Analysis of building deformation in landslide area using multisensor PSInSARTMtechnique

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ContentslistsavailableatScienceDirect

International

Journal

of

Applied

Earth

Observation

and

Geoinformation

j ou rn a l h o m epa g e :w w w . e l s e v i e r . c o m / l o c a t e /j a g

Analysis

of

building

deformation

in

landslide

area

using

multisensor

PSInSAR

TM

technique

Andrea

Ciampalini

∗

,

Federica

Bardi,

Silvia

Bianchini,

William

Frodella,

Chiara

Del

Ventisette,

Sandro

Moretti,

Nicola

Casagli

DepartmentofEarthSciences,UniversityofFirenze,ViaLaPira4,50121Firenze,Italy

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received25March2014 Accepted23May2014 Keywords: Landslide Radar PSInSAR Buildings Damages

a

b

s

t

r

a

c

t

Buildingsaresensitive tomovementscaused byground deformation.Themappingbothofspatial andtemporaldistribution,andofthedegreeofbuildingdamagesrepresentsausefultoolinorderto understandthelandslideevolution,magnitudeandstressdistribution.Thehighspatialresolutionof space-borneSARinterferometrycanbeusedtomonitordisplacementsrelatedtobuildingdeformations. Inparticular,PSInSARtechniqueisusedtomapandmonitorgrounddeformationwithmillimeter accu-racy.TheusefulnessoftheabovementionedmethodswasevaluatedinSanFratellomunicipality(Sicily, Italy),whichwashistoricallyaffectedbylandslides:themostrecentoneoccurredon14thFebruary2010. PSInSARdatacollectedbyERS1/2,ENVISAT,RADARSAT-1wereusedtostudythebuildingdeformation velocitiesbeforethe2010landslide.TheX-bandsensorsCOSMO-SkyMedandTerraSAR-Xwereusedin ordertomonitorthebuildingdeformationafterthisevent.During2013,afteraccurateﬁeldinspection onbuildingsandstructures,damageassessmentmapofSanFratellowerecreatedandthencompared tothebuildingdeformationvelocitymaps.Themostinterestingresultswereobtainedbythe compari-sonbetweenthebuildingdeformationvelocitymapobtainedthroughCOSMO-SkyMedandthedamage assessmentmap.ThisapproachcanbeproﬁtablyusedbylocalandCivilProtectionAuthoritiestomanage thepost-eventphaseandevaluatetheresidualrisks.

Introduction

Landslidesaregloballywidespreadphenomena,causinga

sig-niﬁcantnumberofhumanlossoflifeandinjury,aswellasextensive

economicdamagestoprivateandpublicproperties.InEuropeand

inItalyinparticular,wherehalfamillionactivelandslidesexist,

massmovementsrepresenttheprimarycauseofdeathcausedby

naturalhazards(Guzzettietal.,1999,2012).Recently,signiﬁcant

resultsinthestudyofgrounddeformationwereobtainedusing

spaceborneSyntheticApertureRadar(SAR)sensors,locally

inte-gratedwithground observations(Ferrettiet al.,2001; Guzzetti

et al., 2009; Lauknes et al., 2010; Herrera et al., 2010, 2013; Tomásetal., 2012).This approachallows deliveringinnovative

andaccurateinformation relevanttotheCivilDefense

Authori-ties covering thepre-event, eventand post-eventmanagement

phases.Nowadaysslowgrounddeformationscanbeeasilydetected

andmonitoredusingsatelliteradartechniquesatarelativelylow

∗ Correspondingauthor.Tel.:+390552757785. E-mailaddresses:andrea.ciampalini@uniﬁ.it,

andrea.ciampalini76@gmail.com(A.Ciampalini).

cost.C-bandsatellitesacquireSARimagessince1992,providinga

verywidearchiveofthegrounddisplacementshistoricalevolution

ofaselectedarea.Thesesensorsarecharacterizedbyamedium

spatialresolution(20m×4m)anda35daysrevisitingtime.The

newX-band COSMO-SkyMed (CSK) and TerraSAR-X(TSX)

mis-sionsreduced therevisitingtimeto4 (CSK)and11 (TSX)days,

enhancingthespatialresolutionto1m×1m.Classical

Differen-tialinterferometry(DInSAR)isusedtomeasuretherelativemotion

betweentwoimageacquisitions(Costantinietal.,2000;Crosetto

etal.,2011).Aphasedifferenceimageorinterferogram,whichis

directlyconnectedtogroundmotion,canbeobtained.PSInSARis

anon-invasivesurveyingtechniqueusedtocalculatemotionsof

individualgroundandstructurepoint-liketargetoverwide-areas

(Ferrettietal.,2000).ThePSItechniquetakesconventionalDInSAR

astepfurtherbycorrectingtheatmospheric,orbitalandDEMerrors

inordertoderiverelativelyprecisedisplacementandvelocity

mea-surementsatspeciﬁcpointsontheground.Thiswell-established

techniqueisparticularlyusefulinlandslidesmappingand

mon-itoring(Liu et al.,2013; Bianchinietal., 2014a).SanFratellois

atownlocatedintheMessinaProvince(Sicily,Italy),whichwas

affectedin the last three centuries by atleast three important

landslides:theﬁrsttwo,occurredin1754and1922respectively,

http://dx.doi.org/10.1016/j.jag.2014.05.011

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A.Ciampalinietal./InternationalJournalofAppliedEarthObservationandGeoinformation33(2014)166–180 167

Fig.1.Locationofthestudyareaand1922and2010landslidesboundaries.(a)Stazzonequarter;(b)Rianaquarter;(c)SanBenedettoquarter.

destroyedthenorthernsectorofthetown,whilethemostrecent

onetookplaceonthe14thFebruary2010,affectingthemid-eastern

quartersof thetown.Thislatter landslide,which isstill active,

causedseveredamagestobuildingsandinfrastructureswithan

estimated costof 300 millionEuros for the disastermitigation

andreconstructionprogram.Thisworkpresentsananalysisofthe

SanFratellotownbuildingdeformationvelocities,obtainedbythe

combinedofPSIdataandbuildingsmap,withtheaimof

under-standingthedeformationevolutionoftheinvolvedstructures.In

particular,C-banddatacollectedbeforethe2010event,wereused

toevaluatethepresenceofprecursory symptomsof instability,

whereastheX-banddata,collectedaftertheevent,wereanalyzed

inordertodetecttheresidualrisksinthepost-eventphase.Finally,

theobtainedpost-eventbuildingdeformationvelocitymapwas

compared with the damage assessment map. This application,

basedonthePSInSARtechnique,canbesuitabletoevaluatehow

theabundanceoftheelementsatriskinalandslideaffectedarea

maychangewithtime,contributingtotheriskassessment,whichis

veryimportantfordecisionmakingbyCivilProtectionAuthorities,

especiallyduringthelandslidepost-eventphase.

Geologicalsetting

ThetownofSanFratelloislocatedinthenortheasternsectorof

Sicily(Italy),along theTyrrheniancoastline(Fig.1),closetothe

boundarybetweentheNebrodiandPeloritani mountainchains.

Thestudyareawashistoricallyaffectedbylandslides(Goswami

et al., 2011; Mondini et al., 2011; Del Ventisette et al., 2012;

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Fig.3.DamageassessmentmapofSanFratelloaccordingtothedegreeofdamagesofTable1(a);andtherelatedfrequencyofbuildingsconsideringthedegreeofdamages (b).SomeexamplesofthedamagesofSanFratello(c)class3;(d)class6;(e)class7;(f)class5;(g)class2;(h)class5.

Table1

Rankingschemeofbuildingdamagecategories(modiﬁedafterCooper,2006).

Class

0 Nodamages.

1 Hairlinecrackingnotvisiblefromoutside.Finecracks,generallyrestrictedtointernalwallﬁnishes:rarelyvisibleinexternalbrickwork.Typicalcrackwidths upto1mm.Generallynotvisiblefromoutside.

2 Cracksnotnecessarilyvisibleexternally,someexternalrepointingmayberequired.

Doorsandwindowsmaystickslightly.Typicalcrackwidthsupto5mm.Difﬁculttorecordfromoutside.

3 Cracksthatcanbepatchedbyabuilder.Repointingofexternalbrickworkandpossiblyasmallamountofbrickworktobereplaced.Doorsandwindows sticking,slighttilttowalls,servicepipesmayfracture.Typicalcrackwidthsare5–15mm,orseveralofsay3mm.Visiblefromoutside.

4 Extensivedamagethatrequiresbreaking-outandreplacingsectionsofwalls,especiallyoverdoorsandwindows.Windowsanddoorframesdistorted,ﬂoors slopingnoticeably;somelossofbearinginbeams,distortionofstructure.Servicepipesdisrupted.Typicalcrackwidthsare15–25mm,butalsodependson numberofcracks.Noticeablefromoutside.

5 Structuraldamage,whichrequiresamajorrepairjob,involvingpartialorcompleterebuilding.Beamslosebearingcapacity,wallsleanbadlyandrequire shoring.Windowsbrokenwithdistortion.Dangerofinstability.Typicalcrackwidthsare>25mm,butdependonthenumberofcracks.Veryobviousfrom outside.

6 Partialcollapse.Veryobviousfromoutside. 7 Totalcollapse.Veryobviousfromoutside.

Table2

usedPSInSARdatasets.

Sensor Geometry Timeinterval No.ofscenes Density(PS/km2₎

ERS1/2 Ascending 11/09/92–05/06/00 34 6.55 ERS1/2 Descending 01/05/92–08/01/01 70 46.45 ENVISAT Ascending 22/01/03–22/09/10 65 64.74 ENVISAT Descending 07/07/03–13/09/10 49 20.41 RADARSAT Ascending 30/012/05–26/01/10 47 85.86 RADARSAT Descending 31/01/06–03/02/10 47 85.86 COSMOSky-Med Descending 16/05/11–02/05/12 32 400.62 TERRASar-X Descending 28/10/11–22/09/12 30 813

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Fig.4. (a)BuildingsdeformationvelocitymapobtainedusingERS1/2descendingdata.(b)Detailsofthe1922landslidewiththePSdataset.Theredasteriskstandsforthe PStimeseriesof(c).Theredlinecorrespondstothe2010landslideboundary;theorangelinecorrespondstothe1922landslideboundary.

Bianchinietal.,2014b).Theoldestdocumentedoneoccurredin

1754,destroyingthenorth-easternpartofthetown,medievalin

age.The8thofJanuary1922anotherlargelandslideseriously

dam-agedthenorth-westernquarters,causingthetowndelocalization.

ThemostrecentlandslideoccurredonFebruary14th2010

caus-ingseveraldamagestobuildingsandinfrastructures.Thislandslide

affectedtheeasternsectorofthetown,determiningthe

evacua-tionofmorethan2000people.Fromageomorphologicalpointof

viewSanFratelloislocatedatabout650ma.s.l.alongaN–S

ori-enteddivideonwhich boththewest andeastfacingslopesare

characterizedbyasteepgradientandcreekerosionattheirtoe.

From a geological point of view, the study area is part

of the collisional system, developed since the LateCretaceous,

as the result of the convergence between the European and

African-Adriaticplates(Corradoetal.,2009).Thestudyareais

char-acterizedbythe presenceof two main structuraldomains:the

Kabilian-Peloritan-Calabrian unitsat northeast,overthrusted on

theApenninic-Maghrebiandomaintothesouthwest(Lentinietal.,

2000; Lavecchia et al., 2007). The Kabilian-Peloritan-Calabrian

unitsaremadeofimbricatesheetsofPaleozoicmetamorphicand

igneousrocksandtherelatedMesozoicsedimentarycover(Somma

etal.,2005).Theseunits,croppingoutwithinthePeloritani

moun-tains,arecoveredbyUpperOligocene-LowerBurdigaliandeposits

(Lentini et al., 2000). The Apenninic-Maghrebian domaincrops

outintheNebrodiMountainsandismadeofimbricatesheetsof

Mesozoic-Tertiarysedimentaryrocks.Theboundarybetweenthese

twodomains,correspondstothelimitbetweenthePeloritaniand

Nebrodimountainchains;thisboundaryisrepresentedbyanactive

frontalthrustmarkedbytheLongi-Taorminalineament(Catalano

etal.,2006;Billietal.,2007).SanFratelloislocatedinproximityto

thisfault (Fig.2).TherocksoutcroppingwithintheSanFratello

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Fig.5.(a)BuildingdeformationvelocitymapusingbothascendinganddescendingENVISATdatasets.(b)ENVISATPSdatasetrelatedtothe1922landslidewithtwoselected timeseries(candd).(c)Correspondstotheyellowasterisk;(d)correspondstotheorangeasterisk.

calcareoussedimentarysequence.Inparticular,thewesternand

southernpart ofthestudy areaismostly characterizedby

ter-rigenousterrains,representedbytheAppeninic-MaghrebidUnits

(claysalternatingwithsandstonesandclayey-marlyformations).

Inthenorthernportionofthearea,theuppermostunits

(Kabilo-CalabrideUnits)cropout,occurringasLiassiccarbonateplatform

sequencesoverlappedbyaterrigenousLateEocene-Oligocene

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Fig.6. (a)ENVISATascendingPSdataset;(b)comparisonbetweenthegrounddeformationvelocitymapandthedamageassessmentmap;(c)timeseriescorrespondentto theyellowasterisk;(d)timeseriescorrespondenttotheorangeasterisk.

D’Alunzio Unit) crop out in the N-NE sector of San Fratello

(Giunta et al.,2000; Lavecchiaet al., 2007).The poor

geotech-nicalpropertiesof theclay-siltandﬂyschoidlithotypesareone

ofthetriggeringfactorsofthelandslidingphenomena,combined

withthesteeptopographyandtheoccurrenceofintenserainfall

events.

Methodology

Damageassessmentmap

Thedamageassessmentmapwasperformedafteranaccurate

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Fig.7. Buildingdeformationvelocitymapusingbothascendinganddescending RADARSAT-1datasets.

amountof738buildingsweremappedonthebasisofthedamages

observableontheirwallsandfacades.Buildingsinteriordamages

wereevaluatedbyinterviewingthelocalinhabitants.Thedegreeof

damagesforeachbuildingwasevaluatedusingamodiﬁedversion

ofrankingschemeofbuildingdamagecategoriesofCooper(2006)

(Table1).

Radarinterferometry

SpaceborneInterferometricradarapproachrepresentsa

pow-erfultooltodetectmovementsontheEarth’ssurface.Mapping

geomorphologic processes and monitoringslope instability can

greatly beneﬁt from satellite data analysis due to the great

cost–beneﬁtsratio,non-invasiveness,wideareacoverageandhigh

precision.SpaceborneInSARcanofferausefulsupportinthe

detec-tionandcharacterizationofslowsurfacedisplacements,providing

rapidandeasilyupdatablegroundvelocitymeasurements,along

thesatellitelineofsight(LOS).Multi-interferometricInSAR

tech-niquehasturnedouttobeavaluableandsuccessfultooltodetect

andmeasuresurfacedisplacementswithmillimeteraccuracy,and

alsotoreconstructthedeformationhistoryoftheinvestigatedareas

(Bovengaetal.,2006;ColesantiandWasowski,2006;Cascinietal., 2009;Cignaetal.,2011;Hungetal.,2011;Bianchinietal.,2012; Ciampalinietal.,2012;Raspinietal.,2012;DelVentisetteetal., 2014;Bianchiniet al., 2014b).In thelast 20 years,several

dif-ferentradarsatellitemissionswerelaunched,providingdifferent

typesofSARimagestobeusedforgroundmovementdetection

andmonitoring.Consequently,todaymanysatellite

interferomet-ricdataareavailable,includingbothhistoricalarchives,acquired

sincetheearly1990s (e.g.ERS1/2,ENVISAT), and imagesfrom

currentlyoperatingsatellites(e.g.TerraSAR-X,COSMO-SkyMed),

spanningatimeintervalofmorethan20years,whichcanallow

theanalysisofbothpastandrecentgrounddisplacementsofthe

observedscenes(Bianchinietal.,2012).PSInSARcanbeusedto

ana-lyzearchivalSARdata,providingannualvelocitiesanddeformation

timeseriesongridsofstablereﬂectivepoint-wisetargets,called

PersistentScatterers(PS),characterizedbyacoherent

electromag-neticbehaviorinalltheradarimages(Ferrettietal.,2001).Several

ofthesePScorrespondtohand-madeartifacts,suchasbuildings,

railwaysorhighways,makingthismethodsuitableforthe

moni-toringofurbanizedareaswherethedensityofthePSishigher.This

canprovideexclusiveinformationforanimprovedunderstanding

ofthelongtermbehaviorofslowandvery-slowground

deforma-tionphenomena(DelVentisetteetal.,2013).PSIanalysis,properly

integratedwithauxiliarydata,hasbeenmainlyusedformapping

andmonitoringslow-movinglandslides,andforevaluatingtheir

stateofactivityandintensity(Ferrettietal.,2000;Meisinaetal.,

2008;Herreraetal.,2009,2010;Guzzettietal.,2009;Nottietal., 2010;Righinietal.,2012;Bianchinietal.,2014b).Therecorded

displacementsaremeasuredconsideringagroundpointofknown

coordinates,calledreferencepoint,consideredstableonthebasis

ofthegeologicalknowledgeofthearea.Satellitesensorsare

side-lookingandacquireimagesintwodifferentgeometries,following

anapproximatelyN–Sdirectionandaperpendicularlineofsight

(LOS).Forthisreason,sensorsmovingfromsouthtonorthacquire

inascendinggeometryandaremoresuitabletodetectmovements

locatedonwestfacingslopes.On thecontrary,sensorsmoving

fromnorthtosouthacquireindescendinggeometrymakingthem

suitable forthe observationofeast-facingslopes. Fivedifferent

sensorswereusedtomonitorthebuildingsofSanFratello:ERS

1/2,ENVISAT,RADARSAT-1,COSMO-SkyMed(CSK)and

TerraSAR-X(TSX)(Table2)coveringaverylongintervaloftime,spanning

from1992upto2012.SatelliteSARdatawereprocessedbyT.R.E.

(Tele RilevamentoEuropa). The ground deformationvelocity of

eachbuildingwascalculatedconsideringtheaveragevelocityof

allthePSincludedinthepolygoncorrespondingtothebuilding.

ThecalculationwasperformedforeachC-andX-banddataset.To

avoidproblemsrelatedtoapossibleshiftbetweenthebuildings

andthePSIlayers,duetoanon-extremelyaccurategeoreferencing

process,PSlocatedinarangeof2moutsidethebuildingswere

includedinthecalculationoftheaveragevelocity.Furthermore,

theuseofthese2mwidebufferimprovedthenumberofusablePS.

Consideringthegrounddeformationvelocity,astabilitythreshold

waschosenbetween±1.5mm/yfortheC-banddataand±2mm/y

fortheX-banddata.Thesethresholdsweredecidedonthebasisof

boththeinvestigatedphenomena(slowmoving)andthestandard

deviationofthePSpopulation.

Results

Damageassessmentmap

Thedamageassessmentmap(Fig.3)reportsthedegreeof

dam-agesof a totalof 738buildingsclassiﬁedfollowing thescheme

reportedinTable1.Themapshowsthatmore thanhalfofSan

Fratellobuildingsareaffectedbydamagescausedbyslope

insta-bility;amongstthem20%ofbuildingsarestronglydamaged(more

thanclass4).Themostaffectedareasarelocatedonthetown

east-ernslope,withintheStazzonequarterandEastofthecitycenter

(Fig.3a).In theseareas,alsosomeofthemostrecentbuildings

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Fig.8.(a)RADARSAT-1ascendingPSdataset;(b)comparisonbetweenthegrounddeformationvelocitymapandthedamageassessmentmap;(c)timeseriescorrespondent totheyellowasterisk;(d)timeseriescorrespondenttotheorangeasterisk.

demolished.IntensedamagescanbeobservedalsointheRianaand

SanBenedettoquartersalongthe2010landslidecrown(Fig.3e).

Theﬁeldsurveysrevealedalsothepresenceofseveraldamaged

buildingslocatedinthenorthwesternpartoftown,corresponding

tothe1922landslidecrownarea.

Radarinterferometry

ERS1/2(1992–2001)

The buildings deformation velocity maps using theERS 1/2

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landslideare affected by deformation, however a maximum of

178(24.12%)buildingsweremappedusingthedescendingdataset

(Fig.4a).Alongthe1922landslidecrown,somebuildingsshow

a deformation velocity, which is consistent with the damages

observedontheirwallsandfacades.Inthiscase,theextentofthe

areaaffectedbydeformationisunderestimated.Abetterresultcan

beobtainedbytheobservationofthewholePSdataset(Fig.4b).

PStargetsclearlyshowthepresenceofatleasttwootherareas

characterizedbygrounddeformation,onelocatedalongthe1922

landslidecrown(Fig.4a),wherethemeandeformationvelocityis

upto−9mm/y(Fig.4bandc),andanotheronewithinthelandslide

body(Fig.4b).

ENVISAT(2003–2010)

TheuseoftheENVISATdatasetsallowedthemappingofupto

279(37.80%)ofthetownbuildings.Therelatedbuilding

deforma-tionvelocitymap(Fig.5a)isverysimilartothoseobtainedwithERS

1/2datasets.Resultshighlightasubstantialstabilityofthetown.

Averysmallamountofbuildingsischaracterizedbydeformation

velocitiesoutsidethestabilityrange.Alongtheeasternslope,only

oneediﬁceshowsanaveragevelocityof−10mm/y.Onthe

west-ernslope,alongthe1922landslidecrown,thebuildingsaffectedby

deformationarealmostthesameofthosehighlightedbythemap

obtainedwithERS1/2.TheuseofENVISATsensorincreasedthe

numberofmonitoredbuildings;however,thesePSdatawerestill

notsuitabletodetectpossiblegrounddisplacementstobe

consid-eredaslandslideprecursors.AlsointhiscaseusingthewholePS

datasetsitwaspossibletoretrievemoreinformationaboutslope

instability(Fig.5b).Alongthewesterntownsectortheareaaffected

bygrounddeformationcouldbebetterrecognizedusingPSwhich

highlightedthepresenceofawideareaaffectedbydeformation,

locatedalongintheeasternsectorofthe1922landslide(Fig.5b).

ThemostinterestinginformationobtainedusingtheENVISATdata

concernsthegrounddeformationoftheeasternslope(Fig.6a).Here

twoareas,showingclustersofPScharacterizedbyhighground

deformationvelocities,canberecognized.Eventhoughboththese

areasarelocatedinsidethe2010landslidebody,includingavery

lownumberofbuildings,thenumberofPSissufﬁcientto

high-lightgrounddeformation.ThecomparisonbetweenthePSandthe

damageassessmentmap showsa goodagreementbetweenthe

distributionofthePShavingthehighestvelocitiesandthemost

damagedbuildingsbythe2010landslide(Fig.6b).

RADARSAT-1(2005–2010)

RADARSAT-1datacanbeusedtoconﬁrmtheresultsobtainedby

ENVISATdataduetothepartialoverlappingoftheirtemporal

cov-erage.ThemainadvantageoftheuseofRADARSAT-1isrepresented

bythehigherdensityofPSthatallowsmappingthedeformation

velocityofalmostthehalf(48.37%)ofthemonitoredbuildings.

Resultsclearlyhighlightthreeareaswherebuildingsareaffected

bydeformation(Fig.7).Theﬁrstareaislocatedwithintheeastern

slope,conﬁrmingtheresultsofENVISATdata.Inaddition,the

defor-mationoftheediﬁceslocatedalongthe1922landslidecrownis

conﬁrmed.ThethirdareacorrespondstotheSanBenedettoquarter,

wherefourbuildingsshowvelocitiesslightlyhigherthanthe

stabil-ityrange.ConsideringthewholePSRADARSAT-1ascendingdataset

(Fig.8a),theareaaffectedbygrounddeformationlocatedwithinthe

easternslopeisclearlyrecognizable.Also,inthiscase,the

agree-mentbetweenthegrounddeformationvelocitiesandthedamage

assessmentmapisgood(Fig.8b),conﬁrmingtheusefulnessofthe

C-bandsensorsrelativelytothisarea,wherethebuildingsaffected

by severe damagesare surrounded by PSshowing the highest

velocities.Alsoalongthewesternslope,RADARSAT-1conﬁrmsthe

resultsobtainedbyENVISAThighlightingthepresenceofground

deformationwithinthebodyandalongthe1922landslidecrown.

Fig.9.Buildingdeformationvelocitymapusingbothascendinganddescending COSMO-SkyMeddatasets.Theblacklinescorrespondtothesecondaryscarps devel-opedduringthe2010landslide.

COSMO-SkyMed(2011–2012)

CSKimageswereacquiredafterthe2010landslide,thusthey

canbeusefultoevaluatepossiblepost-eventgrounddeformation

phenomena.Theavailabledatawereacquiredonlyindescending

orbit,makingthemsuitableespeciallytostudythedeformation

ofthewesternslope.Anyway,thehighdensityofthePSallowed

toretrievealsointeresting information abouttheeastern slope

deformation. Thestability range for CSK data wasincreasedto

±2mm/ybecauseofthehigheraveragestandarddeviationofPS

velocity,ifcomparedtothoseoftheC-banddata.Theincreaseof

thestandard deviationisrelatedtotheshortacquisitionperiod

(onlyoneyear)and tothelocationofSanFratello,which turns

outtobefarfromthereferencepointandattheboundaryofthe

usedSAR images.Inthiscase,thebuildingdeformationvelocity

mappermitstorecognizethreedifferentareas(Fig.9),alongthe

2010landslidecrown,whereseveralbuildingsareaffectedbya

considerabledeformation.Theseareascorrespond,fromnorthto

south,totheStazzone,RianaandSanBenedettoquarters,which

wereintenselydamagedduringtheevent.Thedifferenceincolors

betweentheStazzoneandSanBenedettoquarters,thatrepresent

differentdisplacementdirection,isrelatedtothedifferentposition

oftheareaswithinthe2010landslide.Buildings thatare

mov-ingtoward thesatellitecharacterize Stazzone quarter, whereas

inSanBenedetto theediﬁces aremoving away withrespectto

thesensor.Theﬁrstonesarelocatedinside thelandslidebody,

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Fig.10.Standarddeviation(a),grounddeformationvelocitymapwith±7mm/ystabilityrange(b)andbuildingdeformationvelocitymapwith±5mm/ystabilityrange(c) usingthedescendingTerraSAR-Xdataset.

The fractures patterns developed on the building facades

sug-gest that Stazzoneis moving downhill, onthe contrarypartof

thebuilding locatedwithin the SanBenedetto is characterized

bymainlyverticalmovement.Thisisconsistentwiththe

geom-etryof thelandslide,which is rotational intheupper partand

translationaldownward.Anotherinterestingresultisrepresented

bythepresence ofastrip,locatedalong thecrown ofthe1922

landslide,characterizedbyseveralbuildingsshowinghigh

defor-mationvelocities.Theseediﬁcespresentwelldevelopedfractures

ontheirfacades,conﬁrmingthatthisareaisinterestedbyslow

butcontinuousgrounddeformation.Thecomparisonbetweenthe

buildingdeformationvelocitymap,obtainedusingCSKdata,and

thedamageassessmentmapshowsaverygoodagreement,

con-ﬁrming thatthe mostintenselydamaged areaduringthe2010

landslideare,today,characterizedbyresidualmovements,which

compromisetheirstability.Severalofthesebuildingswererecently

evacuatedandsomeofthemcompletelydemolished.The

measure-mentofthevelocitiesrelatedtoresidualmovementsmustbeused

withcautionbecauseofthehighstandarddeviation.

TerraSAR-X(2011–2012)

TheavailableTSXdatawereacquiredbetweenOctober2011and

December2012.Partoftheacquisitionperiodisoverlappedwith

thatoftheCSKdata.Thisdatasetisaffectedbyaveryhighstandard

deviation,especiallyincorrespondenceofSanFratellotownarea,

wheretheaveragestandarddeviationis7.6(Fig.10aandb).These

highvaluesdependonseveralfactors(e.g.thechoiceofthe

refer-encepoint,theshortacquisitionperiod,thetopographicfactorand

thepositionofthearea,locatedclosetotheboundaryoftheSAR

images).Forthesereasons,TSXdatacouldnotbeusedtocalculate

thebuildingdeformationvelocities.However,someconsideration

abouttheground deformation canbe madeconsideringas not

movingtheareasresultingstablefromCSK.Adifferential

analy-siscanbeusefultohighlightareasaffectedbygrounddeformation.

Inthiscase,theappliedstabilitythresholdwasselectedbetween

±7mm/y,consideringthestandarddeviation.Resultssuggestthat

almostthreeareasofthetownareaffectedbygrounddeformation:

thenorthern oneis locatedalong thecrown of the1922

land-slide,thesecondoneisrepresentedbypartoftheStazzonequarter

andthesouthernonecorrespondstotheSanBenedettoquarter

(Fig.10c).GrounddisplacementshighlightedbyTSXconﬁrmthe

resultsobtainedusingCSKbut,consideringthestandarddeviation,

theevaluationofthegrounddeformationvelocitiescanbemade

onlywiththelastone.

ReprocessedX-banddata

ConsideringtheinterestingresultshighlightedbyX-band

sen-sors, the TSX and CSK datasets were reprocessed, choosing a

differentreferencepoint,inordertominimizethestandard

devi-ations.ThenewreferencepointswerelocatedinsideSanFratello,

incorrespondenceofbuildingsconsideredstable.Thisprocedure

allowedamoreaccuratemeasurementofbuildingdeformations

velocities. During the reprocessingof the CSK data, severalPS,

locatedinthewesternpartofthetownwerelost,butthe

build-ing deformation velocity map clearly shows that Stazzone and

SanBenedetto quartersare affectedbyimportant deformations

(Fig. 11). In addition the reprocessed TSX data (Fig. 12)

con-ﬁrmedthatthesetwoareasaresubjectedtogrounddeformations,

especiallySanBenedettowheretheextent,thegeometryofthe

deformations and theaveragevelocities are comparable to the

CSKdata.TSXdatacanbeusefulalsotoobservethedeformations

locatedalongthewesternslope,whereseveralbuildingsshowhigh

velocities.

Discussion

Landslidesmayaffectdifferent kindof buildingsin different

ways.Constructionmethods,withregardtodepthoffoundations

andusedmaterial,cancounteractthedevelopmentofthe

dam-agescausedbygrounddisplacement.Thisfactisfundamentaltobe

takenintoaccountinatownsuchasSanFratello,wherevery

differ-entkindsofbuildingsarepresent.SanFratellotowniscomposed

byhistoricalbuildingsmadeofbricks,havingveryshallow

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Fig.11.(a)CSKgrounddeformationvelocitymap;(b)buildingdeformationvelocitymapobtainedbyusingCSKdata;(c)timeseriescorrespondenttotheorangeasterisk and(d)timeseriescorrespondenttotheyellowasterisk.

areinforcedconcretestructure.Afterthe2010landslidetheﬁrst

onesshowvariousdegreesofdamages,goingfromwell-developed

fracturesontheirwallsand facades,distortion ofwindowsand

doorframes,uptothestructuralpartialortotalcollapse(Fig.3).

Themorerecentbuildingsstoodupwelltothelandslideinduced

deformations,butinsomecases,theyshowarotationofthewhole

structure.ThecombineduseofthebuildingsmapandthePSradar

datasets(Table3)highlightedthatthePSdensityisafundamental

parameterinordertoevaluatethefeasibilityofthisapplicationfor

buildingmonitoringactivities.OnlytheX-banddatasets,bearing

a hightarget density (200PS/km2_), _resulted _useful _to

under-standthedisplacementvelocitiesofatleastmore thanthehalf

ofthebuildingsoftheSanFratellotown.Anyway,some

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Fig.12. (a)TSXgrounddeformationvelocitymap;(b)buildingdeformationvelocitymapobtainedbyusingTSXdata;(c)timeseriescorrespondenttotheorangeasterisk and(d)timeseriescorrespondenttotheyellowasterisk.

monitoredbuildingswassensiblyimprovedconsideringabufferof

2mforeachbuilding.Thesizeofthebufferwasdecided

consider-ingtheproximityamong thebuildings.Awider buffercanlead

toconsider PS belongingto another building, especially in the

citycenter,where severalstreetsareverynarrow.Theuseofa

2mwidebufferallowedaconsiderablePSnumberimprovement,

whichwereusedtocalculatethebuildingsdeformationvelocity

(Table4).Thisimprovementwasobservedforallthedatasetevenif

theC-bandsensors(ERS1/2andENVISAT)werestillcharacterized

byalow percentageofmonitoredbuildings.OnlyRADARSAT-1,

amongtheC-bandsensors,permittedthemonitoringofa

sufﬁ-cient amountofbuildings. Whenthepercentagesof monitored

buildingswerelessthan40%,theuseofthewholePSdatasetwas

consideredmoreappropriateinordertoavoidthelossof

informa-tion.Thecomparisonbetweenthebuildingdeformationvelocity

(13)

Fig.13. Percentagesofbuildingsmonitoredwiththeavailablesensorsconsideringtheusedvelocity(inmm/y)classiﬁcationdescribedinthepreviousparagraphs.ForC-band sensors:class1:<−5;class2:−4.99to−3.00;class3:−2.99to−1.5;class4:−1.49to1.5;class5:1.51–3.00;class6:>3.ForCSK:class1:<−5;class2:−4.99to−3.00;class 3:−2.99to−2.0;class4:−1.99to2.0;class5:2.01–3.00;class6:>3.ForTSXD:class1:<−10;class2:−9.99to−8.00;class3:−7.99to−7.0;class4:−6.99to7.0;class5: 7.01–9.00;class6:>9;forTSXD2:class1:<−10;class2:−9.99to−7.00;class3:−6.99to−5.0;class4:−4.99–5.0;class5:5.01–7.00;class6:>7.

Table3

ResultsoftheinterpolationbetweentheconsideredPSdatasetsandthebuildings mapusingasimpleintersection.

Dataset No.ofPS PS/km2 _No._of_buildings _%

ERS1/2asc 63 35 45 6.10 ERS1/2 desc 247 137.2 128 17.34 ENVISATasc 418 232.2 199 26.97 ENVISATdesc 115 63.89 75 10.16 RADARSAT-1asc 298 165.56 203 27.51 RADARSAT-1desc 480 266.67 259 35.10 COSMO-SkyMeddesc 1447 803.89 405 54.95 TerraSAR-X 1567 870.56 442 59.98 Table4

ResultsoftheinterpolationbetweentheconsideredPSdatasetsandthebuildings mapusinganintersectionwitha2mtolerance.

Dataset No.ofPS PS/km2 _No._of_buildings _%

ERS1/2asc 149 82.22 101 13.69 ERS1/2desc 359 199.44 178 24.12 ENVISATasc 634 352.22 279 37.80 ENVISATdesc 310 172.22 170 23.04 RADARSAT-1asc 653 362.78 357 48.37 RADARSAT-1desc 757 420.56 348 47.15 COSMO-SkyMeddesc 3036 1686.67 506 68.56 TerraSAR-X 4718 2621.11 608 82.38

indicateasubstantialstabilityofthetown(Fig.13).Inparticular,

uptothe96%ofthemonitoredbuildingswithERS1/2are

char-acterizedbyvelocitiesincludedwithinthestabilityrange.Despite

ofthelownumberofmonitoredediﬁces,thegrounddeformation

velocitymapsfortheeasternslopealsoconﬁrmtheseresults:no

signiﬁcantdeformationswereobserved.Thenumberofbuildings

affectedby higher deformation velocities increases considering

ENVISATand RADARSAT-1.This increase can berelated tothe

higherPSdensity.Thegrounddeformationvelocitymapsobtained

usingENVISATandRADARSAT-1suggestthatthearea

correspond-ingtothe2010landslidebodywasaffectedbygrounddeformation.

ThankstothehigherPSdensityofRADARSAT-1,theinstabilitycan

bealsoappreciatedin therelatedbuildingdeformationvelocity

map.Thenumberofbuildingsaffectedbydeformationincreases

sensiblyconsideringtheX-bandsensors,especiallywithregardsto

CSK,whichhighlightsthatonly60%ofthemonitoredbuildingsare

characterizedbyvelocitieswithinthestabilityrange.Theseresults

are conﬁrmed by the comparison betweenthe building

defor-mationvelocitymapandthedamageassessmentmap.Buildings

characterizedbythehighestresidualvelocitiesarelocatedinthe

areaaffectedbythe2010landslide.Adirectcorrelationbetween

thedeformationvelocityandthedegreeofthedamagewasnot

observedbecauseofthetypeofbuildingaffectstheirresponseto

thedeformation.Forexample,modernbuildingshavingreinforced

concretefoundationsbetterresisttothestresswithrespecttothe

historicalbuildings.TheX-banddatahighlightedthepresenceof

considerablyresidualmovementsalongtheeasternslopeeventwo

yearsaftertheevent.TSXdatacanbeusedwithcautionbecause

oftheirhighstandarddeviation,which makesPSvelocitiesnot

reliable.Onlyadifferentialanalysiscanbeperformedtohighlight

movingareaswithrespecttostableareas.TheTSXbuilding

defor-mationvelocitymapconﬁrmedthesameareasaffectedbyground

deformationdetectedbyCSK.Animprovementinthemeasured

velocitiesaccuracywasobtainedthankstothereprocessingofthe

X-banddatasets.Usingthesedatasets,it waspossibletoreduce

thestandarddeviationandtoevaluatethedeformationvelocities

moreaccurately.Interestingresultswereobtainedalsoalongthe

1922landslidecrown.Inthisarea,slowbutcontinuous

deforma-tionwasdetectedsince1992.Inthissector,thecorrelationbetween

thedamagedbuildingsandthebuildingdeformationvelocitymap

isveryhigh.Someevidencesofdeformationsoftheediﬁcescanbe

observedalsousingC-bandsensors,butthelowerPSdensity

sug-geststhattheuseofthegrounddeformationvelocitymapismore

suitable.

Conclusions

PSInSARtechnique,usingﬁvedifferentsensors,was

success-fullyusedtoevaluatethebuildingdeformationvelocitiesofthe

SanFratellomunicipality,which wasaffectedbytwo landslides

occurredin1922 and2010 respectively.TheC-band datawere

acquiredbeforethe2010event,inordertoobservethepresence

oflandslideprecursorysymptoms,whereasX-bandsatelliteradar

datawereacquiredduringtheposteventphase,withtheaimof

detectingthelandslideresidualrisk.

TheadvantagesoftheX-banddataarerepresentedbythehigh

(14)

alargenumberofthesurveyedbuildings(morethan80%).Onthe

contrary,theC-bandsatelliteswerecharacterizedbyalow

per-centageofmappedediﬁces.OnlyRADARSAT-1dataseemedtobe

effectiveinordertounderstandpossiblepresenceofdeformation

areaswithintheSanFratellotown.ThelowdensityoftheC-band

sensorssuggeststhattheuseoftheground deformation

veloc-itymapismoresuitablewithrespecttothebuildingdeformation

velocitymap.Usingthelatter,severalinformationaboutground

deformationcanbelost,incaseofseveralPSdonotcorrespond

tobuildings.UsuallyX-bandSARimagesaremostexpensivebut,

atthelocalscale,theadvantagesofthehigherPSdensityismuch

greaterintermsofobtainedresults.

This work proves the effectiveness of the combined use of

ground and buildingdeformation velocity maps to understand

thebehavioroflandslidephenomenaduringthepre-and

post-eventmanagement phase.In particularX-banddata, appliedto

themonitoringofbuildingdeformation,combinedwiththe

dam-ageassessmentmap,canbeproﬁtablyusedbytheCivilProtection

andlocalAuthoritiesduringthepostevent,inordertoevaluate

theresidualriskandtoplanevacuationproceduresandmitigation

measuresregardingthemostdamagedandunstablebuildings.This

workalsohighlightstheimportanceofthecontinuousmonitoring

ofurbanizedareasaffectedbyslopeinstabilityphenomena.Inthis

case,thelongtermmonitoringwasperformedusingﬁvedifferent

sensors,butthisapproachcanberelevanttothedesignoffuture

productsbasedondataroutinelyacquiredbythenewESASentinel

mission.

Acknowledgements

Theresearchleadingtotheseresultsreceivedfundingfromthe

EuropeanUnion SeventhFramework Program(FP7/2007–2013)

undergrant agreementNo.242212 (ProjectDORIS)and Project

LAMPRE.Wealsothanktwoanonymousreviewersforthehelpful

suggestions.

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