Movements detection of deep seated gravitational slopedeformations by means of InSAR data and photogeologicalinterpretation: northern Sicily case study

M. Saroli,1 S. Stramondo,2 M. Moro2 and F. Doumaz21CNR, Istituto di Geologia Ambientale e Geoingegneria, Sezione di Roma �La Sapienza�, P.le A. Moro 5, 00185 Rome; 2Istituto Nazionale di

Geofisica e Vulcanologia, Sezione di Roma, via di Vigna Murata 605, 00143 Rome, Italy

Introduction

Synthetic Aperture Radar (SAR)interferometry (InSAR) techniquedetects soil movements by calculatingthe satellite-to-ground distanceschange between two satellite acquisi-tions over the same area (Massonnetand Feigl, 1998). Such movementsrepresent the superficial expression ofon-going gravitational or tectonic phe-nomena. In this paper we propose anew InSAR application to identifymovements originated by deep-seatedgravitational slope deformations(DSGSD) within areas where neitherseismogenic faults are identified norsignificant instrumental seismic activ-ity is recorded (Fig. 1). Preliminaryresults are derived from combinedtechniques, involving two differentapproaches such as aerial photographsinterpretation (Volo Italia, 1988–89flight) and SAR image analysis(ERS1–ERS2 data). The investigatedarea is located in the northern-centralsector of Sicily (southern Italy),between the San Calogero Mt (SCM)and Pizzo Dipilo Mt (PDM)–Cervi Mt

(CM), the first belonging to the Ter-mini Imerese structure, the second tothe Madonie Mountains (Fig. 2).

Geological and structuralframework

Sicily (southern Italy) is part of thewestern central Mediterranean regionand evolves along the African–Euro-pean plate boundary. Catalano et al.(2004), after analysing the regionalfacies of Mesozoic to Paleogene rocks,found that they represent the sedi-mentary cover of distinct palaeogeo-graphical domains (platform/slope/basin system: Panormide, Imerese,Trapanese, Madonie domains). Thesedomains belonged to the TethyanOcean and the African continentalmargin prior to the onset of deforma-tion. The Miocene-Pleistocene rockswere deposited during the deforma-tion of the mentioned domains(Catalano et al., 2004).The geological framework of nor-

thern Sicily is interpreted as resultingfrom recent deformations related tothe opening and the evolution of theTyrrhenian Sea, superimposed onolder deformations related to thedevelopment of the Appennine–Maghrebides fold-and-thrust belt(Fig. 2) (Catalano and D’Argenio,1982; Catalano et al., 1993, 1994,1995, 1996, 2004).

The Termini Imerese and Madoniestructures represent a segment of theMaghrebide-Apenninic chain, consti-tuted by a Meso-Cenozoic sequenceand late-orogenic to post-orogenicdeposits (Agnesi et al., 2000a; Cata-lano et al., 2004). The Meso-Cenozoicsequence is characterized by clay,marly and arenitic deposits belongingto the Sicilide domain (Upper Creta-ceous–Lower Miocene); clay andsandstone levels of the NumidianFlysch (Upper Oligocene–LowerMiocene); carbonate deposits of thePanormide Platform (Upper Triassic–Middle Oligocene); radiolarites, sili-ceous claystones, calcarenites, calciru-dites (Lower Cretaceous–MiddleLiassic), dolomitic calcarenites anddolomitized breccias (Lower Liassic–Upper Triassic) of the Imerese Basin.Late-orogenic and post-orogenicdeposits are represented by fan-deltadeposits (Terravecchia Formation,Upper Tortonian–Lower Messinian)and by pelagic deposits (�Trubi� Auct.,Lower Pliocene). The Imerese succes-sion consists of Upper Triassic toOligocene limestones, marly lime-stones and siliceous rocks depositedin a deep sea environment; thePanormide succession is made ofUpper Triassic to Lower Oligocene –mostly shallow water carbonates.Both the Panormide and Imerese suc-cession were covered by the Upper

ABSTRACT

We investigated the northern-central portion of Sicily region(southern Italy) using aerial photographs and Synthetic Aper-ture Radar (SAR) data obtained by ERS1 and ERS2 satellites. Thisarea shows a geological-structural setting generated by thetectonic superposition of Apenninic-Maghrebian carbonaticstructures on terrigenous deposits. Such a structural settingfavoured the development of large-scale gravity-driven phe-nomena (known in the geological literature as deep-seatedgravitational slope deformations) that are mostly responsiblefor the landscape evolution of the whole area. Morphologicalevidences such as landslides, sacking or rock-flow, lateral

spread and block slide can be detected from photogeologicalanalysis. In order to understand the temporal behaviour andspatial distribution of such deformations we applied theinterferometric SAR (InSAR) technique. Interferograms showfringe patterns spatially coinciding with some of the large-scalegravitative phenomena previously identified by means of aerial-photo analysis. The comparison between photogeological dataand InSAR results allows delimiting the active sectors in thestudy area.

Terra Nova, 17, 35–43, 2005

Correspondence: Dr Marco Moro, Istituto

Nazionale di Geofisica e Vulcanologia -

Sezione di Roma, via di Vigna Murata 605,

00143 Rome, Italy. Tel.: +39 06 5186

0517; fax: +39 06 5186 0507; e-mail

moro@ingv.it

� 2005 Blackwell Publishing Ltd 35

doi: 10.1111/j.1365-3121.2004.00581.x

Oligocene–Lower Miocene syntecton-ic Numidian Flysch.The Termini Imerese and Madonie

structures moved southwards alongthrust surfaces during the construc-tion of the Neogene Appennine–Maghrebide thrust system. Contrac-tional deformation was accompaniedby the development of coeval piggy-back basin within the chain (Catalanoet al., 1998, 2004).The thrust-related structural frame-

work have been overprinted by

normal and strike-slip faults linkedto the opening of the southern Tyr-rhenian Sea that determined fault-delimited blocks rotation (Wezel,1982; Boccaletti et al., 1990; Gueguenet al., 1997; Del Ben and Guarnieri,2000; Catalano et al., 2004).During Pliocene–Quaternary, the

thrusts were accompanied by lateralmovements related to right obliquetranspression accompanying latestclockwise block rotations. This back-ground produce a system of anticlinal

folds dissected by reverse, normal andstrike-slip faults responsible for theformation of a �system of monoclinalfaulted blocks� (Catalano et al., 1998,2002, 2004; Agnesi et al., 2000a; Par-otto and Praturlon, 2004).Later, structural inversion of the

half-graben, formed by a previousextensional event, took place duringUpper Pliocene and Early Pleistocene(Lentini et al., 1995; Renda et al.,1999; Catalano et al., 2004). Exten-sional structures and compressive

Fig. 1 Distribution of instrumental seismicity in the study area recorded by the Istituto Nazionale di Geofisica between 1983 and2001. White circles represent the instrumental earthquakes (1.6 < Md < 3.8).

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transpressive deformation have inter-ested the area between 1.4 and0.5 Ma.Vertical tectonic, imposed during

the last 0.5 Myr, generate a local highrelief energy; these later combinedwith the alternation of lithologieshaving different eroding degrees areresponsible for the development ofDSGSD processes.

SAR interferometry results

In the last years InSAR has beensuccessfully applied to study earth-quakes, to detect and evaluate thesurface displacement field (Massonnetet al., 1993; Stramondo et al., 1999);volcanoes, to estimate the pre-eruptivedeflation and post-eruptive inflation(Massonnet et al., 1995) and land-slides, to detect and monitor theirtemporal evolution (Fruneau et al.,1998; Rott et al., 1999). We appliedInSAR to the study area using ERS1–ERS2 satellite data spanning 1999–2000. SAR images have been selectedtaking into account criteria such as ashort orthogonal component of thespatial baseline Bort (i.e. the spatialbaseline B is the satellite-to-satellitedistance while these �look at� the samearea) to reduce phase sensitivity to

topography and to limit the spatialdecorrelation. Moreover meteorologi-cal information from multispectralsensors ranging the VISible-ThermalInfraRed (VIS-TIR) domain havebeen taken into account in the SARdata selection phase. In fact, goodweather conditions and constantamount of water vapour content arekey points to avoid atmospheric arte-facts (Zebker et al., 1997). To this aimwe used meteorological informationfrom Terra (MODIS sensor) andNOAA (AVHRR) satellites. MODISis an instrument that has near-infrared(IR) channels within and around the0.94-mm water vapour band for re-mote sensing of column water vapouramounts. In particular, the channels17–18–19, at 0.905, 0.935 and0.940 lm, are water vapour absorp-tion channels with decreasing absorp-tion coefficients. The strongabsorption channel at 0.935 mm ismost useful for dry conditions, whilethe weak absorption channel at0.905 mm is most useful for veryhumid conditions or low solar eleva-tion. The AVHRR has five channels inthe visible, near-IR and thermal-IR(TIR) regions of spectrum. In partic-ular, the TIR channels 4 and 5 (10.3–11.3 lm and 11.5–12.5 lm, respect-

ively) are used to evaluate the watervapour content in atmosphere. Thespatial resolution of previous datacannot detect very local atmosphericetherogeneities.Three images from descending or-

bits are available: ERS1 (24 Septem-ber 1999) and ERS2 (25 September1999 and 14 October 2000).The 25 September 1999 to 14 Octo-

ber 2000 differential interferogram ischaracterized by a very high (c.4350 m) ambiguity height ha (i.e. thealtitude variation corresponding to 2pphase change) that ensures the almostcomplete absence of topographicphase. In fact, in this sector of nor-thern Sicily the highest relief is MountCervi (1475 m), which is a maximumof one-third topographic fringe in theconsidered interferogram. Moreoverthe very short orthogonal componentof the spatial baseline Bort (c. 2 m)avoids spatial decorrelation becauseof the different sight angle of the twoERS images (Zebker and Villasenor,1992). To clear the topographic con-tribution we used a tandem pair (i.e.1-day temporal span) and a 20 m pixelsize map-derived Digital ElevationModel (DEM). The tandem interfero-gram is obtained by ERS1–ERS224-25/09/1999 images, with an ha ofc. 30 m. As the 25 September 1999image is either in the tandem pair or inthe differential interferogram, itallowed applying the three-pass inter-ferometric approach. Moreover wealso used the map-derived DEM toremove the topographic phase contri-bution. Then we preferred the map-derived DEM as it better fills holesbecause of the lack of coherence insome areas of the SAR tandem pair.rDEM is the height accuracy (20 m) ofthe map-derived DEM, ha, the ambi-guity height (c. 4350 m for the 25September 1999 to 14 October 2000pair), the theoric standard deviationof the displacement measurement dueto the DEM, rDr ¼ rDEM

ha� k2, is in this

case almost negligible. The differentialinterferogram shows very localizedfringe patterns coinciding with someof the large scale gravitative phenom-ena previously and independentlyidentified through aerial-photo analy-sis (Fig. 3). As a result of the complexmorphology of the landscape, largelayover and shadowing areas are alsopresent. The latter problems avoided agood signal in some of the selected

Fig. 2 Simplified geological sketch map modified from Catalano et al. (2004). At theleft bottom of the figure is the kinematic model of the transpressive junction. Dottedwhite rectangles show studied zones of San Calogero Mt (SCM zone), Pizzo DipiloMt (PDM zone) and Cervi Mt (CM zone).

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Fig.3

Differentialinterferogram

spanning25September

1999to

14October

2000.Within

dotted

whiterectanglesare

thethreeareaspointedoutin

Fig.1.Whitearrowsindicate

thephase

changedirectionsandthesurface

movem

ents

onto

thesatellitelineofsight.Thereddotlocalize

theselected

pointto

propagate

thephase

unwrappingin

Fig.4.

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areas. Only CM zone in Fig. 2 showsa clear and well-defined fringe patterncovering a square area of c. 6 · 6 km.White arrows indicate the direction ofphase changes and the correspondingsurface movements, onto the slantrange direction. Each phase cycle ±pis equal to @ 2.8 cm (Fig. 3).The meteorological conditions

should exclude at all the presence ofresidual atmospheric phase contribu-tion. On the other way, the use of asingle interferogram might lead toambiguity on the presence of artefacts,while a large amount of SAR data canadequately compensate for these arte-facts (Delacourt et al., 1998). In par-ticular, because of the hightopographic relief of CM area, �trop-ospheric fringes� (i.e. the effect of thebottom troposphere on the radar sig-nal, producing phase changes highlycorrelated with the relief) could bepresent in the CM area.To improve the robustness of the

interpretation we unwrapped the in-terferogram. The solution propagatesstarting from a selected point in the S–E part of the CM area (Fig. 4a). Fourprofiles have been traced along theunwrapped phase (Fig. 4b), each onecorresponding to one of the whitearrows in Fig. 3, clockwise from the

eastern one. Profiles follow the arrowdirection. Displacements along pro-files do not appear to increase with theheight of the relief. Therefore a corre-lation of the unwrapped phase withthe topography can be excluded. Con-sequently this confirms the interpret-ation of phase changes as surfacedisplacements.To confirm this statement, profiles

point out surface movements dealingwith lateral spreading of DSGSDobserved in the CM area.

Photogeological interpretation

The investigated areas (SCM, PDMand CM zones) are morphologicallycharacterized by significant elevationchanges because of the presence ofhigh mountainous peaks, separatedfrom surrounding depressed areas bysteep escarpments. In particular, ex-tensional faulting during Pliocene–Quaternary increased the relief energythat helped the development of grav-ity-driven deformations. They seem tobe influenced by the inherited struc-tural and tectonic patterns, related tothe sin- and post-thrusting evolution.Mountain slope deformation phe-

nomena (DSGSD) are often influ-enced by inherited structural and

geological setting; in particular, tec-tonic discontinuities have often turnedout to be an important constraint fortheir onset and development (Dramis,1984; Varnes et al., 1989; Dramis andSorriso-Valvo, 1994; Thompson et al.,1997; Agnesi et al., 2000a; Agliardiet al., 2001; Kellogg, 2001; Onida,2001; Di Luzio et al., 2004a,b).Morphological evidences have

been observed along the three zones,testifying the existence of activelarge-scale gravity-driven processesthat have contributed to the land-scape evolution of the whole zone(Figs 5a,b and 6).Aerial photographic observation

evidenced geomorphological indica-tors related to DSGSD (Figs 5a,band 6); they are represented by evidentdouble crest lines, such as scarps andcounterscarps, several trenches andfractures, depression alignments. TheDSGSD-induced trenches togetherwith high production rate of debrisdeposits, landslides, talus slope depos-its, debris flow and alluvial fans havebeen observed along SCM, PDM andCM. The observed upper limit ofDSGSD, appear in most cases closeto the main faults. They are typicallycurviform presenting their concavitytowards downhill. The DSGSD

Fig. 4 (a) Phase unwrapping of the interferogram in Fig. 3b. Displacement profiles along the unwrapped interferogram are given.The absence of a clear correlation with the relief is evident.

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fracture patterns does not seem to berelated to the main tectonic limits,because their distribution is not com-patible. Hence they could be consid-ered as mainly gravity-relatedfeatures. The double crest line, clearlyvisible from aerial photographs andshaded relief, has been an importantelement in identifying DSGSD be-tween the SCM, PDM and CM zones.Morphologic features, such as scarps

and counterscarps, are also present inthese zones, testifying the gravita-tional reactivation of DSGSD.According to Agnesi et al.

(2000a,b), recognized DSGSD in theCM area, using photogeologicalinterpretation, are mainly representedby rotational sagging and lateralspreading in the central part of thestructure, while the external part ismainly represented by block slides

(Fig. 7). These data are confirmed bythe InSAR observations that showcongruent directions (radial fringes)with the type of the recognized defor-mation (i.e. DSGSD) (Figs 3 and 5c).In Fig. 5c,d), we present two differentconfigurations with the same interfer-ometric background. Figure 5c pre-sents a scenario with DSGSDsurficial expression while Fig. 5dillustrates the case with the main

Fig. 5 (a) Aerial photography 2062, Strip 93A, �Volo Italia 1988–89�, 1:70 000; observed features of DSGSD in the CM area fromaerial photography (b) and differential interferogram; (c) main tectonic limits (modified from Agnesi et al., 2000b) on differentialinterferogram. The deformation pattern obtained from ERS1-ERS2 image analysis, fits with the DSGSD surficial expression.

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tectonic limits. The radial-like fringesdistribution seems to fit better withthe DSGSD surficial expression thanthe other expression. The case shouldpresent an abrupt change in corres-pondence to the main fault traces interms of gradient (Stramondo et al.,1999). A third scenario is possible ifthe two previous cases (gravity andtectonic) are merged but it should

produce chaotic fringe patterns thanradial fringes.In two other zones of interest

(SCM and PDM), InSAR showsevidences of deformation but not asthe CM zone, which shows radialfringes. However, photos interpret-ation shows DSGSD style defor-mation (Fig. 6) similar to the CMzone.

Development of this specific type ofDSGSD is because of the particulargeological–structural setting of thearea, associated with the alternationof lithologies presenting differenteroding degree and a high reliefenergy.Evolution of DSGSD is a conse-

quence of the increasing relief en-ergy, related to selective erosion and

Fig. 7 Three-dimensional view of the CM area. A-A¢ and B-B¢ represent simplified geological cross-sections for a DSGSDschematic interpretation. (1) Apenninic-Maghrebian carbonatic units (Meso-Cenozoic); (2) terrigenous deposits (Miocene–Pliocene); (3) debris deposits, talus slope deposits and landslides; (4) thrust; (5) inferred thrust; (6) fault; (7) principal shear surfacesreactivated by gravity; (8) drainage; (9) cross-section.

Fig. 6 (a) Aerial photography 1120, Strip 92A, �Volo Italia 1988–89�, 1:70 000; observed features of DSGSD in the PDM area fromaerial photography; (b) aerial photography 1125, Strip 92A, �Volo Italia 1988–89�, 1:70 000; observed features of DSGSD in theSCM area from aerial photography.

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to the progressively deepening of thehydrographic network (Fig. 7). Thevariation of erosion base level can beattributed to uplift phases and eu-static oscillations, whereas tectonicevents have seemingly reactivatedpre-existent fault planes (Agnesiet al., 2000b).

Conclusions

The InSAR results point out threelocalized zones (SCM, PDM andCM). In particular, the CM area isthe most significant, characterized bya good coherence and showing a clearpattern of displacement fringes. Inthese zones photogeological interpret-ation allowed to identify the surficialexpression of DSGSD, mainly repre-sented by lateral spreading and slidingof carbonatic units (Agnesi et al.,1978, 1984, 2000a,b). On the contrary,within the considered time interval,corresponding to the InSAR timewindow selected images, no significantinstrumental seismicity has beenrecorded in the study area. Hence,we exclude any eventual seismic trig-gered movements. Nevertheless, a seis-mic crisis might have a main role inemphasizing such phenomena.DSGSD might be considered thegravitative adapting to the tectonicstress variations that induced a regio-nal or local increase of relief energy.The deformation pattern obtained

from ERS1–ERS2 image analysisseems to be clustered and not uniformon the considered area. The geometryof the ERS descending orbit, andconsequently the line of sight, detectedradial surface movements in CM area,as shown by the white arrows inFig. 3. The radial-like fringes distri-bution fits better with the DSGSDsurficial expression in comparisonwith the main tectonic limits as givenin Fig. 5c,d.Further studieswill allow to quantify

the displacement rates. The use of alarge amount of SAR data and theapplication of InSAR time series tofollow the temporal evolution of thiskind ofmovements are foreseen.More-over, the application of the suggestedmethodology to areas characterized bythe presence of seismogenic faults andstruck by a seismic sequence could alsoallow to separate tectonic and gravitycontributions from the whole detectedsurface movement.

Acknowledgements

At the outcome of this work, we would liketo thank Francesco Guglielmino that madeSAR data available. Professors A. Biasini,G. Mariotti and E. Lupia Palmieri from�La Sapienza University� for their encour-agements and advices. We thank Dr E.Carminati, for the first lecture and �Goahead� signal Dr F. Lenci and E. Di Luziofor corrections to the manuscript.

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Received 16 June 2004; revised versionaccepted 29 September 2004

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