modeling the post‐rifting deformation on the krafla volcanic … · 2014-09-24 · among...
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Modelingthepost‐riftingdeformationontheKraflavolcanic
system
ChristopherDiCaprio,MarkSimonsCaltech
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Outline• GeophysicalGoals
– Modelingthepost‐riftingdeformationontheKraflafissuresystem• PreviousWork
– ViscoelasticmodelsandGPSdata– ElasticmodelsandInSARdata
• InSARConstraints• ModelingWorkflow
– Whattoolsweuseandhowtheyareapplied• IntuitionBuildingviaFEMModeling• ChallengesandNextSteps
– Whattechnicalissuesneedtobeaddressed
Today’sGoals:- DemonstrateuseofPyLithandmodelingworkflow- Describemodelingchallengesandneededtools
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GeophysicalGoals
• Whatarewetryingtodo?– ConstrainrheologyofnorthernIcelandiclithospherebyobservingresponsetoariftingevent
• Howareweapproachingtheproblem?– AnalysisofthelargeInSARdataset– Structuralmodelsusinggeophysicalandgeologicalconstraints
– TwoandthreedimensionalFEMmodelsofviscoelasticdeformationusingPyLith
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NVZ
EVZ
WVZ
Krafla
SeeSigmundsson,IcelandGeodynamics
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NorthernVolcanicZone
Theistareykir
Krafla
Fremri‐Namur
Askja
Kverkfjoll
volcanicsystems
• 100‐150yearriftingepisodeinterval• Eachevent50‐100kmextent
1618A.D.
1724,1975A.D.
1874A.D.
historicrifting
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Tryggvason,1984,B.Vol.
KraflaFires1975‐1984
Ndistancealongrift(km)
amou
ntofo
pening(m
)
S
Seriesofindividualriftingepisodeslasthourstodays
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GPSRecordofPost‐RiftingDeformation
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PreviousVisco(elastic)Modeling
Foulgeretal1992
HoftonandFoulger,1996 PollitzandSacks,1996
viscous
elastic
Maxwellviscoelastic
standardlinearsolid
€
1.1×1018 Pa s
€
0.3− 2 ×1019 Pa s
€
3×1018 Pa s
€
3×1019 Pa s
Hekietal1993
€
0.3− 2.2 ×1018 Pa s
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Foulgeretal.1992
€
0.3− 2 ×1019 Pa s
• Materialvariesonlywithdepth(Elasticlayeroverviscouschannel)
• Onlyconsideredriftnormaldisplacementdatafrom1987and1990GPScampaign
Solveforamountofopeninganddiffusivity(Elsassermodel)
2metersofopening
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Hekietal.1993
€
0.3− 2.2 ×1018 Pa s
• Materialvariesonlywithdepth(elasticlayeroverviscouschannel)
• Modelhorizontaldisplacementdatafrom1987and1990GPScampaign
• Elasticlayeroverviscouschannel Findthreediffusioncoefficients
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HoftonandFoulger1996
€
1.1×1018 Pa s
• Materialvariesonlywithdepth
• ElasticlayeroverMaxwellviscoelastic½space
• Modelthreedimensionaldisplacementdatafrom1987and1992GPScampaign
Findviscosityof½space Refineestimatesofdikeheight,dip,andelasticlayerthickness
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HoftonandFoulger1996
Horizontaldisplacementsfitfor1987‐1992
Verticalsaremoredifficulttofitbutalsomuchnoisier
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PollitzandSacks1996• Materialvariesonlywithdepth
• ElasticoverSLSoverMaxwellviscoelastic
• Modelthreedimensionaldisplacementdatafrom1987,1990,1992GPScampaign
Invertforamountofopening
FindviscosityofVElayers
€
3×1018 Pa s
€
3×1019 Pa s
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PollitzandSacks1996
Horizontaldisplacementsfitnearrift1987‐1990and1990‐1992
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Krafla
ascendingdescending
InSARCoverage
Lineofsightvectors
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InterferometricPairs
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95/06/19‐92/09/18
deflatingKraflamagmachamber
contraction,cooling?
Broadpost‐riftingsignal(viscoelasticrelaxation)
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ascendingorbit‐T1
95/06/19‐92/09/18 99/10/11‐96/10/21
Rateofdeformationslowingdown
groundtoradar
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96/06/04‐93/07/31 06/06/027‐03/09/16
descendingorbit‐T9 Smallsignal20yearsafterrifting
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Sigmundssonetal.1997
• ModellocaldeformationnearKraflavolcanousing3interferogramsfromtrack9
• Mogisource–drainingorcoolingofshallowmagmachamber
• Linesource–coolingorpost‐riftingductileflow
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Henriotetal.2001
• ModellocaldeformationnearKraflavolcanousing14interferograms
• 3shallowelasticsourcesofdeformation
• 2sills• 1magmachamber
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deZeeuw‐vanDalfsenetal.2004
• Modelwiderregionofdeformationusing4interferogramsfromtrack9
• DeepinflatingMogisource(21km)invokedtoexplainbroaddeformation
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• BigPicture:– Threedimensionalrheologicstructure– Theworkflowallowsustobuildcomplexmodelsthatusegeologicand
geophysicaldatasets– Biggercomputersandparallelprocessingallowustotestmoreand
biggermodels• Whatgroupsofrheologicalstructuresagreewiththedata?
• ThisTalk–intuitionbuilding:– Howdovariousrheologicalstructuresaffectthesurfacedeformation?– Two‐dimensional– LinearMaxwellviscoelasticity– Nogravity– Allriftingoccursinsingleevent
FEMModelingwithPyLith
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Workflow
VTKPythonMatplotlib
ASCIIfiles
images
GMTgrids
ToolswrittenbyEricHetland
WorkflowDiagramfromBradAagaardgeodynamics.org
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GOCAD
• Faroe‐IcelandRidgeExperiment[Staplesetal1997]
• ICECRTb[Allenetal2002]
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Cubit
• Gocadsurfacesareusedtowebcutvolumetoestablishfaultandmaterialboundaries– TSurfsfromGocadaremeshedandanewsurfaceintheACISgeometryengineiscreatedfromthemesh
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• Elasticuppercrust–5kmthick• Maxwellviscoelasticlowercrust• Maxwellviscoelasticmantle• MohofromtheFaroe‐IcelandRidgeExperiment(Staples1997)
Constructmaterialboundariesusinggeophysicaldata
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VariableTimeStepping• Simplesolution:specifyalltimestepsizes– Notadaptive
• PyLith:timesteppinghandledatPythonlevel• CreatednewcomponentImplicitManualdt – ModificationofImplicitcomponent– Readsfromfiletogettimesteppinginformation– MethodstableTimeStep()returnsnewtimestepsizewhencalled
[pylithapp.timedependent] formulation = pylith.problems.ImplicitManualdt
[pylithapp.timedependent.formulation] filename_dt = dt_var units_dt = 1.0*year
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• Inelasticmaterialshavememoryofpastriftingevents• Stressesinmodelmustreachsteadystate• Numberofeventsneededtoreachsteadystatescalesinverselywith
theSavageparameter(HetlandandHager2006)
Spin‐Up
η=1018Pas
η=1020Pas
€
τ 0 = T /2τM
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RepeatedRiftingEvents
• Cannotspecifyrepeatedsliponfaults• LinearrheologyallowsustouseGreen’sfunctionapproach– CalculateresponsetosingleeventusingFEM– Sumsequencewithappropriatetimelags
• Savescomputationtime
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2DModels–IntuitionBuilding
10km
290km
1000km
MaxwellViscoelastic
ElasticRiftingSource
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1Danalysis:
• Profilefromcenterofriftsegment
• Assumesymmetricdeformation
• Assumenegligiblesensitivitytoalongaxisdisplacements onlyvertical&X‐riftcomponents
Simonsetal,AGUFall2001
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AbsoluteVelocities NormalizedVelocities
HomogeneousViscosityvelocitysnapshotsforeachyear:13–20yearsafterrifting
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NomodelwithaHomogeneousviscositystructurecanmatchthedata(includesHoftonandFoulger1996)
ComparetoaverageInSARvelocities
Dataandmodelde‐ramped
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Whataretheeffectsofhorizontalgradientsin
viscosity?50km
100km
200km
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AbsoluteVelocities NormalizedVelocities
Horizontaldeformation:• Modelsaredifficulttodistinguish
Verticaldeformation:• Widerviscositygradientsresultinwiderdeformationfields(notsurprising)
HorizontalViscosityGradientsvelocitysnapshotsforeachyear:13–20yearsafterrifting
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StronglowercurstWeakmantle
WeaklowercrustStrongmantle
Whatistheeffectofstronglowercrustoverweakmantlevs.weaklowercrustoverstrongmantle?
100km
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VerticalViscosityGradientsvelocitysnapshotsforeachyear:13–20yearsafterrifting
AbsoluteVelocities NormalizedVelocities
Horizontaldeformation:• Modelsaredifficulttodistinguish
Verticaldeformation:• Modelseasilydistinguishedbyshapeofdeformationfield
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Impactofalocalizedviscosityanomalybeneatharift
d
MaxwellViscoelastic
Elastic
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Abilitytodetectanomalydependsonviscositycontrastandtimeperiodofobservation
ChangingViscosityofAnomalyvelocitysnapshotsforeachyear
13–20yearsafterrifting3–10yearsafterrifting
€
ηbulk =1020 Pa s, R =10 km, d = 20 km
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• Sensitivitytoanomalydepthnotmonotonic
ChangingDepthofAnomalyvelocitysnapshotsforeachyear:13–20yearsafterrifting
AbsoluteVelocities NormalizedVelocities
€
ηanomaly =1020 Pa s, ηbulk =1020 Pa s, R =10 km
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• NosingletestmodelfitsInSARdata• Wewillrequirelarge,localizedcontrastsinrheology
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Preliminaryconclusions• Verticaldisplacementsareimportantfordistinguishingamongpost‐riftingmodels– InSARhashighsensitivitytoverticals(ERSlookangleis23°fromvertical)
• Itisnotpossibletomatchthepost‐riftingdeformationasmeasuredbyInSARusinganelasticlayeroveranviscoelastichalfspacemodel
• Verticalgradientsinviscosityhavealargeimpactonshapeofdeformationfield.
• Localizedanomalieswilllikelyberequired
• Existingworkflowgreatlyfacilitatesexplorationofdifferentmodels
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NextSteps
• Continueintuitionbuildingexercise– Localizedanomalies
• Three‐dimensionalmodels• Spatiallyandtemporallydiffusediking• Maywanttoinvertfordikingparameters
– Openingmaynotbeuniformwithdepth• Otherrheologies
– Non‐Maxwell– Non‐linear
• InSARProcessing– Reprocesswithoutre‐estimatingbaseline
tokeeplong‐wavelengthdeformation– Timeseriesanalysis
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Challenges• Non‐uniqueness
– Elasticsource(s)– Non‐Maxwellrheologies
• Non‐linearrheologiesremovetheabilitytousetheGreen’sfunctionapproachtomultipleevents– Morecomputationalcost– Needmultipleearthquakesonasinglefault– Largedeformations
• RepeatedEQ ✓1.2• Nonlinearrheologies 1.4• Gravity ✓1.2• Adaptivetimestepping 1.3• Higherordershapefunctions 1.5
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Krafla
Askja
Fremri‐Namur
Accommodationofrift‐tipstress?
• Inelasticmodelsmustbespun‐uptosteadystateoverseveralriftingcycles
• Howdoweaccommodatethestresscreatedatthecracktipduringrepeatedriftingevents?
• St.Venant'sPrinciplemayallowustouse“easiest”mechanism
Thelocalizedeffectscausedbyanyloadactingonthebodywilldissipateorsmoothoutwithinregionsthataresufficientlyawayfromthelocationoftheload.