Strain partitioning and stress rotation at the North Anatolian fault zone from aftershock focal

13 Pages·2006·0.73 MB·English

Geophys.J.Int. (2006)166,373–385 doi:10.1111/j.1365-246X.2006.03027.x
Strain partitioning and stress rotation at the North Anatolian fault
zone from aftershock focal mechanisms of the 1999 Izmit M = 7.4
w
earthquake
Marco Bohnhoff, Helmut Grosser and Georg Dresen
GeoForschungsZentrumPotsdam(GFZ),Telegrafenberg,14473Potsdam,Germany.E-mail:[email protected]
Accepted2006March27.Received2006March27;inoriginalform2005January28
D
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SUMMARY lo
WeinvestigateaftershockfocalmechanismsoftheM =7.4Izmitearthquakeof1999August ad
w e
d
17,onthewesternNorthAnatolianfaultzone(NAFZ).Spatialclusteringandtheorientation fro
of446faultplanesolutionsareanalysed.TheIzmitmainshockoccurredasaright-lateralslip m
h
on an EW-trending near-vertical fault plane. Aftershock clusters define four individual fault ttp
smeagimnsehnotsc.kF(oMcawl=me7c.1h,a1n9is9m9sNsouvreromubnedrin1g2)thinedeicpaitceenptrreedsoomfinthaentIlzymstirtikane-dslsiupbbsuetqaulesnotnDoru¨mzcael s://aca
faulting.AftershocksintheareabetweentheIzmitandDu¨zcesegmentsaremainlyrelatedto de
m
EW-orientednormalfaultingdelineatingasmallpull-apartstructure.Beneaththeeasternmost ic
SeaofMarmara,alignmentsofaftershockssuggestbranchingoftheNAFZintothreeormore .ou
p
activesegmentsthatdiffersignificantlyintermsoftheirfocalmechanismcharacteristics.The .c
o
distributionofaftershockfocalmechanismscorrespondstofaultsegmentationoftheNAFZin m
/g
theIzmit-Du¨zceregionproducedbycoseismicslip.Areaswithlargeamountsofcoseismicslip ji/a
showaftershocksthatarepredominantlystrike-slip,butlow-slipbarriersshowmostlynormal rtic
fsatruSelsttsrineesgssfaotfetlelnorsswohrioncignkvtshe.resiIoznmsitomf athineshafotcekrs.hIonctkhefoIczamlimt-Seacphaanncisamasresah,otwhermotaaxtiiomnusmofhothreizolonctaall mologyle-abstra
c
compressive stress axis is horizontally rotated counter-clockwise by 8◦ with respect to the eist/16
S6
cDou¨szecisemairceaan),dsltornesgs-etesrmarerergoitoanteadlsctrleoscskfiweilsde..TWoweacrodnscthluedeeasttheartntehnedIozfmthiteeruarptthuqruea(kKearcaaduesreed- GJI/1/37
3
significant stress partitioning along the rupture. The direction of stress rotation is related to /6
3
theorientationoftheindividualfaultsegmentsalongtheNAFZ. 3
5
3
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Keywords: aftershocks,faultplanesolutions,NorthAnatolianFault,seismotectonics,stress b
y
tensorinversion. g
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geodeticdataallindicateseparationofthemainshockintosubevents A
1 INTRODUCTION p
TheIzmitM =7.4(1999August17)earthquakeoccurredonthe eotccaul.rr2in0g01o;nBdiasrtkinacettfaaull.t2se0g0m2;enDtesl(oeu.gis.Reteialiln.g2e0r0e2t;aGl.u¨2l0e0n0e;tTiabli. ril 20
w 1
northern strand of the North Anatolian fault zone (NAFZ) in the 2002;Bosetal.2004).ThewesternterminationoftheIzmitrup- 9
GulfofIzmitregion.Theruptureextendedabout140kmbetween ture is located offshorebeneath the Sea of Marmara(e.g. Wright
the Sea of Marmara and the Du¨zce region along a right-lateral, et al. 2001), possibly extending to the area south of the Prince
predominantlyEW-trending,near-verticalfaultplane(Fig.1).Esti- Islands about 20 km southeast of Istanbul (Bouchon et al. 2002;
matesofaveragecoseismicfaultslipvarybetween2.5m(e.g.Tibi O¨zalaybeyetal.2002).Rupturepropagationtowardstheeastended
et al. 2001) and 2.9 m (Bouchon et al. 2002) from inversion of near Du¨zce where a large earthquake occurred 87 days after the
teleseismicdataandrecordsofnear-faultaccelerometers,respec- Izmitevent(1999November12, M =7.1).Nodalplanesoffo-
w
tively. Maximum slip at the surface reached about 5–6 m (Barka calmechanismsfortheIzmitmainevent,themajorsubevent(S2,
etal.2002;Bouchonetal.2002;Bosetal.2004,others).Thedirec- Tibi et al. 2001) and for the Du¨zce earthquake strike dominantly
tionofslipcorrespondswelltotheoverallhorizontalGPS-derived EW(Fig.1b).ThecombinedrupturelengthfortheIzmitandDu¨zce
2–2.5 cm yr−1 westward motion of the Anatolian block with re- earthquakesisestimatedtobeabout200km.TheMarmararegion
specttoEurasia(e.g.Noomenetal.1996;McCluskyetal.2000, westoftheIzmitruptureisconsideredtobeaseismicgap.Recent
Fig.1a).Analysesofsurfacerupture,teleseismic,strongmotionand estimates of Coulomb stress changes and earthquake probability
(cid:3)C 2006TheAuthors 373
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374 M.Bohnhoff,H.GrosserandG.Dresen
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Figure1. (a)LocationmapoftheAegean-Anatolianregion(courtesyofK.Fischer).RedarrowsrepresenttheGPS-derivedhorizontalvelocityfield(after g
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McCluskyetal.2000).TheboldblacklineisthesimplifiedtraceoftheNorthAnatolianfaultzone(NAFZ).TheIzmitsegmentoftheNAFZisindicatedby e
theblackrectangularandenlargedinFig.1(b).(b)CoulombstressmapaftertheIzmitMw=7.4(1999August17)earthquake(afterParsonsetal.2000,their st o
Fig.1).RedandbluecoloursindicateregionsofincreasedanddecreasedCoulombstress,respectively.WhitedotsindicatelocationsofIzmitaftershocks.Fault n
mechanismsareshownfortheIzmitmainshock,IzmitsubeventS2(Mw=6.9,Tibietal.2001)andDu¨zcemainshock(Mw=7.1,1999November12)from 05 A
WesttoEast.Theblackrectangleindicatestheareainvestigatedinthisstudy. p
ril 2
0
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calculations(Parsonsetal.2000;Wrightetal.2001;Parsons2004) 2 ANALYSIS OF AFTERSHOCK 9
suggestthattheIzmitearthquakeincreasedtheseismichazardfor FOCAL MECHANISMS
theMarmaraandDu¨zceregions(Fig.1b).TheDu¨zceearthquake The data set consists of 446 aftershock focal mechanisms out of
asalsothe1992Erzincaneventindicateaneastwardpropagation which254weredeterminedfromrecordingsofa41-stationseismic
ofmainshocks.However,thisisincontrasttoanoverallwestward networkcoveringtheentireIzmitrupturearea.Apartofthenetwork
migrationofstrongearthquakesalongtheNAFZobservedsincethe (29stations)wasinstalledfourdaysaftertheIzmitmainshockand
1939Erzincanevent(e.g.To¨kso¨zetal.1979;Steinetal.1997). wasoperatedbytheGermanTaskForceforearthquakeshostedat
Here,wefocusonaftershocksoftheIzmitearthquakecovering theGeoForschungsZentrumPotsdam(Grosseretal.1998;Baum-
thetimespan1999August17–November12.Weanalysethespa- bachetal.2003).Thesestationsenlargeda12-stationnetworkthat
tiotemporalevolutionofaftershockfocalmechanismsandperform hadbeeninstalledinthatregionin1996(SABONET,Milkereitetal.
stresstensorinversiontodeterminethelocalstressfield.Theresults 2000).Faultmechanismsforthese254eventsweredeterminedbya
are compared to the coseismic rupture and afterslip of the Izmit gridsearchoverallpossiblefaultplanesolutionsassumingadouble-
earthquakeandtotheregionaltectonicsetting. couplemodel(FPFITprogram,Reasenberg&Oppenheimer1985).
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StrainpartitioningandstressrotationattheNorthAnatolianfaultzone 375
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Fsciagluersew2i.th(am)aEgpniictuendter.eTdhiestrIizbmuittioanndofDthu¨ezc4e4m6afainuslhtopclaknsearseoliuntdioicnasteadnablyystehdeiirnftahuilstsmtuedcyhainnismmasp.Nviuewmboefrsthiendloicwaeterh(seumbi)spsehgemree.nTtshealsoinzgeothfethIzembietarcuhpbtaulrles s://a
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zonethatwereidentifiedbasedonspatialclusteringoffaultmechanismsasdiscussedinthetext.(b)DistributionofPandT axesforthe446faultplane ad
e
solutions in polar projection of the lower hemisphere. The size of circles scales with magnitude and the shading indicates the hypocentral time (lighter m
colour≈lateroccurrence).InadditionweplottedthedensitydistributionforthePandTaxesoftheentiredatasetwhereshadingisscaledtomaximumforP ic
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andTseparately. u
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Almostcompletespatialcoverageallowedustorejectalleventsfor rupture trace of the Izmit event. A prominent cluster containing ji/a
whichthegrid-searchresultssuggestedmultiplefaultplanesolu- 169 events is located at about 30.6◦E in the Akyazi area. Trains rtic
tions.Theaccuracyofindividualfaultmechanismsis5◦forstrike, ofaftershocksindicateactivationofsecondaryfaultstructuresin- le
dipandrake. clined by ∼10◦ to the E–W trending NAFZ. In general, P axes -ab
s
Inaddition,192faultplanesolutionswerecollectedfrompub- areeithersubverticalorsubhorizontaltrendingatN90◦E–N180◦E; tra
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lished studies. The data are from seismic networks with different T axes are predominantly subhorizontal with a large scatter t/1
geometriescoveringpartsoftheIzmitrupturearea(Karabulutetal. (N160◦E–N290◦E).ThedistributionofPandT axesindicatesthat 66
2002; O¨zalaybey et al. 2002; Polat et al. 2002). We also include strike-slipandnormalfaultingdominatetheIzmitruptureafterthe /1/3
sourcemechanismsdeterminedbyregionalmomenttensorinver- mainshock. 73
sionofthe30largestIzmitaftershocks(O¨rgu¨lu¨ andAktar,2001). Segment1(28.95◦E–29.30◦E)contains110faultplanesolutions /63
3
Multipleoccurrencesofeventsareexcludedfromthedataset.We covering the western termination of the Izmit rupture located be- 5
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estimatetheaverageorientationerrorofthefaultplanesolutionsin neaththeeasternmostSeaofMarmara(Fig.3).Aftershockclusters 6 b
ourdatasettobe∼10◦.Theeventscoveranareabetween28.95◦– indicate activation of three different branches of the NAFZ. For y g
31.4◦Eand40.5◦–41.0◦Nandextendtoamaximumdepthof18km eachbranch,thefaultmechanismsareverysimilarandoneofthe ue
s
(asingleeventwaslocatedatadepthof23km,Fig.2a). twonodalplanesoftencoincideswithamappedfault.Thenorth- t o
Spatialclusteringoftheaftershocklocations,faultmechanisms ernpart(subsegment11)containsmainlystrike-slipeventslocated n 0
andslipdirectionsareinvestigatedstatistically.Temporaldistribu- ona ∼N305◦E-strikingverticalplane.Aseriesofeventsindicat- 5 A
tionandeventmagnitudearealsoconsidered.Orientationdistribu- ingnormalfaultingisalignedalongaparallelsmallfaultsegment p
tionofaftershockfaultmechanismsisgivenbytherespectivePand (Fig.3).Thesouthernbranch(subsegment12)containsNNE–SSW- ril 2
0
Taxesinlower-hemispherepolarprojectionplots(Fig.2b).PandT extensionalnormalfaulteventsandsomeNW–SW-compressional 1
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axesrepresentthecentreofthedilatationalandcompressionalquad- thrust fault events revealing a complex local fault structure. This
rantsofthefocalmechanisms,indicatingthedirectionofmaximum area largely coincides with the Yalova cluster that showed swarm
compressionalanddilatationaldeformation,respectively. activity covering a time period of several years prior to the Izmit
event(e.g.Gurbuzetal.2000;Barisetal.2002).
Segment2(29.3◦E–30.4◦E)coverstheIzmit-Sapancaareaand
contains107aftershocksandtheIzmitmainshockepicentre(Fig.4).
3 SEGMENTATION OF THE IZMIT
In the western part (subsegment 21), aftershock epicentres are
RUPTURE ZONE
alignedwiththesurfacerupturetracetrendingE–W.PandT axes
AftershockepicentresareclusteredinanE-Wdirectionalongthe indicateawidedistributionoffocalmechanisms.However,inthe
NAFZ between 40.55◦N and 40.90◦N. The distribution of P and easternpart(subsegment22)theaftershocklocationstrend∼N80◦E
T axes suggests a spatial separation of the 446 fault mechanisms and,therefore,strikeatanangleof∼10◦ totherupturetraceand
intofourmajorsegmentsalongtherupturezone(Fig.2a).Between slipdirectionoftheIzmitmainshock.Focalmechanismsindicate
29.5◦E and 30.4◦E, aftershock epicentres follow the strike of the predominantly right-lateral strike-slip and some normal faulting.
(cid:3)C 2006TheAuthors,GJI,166,373–385
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376 M.Bohnhoff,H.GrosserandG.Dresen
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Figure3. DensitydistributionofPandTaxesandmapviewoffaultplanesolutionsforalleventscontainedinsegment1(seeFig.2).Theepicentraldistribution /6
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exhibitstrainswithhighlysimilarfocalmechanismsmatchingoneofthetwopossiblefaultplanes.Thissuggestsfurthersubdivisionintoanorthern(11)and 35
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asouthern(12)subsegment(seetextfordetails). 6
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Paxistrendsandthenodalplanesofindividualstrike-slipevents indicatea25◦clockwiserotationwithrespecttothelocaltrendofthe st o
are rotated counter-clockwise by ∼10◦ with respect to the Izmit Karaderesegment(N65◦E).Paxesoftheeventsareinaccordance n 0
mainshockfocalmechanism. withactivationofanENE–WSWstrikingfaultsegment.Ingeneral, 5 A
Segment3(30.4◦E–30.85◦E)islocatednearthetownofAkyaziat abroaddistributionoffocalmechanismparametersreflectsuniform p
atriplejunctionformedbytheIzmit-SapancaandKaradere-Du¨zce right-lateralstrike-slipwithfrequentlyE–W-orientedsetsofnodal ril 2
0
faults and the ESE-striking Mudurnu fault, where a major earth- planes(Fig.6). 1
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quakeoccurredin1967(Fig.1b).Thissegmentcontains169fault
planesolutions,thatis,40percentoftheentiredataset.Thefocal
mechanismsofadenseclusterofeventsindicateE–Wextensional
4 STRESS TENSOR INVERSION
normalfaulting(Fig.5).Thehighaftershockactivityisinstriking
contrasttothesmallcoseismicdisplacementof<1minthisarea The aftershock fault mechanisms investigated in this study were
suggestedbysurfaceslipandtheinversionofstrongmotiondata used for an inversion of the stress tensor. Stress tensor inversion
(Barka et al. 2002; Bouchon et al. 2002; Langridge et al. 2002). techniques using focal mechanisms have been discussed in detail
TheESE-strikingsouthernbranchoftheNAFZexhibitedfewaf- by various authors (e.g. Gephart & Forsyth 1984; Gephart 1990;
tershocksandnosurfaceslip,possiblyduetostressrelaxationafter Michael1987a,1991;Hardebeck&Hauksson2001;Bohnhoffetal.
the1967event. 2004).WeuseatechniqueproposedbyMichael(1984,1987a).The
Segment4(30.85◦E–31.3◦E)contains60aftershockfaultplane analysisallowsustoestimatetheorientationofthethreeprincipal
solutions, including the epicentres of the Izmit subevent S2 (Tibi stresses (σ = maximum compressive stress, σ = intermediate
1 2
etal.2001)andtheDu¨zcemainshock(Fig.1b).Aftershocklocations andσ =minimum)andthestressratioR=(σ −σ )/(σ −σ ),
3 2 3 1 3
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StrainpartitioningandstressrotationattheNorthAnatolianfaultzone 377
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Figure4. DensitydistributionofPandT axesandmapviewoffaultplanesolutionsforalleventscontainedinsegment2thatincludestheepicentreof 6/1
theIzmitmainshock.ScanningthedatafromWesttoEastallowsidentifyingasignificantvariationinclusteringofsimilarmechanismswithinthissegment. /3
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Theeasternsubsegment(22)consistsdominantlyofstrike-slipmechanismsingoodaccordancewiththemainshockinfirstorderapproximationandcontains 3
anumberofNNE–SSWextensionalnormalfaultingevents;thewesternsubsegment(21)doesnotexhibitapreferredfaultmechanism.Insubsegment22, /63
3
hypocentresfollowatrendthatisinclinedtotheE–WstrikingfaultwestoftheIzmitepicentre;Paxisdistributioninthissubsegmentshowsseveralpreferred 5
orientationssomeofwhicharerotatedby∼10◦withrespecttotheIzmitmainshock. 36
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a0n≤d tRra≤nst1en(wsiiotnhaRl r<eg0im.5ea,nredspRe>ctiv0e.5lyi)n.dTichaetimngetahtordanisspbreassseidonoanl sthtraitksinugggseusbthporreidzoomntianladnitrleycstitorinkeo-fsσlip(dKefiroartmzia2ti0o0n2w;Hitheiadb∼aNch12e5ta◦El. st on
1 0
theassumptionsthat 2004;Reineckeretal.2004,seeTable1)andpointstowardssignif- 5
A
icantvariationsofstressfieldorientationalongtheNAFZafterthe p
(1) intheareaofinvestigationstressesremainuniformintime Izmitevent.Furthermore,thedatasetasawholeisdominatedby ril 2
andspace, 0
thelargenumberofeventslocatedinsegment3showingpredomi- 1
(2) earthquakes represent shear dislocations on existing faults, 9
nantlynormalfaulting.Toinvestigatefurtherlocalvariationsinthe
and
stressregimealongtheIzmitrupturewesubdividedthecatalogue
(3) slipoccursinthedirectionofthemaximumresolvedshear
accordingtothesegmentationidentifiedfromthedistributionofP
stressonthefaultplane.
andT axes(segments1–4).Stressinversionofthefoursegments
Heterogeneity of the stress field is reflected by the misfit level reveals a clear variation of the local stress field orientation along
andthewidthoftheconfidenceintervaloftheinversion(Table1 theIzmitrupture(Fig.7).FortheAkyaziarea(segment3),there-
andFig.7).Foreachstressinversion,2000bootstrapiterationswere sultsindicateaseparationoftheprincipalstresses(blackdots)with
performed. narrow1σ (68percent,darkgrey)and2σ (95percent,lightgrey)
Thestressinversionofallfocalmechanismsresultsinanormal- confidenceintervals.Fortheremainingsegments,however,confi-
faulting regime with clear separation of the principal stresses dence intervals for the largest and intermediate principal stresses
(Fig. 7). The maximum compressive stress σ is almost vertical formagirdle(Fig.7).
1
and σ is horizontal, trending N237◦E. This result is in contrast Stress ratio (R = 0.83) and confidence intervals for segments
3
to estimates of the regional long-term stress field in NW Turkey 1 and 2 reflect the dominating deformation regimes (strike-slip
(cid:3)C 2006TheAuthors,GJI,166,373–385
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378 M.Bohnhoff,H.GrosserandG.Dresen
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Figure5. DensitydistributionofPandTaxesandmapviewoffaultplanesolutionsforalleventscontainedinsegment3.AdominantEW-extensionalnormal m
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faultingregimeisobservedalongthissegmentthatcontainsabout40percentoftheentiresetoffaultplanesolutionsanalysedinthisstudy.Differenttothe ttp
threeothersegmentsnoalignmentisobservedfromthedistributionofaftershockhypocentres;insteadtheaftershockactivitycovertheentireAkyaziplainthat s
isidentifiedasapull-apartstructure. ://a
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Figure6. DensitydistributionofPandTaxesandmapviewoffaultplanesolutionsforalleventscontainedinsegment4thatincludestheKaraderesegmentand st o
theDu¨zcemainshockepicentralarea.Adominantstrike-slipmechanismisobservedforthiseasternmostpartoftheIzmitrupturearea.Eventswithannotated n
magnitudemarkthelargestandearliestaftershocksinthissegmentandarediscussedinthetext.MosthypocentresarealignedonanalmostEW-trending 05
directionthatisinclinedtothelocalfaulttrendofN65◦E(Karaderesegment,seeFig.8)by∼25◦. A
p
ril 2
aslnidghntloyrminatlhfeaiurlmtinagg)n.itTuhdeeparnidncbioptahlssttrreessssedsirσec1taionndsσtr2enddiffreoruognhllyy segTmheenetapsoteirnntimngosttosweagrmdsenthte4Dcou¨vzecresatrheeaNw6h5e◦rEe-atreMndin=g7K.1areaadrethre- 019
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NW–SEwithinbothsegments(N314◦EandN117◦E,respectively). quakeoccurred87daysaftertheIzmitmainshock.Stressorientation
Theminimumcompressionalstressσ isclearlydifferentiatedfrom andconfidenceintervalsforthisareaindicateastrike-slipregime
3
σ and σ , trending subhorizontally NE–SW. The Izmit-Sapanca withastressratioofR=0.63(Fig.7).Themaximumprinciplestress
1 2
area(segment2)includestheIzmitmainshockepicentre.Here,the (σ )issubhorizontalandtrendsN159◦E,indicatingaclockwisero-
1
post-seismictrendofσ indicatesacounter-clockwiserotationof tationwithrespecttotheregionalstressfieldof>30◦.
1
8◦ withrespecttotheregionalstressfieldandthecoseismicstress
fieldderivedfrominvertingthefocalmechanismsofthesixIzmit
subevents(Gu¨lenetal.2002,Table1).IntheAkyaziarea(segment 5 DISCUSSION
3), directions of principle stresses σ − σ are well constrained
1 3
andcorrespondtoanE–Wextensionalnormalfaultingregime.The 5.1 SegmentationoftheIzmitrupturezone
relativestressmagnitude R=0.62indicatesthatthemagnitudeof
TheIzmitearthquakeruptureda140km-longE–Wtrendingseg-
σ isclosetothemeanofσ andσ .
2 1 3 ment of the western NAFZ. Offshore in the easternmost Sea of
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StrainpartitioningandstressrotationattheNorthAnatolianfaultzone 379
Table1. OverviewonstressfieldorientationsalongtheIzmitpartoftheNAFZobservedfromthedifferentsourcesanddeterminedinthisstudy.Datasets
coverdifferentareasandtimeintervals.Azimuth(tr=trend)andangleofincidence(pl=plunge)fortheprincipalstressesaregiveninNoverEandagainst
horizontal,respectively.
Areacovered Timecovered Focalmech. σ1(tr/pl) Std σ2(tr/pl) Std σ3(tr/pl) std Misfit R Source
[no.] [◦] [◦] [◦] [◦] [◦] [◦]
NWTurkey 1943–1999 11 126/3 13 269/86 – 35/3 13 3.3◦(c) 0.5 Kiratzi(2002)
NWTurkey 1943–1999 13(b) 124/-(a) (25) −/90 – 34/− – – – Reineckeretal.(2004)
Heidbachetal.(2004)
ESeaofMarmara 1996Jan–Sep 37 305/1 14 206/86 20 35/4 17 0.31 0.5 Erginetal.(1997)
Izmitrupture CoseismicIzmit 6 124/3 15 231/79 15 34/11 15 0.31 0.5 Gu¨lenetal.(2002)
Izmitrupture Post-seismicIzmit 30 316/13 10 81/68 10 222/18 10 0.16 0.74 O¨rgu¨lu¨&Aktar(2001)
Izmitrupture Post-seismicIzmit 446 66/87 6 327/0 6 237/3 6 0.22 0.64 Thisstudy
(segment0)
ESeaofMarmara Post-seismicIzmit 110 314/23 6 144/57 6 45/4 6 0.20 0.83 Thisstudy
(segment1)
D
IzmitSapanca Post-seismicIzmit 107 117/15 6 347/67 6 212/17 6 0.22 0.83 Thisstudy o
w
(segment2) n
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Akyaziplain Post-seismicIzmit 169 190/74 6 3/15 6 94/1 6 0.09 0.62 Thisstudy a
d
(segment3) e
d
KaradereDu¨zce Post-seismicIzmit 60 339/6 8 49/74 8 251/15 8 0.19 0.63 Thisstudy fro
(segment4) m
standarddeviation(std)referstothetrendforσ1−3. http
((ab))SanHd.shallowboreholes. s://a
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(c)misfitasgivenbytheFMSIroutine(Gephart&Forsyth1984). ad
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Figure7. Resultsofstresstensorinversionfortheentiredatabasecontaining446faultplanesolutionsandthefoursegments(seeFigs3–6formapview ril 2
offaultmechanismscontainedtherein).Boldblackdotsrepresentthebestfittingorientationsforthethreeprincipalstresses(σ1=maximumcompressional 01
stress,σ2=intermediate;σ3=minimum).Smallandlargedotsrepresentthe1σ (68percent)and2σ (95percent)confidenceintervals,respectively. 9
Marmara,theNAFZsplaysintodifferentbranchesproducingacom- formedasalargepull-apartstructure(e.g.Armijoetal.1999)ina
plexnetworkcomprisingtheIzmit-Sapanca,Du¨zce,Iznik,Geyve transtensionalenvironment.Onasmallerscale,faultsegmentsare
and Mudurnu faults (Fig. 8). The width of this network is about separatedbyreleasingbendsandsmall-scalepull-apartstructures
30 km and thus on the order of the thickness of the seismogenic suchasIzmitBayandLakeSapanca.
upper crust. Regional tectonics in western Turkey are dominated AnalysisofaftershockfocalmechanismsalongtheIzmitrupture
by right-lateral strike-slip along the ∼EW-trending NAFZ and a revealspredominantlystrike-slipandnormalfaultingeventsincom-
NE–SW extensional regime associated with a >3 cm yr−1 SSW- binationwithfewthrustevents,indicatingthatstrainpartitioning
wardmigrationoftheSouthAegeandomainwithrespecttostable andvariationsinthelocalstressfieldisadominantfeaturewithin
Eurasia(e.g.McCluskyetal.2000;Fleritetal.2004).AttheNAFZ, thisregion.ThrustfaultingisrestrictedtoasmallareanearYalova
GPS data indicates westward motion of northwestern Anatolia at wherepronouncedseismicitywasobservedpriortotheIzmitmain-
about2–2.5cmyr−1.Inthistectonicregime,theSeaofMarmara shock(Gurbuzetal.2000;Barisetal.2002).IntheIzmit-Sapanca
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Figure8. Upperpart:TopographicmapoftheIzmitsegmentoftheNorthAnatolianfaultzone(afterFieldingetal.1999).Reddottedlinesindicatethe tra
c
simplifiedsurfaceruptureoftheIzmitevent(afterBarkaetal.2002).TheboldreddotindicatestheepicentreoftheIzmitmainshock.Middlepart:Mainfaults t/1
oftheNAFZalongtheIzmitrupturearea.BluelinesalongthethreeonshoresegmentsrepresentlateraldistributionofsurfaceslipafterBarkaetal.(2002) 6
6
scaledtotheslipmaximumof∼6m.Lowerpart: Distributionofthe446focalmechanismsanalysedinthisstudy(seealsoFig.2)andsegmentationidentified /1
basedonspecialclusteringoffocalmechanisms. /37
3
/6
3
andKaradere-Du¨zceareas(oursegments2and4),aftershockfo- seeFig.8).Fromwesttoeast,thesegmentsaretheYalova-Hersek 35
cal mechanisms reflect mainly strike-slip and normal faulting. In (strikingN80◦E),Karamu¨rsel-Go¨lcu¨k(N70◦–80◦E),Izmit-Sapanca 36
general,theepicentresofindividualeventsformtrailsdelineating (∼N90◦E), Sapanca-Akyazi (N75◦–85◦E) and Karadere (N65◦E). by
g
faultsegmentsthatcorrespondtoonesetofaftershocknodalplanes Step-oversbetweensegmentsexhibitverticaloffsetsofupto2.4m, u
e
(Fig.2andFig.8).E–Wextensionalnormalfaultingtypifyingthe indicating a significant component of normal faulting that is in st o
Akyaziplainofsegment3(Fig.5)isconsistentwithlocaltopog- agreement with aftershock fault mechanisms at several locations n
raphy at 30.6◦E, which exhibits >500 m subsidence with respect alongtherupturetrace. 05
A
to the surrounding area (Fig. 8). We interpret this plain to repre- Coseismicrupturestartedatthemainshockepicentrejustwest p
sent a small pull-apart basin similar to Lake Sapanca, Izmit Bay ofGo¨lcu¨kandpropagatedwestwardsalongtheKaramu¨rsel-Go¨lcu¨k ril 2
0
andthesomewhatlargerSeaofMarmara.Inthefollowingwere- segmentwithlittlemomentrelease(Bosetal.2004).Eastwardprop- 1
9
lateaftershocksegmentationalongtherupturezonetothespatial agationwastriggeredontheIzmit-Sapancasegmentwithashortde-
distributionandtemporalevolutionofcoseismicslipoftheIzmit layresultinginasymmetricruptureofthemainsource(Delouisetal.
earthquake. 2002;Lietal.2002).Rupturepropagationwascomplex,involving
severalsubeventspossiblytriggeredondifferentfaultsegments.To-
talsourceduration,thenumberofsubeventsandthedistributionof
5.1.1 CoseismicslipalongtheIzmitrupturetrace momentreleaseafterthefirst∼20sremaincontroversial(Tibietal.
AlongtheIzmitrupture,seismicmomentreleaseandcoseismicslip 2001;Delouisetal.2002;Gu¨lenetal.2002;Lietal.2002;Bos
varyfrom>12*1019Nmand>5minhigh-slipzonesto<1*1019 etal.2004).Usingsource-timefunctionsofupto90s,3–6subevents
Nmand<1matlow-slipbarriers,respectively(e.g.Bouchonetal. havebeenidentified(Tibietal.2001;Gu¨lenetal.2002).
2002;Papageorgiou2003).Mappedsurfacerupturesindicatefive Inversionsofteleseismic,groundmotionandspacegeodeticdata
separatefaultsegmentsreachingfromwesttoeastseparatedbyre- also indicate distributed slip on separate fault segments with two
leasing (pull-apart) step-overs of about 1–4 km width (Tibi et al. slipmaximaof5–6mnearGo¨lcu¨kandLakeSapanca(e.g.Reilinger
2001;Barkaetal.2002;Langridgeetal.2002;Polatetal.2002, etal.2000;Bouchonetal.2002;Gu¨lenetal.2002;Lietal.2002).
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StrainpartitioningandstressrotationattheNorthAnatolianfaultzone 381
Table2. Energyrelease,ruptureareaandaverageslipfortheIzmitand increasinglyrevealtheorientationoftheregionalstressfieldorien-
Du¨zce mainshocks (coseismic) and the Izmit post-seismic time (after tation.Thiseffecthasalsobeenobservedforfluid-injectioninduced
Bouchonetal.2002;Bu¨rgmannetal.2002;Ergintavetal.2002;O¨rgu¨lu¨.& microseismicity(Bohnhoffetal.2004),butatalowermagnitude
Aktar2001;Reilingeretal.2000;Tibietal.2001;Umutluetal.2004).See level(−1<M <1).
w
textfordetails.
Mw Mo Rupturearea Averageslip
[1020Nm] [km2] [m]
5.2 EvolutionofthestressfieldattheNAFZ
Izmitmainshock 7.4 1.4–2.0 140×20 2.5–2.9
Du¨zcemainshock 7.1 0.5 60×20 0.8–1.0 Toanalysespatialandtemporalvariationsofthestressfieldalong
theNAFZandespeciallyalongtheIzmitrupturewecollectedall
Cumulativegeodetic 7.0 0.3 140×20 0.43
availableinformationonthelong-termregionalstressfieldinNW
moment(Izmit-Du¨zceintereventtime)
Turkey covering the past ∼5 decades. In addition, we performed
Izmit-Aftershocks 6.6 0.012 140×20 0.017
stresstensorinversionoffocalmechanismsofeventsprior(‘pre-
(30largest)
Izmit-Aftershocks 6.8 0.025 140×20 0.036 seismic’)andduring(‘coseismic’)theIzmitmainshock.Theentire
setofinformationonstressfieldorientationintheareaofinvestiga- D
(4400largest) o
tionissummarizedinTable1.Informationonthelong-termregional w
n
stressfieldinNWTurkeywastakenfromtheWorldStressMapdata lo
a
Slipmaximawerefoundbetweenthesurfaceandabout12kmdepth. base(Heidbachetal.2004;Reineckeretal.2004)whichcontains ded
Dteeleloseuiissmeitcaaln.d(2s0tr0o2n)gpmreosteinotneddaatajo(isneteianlvseorsSioalnicohfonIneStAaRl.,2G0P03S), 1in3gsatremsseaonrimenatxatiimonummehaosruizreomnteanltsstrfeosrsth(SeHa)retareonfdiinntgerNes1t2i4n◦dEic.aItn- from
addition,Kiratzi(2002)performedstresstensorinversionofthe11 h
d6pe–lal1iin2nek(a3mt0in.d4ge–fp3ot0hu.r7bf◦eaEluo)lwtissGecgoo¨mlncseu¨inkstt-seI.nzMmtlyiatxiadinmednutLmifiaeksdleipaSsiaspaunapnactroae.a8TomhferaAetdakubycoaeuzdti lraervgeeasltedeaartshtqriukaek-esslipthfaatuhlatidngocrceugrimreedwinitthhesurebghioornizsoinntcael1N9W43–SanEd- ttps://ac
slip(<1m). trending orientation of the maximum principal stress and a near- ad
e
vertical intermediate principal stress. Further information on the m
pre-seismic stress field was obtained from inverting focal mech- ic.o
5.1.2 Aftershockactivityandpost-seismicslipalongtheIzmit aMnaisrmmsaroabdtuairninegd1w9i9th6Jaanloucaaryl–nSeetwptoermkbienrt(hEergeinasetetranlm.1o9s9t7S)e.a of up.co
rupturetrace m
The‘coseismic’stressfieldduringtheIzmiteventwasdetermined /g
Post-seismicslipestimatedfromGPSoverthe75daysfollowing byinvertingthefocalmechanismsofthesixIzmitsubeventsconsti- ji/a
the Izmit earthquake was about 0.43 m. The corresponding total tutingtheentireIzmitrupture(Gu¨lenetal.2002).Furthermore,the rtic
geodeticmomentisequivalenttoM =7.0(Reilingeretal.2000; 30largestaftershocks(O¨rgu¨lu¨ &Aktar2001)wereusedtoderive le
w -a
Bu¨rgmann et al. 2002; Ergintav et al. 2002) and, therefore, one the regional ‘post-seismic’ stress field. Interestingly, this data set b
s
orderofmagnitudehigherthanthemomentliberatedbythe4000 containsonlythreeEW-extensionalnormalfaultingeventswithin tra
c
largest aftershocks (see Table 2). This indicates that post-seismic the Akyazi Plain and yields a well-defined stress regime almost t/1
deformationwaslargelyaseismic.Modellingofafterslipsuggests identicaltothe‘pre-seismic’regionalstressfield(Table1). 66
that maximum creep is expected at depths of >20 km (Reilinger InFig.9thestressfieldorientationsofthedatasetsasdescribed /1/3
etal.2000). aboveareshowninchronologicalorder.Theanglebetweenthetrend 73
The maxima of coseismic and post-seismic slip and the after- ofσ1 andtheregionaltrendoftheNAFZ(N90◦E)isindicatedby /63
3
shockactivityareanti-correlated.Forexample,theminimaofafter- theblacklinesandthegrey-shadedareasrepresenttheirstandard 5
3
shockactivityintheHersekDelta(29.6◦E)andintheLakeSapanca deviation.ThestressfieldorientationsofKiratzi(2002)andfrom 6 b
(30.2◦E–30.3◦E)insegment2(Fig.4)andnearKaradere(30.8◦E) theWorldStressMapareidenticaltowithin≤2◦.Weconsiderthis y g
in segment 4 (Fig. 6) coincide with the corresponding coseismic orientationtorepresentthelong-termregionalstressfieldinNW ue
s
slipmaxima.HighcoseismicslipintheIzmit-Sapanca(segment2) Turkey.Notably,theregionalstressfieldorientationisalmostiden- t o
andKaradere-Du¨zce(segment4)areasiscorrelatedwithapredomi- tical with the local stress field orientation during the Izmit earth- n 0
nantlystrike-slipfaultingregimeoftheaftershocks.Incontrast,high quakewithbothconfidenceregionsintherangeof12◦.Thelocal 5 A
ratesofaftershockactivityandabundantnormalfaultingeventsin stress field within the eastern Sea of Marmara in 1996 differs by p
theAkyaziareaandatthewesternendoftherupturetracecoincide ∼10◦withtheregionalstressfieldwhilethe68percentconfidence ril 2
0
withlowslipatbarriersandstep-oversbetweenfaultsegments. regionsoverlap. 1
9
SignificantafterslipoccurredatdepthalongtheeasternKaradere ThestressfieldaftertheIzmitmainshockisspatiallyheteroge-
segmentclosetotheepicentreoftheDu¨zcemainshock(M =7.1, neousalongtherupture.Orientationsoftheprincipalstressesvary
w
1999November12).Interestingly,weobservearelativelyhighrate significantlybetweensegments1–4reflectingfaultcomplexity.For
ofaftershockactivityinthisregion.Focalmechanismsofthethree segments1,2and4themaximumcompressivestressdirectionσ is
1
largestaftershocksonthissegment(M≥4,Fig.6)areverysimilar subhorizontalandtrendsN117◦EtoN159◦E.We,therefore,approx-
toIzmitsubeventS2andtheDu¨zceevent(bothindicatedinFig.1), imateitsazimuthbytheazimuthofthelargestnear-horizontalstress
whichoccurred30sandaboutthreemonthsaftertheIzmitearth- (SH)andthusequateσ withSHinsegments1,2and4(insegment
1
quake,respectively.Thethreeaftershockeventsalloccurredwithin 3,σ isalmostvertical,seeFig.7,Table1).Note,thattheaccuracy
1
6hoursaftertheIzmitmainshock.Incontrast,faultplanesolutions fortheorientationsofσ1−3isintherangeof6–8◦foronestandard
of small aftershocks show a larger variability, which was not ob- deviation(68percent)and,therefore,unprecedentlysmallforthis
servedonanyofthethreesegmentstothewest.Thisobservation region.Weinterpretthestrongpartitioningofthepost-seismicstress
suggeststhatsmalleventsalsoreflectsmall-scalestructuralcom- fieldtoreflectthelocalfaultstructurealongtherupture.Astriking
plexityoftheNAFZ.Withincreasingmagnitude,focalmechanisms result of the stress inversion of the aftershock focal mechanisms
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Figure9. EvolutionofstressfieldorientationinNWTurkeyalongtheIzmitrupture.Thediagramdisplaystheanglebetweenthetrendofthemaximum b
s
compressionalstress(σ1)andtheregionaltrendoftheNAFZ(N90◦E)plottedversustimeforthedifferentdatasetsconsideredinthisstudy(seeTable1for tra
c
details).Thehorizontalextensionoftherectanglesmarksthetimeintervalcoveredbytheirdatacontent.Blacklinemarksthebestresolvedorientationwhereas t/1
greyintervalsrepresenttheconfidenceregion(68percent). 6
6
/1
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intheIzmit-Sapanca(segment2)andKaradere-Du¨zce(segment4) to∼5◦ atbest,whichismainlyduetotheuncertaintyinorienta- 73
areasisthatthelocalstressesarerotatedcomparedtotheregional tionoffocalmechanismsusedforthestressinversion.Often,these /63
3
stressfieldandlocalfaulttrend.Achangeofthestateofstressinthe errors in stress field orientation are of the same order of or only 5
3
vicinityofcoseismicslipmaximamayhavereactivatedsecondary slightly less than the magnitude of the expected rotations. In the 6 b
faultsandmodifiedthelocalfaultstructure.Inparticular,shearfail- Izmit-Sapanca area (segment 2) we observe a counter-clockwise y g
ureandtheassociateddropinshearstressmayresultinarotation rotation of the post-seismic stress field with respect to the long- ue
oftheprincipalstressesactingonthefault.Thishasbeenobserved termregionalandcoseismicstressfieldofabout8◦.Inthispartof st o
for some earthquakes at strike-slip and reverse faults in Califor- theNAFZamoreorlessuniformEW-trendingfaultwasactivated n 0
nia such as the 1983 Coalinga (Michael 1987b), 1986 Oceanside during the Izmit mainshock. Interestingly, the hypocentral distri- 5 A
(Hauksson&Jones1988),1989LomaPrieta(Michaeletal.1990; bution of aftershocks in this area shows the same ∼10◦ counter- p
Zoback&Beroza1993),1992Landers(Hauksson1994;Hardebeck clockwise rotation with respect to the local coseismic fault trend ril 2
0
&Hauksson1999,2001),and1994Northridge(Zhaoetal.1997) (Fig.10)indicatingthatthemainshockstressdropinducedalocal 1
9
earthquakes.Stressratiosvariednotablyindicatinglocaltransten- rotation of the stress field that resulted in activation of optimally
sionalfaulting(R=0.43–0.65,Hauksson1994)andtranspression oriented secondary faults. Stress rotation is also observed at the
(R=0.2–0.55,Michaeletal.1990).Principalstresseswerehorizon- easternterminationoftheIzmitrupture(Karadere-Du¨zcearea,seg-
tallyrotatedsignificantlybetweenabout10◦–20◦inaclockwiseand ment 4). Here, the local fault trend of the NAFZ is N65◦E along
counter-clockwisedirection.Insomecases(Northridge,Coalinga, theKaraderesegmentwheremostoftheIzmitaftershocksinseg-
Landers) stress rotations reversed with time and varied along the ment4occurred.Thepost-seismicorientationsofthethreeprincipal
rupturetrace.Notwithstandingthatstressrotationsduringtheseis- stressesinthisareaarewellresolved(seeFig.7).Withrespecttothe
miccycleandespeciallyinconnectionwithlargeearthquakesare regionalstressfieldweidentifya34◦clockwiserotationofthelocal
inaccordancewiththepresentunderstandingoffaultingkinemat- stressfieldalongtheKaraderesegmentduetothemainshock.As
ics,observedstressrotationsshouldbeinterpretedwithcareaswas intheIzmit-Sapancaarea,therotationofthelocalstressesiscon-
shownforthecaseofLandersbyTownend&Zoback(2001). sistentwiththelocaltrendformedbythedistributionofhypocen-
Localrotationsofthestressfieldatafaultareextremelydiffi- tresthatindicatearotationof∼25◦(Fig.10).However,attheKa-
culttodetect.Theaccuracyofstressfielddeterminationislimited raderesegmentthestressrotationoccurredinaclockwisedirection
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