Tsunamigenic potential of a Holocene submarine landslide along the North Anatolian Fault

26 Pages·2017·10.82 MB·English

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Tsunamigenic potential of a Holocene submarine landslide along the
North Anatolian Fault (North Aegean Sea, off Thasos Island):
insights from numerical modeling
AlexandreJANIN1,MathieuRODRIGUEZ1,DimitrisSAKELLARIOU2,VasilisLYKOUSIS2,and
ChristianGORINI3
1LaboratoiredeGéologiedel’EcolenormalesupérieuredeParis;PSLresearchuniversity,CNRSUMR8538,24rue
Lhomond,75005Paris,France.
2InstituteofOceanography,HellenicCenterofMarineResearch,GR-19013Anavyssos,Greece
3SorbonneUniversités,UPMCUniversitéParis06,UMR7193,ISTeP,F-75005,Paris,France.
Correspondenceto:AlexandreJANIN([email protected])
Abstract.TheNorthAnatolianFaultinthenorthernAegeanSeatriggersfrequentearthquakesofmagnitudeuptoMw 7.
∼
This seismicity can be a source of modest tsunamis for the surrounding coastlines with less than 50 cm height according
to numerical modelling and analysis of tsunami deposits. However, other tsunami sources may be involved, like submarine
landslides. We assess the severity of this potential hazard by performing numerical simulations of tsunami generation and
5 propagationfromaHolocenelandslide(1.85km3involume)identifiedoffThasosisland.Weuseamodelcouplingthesimu-
lationofthesubmarinelandslide,assimilatedtoagranularflow,tothepropagationofthetsunamiwave.Theresultsofthese
simulations show that a tsunami wave of water height between 1.10 m and 1.65 m reaches the coastline at Alexandroupolis
(58.000inhabitants)onehourafterthetriggeringofthelandslide.Inthesameway,tsunamiswavesofwaterheightbetween
0.80mand2.00mreachthecoastlineoftheAthospeninsula9minafterthetriggeringofthelandslide.Landslidetsunamis
10 should not be neglected as a potential source of tsunami in the area. Despite numerous earthquakes of Mw>7 and strong
detritalinput(ontheorderof30cm.ka 1),onlyafewHolocenelandslideshavebeenrecognizedsofar,askingthequestion
−
oftherelationshipsbetweenseismicityandlandslidefrequencyinthearea.
1 Introduction
Tsunamis constitute a major natural hazard for coastal populations and infrastructures. Tsunamis result from an impulsive
15 perturbation of the seafloor, which generates waves with a wavelength λ 100 km and a period in the 10 30 min range.
∼ −
Accordingtotheshallow-waterapproximation,thephasevelocityoftsunamiwavesisexpressedbyc=√ghatlongperiod,
wherehisthebathymetryandgthegravitationalacceleration.Althoughmostofthedeadliesttsunamisinthelastdecadesre-
sultfromearthquakesnucleatedalongsubmarinefaults(Okal,2015),othersourcesofdamagingtsunamishavebeenidentified,
includingsubmarinelandslides(Tappinetal.,2008;tenBrinketal.,2014).Oneofthemostconspicuouscase-studyistheJuly
20 1998eventthatstrokethecoastlineofPapua-NewGuinea.There,aMw 7earthquaketriggereda 4km3 submarineland-
∼ ∼
slideoffshoretheSissanoBay.Theslidemotiongeneratedatsunamiwithrun-upvalueslocally>10m,whichcausedmore
1
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than2200casualties(Heinrichetal.,2000,2001;Tappinetal.,2001,2008).Inthewakeofthe1998Sissanoevent,numerous
studiesinvestigatedthedistributionofsubmarinelandslidesalongcontinentalslopes,islandsandbathymetrichighs,aswellas
theirtsunamigenicpotential(Canalsetal.,2004;Massonetal.,2006;McAdooetal.,2000;Chaytoretal.,2009;Twichelletal.,
2009;Mulderetal.,2009;Tappin,2010;Iaconoetal.,2014;UrgelesandCamerlenghi,2013;Macíasetal.,2015;Palomino
5 etal.,2016;Rodriguezetal.,2012,2013,2017).Submarinelandslide-generatedtsunamisdisplayspecificcharacteristicscom-
paredtoothersources(TrifunacandTodorovska,2002;Harbitzetal.,2006,2014).Verticaldisplacementsinducedbytheslide
canbelargeraccordingtoitsvolumeanditsinitialacceleration,whichmayproducewaveamplitudeshigherthaninthecaseof
anearthquakesource(OkalandSynolakis,2003).Landslidemotioncanlastoverseveralminutestohours,leadingtocomplex
patternsofwaveinteractionsthateitheramplifyorattenuatethewaveamplitude(Haugenetal.,2005;Harbitzetal.,2006;Ma
10 etal.,2013;Løvholtetal.,2015).Forlandslidevolumesontheorderofafewkm3,frequencydispersionofthetsunamiresults
inshorterwavelengthandfasterwaveamplitudeattenuation,andlimitsthefar-fieldpropagationofthetsunami.
The North Anatolian Fault (Fig. 1) is a major, 1200-km-long continental strike-slip boundary, which is taking up the
∼
dextralrelativemotionbetweenAnatoliaandEurasiaatacurrentrateof 25mm.yr 1(Reilingeretal.,2006,2010;Perouse
−
∼
et al., 2012; Müller et al., 2013). The North Anatolian Fault is one of the most active fault system in Europe with several
15 earthquakesaboveMw7recordedfrominstrumentalseismologyinthelastdecades,amongwhichthedeadlyIzmitandDuzce
eventsin1999(Hubert-Ferrarietal.,2000).Detailedhistorical(Altinoketal.,2011)andfieldstudies(Meghraouietal.,2012)
additionallyrevealedthegeologicalsignatureofnumerouspastHoloceneearthquakesandtsunamis.IntheMarmaraSea,the
response of sedimentary systems to the seismic activity has been investigated throughout detailed stratigraphic comparisons
betweenthetimingofmasswastingeventsandthetimingofearthquakes(McHughetal.,2006;Becketal.,2007;Drabetal.,
20 2012,2015).Multibeamandseismicreflectiondatarevealedthesignatureofnumeroussubmarinelandslides(Gralletal.,2012,
2013),amongwhichsomemayhavetriggeredtsunamis(Hébertetal.,2005;Özerenetal.,2010;Alparetal.,2001;Yalçıner
etal.,2002).
Although numerous tsunami deposits have been identified in the geological record along the northern aegean coastlines
(Reicherter et al., 2010; Papadopoulos et al., 2014), the tsunamigenic potential of submarine landslides in this area remains
25 only preliminary investigated (Karambas et al., 2012). The North Aegean Trough is a strike-slip basin emplaced along the
NorthAnatolianFault.Itstectono-sedimentarycontextisverysimilartotheMarmaraSea,withsplaysoftheNorthAnatolian
FaulttriggeringfrequentearthquakesuptoM 7(suchasthe24thMay2014M 6.9eventintheGulfofSaros),andlocally
w w
gas-chargedsediments(Papatheodorouetal.,1993).However,onlyoneconspicuoussubmarinelandslidehasbeenidentified
sofaralongtheedgesoftheNorthAegeanTrough(Lykousisetal.,2002).Thissubmarinelandslideemplacedat40 15 N,
◦ 0
30 24◦ 520 E,offThasosIsland,somewherebetween5500and7500 8500yearsago,andremovedseveralkm3 ofsediments
−
(Lykousisetal.,2002).Theslidewastriggeredat300-m-depth,lessthan50-kmawayfromthesurroundingcoastlines,i.e.a
configurationverysimilartothe1998SissanoeventinPapua-NewGuinea.Forsimplicity,thislandslideishereafterreferred
astheThasoslandslide.TheobjectiveofthispaperistoinvestigatewhethertheThasoslandslidecouldhavebeenapotential
source of tsunami along the Aegean coasts by the mean of numerical modeling of tsunami generation and propagation. A
35 particular emphasis is put on the possible wave interactions related to the complex morphology of the Aegean coastlines,
2
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withnumerousislands,baysandpeninsulas(Fig.1).Weadoptadeterministicapproachbasedontheevaluationofcredible
scenariosdefinedandvalidatedonthebasisofthesimilaritybetweentheobservedandthemodeledlandslideextent.
2 GeologicalbackgroundoftheNorthernAegeanSeaanddescriptionoftheThasossubmarinelandslide
2.1 TectonicsandMorphology
5 ThegeologicalhistoryoftheAegeanSeatakesplaceinthecomplexrealmofthecollisionbetweenAfricaandEurasiaduring
theCenozoic.TheAegeandomainusedtobeamountainbelt(Hellenides)duringtheEarlyCenozoic,whichcollapsedsince
theLateEocene-EarlyOligocene,incloserelationshipwiththesouthwardsretreatoftheHellenictrench(JolivetandFaccenna,
2000; Jolivet and Brun, 2010; Jolivet et al., 2013; Brun et al., 2016). Since the Late Miocene, strike-slip tectonics along the
North Anatolian Fault system accommodates the lateral escape of the Anatolia (Armijo et al., 1999; Faccenna et al., 2006;
10 Hubert-Ferrarietal.,2009;LePichonetal.,2015).IntheNorthAegeanSea,theNorthAnatolianFaultsystemisdividedinto
two main strike-slip systems, shaping the morphology of the seafloor. The age of inception of these strike-slip segments is
estimatedinthePlio-Pleistocenefromtiesofavintageseismicdatasetwithindustrialwells(Beniestetal.,2016).
Thefirst,mainfaultsegmenttothenorthrunsalongtheSarosGulf,whereitformsanen-échelon,negativeflowerstructure
systemthatconnectstheNorthAegeanTrough(Fig.1)(Kurtetal.,2000;McNeilletal.,2004).Thissegmentaccommodates
15 dextralshearingata 20mm.yr−1rate(LePichonandKreemer,2010;Perouseetal.,2012).Itcorrespondstotheprolonga-
∼
tionoftheMainMarmaraFault(RoussosandLyssimachou,1991;LePichonetal.,2001)westoftheDardanellesStrait(Fig.
1). Numerous oblique NW-SE to E-W splays connects the North Anatolian Fault and form a horsetail structure, typical of
strike-slipfaultterminations.Thehorsetailstructureformsa 150-km-long, 60-km-wide, 1500m-deepbasin,referred
∼ ∼ ∼
as the North Aegean Trough (Papanikolaou et al., 2002; Sakellariou and Tsampouraki-Kraounaki, 2016; Sakellariou et al.,
20 2016), where the oblique splays isolate a series of half-grabens, tilted over a crustal detachment at 10-km-depth (Laigle
∼
etal.,2000).Thiscomplexstructuralpatternresultsinaveryunevenbathymetry,withaseriesofbathymetrichighsandlows
within the North Aegean Trough. Southwards, the second strike-slip splay corresponds to the southern branch of the North
AnatolianFault,activeata 5mm.yr 1 rate(LePichonandKreemer,2010;LePichonetal.,2014).Thisstructureendsoff
−
∼
SkyrosIsland,whereitformstheEdremitTrough(Fig.1).
25 2.2 Sedimentology
DuringtheQuaternary,thedepositionofdetriticsedimentscomingfromthesurroundinglands(Sucetal.,2015)hasbeencon-
trolledbyfastsubsidence(0.3to1.5m.Kyrs 1)(PiperandPerissoratis,1991)andglacio-eustaticvariationsofsea-level(˙Is¸ler
−
etal.,2008).InthevicinityoftheNorthAegeanTrough,theLateQuaternary(i.e.last150kyrs)sedimentsarecomposedof
pro-deltaformations,composedofsandytosiltyturbidites(PiperandPerissoratis,1991;Lykousisetal.,2002).Theuppermost
30 sediments(2mbelowseafloor)consistinnormallyconsolidatedorslightlyover-consolidatedmuddytosiltysediments,with
abulkdensityofsedimentsrangingbetween1.4and1.5g.cm 3(Lykousisetal.,2002).
−
3
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2.3 TheHoloceneThasossubmarinelandslide
Onthebathymetry,theslideisrecognizedbya 30-km-longscar,enclosinganareaof85-km2andbyanupto75 m-thick
∼ −
lobe-shapedepositspanninganareaof 50km2 (Fig.2(a))(Lykousisetal.,2002).Themasstransportdepositischaracterized
∼
byahummockyfaciesontheseaflooranddisplaysatypicalfaciescomposedofchaoticandhyperbolicreflectorsontheseismic
5 profiles(Fig.2(b)).Asecond,minormasstransportdepositunitextendsover8.7-kmfromthetopthescarpoutwardsandis
characterizedbya5-m-thickdeposit.Theinitialvolume,mobilizedduringthefirststepofthefailure,isestimatedfollowing
themethoddescribedinTenBrinketal.(2006).Itconsistsinfilling-inthefailedareaaccordingtotheadjacentscarheight.
Thedifferencebetweenthefilled-ingridandthegriddisplayingtheslidescarsgivesafailedvolumeat1.85km3inthecaseof
theThasosslide.Thevolumeofthemasstransportdepositlyingatthebottomofthescarisestimatedat 3.8km3(Lykousis
∼
10 etal.,2002).Theslideevolvedasatranslationalslideofwell-beddedsediments,overaglideplanecorrespondingtoamuddy
weak layer deposited 170 245 kyrs ago (Lykousis et al., 2002). Although earthquake shaking related to the activity of the
−
North Anatolian Fault may be considered as the most likely trigger of submarine landslides, recent M 7 events did not
w
∼
triggersignificantlandslidesandtsunami,despitetheunstablestateoftheNorthAegeanTroughslopes(Lykousisetal.,2002).
This discrepancybetween strong earthquakesand landslide frequencieshas been highlightedin various tectono-sedimentary
15 contexts (Völker et al., 2011; Strozyk et al., 2010; Pope et al., 2015, 2016). Various process may explain this discrepancy,
includingtheamountandthenatureofsedimentarysupply,localvariationsinphysicalpropertiesofthesediments(Lafuerza
et al., 2012) or the over-compaction of the sediments subsequent to the fluid release induced by the seismic wave shaking
(Hamptonetal.,1996;Strozyketal.,2010).Somemasstransportdeposits,probablyolderthanHolocene,canbeguessedon
sparkerlinesintheSporadhesbasin(BrooksandFerentinos,1980;Ferentinosetal.,1981)andonsub-bottomprofilescrossing
20 the Saros Gulf (McNeill et al., 2004). However, a comprehensive map of the spatial distribution of landslides and precise
stratigraphicconstraintsontheirrecurrencearelackingtofurtherdescribehowthesedimentarysystemreactstotheseismic
activityoftheNorthAnatolianFault.
3 Numericalmodelingofthesubmarinelandslideandtheassociatedtsunami
3.1 Bathymetricdataset
25 Thebathymetricgridusedfortsunamicalculationsisacompilationofseveralgridsofdifferentresolution.Weusethemulti-
beamgridpreviouslypublishedinPapanikolaouetal.(2002),acquiredwithaseabeam2120workingat20kHz.Thehorizontal
resolutionis 100m,foraverticalresolutionontheorderof1m.Attheslidelocation,weincludethehigh-resolution(30-
∼
m horizontal resolution) grid acquired by HCMR in 2013-2015 onboard the Aegeo, in the frame of the YPOTHER project
(Sakellariouetal.,2016).Theremaininggapshavebeenfilled-inwiththeSRTMPLUSbathymetry(Beckeretal.,2009).
4
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3.2 Slidemodeling:physicalbackgroundandlimits
WeusetheAVALANCHEcodethathasbeensuccessfullytestedfromcomparisonswithtidalmeasurementsforthe1998Papua
NewGuineatsunami(Heinrichetal.,2000)andthe1979Niceevent(Labbéetal.,2012).Twoapproachesarecommonlyused
to model the dynamics of a submarine landslide. A first approach assimilates the slide to a viscous flow, with a Bingham
5 rheology.Theslideisdividedintoabottomlayersubmittedtoshearalongtheglideplane,andatoppluglayerwithauniform
velocity profile (Jiang and LeBlond, 1993). This approach efficiently reproduced some landslides in the Mediterranean Sea
(Iglesiasetal.,2012;Løvholtetal.,2015).However,inourcase,allthesimulationscarriedforareasonablerangeofdynamic
viscosities(25 500m2.s 1)failedtoreproducethefirst-ordermorphologyoftheThasosslide,andleadtoexcessiverunout
−
−
and volume values. The second approach, which assimilates the propagation of the landslide to a granular flow (Savage and
10 Hutter,1989),producedmoreconvincingresultsinourcase,whichmaybeconsistentwiththedetriticnatureofthesediments
off Thasos (Piper and Perissoratis, 1991; Lykousis et al., 2002). Modelling the slide as a granular flow implies the sediment
loses its cohesion immediately after failure occurred at the glide plane. This model assumes that the energy dissipated at
the glide plane is much higher than the energy dissipated within the sedimentary flow itself. Although large deformations
throughoutthethicknessoftheflowareinevitable(tenBrinketal.,2014)andmayhaveimportanteffectsontheevolutionof
15 thetsunamiwave(Maetal.,2013),theconfigurationoftheslide(thicknesssmallerthanitslengthanditswidth)allowsthe
use of the shallow water assumption (Savage and Hutter, 1989), and therefore the simplification of the mechanical behavior
withinthesedimentaryflow.Deformationswithintheflowintheearlieststagesofcollapsearealsoneglected,aswellasthe
addedmasseffect(Løvholtetal.,2015),whichmayresultinslightlyoverestimatedinitialacceleration(ontheorderof10 2
−
m.s 2). The velocity of the flow in the direction parallel to the slope is considered constant over the entire thickness of the
−
20 slide.BasalfrictionattheglideplaneismodeledbyaCoulomb-typefrictionlaw(SavageandHutter,1989).Thislawassumes
aconstantratiooftheshearstresstothenormalstressatthebaseoftheslideandinvolvesadynamicfrictionangleφbetween
theroughbedandtheslidingmass.Whentheflowisclosetorest,thefluidvelocityissetto0assoonastheCoulombforce
islargerthanthealgebraicsumoftheforcesduetogravityandheightgradient.Granularflowscommonlyoccurforinternal
frictionangles(φ)around30 40 .However,thisrangeofvaluesisnotappropriateforsubmarinelandslides,whichinvolve
◦
−
25 φvalues<15◦ (Canalsetal.,2004;TenBrinketal.,2009),andevenφvalues<2◦ forlowslopegradient(Rodriguezetal.,
2012;Urlaubetal.,2015).Therefore,weperformsimulationsonlyforfrictionanglesrangingbetween1and5 tosimulatethe
◦
Thasoslandslide,whichspanstherangeofslopevaluesinthisarea.Thenonlinearandnondispersiveequationsfromshallow
waterassumption,writteninacoordinatesystemlinkedtothetopography,giveus(Heinrichetal.,2000):
fluidmassconservation:
∂h ∂ ∂
30 + (hu)+ (hv)=ew(u+v) (1)
∂t ∂x ∂y
momentumconservationequations:
∂ ∂ ∂ 1 ∂
(hu)+α (hu u)+α (hu v)= κ (gh2cosθ)+κghsinθ τ (2)
∂t ∂x · ∂y · −2 ∂x x− xz(z=0)
5
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∂ ∂ ∂ 1 ∂
(hv)+α (hv u)+α (hv v)= κ (gh2cosθ)+κghsinθ τ (3)
∂t ∂x · ∂y · −2 ∂y y− yz(z=0)
sedimentmassconservation:
∂ ∂ ∂
(ch)+ (chu)+ (chv)=0 (4)
∂t ∂x ∂y
5 where:
– h(x,y,t)theperpendicularthicknessofthelandslide
– utheveloctiyvectorwithitscomponents(u,v)inaccordancewithxandyaxis,respectively
– κ=1 ρw/ρs with ρs the sediment density; ρs=1450 kg.m−3 according to in-situ measurements (Lykousis et al.,
−
2002)andρ ,thewaterdensity.Hereweconsiderρ =1000kg.m 3
w w −
10 – cthesedimentconcentrationatameandepth
– θ(x,y)thelocalslidinganglewithθx,θy respectivelyitscomponentsinaccordancewithxandyaxis
– τ andτ aretheshearstressesatthebedsurface.Theconstitutivelawgoverninggranularflowsisgenerally
xz(z=0) yz(z=0)
theCoulomb-typefrictionlawdefinedinthex-directionby:τ =κghcosθtanφ u
xz(z=0) u
k k
– ew thewaterentrainmentcoefficient(ew [0,0.01])(Fukushimaetal.,1985;KosticandParker,2006;Sequeirosetal.,
∈
15 2009).Itcorrespondstothedilutionofthesedimentarymassbywaterincorporationattheinterfacebetweentheslide
andthewater.
– α is a parameter characterizing the deviation of the velocity profile compared to a homogeneous distribution profile.
Modellinglandslideasagranularflowrequiresalinearprofileandα=4/3(Heinrichetal.,2001).
Themainlimitofthesimulationofapastsubmarinelandslideistheimpossibilitytoprovideconstraintsonthepatternof
20 velocityandaccelerationoftheslidefromthegeologicalrecord(Harbitzetal.,2006;Maetal.,2013;Løvholtetal.,2015).
Wethereforeconsiderscenarioswhereslidevelocitiesarecompatiblewiththefewconstraintsavailablefromsubmarinecable
breaksrecordedinthe20thcentury(i.e.,withmeanvelocity<20 30m.s 1 andinitialacceleration<0.6m.s 2 (Heezen
− −
−
andEwing,1952;Mulderetal.,1997;Fineetal.,2005).
3.3 Tsunamimodeling
25 Simulationsofthetsunamiwavesarealsobasedontheshallow-waterapproximation,whichdealswiththefullinteractionof
landslideandwater,includingthedeformationofthesedimentbody.Equationsgoverningthelandslideandthetsunamiprop-
agationaresimilarandarethussolvedusingthesameGodunov-typescheme,extendedtosecondorderbyusingtheconcept
ofVanleer(AlcrudoandGarcia-Navarro,1993;Mangeneyetal.,2000).Thismodelisparticularlyadaptedtononlinearwaves.
Thetimehistoryofseabottomdeformationresultingfromthelandslideisintroducedasaknownforcingterm(cosθ) 1∂h/∂t
−
30 inthemassconservationequationofthetsunamimodel(Heinrichetal.,2000).
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4 Results
4.1 ElaborationofLandslideandTsunamiScenarios
We simulate the tsunami that would be triggered in the present-day context by a slide similar to the Thasos slide. Credible
scenariosaredefinedaccordingtotherangeofparametersabletoreproduceascloselyaspossiblethefirst-ordermorphology
5 oftheobservedmasstransportdeposit(i.e.fittingthefollowingcriteria:areaofthedeposit,volume,runout).Crediblescenarios
(Table.1)areobtainedforarangeofbasalfrictionangleφbetween1.5 1.8 andaninitialvolumeoffailureof1.85km3
◦
−
(Fig. 3). The selection of the most credible scenarios is buttressed on the basis of normalized inverse distance weighting
(Shepard,1968)(AppendixA).
4.2 PropagationoftheTsunamipotentiallygeneratedbytheThasoslandslide
10 For each selected scenario, we present maps of tsunami propagation at different, representative time steps (10, 16, 28 and
43 minutes ; Fig. 4, Fig. 7). We also plot maps of the maximal water heights reached after a 2 h propagation time in
the entire study area (Fig. 5, Fig. 8), which helps to identify bathymetric forcing effect on the tsunami elevation. The
propagation of the tsunami at the source displays the same pattern in every scenario, typical of landslide generated tsunami
(Ward,2001;MohammedandFritz,2012;Maetal.,2013).Atthesource,theincipientradiatedwavehastwopeaksandone
15 trough (Fig. 4). The water ahead of the front face of the slide (i.e. the out-going wave) is pushed away, creating a leading
positivewaveintheslidedirection(i.e.towardstheChalkidikiPeninsulaandtheThermaikosGulf).Thetroughfollowingthe
crestissimultaneouslycreatedbytheslideexcavation,andisfollowedbyalargesecondpositivepeakcreatedbytheinfilling
of the trough. In contrast, the front of the wave propagating in the opposite direction of the slide (i.e. the back-going wave)
formsatroughpropagatingtowardsAlexandroupolis,followedbyasecondpositivewave(Fig.4).Amongthe40simulations
20 carriedout,wepresenttwoofthemwherethedepositiswellreproduced,butleadingtodifferenttsunamielevation(Table.2).
4.2.1 Firstscenario
Thefirstisobtainedforabasalfrictionangleφof1 andawaterentrainmentcoefficientof10 3.Althoughtherunoutofthe
◦ −
modeledslideisslightlyoverestimatedby 2km,themodeledvolumeis3.7km3,whichcomparescloselytotheobservations
∼
(Fig. 3). The thickness of the modeled deposit, between 20 and 40 m, is also in fair agreement with the observations from
25 theseismiclinecrossingtheThasosslide(Fig.3).Inthisscenario,themaximalvelocityoftheslidereachesthevalueof30
m.s 1 about6minutesaftertheslidebeginning.Duringthisphase,theaccelerationoftheslideisontheorderof0.2m.s 2.
− −
Wedescribethepropagationofthetsunamifollowingthetimingofarrivaloftheleadingwaveatthecoastline.Atthesource,
theslidegeneratesatsunamiwithamaximalwaterelevationat3.6m(Fig.5).After8minutesofpropagation,theleading
out-goingwavereachestheChalkidikiPeninsulaandtheislandofLemnos,withlocalamplificationupto1.25m.Thefront
30 oftheout-goingwavethensweepsthecoastoftheAthosPeninsulatowardstheOrfaniGulf.There,theleadingwavereaches
an elevation up to 1.20 m-high after 40 minutes of propagation (Fig. 4). The tsunami then propagates into the multiple
∼
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gulfformedbythe’finger-like’patternoftheChalkidikiPeninsula(Fig.4).ThetsunamienterstheSigitikosGulfafter 15
∼
minutesofpropagation,thentheKassandraGulfafter 28minutesofpropagationandeventuallytheThermaikosGulfafter
∼
40 minutes of propagation. Tsunami elevation reaches 0.8 0.9 m in the Sigitikos Gulf, but remains below 0.4-m in the
∼ −
KassandraGulf(Fig.4).ThecoastlinerunningfromtheSporadestotheThermaikosGulfisstrokebya 0.5-m-hightsunami
∼
5 (Fig.4).AbathymetricthroughrelatedtoasplayoftheNorthAegeanTroughoffKassandraPeninsulaisresponsibleofthe
focalizationandtheamplificationofthetsunamibeforeitenterstheThermaikosGulf(Fig.4).Southofthesource,theleading
out-goingwavereachestheNorthofLemnosafter 10minutesofpropagationandAgiosEfstratiosislandatt=26min(Fig.
∼
4), where complex phenomenons of wave interactions lead to tsunami elevation on the order of 1.4 m at the coastline. The
leadingback-goingwaveissplitinseveralfrontspropagatingatdifferentspeedswhenitreachestheSamothrakiandGokceada
10 Islandsafter 16minutesofpropagation.A1.65-m-hightsunamistrikesAlexandroupolisafter1h07minofpropagation(Fig.
∼
4).ThecomplexpatternoftheAegeancoastlineresultsinacomplexpatternofwaveinteractionsthateitherdestroyoramplify
thetsunami(Fig.6).AmplificationthroughresonanceisexpectedwithintheSigitikosBay,theOrfaniGulf,andthePournias
Bay north of Lemnos (Fig. 4). South East of Agios Efstratios and north of Samothraki, we observe lineaments where the
tsunami wave is amplified (on the order of 50 cm), despite the lack of bathymetric feature that could explain local forcing.
15 Constructiveinterferencesbetweenthewavesthatsurroundedtheseislandsmayproducethisamplification.Atbothlocations,
the leading wave is split in two wave fronts that rotate along the islands by a coastal attraction phenomenon, until the two
wave fronts merge together, leading to wave amplification. For instance, the back-going wave of the tsunami is split in two
wave fronts when it strikes Samothraki at t=28 min (Fig. 6). After their propagation around the island, both fronts collide
att=36min,whichresultsinconstructiveinterferencesthatamplifythewaveupto 0.5 0.6m.Asecondpositivewave
∼ −
20 front,whichhasbeenamplifiedalongtheThraciancoast(Fig.6at43min),collideswiththeback-goingwavefrontformed
east of Samothraki. This interference cancels the negative polarity of the back-going wave, and even amplifies the tsunami
frontupto1.65matAlexandroupolis.
4.3 Secondscenario
Thesecondselectedscenarioisobtainedforabasalfrictionangleφof1.5 andawaterentrainmentcoefficiente of5.10 4.
◦ w −
25 In this model, the area of the mass transport deposit is larger than the observations, resulting in an overestimated but still
reasonablevolumeof4.1km3 (Fig.3).Inthiscase,themaximalvelocityoftheslide,ontheorderof25m.s 1,isreached
−
about3minutesaftertheslidebeginning.Duringthisphase,theslideaccelerationisontheorderof 0.15m.s 2.Thissecond
−
∼
scenarioleadstothesametimingandmodeofpropagationofthetsunami,withsimilarprocessesofwaveamplificationsand
interactionsaroundtheislandsofAgiosEfstratiosandSamothraki(Fig.7).However,thetsunamielevationislessimportant
30 everywhere.Themaximumofwaterelevationisreducedat 85cmalongtheChalkidikiPeninsula,at 1.10matAlexan-
∼ ∼
droupolis,andaround50cmatAgiosEfstratiosandnorthofLemnos.AmplificationwithinthebayofSigitikos,Kassandra,
andtheGulfofThermaikosisalsoreduced,withwaterheightslessthan20cm(Fig.8).
8
Nat.HazardsEarthSyst.Sci.Discuss.,https://doi.org/10.5194/nhess-2017-405
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5 Discussion
TheresultsofoursimulationsshowthattheThasosslidewasprobablytsunamigenic.Ifasimilarslideoccursinthecurrent
sea level, the areas of Alexandroupolis, the Chalkidiki Peninsula and Agios Efstratios would be the most threatened, with
tsunami elevations at the coastline ranging between 1 and 2 m according to the slide parameters. Most of the coastlines of
5 the northern Aegean Sea are stroke by the tsunami after less than 1 hour of propagation (Table. 2). The Athos Peninsula is
reached after just 10 minutes, but is not densely populated and the topography does not allow large run up distances. The
tsunamisimulationshavesomelimits.Thefirstoneisthelackofhigh-resolutionbathymetryalongthecoast,whichdoesnot
allowaccuratecomputationofthefloodingonland.Itremainsunknowniflocalbathymetricfeaturesorharborswouldresult
infurtheramplificationordispersionofthetsunamiwave.Thisisespeciallycriticalforthenumerousgulf(Orfani,Sigitikos
10 and Kassandra) where resonance is expected. Run-up values being often more important than the water height calculated a
fewkilometersfromthecoastline(Synolakis,1987),thecomputedtsunamielevationsatthecoastlineshouldbeconsideredas
minimumvaluesoftheactualrunup.Moreover,theinitialslideaccelerationanditsspeedstronglyimpingeontheinitialpattern
ofthetsunami,includingthepatternofshortwavelengths,andthereforeonthesubsequentfrequencydispersion(Haugenetal.,
2005).Evenifthemodeledvaluesofslidevelocityandaccelerationfitwithvaluesmeasuredonslidestriggeredinthe20th-21st
15 centuries(HeezenandEwing,1952;Mulderetal.,1997;Popeetal.,2016),geologicalrecordsarenotsufficienttodetermine
theseparametersforHoloceneperiod.
A second-order source of simplification is the non-dispersive numerical model that is not able to account for frequency
dispersioninherentintsunamiswithshortwavelengths.However,thedensityofthemobilizedsedimentsisquitelow( 1500
∼
kg.m 3).Frequencydispersionatthesourceisexpectedtobenegligibleforthisrangeofdensities(Maetal.,2013).
−
20 Other studies have investigated the tsunamigenic potential of landslides in the Aegean Sea. Karambas et al. (2012) used
a model based on Boussinesq type of equations for the waves propagation. The internal dynamic of slide is not taking into
account in this type of simulation. However, all studies agree that the Greek coasts in the north of Aegean Sea are the most
threatenedareabytsunamiifaThasos-likeslideoccursnowadays.
Despite these limits, the results of our simulations show that the expected tsunami wave from the Thasos slide are higher
25 thanvaluesexpectedinthecaseofanearthquakealongtheNorthAnatolianFault,andhigherthanrunupvaluesdocumented
fromtsunamidepositsintheThermaikosGulf(Reicherteretal.,2010).Thestudyhighlightstheneedtobuildacomprehensive
mapofthedistributionoflandslideswithintheNorthAegeanTrough,aswellasafullquantificationoftheirvolumes,tobetter
estimatethevarietyoftsunamiscenariosinthearea.Thisdiscrepancybetweenearthquakeandlandsliderecurrenceasksthe
questionoftheresponseofsedimentarysystemstogroundshaking.However,moredangeroussourcesoftsunamihavebeen
30 recognizedinthesouthernpartoftheAegeanSea,includinglandslidestriggeredduringthe1956Amorgosevent(Perissoratis
andPapadopoulos,1999;Okaletal.,2009);ortsunamitriggeredduringorsubsequenttovolcanicevents(e.g.Santoriniinthe
Bronzeage;(Novikovaetal.,2011).
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Nat.HazardsEarthSyst.Sci.Discuss.,https://doi.org/10.5194/nhess-2017-405
ManuscriptunderreviewforjournalNat.HazardsEarthSyst.Sci.
Discussionstarted:7February2018
c Author(s)2018.CCBY4.0License.
(cid:13)
AppendixA: SelectionofTsunamiscenario
In order to quantify the gap between landslide simulation results and data from bathymetry and seismic profiles we used an
’normalizedinversedistanceweighting’function.Therebywedefineaweightfunctionωas:
4
1
ω= (A1)
d (x ,xth)p
k=1 k k k
X
5 Where:
– xk areparametersusedfordeterminedthedifferencesbetweenthenumericalsimulationandthebathymetry.Weused
forx :(1)theslidingdirection,(2)thetotalsurfaceofdeposit,(3)theamplitudeofthediscontinuityinthethicknessof
k
sedimentsbetweenunits1and2(Fig.2)and(4)thefinalvolumeofthedeposit.
– xtkharetheusedobservableparametersfromthebathymetryandtheseismicprofilescorrespondingtoxk
10 – dk(xk,xtkh)aredistancefunctionsbetweenxk andxtkh.Wedefineheredk asdk=|xtkh−xk|
– pisarealpositivenumber,calledthepowerparameter.Ifp>1,thecontributioninωofthesmalldistanceisamplified
(weusedherep=1).
The higher the ω, the closer the scenario is to bathymetry and seismic data. The function ω is calculated for all scenarios
andallowsustohighlightthetwoscenariospresentedinthisstudy.Thecomputationofω forthe2crediblescenariosgives
15 ω=181.52andω=2.56(Fig.3)
Acknowledgements. Weacknowledgethesupportofthe"YvesRocard"JointLaboratorybetweentheÉcoleNormaleSupérieure,theComis-
sariatàl’ÉnergieAtomique,andtheCNRS.
10