or minor faults. Such a trenchis dormant and extends down to the thalweg of Scotrapiti Torrent,whose left side slope forms a steep cliff (Fig. 17K). This trench clearlyshows NNE–SSW trending listric shear surfaces of gravitative origin,with horizontal extension toward the WNW–ESE. These structures, bi-furcated in the south end (Fig. 17K), form a single graben-like trenchto thenorth (Fig. 10). Such a graben-like trench, typical of a translationalsliding, has caused the spreading of the limestone bank and the sinkingof the overlying Pliocene deposits (Fig. 17K–M) that, caused by the dis-placement suffered, shows a relevant and pervasive subvertical fractur-ing obliterating the sedimentary structures. Furthermore, immediatelySE of the trench, a decimetric brownish-red and green laminated marllayer, interbedded with the evaporitic limestone (Fig. 17N), shows

Fig. 15. Photos of the study case: (E)–(H), intermediate sector (location in Fig. 13). (E) Very large rock block of evaporitic limestone, little deformed, translated downstream; the olive treesin the background lie on this block; (F) laminated marl strata, interbedded inside the evaporitic limestone, stretched and deformed with asymmetrical centimeter-size a fold, vergingtoward the direction of the landslide movement, locally doubled and wedged into the cracks of the rafted blocks; (G) secondary sliding surfaces exposed on the remnant marl strata;(H) toe of the failure surface of the Maierato landslide daylighting (for about 20 m) in the terminal part of the intermediate sector, inside the hemipelagic marls where slickensidesand reddish-brown coloration are observed.

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evidence of ductile deformation (Fig. 17O) that could have been causedby a translational movement of the overlying limestone bank.

In the 2010 event, only the SE portion of the Draga block-slideunderwent reactivation highlighted on the ground surface by a morethan 100-m-long, freshly-open fracture system (Fig. 10). The SE portionis characterized by an evident graben-like depression containing thecollapsed Pliocene clayey–sandy succession. According to the data

Fig. 16. Photos of the study case: (I)–(J), lower sector (location in Fig. 13). (I) Chaotic mixture omarls of the flow tongue filling the valley bottom of the Scotrapiti Torrent; (J) particular of the

collected on thefield, the succession is displaced by about 20 m. At pres-ent, some of the ground fractures display continuous, slow enlargementtestifying to a residual post-paroxysmal activity.

Between the NE Maierato main scarp and the upper trench of theDraga landslide, some ground cracks and conjugate small scarps(about 50 cm high) related to the movement of the 15 February land-slide have been detected (Fig. 10).

f yellowish-white limestone, ranging in size from sand to small blocks, and reworked blueflow tongue overlapping the in situ Tortonian sandstones.

Fig. 17. Steep cliff along the left side slope of Scotrapiti Torrent (photo location in Fig. 13): (K) panoramic viewof the trenches, typical of a translational sliding, that caused the spreading ofthe limestone bank and the sinking of the overlying Pliocene deposits (K)–(M); (N) decimetric brownish-red and green laminatedmarl layer, interbeddedwithin the evaporitic limestone,and showing evidence of ductile deformation (O) with a folding that could have been caused by a translational movement of the overlying limestone bank.

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5.2.2. In situ investigationsIn order to acquire more information on the internal structure of the

landslide and the underground geology of the mobilized slope, 15 geo-technical core drillings have been carried out inside the landslide bodyand the surrounding areas (Fig. 10) (Gullà et al., 2012).

The stratigraphy of the drilling and the pictures of the core areshown in Fig. 18. Drilling depth varies from about 20 m (B15) toabout 100 m (B8). The thickness of the geological units found in depthvaries from 4 to 6 m for Quaternary gravelly sands (Fig. 18A); from 3to 17 m for Pliocene fossil sands (Fig. 18B); from 1 to 12 m for the Plio-cene silty clays (Fig. 18C); from 9 to 38 m for the Messinian evaporiticlimestone (Fig. 18D); from 2 to 19 m for the upper Tortonian/earlyMessinian hemipelagic marls (Fig. 18E); and from 15 to 42 m for theTortonian sandstones (Fig. 18F); the Tortonian conglomerate found

only in the B7 drilling has a thickness of about 6 m. The depth of thePaleozoic gneissic basement (Fig. 18G), found only in some drillings,varies from about 50 m (B1) to about 90 m (B3). Deep gneiss is weath-ered to a degree ranging from highly weathered rocks to completelyweathered rocks (Gullà and Matano, 1997; Borrelli et al., 2012). Inparticular, in the B8 borehole the gneiss is completely weathered andreduced to saprolite for its whole thickness of about 18 m (Fig. 18H).

The two boreholes performed in the landslide body (B8 and B13)show that the actual thickness of the same is about 25 m (Fig. 18).

A thorough analysis of core materials revealed that at the contactlayer between the landslide debris and the unaffected hemipelagicmarls, striated surfaces and sharp fragments of evaporitic limestone –

randomly packed and dipped into the marls for a few centimeters –are present. Such a layer identifies the sliding surface of the landslide.

Fig. 18. Stratigraphy from the geotechnical boreholes carried out inside the landslide body and in the surrounding areas (Fig. 10) and photos of the principal rock types detected (A–H).Legend: 1)Maierato landslide debris; 2) anthropic cover; 3) Pleistocene gravels and sands (photo A); 4) Pliocene sands (photo B); 5) Pliocene silty clays (photo C); 6)Messinian evaporiticlimestone (photo D); 7) upper Tortonian/early Messinian hemipelagicmarls (photo E); 8) Tortonian sandstones (photo F); 9) Tortonian conglomerates; 10) Paleozoic gneissic basement,moderately (photo G) and completely (photo H) weathered; 11) sliding surfaces.

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Fig. 19. Geological model of the Maierato landslide (A–A' section) and Draga translational slide (B–B' section): 1) Maierato landslide debris; 2) Pleistocene gravels and sands; 3)Pliocene sands; 4) Pliocene silty clays; 5) Messinian evaporitic limestone; 6) upper Tortonian/early Messinian hemipelagic marls; 7) Tortonian sandstones; 8) Paleozoic gneissicbasement; 9) fault; 10) sliding surface the February 2010 landslide, presently inactive; 11) active block-slide (Draga SE portion); 12) dormant sliding surface of the Draga block-slide; 13) versus of displacement of Draga slide; 14) borehole (a) and projected borehole (b).

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In particular, in the B8 drilling, the sliding surface has been located at adepth of about 25 m; while in the B13 at a depth of 23 m. In the B10drilling, carried out in the area of the Draga translational block-slide,the same features have been found at a depth of 23 m in the contactzone between the evaporitic limestone and the underlying marls.

5.2.3. Geological model of the slopeIntegrating surface and subsurface data have allowed us to recon-

struct the geological model of the slope.The A–A' cross section of the Maierato landslide (Figs. 10 and 19)

shows the failure surface of 15 February 2010 in its complete develop-ment, considering also the retreat of the scarp of 20 February, 2010.The failure surface, which is almost planar, has a total length of about500 m and amaximumdepth of about 50 m in the upper part, progres-sively decreasing toward the lower part with respect to the pre-eventslope profile. This surface crops out in correspondence with theScotrapiti Torrent. The difference in elevation between the edge of thecrown and the toe is about 105 m, while the assessed volume of thewhole landslide body is about 5 × 106 m3.

The failure surface of 15 February developed mainly within thehemipelagic marls; it has a planar geometry with an approximate dipof 8° toward the SE. The retreat of the 20 February failure surfacemainlyoccurred in the evaporitic limestone (Fig. 19).

Yielding of marls is an expected item, being the formation with thelowest resistance parameters among those actively involved in the rup-ture (Gullà et al., 2012; Gattinoni and Scesi, 2013). Less expected is theyielding of the evaporitic limestone; but this fact, as already discussed,depends on the peculiar conditions of this formation.

Evidence disagrees with previous works on the Maierato landslidestating that the failure surface develops into the Tortonian sandstonesat a depth of 60–70 m (Gattinoni et al., 2012; Guerricchio et al., 2012;Gattinoni and Scesi, 2013).

The B–B' cross section of the Draga landslide (Figs. 10 and 19) showstwo graben-like trenches delimiting the upper part of the translationalblock-slide. The structure of the main trench, activated during the Feb-ruary 2010 event, has been verified by the B10 core boring. Here, as de-scribed in Section 5.2.1, the limestone slid on the marls thus leaving agap into which Pliocene terrains sank, losing the evidence of their geo-logical structure (Fig. 19). The competence of the slid limestone bankwas estimated to be about 25 m; the volume was estimated to beabout 8 × 105 m3. The failure surface is very close to the roof of thehemipelagic marls and nearly horizontal. It does not correspond in ele-vation and depth with the failure surface of the Maierato body (Fig. 19,section C–C'), being some 15 m higher, thus suggesting the separationof the Maierato and Draga phenomena into two different landslides —though they probably share the same structural discontinuities as con-trol features for head scarps.

Upslope, the B–B' section shows the presence of a second dormanttrench responsible for the geological and geomorphological evidenceshown in Fig. 17. In the depth, the upper trench shows a dormantlistric-like surface of rupture directly linked with the active one, whichdevelops always in contact with the marls (Fig. 19).

5.3. February 2010 dynamics of landsliding

Based on the information collected, 8:00 AM is the time of reactiva-tion of the landslide, when a 10-cm displacement occurred along thescarp at 270 m elevation. Because of its potential consequences, themayor of the Maierato town municipality ordered the evacuation ofthe people living in the area affected by the landslide and forbade trafficon the S.P. 55 road.

Some photos taken at about 1:30 PM (Fig. 20A), clearly show the re-activation of the scarp at 270 m elevation (maximum height about5 m), located between the Corvolì Torrent and the area of Giardino

Fig. 20. Photos pre- (A) and post- (F) paroxysmal phase and frames (B) to (E) from the amateur video filmed almost perpendicular to the movement direction, using a mobile phone,during the collapse of 15 February 2010 (Fig. 13). For explanation see the text.

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Fig. 21. Deformation of the S.P. 55 road a few hours before the paroxysmal phase of the landslide occurred on 15 February 2010. Photo by F. Lo Bianco (fireman in Vibo Valentia).

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(Fig. 7). Such reactivation has caused a ground bulging in the centralpart of the slope between this scarp and the S.P. 55 road, as well asthe formation of cracks and bulgings along the same road (Fig. 21).These cracks correspond to those shown in the photos taken from the2009 images of Google Street View® (Fig. 8C, D).

Starting from 4:30 PM, two videos were filmed that clearly displaythe dynamics of the movement.

The first video, lasting about 15 min, was filmed by a citizen (courtesyof V. Cirillo) by means of a mobile phone from a shooting point locatedalmost in front of the slope affected by the movement (Fig. 13). Observa-tion of some single frames shows that at about 4:55 PM (Fig. 20B), afurther increase of the scarp at 270 m elevation occurred (maximumheight about 15 m) after a significant increase of the landslide velocity;at the same time, another ground rupture started to delineate (maximumheight about 2 m) precisely following the previous scarp at 300 m eleva-tion (hereinafter referred to asmain scarp), identified on the aerial photospredating the event (Fig. 7). Simultaneously with the reactivation of themain scarp, immediately downstream of the same, a third rectilineartransversal fracture came to evidence involving an electricity pylon,which was reclined by about 20° (Fig. 20B).

At 5:04 PM, themovement along themain scarp extended toward thehairpin turn of the bypass roadwhere the displacementwas such as to cutthe electric cables located at the edge of the road, as indicated by the flashin Fig. 20C. The main scarp near the hairpin turn of the bypass road coin-cides with the two subvertical shear fractures located on the road scarpand observed through Google Street View images (Fig. 8A, B), one ofwhich had already shown signs of reactivation in March 2009 (Fig. 8B).From now on, a further increase of the landslide activity occurred andthe landslide completed this fast diastrophic phase in the time span ofabout 4 min. At 5:06 PM, starting from the frame of the video inFig. 20D, an arrest of the activity was initially recorded along the scarp at270 melevation, and subsequently a reduction of its heightwas observed.At the same time, a progressive increase in the height of the main scarp

was recorded (maximum height 20 m), while the scarp at 270 m eleva-tion continued to decrease until it disappeared. The sliding of the landslidebody, included in the portion between the two scarps, produced the disar-ticulation of the rock mass involved in the movement (Fig. 20D). At thistime, almost contemporaneously to the increase in height of the mainscarp along the left flank of the landslide (Fig. 20D), a secondary landslidemovement exclusively affecting the evaporitic limestone started; this wasshown by a video filmed by the video camera of a private broadcast com-pany, subsequently analyzed (www.youreporter.it). This landslide rapidlyevolved into flow. At about 5:08 PM, the main scarp was almostcompletely developed (maximum height 50 m); while in the lowerpart, a generalized ground-surface lowering, the dissection and fragmen-tation of the landslide body, and the consequent translation downward oflarge rock blocks are shown in the movie. During this phase, the moviealso shows the extrusion of viscous masses of limestones and marlsthroughout the gaps between the dissected large rock blocks.

The average velocity assessed on the basis of the observation of thevideos, in the time frame between 4:55 PM and 5:08 PM, is about0.5 m/s. We can approximately assume that, during the movement,energy equivalent to 43 × 1011 J was released.

Fig. 20F (March 2010) shows the landslide phenomenon in its com-plete evolution, including also the retreat of the 20 February scarp.

Fig. 22 shows a photo taken at about 1:26 PM (Fig. 22A) and someframes (Fig. 22B, E) taken from the second video (lasting about 3 min)filmed by the operator located almost orthogonally to the movementdirection (Fig. 13). In particular, the video enabled us to observe thedebris and earth flow dynamics triggered in correspondence with theleft end of themain scarp, almost contemporaneouslywith themain phe-nomenon (Figs. 9B and 20D). The assessed velocity of this phenomenonwas 5 m/s on average. Moreover, this video shows more clearly howthe scarp at 270 m elevation was progressively destroyed (Fig. 22C, D)starting from 5:06 PM, while the scarp at 300 m elevation (main scarp)started to be outlined (Fig. 22E).

Fig. 22. Photos taken during the paroxysmal phase (A) and frames from the video (B)–(E) filmed by an operator of a private broadcast company (www.youreporter.it) located almostorthogonally to the movement direction (Fig. 13). For explanation see the text.

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6. Discussion

The research illustrated in this paper was aimed at defining thegeomorphological features and the dynamics of the 2010 Maierato

landslide. First of all, we had to make clear whether this event wasrelated with a DSGSD or not. Indeed, preliminary geomorphologicalobservations from aerial photos, as well as papers published on theMaierato landslide (Gattinoni et al., 2012; Guerricchio et al., 2012;

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Gattinoni and Scesi, 2013) indicated that the study area could have beenaffected by a DSGSD. In particular, Guerricchio et al. (2012) indicatedthat the presence of a DSGSD represents an important predisposing fac-tor for the activation of the Maierato landslide, which could be a conse-quence of the same DSGSD. For this reason this hypothesis wassubmitted to analyses that led to its rejection owing to the followingreasons: (i) the drainage network is controlled by the recent tectonicfeatures and the current drainage pattern (trellis type essentially) ismost probably caused by the combination of structural control andstream capture phenomena (Fig. 5); (ii) along the flanks of the supposed

Fig. 23. Dynamics of the 15 February 2010 Maierato landslide (for complete explanations see th(B) failure phase of the landslide body delimited by the scarp at 300 m elevation; (C) beginnincumulated displacements produced by the translational slide; (D) transition from slide into v20 February 2010. Key to the symbols: 1) Pleistocene gravels and sands; 2) Pliocene sands; 3) Phemipelagic marls; 6) Tortonian sandstones; 7) Paleozoic gneissic basement; 8) chaotic mixtu9) fault; 10) dormant sliding surface; 11) active sliding surface; 12) secondary yielding surface

DSGSD, some left-lateral strike slip faults – reactivated (as normal) byrecent extensional tectonics – are present; at the same time, neither evi-dence of gravity-derived transcurrence along the flanks nor gravitativedisplacement ofmorphological or geostructural elementswere observed;(iii) the B8 deep borehole, driven down to a depth of 50 m below thelevel of the bed of the Scotrapiti Torrent and, therefore, lower than thepossible depth of the DSGSD, did not show any deformation zone in thecrossed rocks below the sliding surface of the Maierato landslide.

The control of tectonic structure and geology on the morphology ofthe study area is important with respect to the landslides spreading

e text): (A) failure phase of the landslide body delimited by the scarp at 270 m elevation;g of the structural collapse of the evaporitic limestone in its basal portions caused by theery rapid flow and translation downstream of large rafted rock blocks; (E) retreat of theliocene silty clays; 4) Messinian evaporitic limestone; 5) upper Tortonian/early Messinianre of destructurated and fluidized evaporitic limestones and reworked hemipelagic marls;s.

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on the SW- andNE-facing slopes of the study area. Such structures deter-mined the strong asymmetry of the Scotrapiti valley, showing a muchlarger development of the left-side slope, which is the site of the land-slides under investigation where layering displays a dip-slope attitude.In addition, the strongly convex shape of the slope and the sharp deepen-ing of the valley in correspondence of the Maierato landslide are both infavor of slope instability. Here, and to a lesser extent across the wholestudy area, such conditions are confirmed by the presence of preexistentlandslides and related morphological features (including scarps, niches,step terraces, graben-like depressions, and slope ruptures; Fig. 6).

The February 2010 event consisted of the reactivation of a preexistinglandslide. Themain landslide scarp at 300 m elevation corresponds to anold degraded and discontinuous scarp (Figs. 6 and 7) of probable seismicorigin, according to the evidence that several coseismic landslides oc-curred during the last three centuries in the areas close to Maierato orin other geologically similar areas, and have remained in a stage of incip-ient failure (Sarconi, 1784; Vivenzio, 1788; Cotecchia et al., 1986; Sorriso-Valvo and Tansi, 1996). Google Street View® 2009 images showed thatthe eastern section of this old scarp coincided with two subverticalshear surfaces, one of which has been recently activated. In particular,the fall in correspondence of the shear fracture cropping out on theroad scarp of the bypass road (Fig. 8B) and the activation of the twosmall landslide phenomena downstream from the road (Fig. 7) – bothoccurring in March 2009 – are clear evidence of the reactivation of thisportion of the landslide, as well as of the fact that the full reactivationcovered nearly the entire subsequent humid season.

Thanks to the integration of field survey data with drilling logs, wecould define the relevant characteristics of the study event which canbe summarized as follows:

• The 15 February 2010Maierato landslide is a complexphenomenon ofvery rapid rock and earth slide and debris and earth flow (Cruden andVarnes, 1996). The slide is a compoundonewith retrogressive evolutionand transformation into debris and earth flow during the collapse(Fig. 9A). The initial volume of the mass movement was about2.2 × 106 m3 delimited by a first landslide scarp at 270 m elevation,followed by further retrogressive expansion of the movement, upto 300 m asl, involving extra material whose volume was about3 × 106 m3.

• The main basal failure surface developed inside the Tortonian/earlyMessinian hemipelagic marls (Figs. 15H and 18) where slickensides,chaotic texture, and also fraction banding induced by shearing arepresent. The maximum depth of the failure surface was about 50 m;the entire landmass slid at once on a sliding surface dipping 8°.

• The landslide clearly shows three evolution phases, each characterizedby different failure mechanisms. The first phase was a rapid rotationalslide where back-tilted blocks of rocks were preserved. The secondphase included very rapid translational slide, along gentle slip surfaces,of large rock blocks with minor little rotations. The third one includedsudden and extremely rapid flow-slide where the slide material hasbeen destructurated and subsequently disaggregated while it wasflowing along a gentle slip surface.

Videos, pictures, and information drawn from interviews allowed adetailed reconstruction of the dynamics of the Maierato landslide(Fig. 23).

The first movements of the paroxysmal phase were observed on thescarp at 270 m elevation starting from 8:00 AM; at 1:30 PM the scarpwas clearly outlined with a maximum height of 5 m (Fig. 23A). At4:55 PM the movement expanded toward the lower part of the slopewith an increase in the height of the scarp; at the same time, the retro-gressive evolution of the phenomenon started caused by the loss of sup-port by themovingmass and testified by the reactivation of the scarp at300 m elevation (Fig. 23B). Based on that, we can assume that the rup-ture phase of the landslide delimited by the scarp at 270 m elevationand the prerupture phase of the landslide mass delimited by the scarp

at 300 m elevation, both occurred and completed from 8:00 AM to4:55 PM. At 5:06 PM the movement clearly expanded up to the scarpat 300 m elevation which reached the height of about 20 m (Fig. 23C).Between 5:06 PMand5:08 PM, the displacement caused by the transla-tional slide caused the structural collapse of large volumes of evaporiticlimestone in its basal portion and, as a consequence, the transition of theevent from very rapid slide into extremely rapid flow. At the same time,large rock blocks rafted on the destucturated and fluidized limestone ofthe lower layers, translated downstream. Both post-eventfield evidenceand movies show how, during this phase of movement, part of thevoids-rich limestonewas squeezed out on the ground surface (togetherwith the sheared hemipelagic marls) through the jags between theseveral rafted rocky blocks and along the natural levees of flow tongue(Fig. 9). At 5:08 PM the upper main scarp was completely developedand reached the height of about 50 m. It increased in height becauseof the thinning of the moving mass owing to the collapse anddestructuration of large basal portions of limestone during the move-ment (Fig. 23D). From 5:08 PM until the end of the landslide bodymovement, the extremely rapid flow developed; and the flowingmate-rial, essentiallymade up of destructurated and fluidized evaporitic lime-stone and reworked hemiplegic marl materials, filled the valley bottomof the Scotrapiti Torrent.

The evaporitic limestone, thus, played a key role in the evolution of thelandslide phenomenon, like during the rupture phase of the landslide. Thepresence of several voids caused by the dissolution of the halite betweenthe aggregates of calcite microcrystals (Fig. 12) favored the progressiveand complete destructuration of relevant rock volumes located close tothe failure surface. The destructuration, or the transition from rock-liketo soil-like behavior (Leroueil and Vaughan, 1990), occurred graduallyas the bonds break. This structural collapse was probably favored by thedeformations cumulated in the reactivations prior to February 2010. Fur-thermore, in 2010 thepresence ofwater favored the transformationof themechanical behavior of relevant rocky volumes,which displayed a behav-ior similar to that of a viscous fluid. In particular, the rheological behaviorof the evaporitic limestone of the basal portions of the unit justifies thevery rapid velocity of the event during the paroxysmal phase and the rel-evant translation (about 400 m) of large rock blocks.

As a consequence of the reactivation and development of theMaierato landslide, the block-slide in Draga showed partial reactivationin its lower sector (Fig. 10). This phenomenon developed along a nearlyplanar failure surface. The comparison of the data from field observationwith the borehole data allowed demonstrating that the landslide massis principally made up of evaporitic limestone and that the rupturesurface is located within the marls, very close to the contact with thelimestones (Fig. 19). The presence of the graben-like trenches in the up-stream sector of the block slide, the ductile deformations of the marlylayer interbedded in the limestone bank, and the subhorizontal failuresurface suggest a seismic origin of this phenomenon (Hansen, 1965;Chigira et al., 2003; Chang et al., 2005; Chigira and Yagi, 2006; Páneket al., 2008, 2012; Miyagi et al., 2011; Tsou et al., 2011).

Notably, the failure surfaces of theMaierato and Draga landslides donot correspond in elevation and depth: the Maierato failure surface issome 15 m lower and 8° dipping, while the one in Draga is nearlyhorizontal. This evidence suggests that the phenomena of Maieratoand Draga are two different landslides (Fig. 19, section C–C'), thoughboth headscarps are probably controlled by the same structuraldiscontinuities.

7. Conclusions

In conclusion, we can affirm that the following elements were themain predisposing factors for the study event: (i) the geological–struc-tural setting of the area; (ii) the convexmorphology of the slope and itsstratigraphic asset; (iii) the topographic marking of old morphologicalfeatures linked to seismic events; (iv) the porosity of the limestonecausedbydissolutive diagenesis andprobably enhancedby the secondary

72 L. Borrelli et al. / Geomorphology 208 (2014) 50–73

dissolution eased by the deformations cumulated that occurred duringthe reactivations prior to February 2010; and (v) the poor mechanicalproperties of hemipelagic marls.

A fundamental role for the triggering of the landslide was likelyplayed by pore-water pressures connected with the complex system ofgroundwater circulation that was critical in the preceding wet seasonsand furthermore increased by the continuous rainfalls that occurred inthemonths prior to the landslide, which had clearly proven to be alreadyunstable the year before the event. However, data relative to the ground-water conditions prior to and during the failure are not available.

This event is a rather frequent combination ofmassmovementsmadecomplicated and spectacular by the fluidization of the weak and porouslimestone that imparted great dynamics to themovements. Suchfluidiza-tion is a rare event in this geological and geomorphological context.

The study carried out for the landslide in Maierato shows, in con-texts characterized by similar conditions, the paramount importanceof the recognition of geomorphological features that could be ascribedto incipient landslides, which, as a consequence of heavy cumulativerainfalls, could be reactivated as deep-seated landslides and evolve intorapid or extremely rapid flowswith potentially disastrous consequences.

This study case, particularly interesting from a scientific point ofview, also provides general indications for territorial planning andallows us to define geotechnical models aimed at risk mitigation.

Acknowledgments

Authors are grateful to Dr. Gino Cofone and Dr. Saverio Vigliarolofor their precious contribution during the field surveying and toDr. Francesco Perri for having provided the SEM images. The authorsalso thank Ms. V. Cirillo for the video, the Mayor of Maierato, and thefiremen of Vibo Valentia.

This work was carried out under the Commessa TA.P05.012Tipizzazione di eventi naturali e antropici ad elevato impatto socialeed economico of the CNR Department Scienze del sistema Terra eTecnologie per l'Ambiente.

Data and information used have been collected within the frame-work of the activities carried out for the action Regione Calabria —

Supporto tecnico-scientifico al Commissario delegato O.P.C.M. n. 3862/2010.

The authors wish to thank the anonymous referee for providinghelpful suggestions to improve the initial version of this paper. Wewould like to say a specialword of thanks and appreciation to the Editor,Prof. Richard Marston, for his constructive comments and for the excel-lent work he has done for the editing of the manuscript.

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