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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.
Echoes of ancient tsunamis �
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.
A
B
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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
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