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Echoes of ancient tsunamis �

Echoesof ancient tsunamis

New research will help gauge the tsunami hazard to AustraliaAmy Prendergast

Geologicalsignaturesoftsunamisprovidecluestotsunamihazardsthatareunknownorpoorlyunderstoodfromwrittenandinstrumentalrecordsalone.InnortheastJapan,westernNorthAmerica,Norway,andScotland,tsunamidepositsserveaslong-termwarningsofunusuallylargetsunamisthatcouldotherwisetaketheseareasbycompletesurprise(Nanyamaetal.2003;Atwateretal.2005;Bondeviketal.2005).

BecausetherewasnohistoricalprecedentforaneventthesizeoftheIndianOceantsunamiof26December2004alongtheAceh–Andamansubductionzone,countriesaffectedbythetsunamiandtheirneighbourswerenotadequatelypreparedforthedisaster.IfgeologicalrecordsoftsunamisintheIndianOceanregionhadbeenstudiedbeforetheevent,theregionaltsunamihazardmayhavebeenrecognisedandtheimpactmayhavebeenreducedthroughtheimplementationofeducationprogramsandearlywarningsystems.

Deposits from ancient tsunamisHistoricalandinstrumentalrecordsoftsunamishavebeengatheredforamuchshorterperiodthantherecurrenceintervalsoflargetsunamis.Studyingthegeologicalsignaturesofpasttsunamisthereforeextendsthetsunamirecordbythousandsofyears,leadingtoabetterunderstandingoftsunamifrequency,magnitudeandflowdynamics,andagreaterappreciationoftsunamihazardandrisk.

Geologicalevidencefortsunamisvariesfromlargeboulderstoerosionalfeatures.Themostcommontsunamisignaturesarelandward-tapering,higherenergysedimentsheetspreservedwithinlowerenergydepositionalenvironments(figure1).Thecompositionofthesedimentsheetsvarieswiththeavailableonshoreandoffshoresediments,butfinetomediumsandgenerallydominates.

Tsunamisedimentsheetsrangeinthicknessfromafewcentimetrestotensofdecimetres,andmantlebeach-ridgeplain,estuarinemarshorlakebottomsediments.Theycharacteristicallyhaveasharp,erosionalcontactwiththelowerunit(usuallysoil),indicatingsomescouringbeforedeposition(figure2).Thesedimentsmaycontainlocalorfar-fieldgravel,mudandsoilrip-upclastsmixedwithsandandsilt(figure3).Multiple,normallygradedlayersareevidentinsomedeposits,allowingthedifferentiationofspecificwavesinthetsunamiwavetrain.

Microfossilassemblages(ostracods,diatoms,foraminiferaandpollen)provideevidenceofsedimentstransported

Figure 1. Theformationoftsunamidepositsduringasubductionzoneearthquake.Co-seismicsubsidenceoccursduringtheearthquake,loweringthelandlevelanddrowningcoastalmarshdeposits.Minutestohoursaftertheearthquake,severalsediment-ladentsunamiwaveswashoverthedrownedmarshlands,leavingbehindsedimentsheets.Overthenextfewdecades,thelandstabilises,allowingvegetationtorecolonisetheareaandasoilprofiletodevelop.

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Figure 3. Anexampleoftsunamisedimentsheets,soilprofilesandtidalflatdepositsinstratigraphicsequencefromMaullin,Chile.Thetsunamieventshavebeenconstrainedbyradiocarbondating(Cisternasetal2005).Notethesharpcontactbetweentsunamisandsheetsandunderlyingsoil,indicatingscouringbeforedeposition.Thebioturbatedpre-1575soilprofileindicatessubsidence.


anddepositedbytsunamis.Tsunamidepositsgenerallycontainamixtureoffossilsfromterrestrial,tidalanddeepwaterenvironments,indicatingbothlandwardandseawardtransportofsedimentsduringinundationandbackwash.Geochemicalsignatures,suchasstableisotopesofcarbonandoxygen,arealsousefulindistinguishingsedimentsources,astheycanbeusedasindicatorsoffreshandsaltwaterinfluxes.

Ifseveraltsunamidepositsoccurinstratigraphicsequence,datingofthedepositsusingradiogenicorluminescencetechniquesallowsestimatesoftsunamifrequency(Cisternasetal2005).Thisinformationcanprovidethebasisfortsunamihazardassessments.Detailedstudiesofthesedimentologyoftsunamidepositscanyieldconstraintsontsunamibehaviour,suchasflowdepthandvelocity(JaffeandGelfenbaum2002,Atwateretal2005),providingempiricaldatafortsunamimodellingandallowingbetterhazardestimation.Themappedgeographicalextentoftsunamidepositscancontributetoprobabilistichazardmapsandtocalibration,testing

andenhancementoftsunamirun-upmodelling.Furthermore,tsunamidepositscanbeafocusforpubliceducationabouttsunamihazards.

Identificationoffar-fieldtsunamidepositsisoftenmoredifficultthanidentificationofearthquake-generateddepositsclosetoatsunamisourceregion.Inplatemarginsettings,tsunamisedimentsheetsarepreservedinconjunctionwithevidenceforco-seismicsubsidencelandwardofthesubductionzone.Suchevidenceincludesdrownedtreesingrowthposition,achangeinbiotafromsupratidaltosubtidalassemblages,highlybioturbatedsoilprofiles,andachangeindepositsedimentologybetweentheupperandlowerunits(figure2).Thisadditionalevidencemakesidentificationofasedimentsheetastsunamigenicmorecertain.Furthermore,co-seismicsubsidencemakesitmorelikelythatthetsunamisedimentsheetwillbepreserved.

Incoastspronetoseverestormevents,tsunamihazardassessmentiscomplicatedby

Figure 2. Soilandsedimentrip-upclastsinthe1960ChiletsunamidepositnearMaullin,Chile.gc=soilrip-upclast;rc=sedimentrip-upclast.

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thepotentialforstormsurgestodepositsedimentsheetsthatmaybedifficulttodistinguishfromthoseleftbytsunamis.Recentstudiesofhistorictsunamiandstormdepositshavesuggestedsomecriteriafordistinguishingbetweenthem.Theseinclude:

tsunamidepositsaregenerallyofgreaterlateralextent

stableisotopicanalysisofoffshoresedimentscanbeusedtoidentifyfreshwaterfluxtothecontinentalshelvescausedbystormevents.

Nonetheless,thedifferentiationofpalaeotsunamiandpalaeostormdeposits,particularlyfordistantlygeneratedevents,remainsproblematic.Moreworkisneededtolinkthesedimentologyoftsunamiandstormdepositswiththephysicsofsedimenterosion,transportanddeposition(Tuttleetal2004,Atwateretal2005,Rhodesetal2006).ItisthereforeimportantforthecharacterisationofthetsunamithreattoAustraliathatevidencebeconsiderednotonlyfromtheAustraliancoastlinebutalsofromneighbouringsubductionzoneswherethereisabetterchanceofpreservinglessequivocaltsunamisignatures.

SeveralauthorshavereportederosionalanddepositionalfeaturesalongtheAustraliancoastlinepurportedtobetsunamigenic(BryantandNott2001,Switzeretal2005).However,mostarelargebouldersanderosionalfeatures,ortheiroriginisenigmatic.Theyarenotasusefulfortsunamihazardestimationbecausetheydonotyieldinformationabouttsunamifrequency.

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Hazards from the north…Tsunamiscanbegeneratedbyanyprocessthatverticallydisplacestheseasurface,includinglandslidesintothesea,underwaterlandslides,volcaniccollapsesandbolideimpacts.However,underseasubductionzoneearthquakesarethemostcommonmechanism.TsunamisgeneratedfromearthquakesaroundtheAustralianmargincouldpotentiallyreachAustralianshoreswithinhours(figure4).

TheSundaArcsouthofIndonesia,wheretheAustralianPlateissubductingbeneaththeSundaPlate,posesthegreatesttsunamithreattoAustralia’snorthwestcoast.Althoughpopulationdensityisfairlylow,ironoreproductionfacilitiesandextensiveoilandgasinfrastructureareconcentratedinthisregion(figure4).Furthermore,ifalargetsunamioccursinthisregion,theremotenessofthesettlementsalongthenorthwestcoastlinemayhamperthedeliveryofaid.Tsunamiinundationalongthiscoastline,therefore,hasthepotentialtocauseconsiderablehumanandeconomicloss.

The2004eventconfirmedthatthewesternSundaArciscapableofgeneratingtrulygiantearthquakes.OntheWesternAustraliancoast,the2004tsunamidisplacedboatsfromtheirmooringsanddraggedswimmersouttosea.However,duetotheorientationofthe

Figure 4. TheAustralianregion,showinglocationsoftsunamigenicearthquakesandvolcaniceruptions,oilandgasproductionfacilities,andpalaeotsunamidepositsontheAustraliancoastline(reportedfromBryantandNott2001).Knowntsunamirun-upheightsarescaledandcolour-codedwiththeirsources.

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arcinrelationtotheAustraliancoastline,mosttsunamienergywasdirectedawayfromAustraliaandtowardstheIndianOceanBasin(figure5a;Dominey-Howeetal,inpress).

Open-oceantsunamipropagationmodellinghasshownthatlargeearthquakesintheeasternSundaArccouldhaveasignificantimpactalongAustralia’snorthwestcoastline(figure5b;BurbidgeandCummins,inpreparation).The1977Sumbawaearthquakeandthe1994JavaearthquakeintheeasternSundaArcgeneratedfour-metretosix-metretsunamisonthenorthwestAustraliancoastline.ThetwoearthquakeswereratedatMw8.3andMw7.8respectively(Mwisalogarithmicmeasureofearthquakesize,similartotheRichterscalebutbettersuitedtoverylargeevents).The2006WestJavaearthquake(Mw7.7)alsohadasignificantimpactonpartsoftheWesternAustraliancoastline.

ThereisstilldebateaboutwhethertheeasternSundaArciscapableofgeneratingearthquakesgreaterthanMw9,whichcouldpotentiallycausealargetsunamialongtheentirewestAustraliancoast(BurbidgeandCummins,inpreparation).ThiswillbeimportantinthefuturecharacterisationoftsunamihazardtoWesternAustralia.

… and from the eastAlongtheeasternAustraliancoastline,wheremostAustralianslive,thetsunamithreatcomesfromseveralsources.Althoughtheyhaveproducedfewhistoricaltsunamis,theSolomonstrench,theNewHebridestrenchoffVanuatu,theTonga–KermadectrenchnorthofNewZealand,theAlpinefaultinNewZealandandthePuysegurtrenchsouthofNewZealandmayallhavethepotentialtoproduceearthquake-generatedtsunamiscapableofreachingAustralianshores.Moreworkneedstobedonetocharacterisetheearthquakemechanismsintheseregions,includingassessmentsofthemaximummagnitudeearthquakethateachzonemightgenerateandtheexpectednatureofearthquakerupture.

ThesteepslopesofthecontinentalshelfontheeasternAustralianmarginmayinduceunderwaterlandslidescapableofproducinglocalisedtsunamis.IntheAustralianTsunamiDatabase(Allport&Blong1995),severallargewavesofunknownsourcearedocumentedalongtheeasterncoastbetweenHobartandNewcastle.Ithasbeensuggestedthatthesewaves—recordedonotherwisecalmandcleardays—maybelocalisedtsunamisgeneratedbysubmarineslumps.ThemostfamoussuchincidentoccurredonSydney’sBondiBeachin1938,whenthreewavesencroachedonthebeachinquicksuccession.Thebackwashwasstrongenoughtodragswimmersouttosea.Morethan200bathersrequiredassistanceandfivepeopleweredrownedonadaythatbecameknownasBlackSunday.

Overahundredfeaturessuggestedtobeslumpscarshavebeen

identifiedalongthesoutheastcoastbetweenSydneyandWollongong.Higherresolutionbathymetrydataandoffshorecoringanddatingarenecessarytocharacterisetheage,magnitudeandtsunamigenicpotentialofthesefeatures.

AnothersourceoftsunamihazardfortheAustralianregionisthearcofmanyactivevolcanoes,inIndonesiaandthe

Figure 5. Open-oceantsunamipropagationofMw9earthquakesontheSundaArc.A:The2004SumatratsunamididnotsignificantlyaffectAustralia(Dominey-Howeetal,inpress).B:AnearthquakeinJavawouldhaveagreaterimpactonnorthwesternAustralia(BurbidgeandCummins,inpreparation).Thismodellingisaccuratefortsunamipropagationindeepwater.Run-upofthetsunamiontotheshorelineislikelytoincreasethetsunamiamplitudeseveralfold.FigurescourtesyofDavidBurbidge.

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Pacific,thatencircletheAustralianmargin.ThefamousKrakataueruptionof1883caused36000deathsinIndonesiaandgeneratedafour-metretsunamiinnorthwesternAustralia.The1453eruptionofTongolainVanuatuisreportedtohavebeenfourtimesaspowerfulasKrakatau,butitisnotknownwhetherthetsunamigeneratedbythiseruptionreachedAustralia.

Geoscience Australia’s role in palaeotsunami researchGeoscienceAustraliahasbeenbuildingexpertiseintsunamigeologyandiswellplacedtotakealeadingroleinpalaeotsunamiresearchandhazardandriskestimationintheregion.

InFebruary2006,staffparticipatedinafield-basedtrainingcourseinChile,thesourcelocationfortheMw9.5earthquakeandsubsequenttrans-Pacifictsunamiof1960.Thecourseenabledustodevelopourexpertiseintsunamigeologyandfosteredcollaborativecontactswithtsunamigeologistsfromothernations.InMay2006,wecontinuedourcollaborationwithIndianOceanandUnitedStatesscientiststhroughparticipationinatsunamidepositreconnaissanceprograminJava.ItisexpectedthatcontinuingcollaborationinthisregionwillhelptocharacterisethetsunamihazardfromtheenigmaticeasternSundaArcsubductionzone,whichpotentiallyposesthegreatesttsunamihazardtoAustralianshores.

Overthenextyear,GeoscienceAustraliawillconductapilotprojectfocusingonthesoutheastcoastofAustralia,wheretsunamismightbegeneratedbysubmarineslumpsoffthesteepcontinentalshelfandbyearthquakessouthofNewZealand.Thisprojectwillcomplementtsunamipropagationandinundationmodellingandahigh-resolutionstudyofthepotentialofthecontinentalmargintogenerateunderwaterlandslides.

FutureworkinthecharacterisationoftsunamihazardtotheAustralianregionwillrequireonshoreandoffshoreinvestigationsalongtheAustraliancoastline,aswellascollaborationwithregionalneighbours,inordertobettercharacterisethethreatfromplatemarginearthquakes.Theworkwillinvolveinterdisciplinarycollaborationbetweensedimentologists,geomorphologists,micropalaeontologists,tsunamimodellersandemergencymanagers.Throughanunderstandingofthemagnitude,frequencyandflowdynamicsofpasttsunamis,tsunamidepositscanimproveourunderstandingofthetsunamihazardandprovideameansofassessingfuturerisk.

More information

phone AmyPrendergaston+61262499292email [email protected]

References

AllportJK&BlongRJ.1995.TheAustralianTsunamiDatabase(ATDB).NaturalHazardsResearchCentre,MacquarieUniversity,Sydney.AtwaterBF,BourgeouisJ,YehH,AbbottD,CisternasM,GlaweU,HigmanB,HortonB,PetersR,RajendranK&TuttleMP.2005.Tsunamigeologyanditsroleinhazardmitigation.Eos86:400.BondevikS,LøvholtF,HarbitzC,MangerudJ,DawsonA&Svendsen,JI.2005.TheStoreggaSlidetsunami—comparingfieldobservationswithnumericalsimulations.MarineandPetroleumGeology22:195–208.BryantEA&NottJ.2001.GeologicalindicatorsoflargetsunamiinAustralia.NaturalHazards24:231–249.BurbidgeDR&CumminsP.Inpreparation.AssessingthethreattoWesternAustraliafromtsunamigeneratedbyearthquakesalongtheSundaArc.CisternasM,AtwaterBF,TorrejonF,SawaiY,MachucaG,LagosM,EipertA,YoultonC,SalgadoI,KamatakiT,ShishikuraM,RajendranCP,MalikJK,RizalY&HusniM.2005.Predecessorstothegiant1960Chileearthquake.Nature437:404–407.DomineyHoweD,CumminsP&BurbidgeD.Inpress.HistoricrecordsofteletsunamiintheIndianOceanandinsightsfromnumericalmodelling.NaturalHazards.JaffeBE&GelfenbaumG.2002.Usingtsunamidepositstoimproveassessmentoftsunamirisk.ProceedingsofSolutionstoCoastalDisasters2002,AmericanSocietyofCivilEngineers,836–847.NanayamaF,SatakeK,FurukawaR,ShimokawaK,AtwaterBF,ShigenoK&YamakiS.2003.UnusuallylargeearthquakesinferredfromtsunamidepositsalongtheKuriltrench.Nature424:660–663.RhodesB,TuttleM,HortonB,DonerL,KelseyH,NelsonA&CisternasM.2006.Palaeotsunamiresearch.Eos87(21):205.SwitzerAD,PucilloK,HaredyRA,JonesBG&Bryant,EA.2005.Sealevel,stormortsunami:enigmaticsandsheetdepositsinashelteredcoastalembaymentfromsoutheasternNewSouthWales,Australia.JournalofCoastalResearch21(4):655–663.TuttleMP,RuffmanA,AndersonT&JeterH.2004.DistinguishingtsunamifromstormdepositsineasternNorthAmerica:the1929GrandBankstsunamiversusthe1991Halloweenstorm.SeismologicalResearchLetters75:117–131.

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