Physics of the Earth and Planetary Interiors Seismic anisotropy in central north Anatolian Fault ...

15 Pages·2017·3.06 MB·English

by  A. Licciardi

Physics of the Earth and Planetary Interiors 277 (2018) 99–112
ContentslistsavailableatScienceDirect
Physics of the Earth and Planetary Interiors
journal homepage: www.elsevier.com/locate/pepi
Seismic anisotropy in central north Anatolian Fault Zone and its implications
T
on crustal deformation
A. Licciardia,⁎, T. Ekenb, T. Taymazb, N. Piana Agostinettic, S. Yolsal-Çevikbilenb
aGeophysicsSection,SchoolofCosmicPhysics,DublinInstituteforAdvancedStudies,Dublin,Ireland
bDepartmentofGeophysicalEngineering,FacultyofMines,IstanbulTechnicalUniversity,Istanbul,Turkey
cDepartmentofGeodynamicsandSedimentology,FacultyofEarthSciences,GeographyandAstronomy,UniversityofVienna,Vienna,Austria
A R T I C L E I N F O A B S T R A C T
Keywords: WeinvestigatethecrustalseismicstructureandanisotropyaroundthecentralportionoftheNorthAnatolian
Seismicanisotropy FaultZone,amajorplateboundary,usingreceiverfunctionanalysis.Thecharacterizationofcrustalseismic
Crustaldeformation anisotropyplaysakeyroleinourunderstandingofpresentandpastdeformationprocessesatplateboundaries.
Seismology Thedevelopmentofseismicanisotropyinthecrustarisesfromtheresponseoftherockstocomplicatedde-
Receiverfunction
formation regimes inducedbyplate interaction.Through theanalysis of azimuthally-varying signals oftele-
seismicreceiverfunctions,wemaptheanisotropicpropertiesofthecrustasafunctionofdepth,byemploying
theharmonicdecompositiontechnique.AlthoughtheMohoislocatedatadepthofabout40km,withnomajor
offsetacrossthearea,ourresultsshowaclearasymmetricdistributionofcrustalpropertiesbetweenthenorthern
andsouthernblocks,dividedbytheNorthAnatolianFaultZone.Heterogeneousandstronglyanisotropiccrustis
presentinthesouthernblock,wherecomplexintra-crustalsignalsaretheresultsofstrongdeformation.Inthe
north,asimplerandweaklyanisotropiccrustistypicallyobserved.Thestrongestanisotropicsignalislocatedin
thefirst15kmofthecrustandiswidespreadinthesouthernblock.Stationslocatedontopofthemainactive
faultsintheareaindicatethehighestamplitudes,togetherwithfault-parallelstrikesofthefastplaneofani-
sotropy.Weinterprettheoriginofthissignalasduetostructure-inducedanisotropy,androughlydetermineits
depthextentupto15–20kmforthesestations.Awayfromthefaults,wesuggestthecontributionofpreviously
documentedupliftedbasementblockstoexplaintheobservedanisotropyatupperandmiddlecrustaldepths.
Finally,weinterpretcoherentNE-SWorientationsbelowtheMohoasaresultoffrozen-inanisotropyinthe
uppermantle,assuggestedbypreviousstudies.
1. Introduction The determination of the directional dependence of seismic wave
speed, also known as seismic anisotropy, plays a fundamental role in
Asanintercontinentaldextralstrike-slipfaultwithsignificantstrain the elucidation of the complicated deformation regimes induced by
localization,the1600-km-longNorthAnatolianFaultZone(NAFZ)re- plateinteractionalongsuchplatemargin.
presents a major plate boundary between the Eurasian plate in the Crustalseismicanisotropyisgenerallyattributedtothealignmentof
northandtheAnatolianplateinthesouth.Althoughcollisionbetween joints or microcracks, to lattice preferred orientation (LPO) of aniso-
theArabianandEurasianplates(intheeast)wasinitiallythoughttobe tropic minerals, or to highly foliated metamorphic rocks (e.g.,
themaindrivingforceforthewestwardmotionoftheAnatolianplate Sherringtonetal.2004).Intheuppercrust,possiblesourcesofseismic
(e.g.DeweyandŞengör,1979),recentadvancesinhigh-resolutionGPS anisotropy can be either stress-induced or structure-induced (Boness
datahaverevealedaclearroleofthesouthwest-trendingrollbackofthe andZoback,2006).Stress-inducedanisotropycanbegeneratedeither
Hellenic subduction zone in the south Aegean Sea for the rapid de- bytheextensivedilatancyoffluid-filledmicrocracks(Crampin,1987)
formationoftheAegean-Anatolianregion(e.g.McCluskyetal.,2000; or by the preferential closure of fractures by the in situ stress field
Reilinger et al., 2006). In this respect, the deformation history of the (Boness and Zoback, 2006). In the latter case, the orientation of fast
rocks atvariousdepth rangesremainsenigmatic withinthecrustand waves of vertically propagating shear waves aligns parallel to the
mantle ofthis complextectonic setting. Adetailed sketchoftheAna- maximum horizontal stress (SH ). When structure-induced mechan-
max
toliantectonicsettingcanbefoundinFig.1. isms are dominant, seismic anisotropy may be associated to the
⁎Correspondingauthorat:Géosciences,UniversitédeRennes1,Rennes,France.
E-mailaddress:[email protected](A.Licciardi).
https://doi.org/10.1016/j.pepi.2018.01.012
Received4October2017;Receivedinrevisedform14December2017;Accepted26January2018
Available online 06 February 2018
0031-9201/ © 2018 Elsevier B.V. All rights reserved.
A.Licciardietal. Physics of the Earth and Planetary Interiors 277 (2018) 99–112
Fig.1.Asketchmapofactivetectonicboundariesinthe
studyareaandsurroundings.Forthesourceofcompiled
data,pleaseseeTaymazetal.(1990,1991,2004,2007a,b),
Yolsal-Çevikbilen and Taymaz (2012) and references
therein. Abbreviations: AS: Apşeron Sill, ASM: Anaxi-
mander Sea Mountains, BF: Bozova Fault, BGF: Beyşehir
Gölü Fault, BMG: Büyük Menderes Graben, BuF: Burdur
Fault, CTF: Cephalonia Transform Fault, DSF: Dead Sea
Transform Fault, DF: Deliler Fault, EAF: East Anatolian
Fault, EcF: Ecemiş Fault, EF: Elbistan Fault, EPF: Ezine
Pazarı Fault, ErF: Erciyes Fault, ESM: Eratosthenes Sea
Mountains,G:Gökova,Ge:GedizGraben,GF:GarniFault,
HB:HerodotusBasin,IF:IğdırFault,KBF:KavakbaşıFault,
KF: Kağızman Fault, KFZ: Karataş-Osmaniye Fault Zone,
MF: Malatya Fault, MRF: Main Recent Fault, MT: Muş
Thrust, NAF: North Anatolian Fault, NEAF: North East
Anatolian Fault, OF: Ovacık Fault, PSF: Pampak-Savan
Fault,PTF:PaphosTransformFault,RB:RhodesBasin,SaF:
SalmasFault,Si:SimavGraben,SuF:SultandağFault,TeF:
TebrizFault,TF:TatarlıFault,TGF:TuzGölüFault.Black
arrowsexhibitrelativeplatemotionswithrespecttoEur-
asia(McCluskyetal.,2003;Reilingeretal.,2006).
alignment of macroscopic features due to shear-induced deformation et al., 2010; Park and Levin, 2016) has proven to be effective to
nearactivefaults(ZhangandSchwartz,1994;ZinkeandZoback,2000; quantifyseismicanisotropyinvariousgeodynamical settingsoverthe
Tadokoro et al., 2002), sedimentary bedding planes (Kern and Wenk, last decade and at different scales of investigation (Piana Agostinetti
1990),andpreferredmineralalignments(Sayers,1994). et al., 2011; Bianchi et al., 2015; Olugboji and Park (2016); Vinnik
Local shear waves splitting analyses, performed over detected et al., 2016) including the shallow crust (Licciardi and Piana
micro-seismic earthquakes along various segments of the NAFZ, have Agostinetti, 2017; Piana Agostinetti et al., 2017). In particular, RF
indicatedaspatialcorrelationbetweenthefastpolarizationdirections harmonicshavebeenusedtomapthedepth-dependentdistributionof
(FPDs)andstationdistancefromthefault(e.g.Tadokoroetal.,2002; seismicanisotropyinareasofintensecrustaldeformation,e.g.,around
PengandBen-Zion,2004;HurdandBohnhoff,2012;Ekenetal.,2013). the San Andreas Fault (SAF) (Audet 2015), the Tibetan Plateau (Liu
LateralvariationsoftheFPDsinferredfromlocalshearwavesimplied et al., 2015), the Cyclades (Cossette et al., 2016), the Canadian Cor-
thepresenceofbothstress-andstructureinducedmechanismscausing dillera(Tarayounetal.,2017)andtheAppennines(Bianchietal.,2010;
seismicanisotropyintheupper8–10kmofthecrust.Inaddition,these 2016).
local splitting studies have highlighted the structural control of the In this work, we analyse RFharmonics using data from theNorth
complexgeologicandtectonicenvironmentalongthewesternsegments Anatolian Fault passiveseismic experiment([dataset]BeckandZandt,
oftheNAFZonthespatialvariationofFPDs. 2005;Biryoletal.,2010),inordertodelineatethefirst-orderseismic
For the deeper part of the crust, i.e.>20–25km, several studies structure of the crust and to map crustal anisotropy as a function of
have shown that aligned minerals are the most likely cause of aniso- depth.Inparticular,ourmainobjectivesaretoelucidatei)orientation
tropy (Sherrington et al., 2004, and references therein) and that hex- andstrengthofdeformationinthecrustatvariousdepthrangesii)how
agonal anisotropy with a unique slow symmetry axis can explain se- much the strain fields within crust and upper mantle are coupled iii)
ventypercentoftheobservations(Brownleeetal.,2017).Inparticular, possiblelinkbetweenlateralvariationofcrustalanisotropyparameters
thealignmentofmicasalongtheplaneoffoliationisoftentheprimary and existing lithology contrast across the NAF. These results yield in-
causeofbulkanisotropyinthisdepthrange(Sherringtonetal.,2004; sight into the poorly known role of crustal seismic anisotropy in the
Audet2015). area.
Overthelastdecade,receiverfunction(RF)datahavebeenwidely
usedforthecharacterizationofseismicanisotropy.RFsaretimeseries
2. GeologicalsettingofNorth-CentralAnatolia
that represent the impulse response of the near receiver structure in
terms of P-to-S conversions contained in the P-coda of teleseismic
Thestudyregionislocatedinanimportantareaoforogenicamal-
events (Vinnik, 1977; Langston, 1979). After deconvolution of the
gamation of Anatolia, a transition zone between compressional-de-
verticaltracefromthehorizontalones,P-to-SVandP-to-SHconverted
formed eastern Anatolia and extensional western Anatolia. There are
phasesareisolatedontheRadial(R)andTransverse(T)componentsof
numerous key structures developed under the complex deformation,
theRFs,respectively.Inparticular,P-to-SHconversionsaregenerated suchastheEzinePazarı–SungurluFault,theİzmir–Ankara–Erzincan
fromtherotationoftheenergyoutofthesource-receiverplaneinduced (IAESZ) and Intra – Pontide Suture Zones, the İstanbul Zone, the
byanisotropyand/ordippingvelocitycontrastsatdepth(Sherrington Sakarya Continent, the Central Pontides, the Kırşehir Massif and the
etal.,2004;MaupinandPark,2007;PianaAgostinettiandChiarabba, ÇankırıBasin(OkayandTüysüz,1999;Fig.2).Itisreportedthatsome
2008; Schulte-Pelkum and Mahan, 2014a). The analysis of the azi- ofthesestructures(e.g.,theIstanbulZone;Şengör,1979)werepartsof
muthally varying characteristic of the P-to-S conversions (amplitudes
Eurasia, while other fragments were separated from the Arabian-
anddelaytimes)canproviderobustinformationaboutthelocationof
AfricanPlate.Görüretal.(1998)furtherinformthatthemajorbasinsin
anisotropy at depth (e.g. Rümpker et al., 2014; Licciardi and Piana
Central Anatolia were formed on continental units; i.e., the Sakarya
Agostinetti, 2016). RFs provide complementary depth-dependent in- Continent andtheKırşehirMassifadjacent to thesuture zones. These
formationaboutseismicanisotropythatisdifficulttoobtainwithother structures have significant importance on understanding the tectonic
common seismological data (e.g. shear wave splitting and surface evolutionoftheregion.Forexample,theİzmir–Ankara–Erzincansuture
waves dispersion), since RFs are strongly sensitive to the depth of
zone(IAESZ)isaremnantoftheNeo-TethysOceanandhenceconsists
contrastsinanisotropicproperties.
ofophioliticunits(Rojay,2013).ItseparatesthePontidestothenorth
Moreindetail,theRFharmonicdecompositiontechnique(Bianchi from the Anatolide–Tauride and the Kırşehir blocks to the south
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A.Licciardietal. Physics of the Earth and Planetary Interiors 277 (2018) 99–112
Fig.2.Mapwiththelocationofthe38broadbandseismic
stations used in this study. Stations belong to the North
AnatolianFault(NAF)passiveseismicexperiment(Biryol
etal.,2010),whichoperatedfromJanuary2006toMay
2008. Major faults are indicated in black. Suture zones
modifiedafterOkayandTüysüz(1999)areindicatedwith
redlines.TracesoftheprofilesshowninFig.7areindicated
inblueandlabelled.StationsusedinFigs.4and5(ALIC
and DOGL) are indicated by red triangles. CB, Çankırı
Basin; CP, Central Pontides; ESF, Ezinepazari-Sungurlu
Fault; IAESZ, Izmir-Ankara-Erzincan Suture Zone; IPS,
Intra-PontideSuture;IZ,IstanbulZone;ITS,Intra-Tauride
Suture;KM,KırşehirMassif;NAF,NorthAnatolianFault;.SZ
SakaryaZone.
(Koçyiğit, 1991; Okay and Tüysüz, 1999; Taymaz et al., 2007a,b). 2005;Biryoletal.,2010).Foreachstation,weselectedtheeventswith
Yolsal-Çevikbilen et al. (2012) identified three major high-velocity high signal-to-noise ratio (SNR) waveforms using a visual inspection
blocks in the mid-crust of north-central Anatolia, separated by the process. These waveforms were extracted from a list of about 1000
İzmir-Ankara-Erzincan SutureZone,andtheyinterpretedtheseblocks teleseismic events with magnitude≥5.5 that occurred at epicentral
to be continental basement fragments that were accreted onto the distances(Δ)between30°and100°.Mostoftheeventscomefromthe
margin, following the closure of Neo-Tethyan Ocean. They also pro- first quadrant for the selected Δ range, where the major seismogenic
posedthatthesehigh-velocitybasementblocksmightcontroltherup- zonesarelocated.Nonetheless,agoodazimuthalcoverageisachieved
turepropagationsofthehistorical1939,1942and1943earthquakes. foralmostallstations.AnexampleofsuchcoverageisshowninFig.3
Similarly, the Çankırı basin is a natural laboratory for studying the forstationDOGL.Theselectedthree-componentseismogramsarethen
subduction and collision processes, and therefore it is accepted as an rotatedintotheZRTreferencesystem,whereRistheRadialdirection
essentialunitshapingactivetectonicsofthenorth-centralAnatolia.It along the great circle path from the epicenter to the station, T is the
locatesatthecontactregionbetweenthePontidesandtheAnatolide- TransversedirectionperpendiculartoRinthehorizontalplane,andZis
Tauride Block, where the 1200km long North Anatolian Fault Zone the vertical component. In RF analysis, the Z component of the
bifurcates into a number of small-scale splay faults deforming the
Anatolian Block internally, with a complex rotational deformation
(Seyitoğlu et al., 2004; Kaymakçı et al., 2009; Lucifora et al., 2013).
Recently, Lucifora et al. (2013) have obtained asymmetric tectonic
rotations along the opposite edges of the Çankırı Basin from palaeo-
magnetic data. They have correlated that complex pattern of paleo-
magneticrotationswithalocalblockrotationmechanismcausedbythe
strike-slipfaultactivityalongthemarginsoftheBasin.
Theotheressentialtectonicunitsinnorth-centralAnatoliaarethe
Sakarya Continent andthe KırşehirMassif, also known astheCentral
Anatolian Crystalline Complex (Akiman et al., 1993). The Kırşehir
Massif has a triangular shape that spans over an area of ∼300km×
∼200km (Şengör and Yilmaz, 1981), and is bordered by the right-
lateralTuzGölüFaultZoneonthewest(Çemenetal.,1999)andbythe
left-lateralstrikeslipEcemişFaultZoneontheeast(seeFig.2).Itisalso
surrounded by ophiolitic melanges associated with subduction-accre-
tion complexes of the Izmir-Ankara-Erzincan and the Inner Tauride
suture zones. This Massif is dominated by Late Cretaceous age high-
grademetamorphism,intrudedbybatholiths(Ketin,1966;Göncüoğlu,
1977;WhitneyandHamilton,2004).Itconsists ofmetamorphiccrys-
talline rocks, i.e., metasediments, ophiolitic sequences, and magmatic
intrusions(Kaymakçıetal.,2009).Similarly,theSakaryaContinenthas
a high-grade metamorphic basement which consists of gneisses, mar-
bles, amphibolites, and metaperidotites, with granodiorite intrusions
(Okay,1996;Görüretal.,1998).
3. Dataandmethods
Fig.3.AzimuthalequidistantprojectioncenteredatstationDOGLwiththesourceloca-
WeusedteleseismicdatarecordedbetweenJanuary2006andMay tionoftheteleseismiceventsusedtocomputetheRFdatasetforthisstation(blacktri-
2008at38broadbandseismicstationsbelongingtotheNorthAnatolian angle).Onlyeventsfrom30°to100°ofepicentraldistancehavebeenused.Sizeofcircles
Fault (NAF) passive seismic experiment ([dataset]Beck and Zandt, isproportionaltoeventmagnitude(M)asperlegend.
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A.Licciardietal. Physics of the Earth and Planetary Interiors 277 (2018) 99–112
observedseismogramsistakenasaproxyofthesourcewavelet,while (red) color is used for positive (negative) pulses associated with an
horizontal components hold information about the near receiver S- increase (decrease) of velocity with depth. On the RRF sweeps, the
wavevelocitystructure.Thelattercanberecoveredafterdeconvolution strongpositivepulseatabout5.5scanbeassociatedwiththeconver-
oftheverticalcomponentfromthehorizontal ones.Inthiswork,RFs sionoriginatedattheMoho.InthetimewindowprecedingtheMoho
are computed through the frequency domain deconvolution method arrival, crustal complexities beneath both stations are represented by
developedbyDiBona(1998). high-amplitude pulses (both positive and negative). High amplitudes
AGaussianfilterwithlow-passfrequencyof∼1Hz(Gaussianfilter are also observed on the TRF panels at both stations for delay times
width a=2) is applied to each RF, in order to rule out the effect of between 0 and 6s. This observation indicates that strong P-SH con-
high-frequency noise. This choice limits the vertical resolution of the versions take place at crustal depths. Moreover, clear patterns and
RFs to about 2–4km (Piana Agostinetti and Malinverno, 2017). Both periodicity canbe observed inthewaveformson theTRF panels sug-
radial receiver function (RRF) and transverse receiver function (TRF) gestingthepresenceof3-Dfeaturesatdepth.Insuchcomplextectonic
arecomputedfromthethree-componentseismogramsofasingleevent. settings,usefulinformationaboutthesubsurfacecanbeextractedfrom
AfterRFsareobtained,anothervisualinspectionisperformed,andonly bothRRFandTRF.
RFwithacceptableSNR,small“ringing”andsmallamplitudesfordelay Inthenextsection,asimple,yeteffectivemethodispresentedfor
times<0s areretained.Thisproduces atotalnumber of∼7100 RFs theanalysisofboththe“isotropic“and“anisotropic”componentsofthe
withanaveragevalueof∼180RFsperstation. RFdataset,i.e.theenergycontainedintheRFdatasetgeneratedfrom
Inaflat-layeredandisotropicmedium,onlyP-to-SVconversionsare bulk S-wave velocity discontinuities, also referred as “isotropic struc-
generated and no energy should be observed on the TRF (Savage, ture”, and from anisotropic materials at depth, also referred as “ani-
1998). However, the presence of anisotropy and/or dipping velocity sotropicstructure”.
contrasts at depth (called 3-D features hereinafter) rotates the energy
outofthesource-receiverplaneandproducesP-to-SHconversionsthat
arecommonlyobservedontheTRF(ParkandLevin,2016).Inthiscase, 3.1. Harmonicdecomposition
two-lobed (360°) or four-lobed (180°) periodicity in amplitudes and
delaytimesasafunctionofback-azimuth(ф)areexpectedonbothTRF Classical RF studies assume isotropic and flat-layered media for
andRRF(LevinandPark,1998;Audet,2015;Bianchietal.,2015). which the response of the subsurface is invariant with the back-azi-
InordertoassessthequalityofthefinalRFdatasetandtohavea muth,resultinginalltheP-to-Sconvertedenergytobeconfinedinthe
first glimpse atthecontribution of3-D features beneath each station, RRF.Anisotropyand/ordippingvelocitycontrastsintroduceamplitude
weplotboththeRRFandTRFasafunctionofф,afterstackinginфand and delay time variations as a function of back-azimuth on both RRF
Δwithbinsof20°and40°widthrespectively,and50%overlaptoin- and TRF with distinct periodicity (either π or 2π). These effects are
creasetheSNR.InFig.4weshowtwoexamplesforstationsALICand coupledontheRRFandTRF,drasticallyincreasingthecomplexityofa
DOGL,whicharelocatedontopofthesurfacetraceoftwomajoractive given RF dataset. This makes even a qualitative analysis of raw RF
faultsofthearea,i.e.theNAFandtheESF,respectively.Althoughthe datasets (as those shown in Fig. 4) difficult, leading to strongly non-
finalback-azimuthalsweepsaredifferentbetweenthesestations,both unique interpretations. To overcome this difficulty, RRF and TRF
ofthemshowazimuthalvariationsinamplitudeanddelaytimesofthe should be analysed jointly. The harmonic decomposition technique
conversions along the RRF and TRF components. In this work, blue providesasimplewayforthesimultaneousanalysisoftheRRFandTRF
(Bianchietal.,2010;Audet,2015).Thistechniqueaimsatseparating
Fig.4.BinnedRadialandTransverseRFsforstations(a)ALICand(b)DOGL,plottedasafunctionofback-azimuth.Graynumbersindicatethecentralepicentraldistanceusedinthe
binningprocedure.SmallnumbersindicatethetotalnumberofRFsmakingupeachindividualbin.Blue(red)pulsescorrespondtoaninterfacewithapositive(negative)downward
velocityjump.(Forinterpretationofthereferencestocolourinthisfigurelegend,thereaderisreferredtothewebversionofthisarticle.)
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A.Licciardietal. Physics of the Earth and Planetary Interiors 277 (2018) 99–112
the isotropic response from the response induced by 3-D features at (derivativepulses)onbothk=1componentsinthefirst4s,although
depth exploiting the azimuth-dependent signatures produced by the station DOGL is dominated by the derivative pulse on the cos(ф)
latter. Thisrepresents a clearadvantage over classical RFanalysis, as componentwithpeaksat1.6and3s.Smallamplitudesareobservedon
the RF dataset is partitioned into azimuth-invariant and azimuth-de- the k=2 harmonics compared to k=1. In addition, for the k=2
pendentcomponents,thusfacilitatingsubsequentinterpretations. harmonics, comparable amplitudes are observed between the “mod-
Moreindetail,themethodisbasedontheassumptionthatanyRF eled”and“unmodeled”terms.Theseobservationsholdforthemajority
dataset can be approximated as a series of cos[kф(t)] and sin[kф(t)], ofthestations,thus,inthefollowing,weexcludethek=2harmonics
withk=0,1,2,…Inthisway,theazimuth-dependentfeaturescanbe and restrict the analysis only to the first two order harmonics. We
isolatedfromtheazimuth-invariantcomponents.Thek=0component displaythecomponentsofcos(ф)(orientedN-S)andsin(ф)(orientedE-
istheconstantazimuthtermanddescribestheisotropicstructure(i.e. W) separately for k=1, giving a total of three harmonic coefficients
contrastsinVS).AtthecoreofthemethodisthattheSNRofthetwo- (i.e.threetimeseries).
lobed or four-lobed periodicity is enhanced on the k=1 and k=2
harmonics,respectively,bystackingtheRRFandTRFwithapositive 3.2. Energyandmap-orientationof3-Dfeatures
shiftinback-azimuth(+π/2k),asshownbyShiomiandPark(2008).
The two-lobed periodicity (k=1) is associated with the presence of The main aim of this work is to map the position of anisotropic
dipping velocity contrasts or hexagonal anisotropy with plunging bodies and/or dipping velocity contrasts at crustal depths and to de-
symmetryaxisatdepth.Ontheotherhand,thefour-lobedperiodicity terminetheirorientations. Basedontheharmonicsdatasetpreviously
(k=2)isuniquelyproducedbyanisotropywithhorizontalsymmetry described,wefirstperformatime-to-depthconversionofthek=0and
axis(GirardinandFarra,1998). k=1harmonicsateachstation.ThestandardIASP91velocitymodel
FollowingBianchietal.(2010)andLiuetal.(2015),foreachtime (KennettandEngdahl,1991)isusedforthispurpose.Weusethemean
step we can recover five coefficients, A(t), B(t), C(t), D(t), E(t), by slowness value at each station to perform the depth conversion. In
solvingthefollowinglinearsystemofequationsforeachtime-steptin principle,thischoicecouldproducedefocusedresultsintheharmonics,
theRFdataset: if the range of slowness considered is broad. In order to estimate the
⎡⎢R1…(t)⎤⎥ ⎡⎢…1 cos(ϕ1) sin(ϕ1) cos(2ϕ1) sin(2ϕ1) ⎤⎥⎡⎢AB((tt))⎤⎥
⎢⎢RTN((tt))⎥⎥=⎢⎢10 cosc(ϕos(+ϕNπ)/2) sin(sϕin(+ϕNπ)/2) cos(c2o(ϕs(2+ϕNπ)/4)) sin(2si(nϕ2(+ϕNπ)/4))⎥⎥⎢⎢C(t)⎥⎥,
⎢ 1… ⎥ ⎢… 1 1 1 1 ⎥⎢D(t)⎥
⎣⎢TN(t)⎦⎥ ⎣⎢0 cos(ϕN +π/2) sin(ϕN +π/2) cos(2(ϕN +π/4)) sin(2(ϕN +π/4))⎦⎥⎣⎢E(t)⎦⎥ (1)
whereNisthetotalnumberofeventsconsideredateachstation.A(t) error associated with this choice, we compare our results with an al-
corresponds to the k=0 harmonic, B(t) and D(t) are the cos[kф(t)] ternativeapproachinwhichtheharmonicdecompositionisperformed
components and C(t) and E(t) are the sin[kф(t)] components of the in depth domain after time-to-depth conversion of the RF dataset
k=1andk=2harmonics.Withthisconfiguration,cos[kф(t)]andsin (Audet 2015; Cossette et al., 2016; Tarayoun et al., 2017), thus pre-
[kф(t)] for k=1 are orthogonal and oriented N-S and E-W, respec- serving the information about slowness. The comparison points out
tively. The same linear system can be solved using a negative phase
shift(−π/2k),inwhichtheeffectofdippingstructuresandanisotropy
issuppressed(Liuetal.,2015).Inthiscase,theretrievedcoefficients
quantifytheeffectofcomplex3-Dstructures,scattering,orincomplete
geometrical coverage (“unmodeled” components hereinafter). The
“unmodeled”coefficientscanbeseenasanestimateoftheerrorasso-
ciated with the harmonic decomposition. For further details on the
method,thereaderisreferredtoParkandLevin(2016).
The harmonic decomposition is applied to the RF dataset at each
station, after stacking in ф and Δ with bin-width of 10° and 20° re-
spectively, and with no overlap. This choice prevents the use of the
same RF twice in the analysis, thus reducing any possible bias in-
troduced by smoothing. As an example, we show the harmonic de-
compositionoftheRFdatasetsatstationsALICandDOGLinFig.5.The
values ofф andΔofeachbinusedtocomputetheharmonicsarein-
dicatedwithredstarsforeachstation.
The two stations show different isotropic responses on the k=0
plot.TheP-to-SconversionoriginatedattheMohocanbeidentifiedas
the positive pulses at about 5.5 and6.0s on ALIC andDOGL, respec-
tively.Atearlierdelaytimes,ALICshowspositivepulsesatabout1and
2.4swhileabroadnegativepulsecanbeobservedat4s.Onthecon-
trary, the pulse with highest amplitude at station DOGL is found at
about0.8s,indicatingthepresenceofastrongandshallowimpedance
contrast,followedbyanegativepulseat2.3s.
The k=1 harmonics (cos(ф) and sin(ф) traces) show higher am-
plitudes on the “modeled” part than on the “unmodeled”, thus in- Fig.5.HarmonicdecompositionofRFsforstations(a)ALICand(b)DOGL.k=0trace
dicatingasignificantandcoherentcontributionfromtheazimuthally- correspondstothe“constant”,zero-orderharmonic.cos[kф(t)]andsin[kф(t)]harmonics
fork=1,2areshowninthebottompanelsformodeled(+π/2kshiftinthestack)and
varying structure beneath the two stations. Higher amplitudes are unmodeled(−π/2kshiftinthestack)parts.Seetextfordetails.Toprightpanelshowsthe
foundinthefirst6sforbothstations,suggestingacrustaloriginofthis geometricaldistribution(inback-azimuth,фandepicentraldistance,Δ)ofRFsusedfor
signal.Bothstationsdisplayaseriesofpulseswithoppositepolarities theharmonicdecomposition.Thesameamplitudescaleisusedinallplots.
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A.Licciardietal. Physics of the Earth and Planetary Interiors 277 (2018) 99–112
that, given the distribution of events for the NAF experiment, small interpretations.Itcould,forexample,representthetrendoftheplun-
errorsareproducedbyusingthemeanrayparameterandthiseffectis gingsymmetryaxisofhexagonalanisotropy.Inthiscase,theaxiswould
negligibleinthecrust(seeAppendixA). plungetowardseitherNWorSE.Someambiguitiesariseinthiscase,as
Consideringthattheinter-stationdistanceisgenerallygreaterthan noinformationaboutthenatureofthesymmetryaxis(fastorslow)is
30km, we build depth-profiles by juxtaposing single-station data and available (see Sherrington et al., 2004 for details). A second possible
focusourattentiononverticalandlateralvariationsofcrustalstructure interpretation provides the dipping direction of a dipping velocity
(k=0harmonics)andpositionof3-Dfeatures(fromk=1harmonics) contraststrikingNE-SW.Thisambiguityisstrongerintheuppercrust,
alongtheprofiles. butitdiminisheswithincreasingdepth,wherestronglydippingvelocity
Asaproxyforthepositionof3-Dfeaturesatdepth,wecomputethe jumps(morethan30°)arelesslikelytooccur,andhenceanisotropyis
sumofthesquaresofthe(depthconverted)“modeled”cos(ф)andsin assumedasthemaincauseoftheobservedsignals.
(ф)termsforthek=1harmonics(PianaAgostinettiandMiller,2014) In the next section, to facilitate our interpretation, we follow
and subtract the contribution of the “unmodeled” counterpart ac- Schulte-Pelkum and Mahan (2014b) and plot the direction normal to
cordingto: the retrieved trend direction (i.e. the strike). Therefore, for a dipping
velocitycontrastourmap-projectedstrikecanbedirectlyinterpretedas
E(z)=[B(z)2M +C(z)2M]−[B(z)U2 +C(z)U2] (2) the strike of the dipping velocity contrast. On the other hand, for
hexagonalanisotropy,thisgivesusthemap-projectedorientationofthe
wherethesubscriptsMandUreferstothe“modeled”and“unmodeled”
planeperpendiculartotheplungingsymmetryaxis.Incaseofnegative
harmoniccoefficientsandzisthedepthaxis.TheresultingquantityE(z)
anisotropy (unique slow symmetry axis), this is the strike of the fast
(referredas“energy”hereinafter)highlightsthepositionof3-Dfeatures
plane,whichcanbeinterpretedasthestrikeoffoliationorthestrikeof
atdepthinanintuitiveway,thusmakingtheinterpretationoftheresults
theplaneofpreferredcracksormineralalignment.Byplottingthemap-
simpler. High energy at a certain depth may suggests the presence of
projected strike together with the energy in a given depth range, we
either a contrast in anisotropic properties (top or bottom of an aniso-
buildenergymapswithorientationofthe3-DfeaturesasseenbytheRF
tropicbodywithplungingsymmetryaxis)oradippingvelocitycontrast.
data. Thisprovides uswith spatialinformation abouttheazimuthally
In addition, the map-projected orientation of 3-D features can be
varyingstructureandtheirfirst-orderdepthdependence.
retrievedfromthedepth-convertedk=1harmonics.Infact,fromthe
particlemotionofthecos(ф)(N-Scomponent)andsin(ф)terms(E-W
4. Resultsanddiscussion
component)itispossibletoinferthetrendoftheuniquesymmetryaxis
ofhexagonalanisotropy(orthedippingdirectionofadippingvelocity
Weapplytheharmonicdecompositiontechniquetoallthestations
contrast)responsiblefortheobservedsignal(Bianchietal.,2010).Ina
inthearea,andplottheresultssidebysidealongsevenprofiles(Fig.7).
simplelinearregressionoftheparticlemotionateachstation,theor-
IneachpanelofFig.7,weplotthetopographyofeachprofilewiththe
ientation of 3-D features is recovered from the slope parameter. In
locationofthemainactivefaults(NAFandESF)togetherwiththere-
principle, we could estimate a value of the slope parameter at each
sultsoftheharmonicanalysis.Foreachstation,thewaveformsofthe
depthpoint;however,asinAudet(2015),wefoundmorestableresults
resulting harmonics are plotted individually. Blue lines are used to
ifarangeofdepthsisconsidered.Anexampleoftheparticlemotionfor
highlightourobservationsontheconstantcomponent.Inthelastplot
the0–15kmdepthrangeatstationALICisshowninFig.6.Weestimate
of each panel, the energy associated with the k=1 harmonics is dis-
the trend direction (green arrow) from the slope parameter, together
played in grayscale. Here, green and orange boxes indicate areas of
withitsuncertainties(grayarrows),foreachstation,findingerrorsof
strongenergyatdepthssmallerandgreaterthan30km.Locationofthe
usuallylessthan15°(95%confidenceinterval).
profiles is shown in Fig. 2. Five profiles (from AA′ to EE′) follow S-N
The retrieved orientation (green arrow) can have multiple
direction and cross themain active faults inthe area (NAF and ESF).
The last two profiles, FF' and GG', run W-E and are designed to in-
vestigatethecrustalsettingofthenorthernandsouthernblocksdivided
bytheNAFZ.Lateralvariationsinbothk=0andk=1harmonicsare
evidentinthemajorityoftheprofiles.
4.1. CrustalstructureandMohotopography
Itisbeyondthescopeofthispapertodiscussindetailtheisotropic
structureofthearea,asthefocusismainlyon3-Dfeatures.Giventhat
theinter-stationdistancedoesnotallowforoverlappingzonesofsen-
sitivityinthecrust,ourobservationsarethereforegenerallylimitedtoa
single-station approach. With a vertical resolution of 2–4km, k=0
harmonicsatthestationsanalysedinthisstudyindicatethattheMoho
topography is generally a continuous feature at about 40km (Fig. 7).
Thisresultisingoodagreementwithearliercrustalthicknessestimates
fromRFsinthesamearea,whichreportedfairlyconsistentvalueswith
thepresentstudy,i.e.∼35–40km(Özacaretal.,2010;Vanacoreetal.,
2013).Nevertheless,ourresultsshowafewlocalexampleswherethe
identificationoftheMohoisdifficultorambiguous.AlongprofileCC′
(Fig.7c)forexample,apositivepulseislocatedatabout40kmdepth
foralmostallstations.However,thecontinuityoftheMohoPsphaseis
brokenattwoneighbouringstations(CAYAandPANC)between75and
Fig.6.Particlemotionforthe0–15kmdepthrangeatstationALIC.Particlemotion(red
points)oftheN-SandE-Wcomponentofthefirstorderharmonic.Trendofthesymmetry 105kmalongtheprofile.StationCAYAshowsalmostnosignalbetween
axis(greenarrow)isestimatedthroughlinearregressionoftheparticlemotion.Grey 25 and 50km depth. The following station to the north, PANC, pos-
arrows indicate three standard deviations of the estimated slope parameter from the sessesapositivepulseatabout30km,whichisquiteshallowerthanthe
linearfit.(Forinterpretationofthereferencestocolourinthisfigurelegend,thereaderis general Moho depth along this profile. These two contiguous stations
referredtothewebversionofthisarticle.) arelocatedbetweentheESFtothesouthandtheIAESZtothenorth.In
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Fig.7.Harmonicdecompositionresults.Time-to-depthconvertedharmonicsareshownalongsevenprofiles.ThesurfacetraceofeachprofileisindicatedinFig.2.Foreachprofile,the
toppanelindicatesthesurfacetopographyforreference.Thefollowingthreepanelsdisplaythek=0harmonic,andthecos[kф(t)]andsin[kф(t)]componentsofthek=1harmonic.
Wigglesareplottedontopofaninterpolatedimageforclarity.Thebottompanelshowstheenergycalculatedfromthek=1harmonics,asexplainedinthemaintext.Redarrows
indicatethepositionofthesurfacetracesoftheNorthAnatolianFault(NAF)andtheEzinepazari-SungurluFault(ESF).Thesamescaleisusedforharmonicsandenergyamplitudes
betweendifferentpanels.
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Fig.7. (continued)
addition,thek=0harmonicatstationPANCshowsthehighestenergy Çankırı Basin and its sedimentary infill is likely responsible for pro-
atabout5kmdepth,andverybroaddirectP-wavepulsesareobserved ducing reverberations in the sedimentary pile that can interfere with
inthefirst110kmoftheprofile.StationsCAYAandPANCarelocated theprimaryPsconversionfromtheMoho,thusproducingtheobserved
within the Kırşehir Massif tectonic unit. Here, the presence of the broaddirectP-wavepulse.
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A.Licciardietal. Physics of the Earth and Planetary Interiors 277 (2018) 99–112
Themainstructuraldifferencesbetweenthecrustoftheblockslo-
catedsouthandnorthoftheNAFZareevidentwhencomparingthetwo
longitudinalprofilesFF′andGG′showninFig.7gandf,respectively.In
FF′, the Moho Ps pulse is continuous along the length of the profile,
althoughittendstogetshallowerandbroadertowardstheeast.InGG′,
the pulse related to the Moho is located at a depth of about 40km,
exceptforstationCAYA,whichdoesnotshowaclearMohosignal,and
forstationDOGL,whichhasadeeperMoho(50kmdepth).Atthesame
time,alongGG′,nocontinuityofintra-crustalsignalscanbeobserved,
indicating a more heterogeneous setting in the crust. The intense de-
formation history that affected the southern block, together with its
tectonic activity, result in a more heterogeneous crust, in accordance
with3-Dlocalearthquaketomography(Yolsal-Çevikbilenetal.,2012).
This dramatically increases the complexity of the RF waveforms with
respecttothemoregeologicallystablenorthernblock.
Finally, there is no clear evidence of Moho offset related to the
NAFZ,whichconfirmspreviouslower frequencyobservations from S-
RF(Kindetal.,2015).However,theNAFZclearlyaffectsintra-crustal
structures in the first 20km (see Fig. 7. profiles AA′, EE’). A smaller
inter-station distance would be required in order to study the lateral
continuityofintra-crustalPsconversionsacrossthismajorfaultsystem.
4.2. Seismicanisotropy
IntheprofilesshowninFig.7,highvaluesoftheenergyassociated
withthek=1harmonicsindicateawidespreadcontributionfrom3-D
featuresthroughoutthecrustofthestudyarea.Twomainobservations
canbedrawnfromtheseresults.First,asubstantialenergydropfrom
southtonorthoftheNAFZisobservedalongprofilesBB′,DD′andEE′.
Inaddition,highestenergywithcomplexpatternsislocatedinbetween
activefaultswherecrustalstructureisalsocomplex(profilesAA′,CC′
andDD′ofFig.7)and,atleastlocally,theNAFcontrolsthetransitionof
theenergyfromsouthtonorth(profileBB′inFig.7).Second,themain
sourceoftheenergysignalislocatedwithintheupperandmiddlecrust,
wherethehighestamplitudesareobserved.Deeper(greaterthan30km
depth)sourcesoftheobservedenergyshowlowestamplitudesandless
continuity.Finally,strongdifferencesintermsofenergyareobserved
between profiles FF′ and GG′, both oriented W-E, and located respec-
tively in the northern and southern blocks divided by the NAF. GG'
showsmuchstrongerenergythanFF′throughoutthecrust.
We complement the depth-dependent information about 3-D fea-
turesshowninFig.7withaspatialmappinganalysisonthreediscrete
depthranges:0–15,15–30and30–60km,roughlycorrespondingtothe
uppercrust,middlecrustandlowercrustplusuppermantle.Foreachof
these depth ranges, we build interpolated energy maps for the k=1
harmonicsbysummingthecontributionofeachpointinagivendepth Fig.8.Mapsofthetotalenergyatthreedifferentdepthranges.(a)0–15km,(b)15–30km
range.Atthesametime,wecalculatethedirectionofthe3-Dfeatures, and(c)30–60kmdepth.Thecolorscaleindicatesthesumoftheenergyonthek=1har-
monicsinthecorrespondingdepthrange.Strikeoftheplanenormaltotheaxisofsymmetry
as explained in the previous section, from the slope parameter of the
isindicatedwithagraybarateachstationandscaledbythetotalenergyinthesamedepth
linear regression of the particle motion in the same depth range. The
range.Themajortectonicfeaturesintheareaaredrawnwithblacklines(seealsoFig.2).
resultsareshowninFig.8andsummarizedinTableS1ofthesupple-
mentarymaterial. energyvalues,thusgivingrisetoahigherspatialfrequencyvariation
In the following, we discuss our results regarding the energy dis- when compared to north of the NAF. A second order observation in-
tribution,intermsofdepth-dependentandspatialfeatures. dicatesthatalmostallstations(13outof15stations)withhighenergy
(greater than 0.5) are located on top or in close proximity of active
4.2.1. Uppercrust faults(NAFandESF)orancientsuturezones.
The strongest azimuthally varying signal is generated in the first Thisisperhapsthemoststrikingfeatureofourresults:thespatial
15kmofthecrust.Overall,theenergyobservedinthisdepthrangeis distribution oftheenergy associated with the k=1 harmonics inthe
higherthanthatoriginatedinthedeeperportionofthecrustandupper uppercrustisclearlyinfluencedbythestructuralsettingofthearea.In
mantle(comparethetotalenergyinFig.8atdifferentdepths).Between particular, we identify the well defined partitioning between low-en-
0 and 15km depth (Fig. 8a), two spatial zones with distinctive char- ergystationsnorthoftheNAFandhigh-energystationsontopofthe
acteristics are separated by the NAFZ, confirming what was already NAFandsouthwards(Figs.7and8).
observedalongtheprofilesofFig.7.StationslocatednorthoftheNAF InourRFanalysis,bothdippingvelocityjumpsandseismicaniso-
show consistently low energy in the k=1 harmonics. Conversely, on tropycontributetotheobservedenergy,especiallyintheuppercrust,
topoftheNAFandsouthwards,higherenergyisfoundformorethan wherethecontributionofdippinglayerscanbestrong.In oursimple
halfofthestations(15outof28stations).Inthisregion,inaddition, approach, the discrimination between the two effects is not easy and
the pattern is more heterogeneous, with alternating low and high evenaninversionapproachwouldproducenon-uniqueresults.Adirect
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A.Licciardietal. Physics of the Earth and Planetary Interiors 277 (2018) 99–112
interpretationofourresultsintheuppercrustisfurthercomplicatedby High-energystationslocatedontoporincloseproximityofancient
thepossiblepresenceofreverberationsinducedbyshallowlow-velocity suture zones (IPS, IAESZ, and ITS) are more difficult to interpret, as
layers,asthosepresentinsedimentarybasinswithcomplexgeometries they show both parallel (ALIN and CUKU) and oblique (KIZIK and
(seeAppendixB). CAKM)strikes.
Despite the inherent non-uniqueness of our results, shear wave Away from active faults, anisotropy is generally lower, but some
splitting analyses performed on local seismicity across the different local high-energy anomalies are observed at different depth ranges,
portionsoftheNAFZyieldedasignalwithamaximum∼0.2–0.3sof south of the NAF. We suggest that some of these spots of high aniso-
time delay, which has been attributed to the presence of anisotropy tropyarecorrelatedwiththepresenceoftheupliftedblocksofaccreted
within the first 8–10km of the upper crust (e.g., Peng and Ben-Zion, andstronglydeformedbasementrecognizedbyYolsal-Çevikbilenetal.
2004;HurdandBohnhoff,2012;Ekenetal.,2013).Inaddition,are- (2012),above30kmdepthinthecentralandeasternpartofthestudy
cent3-Dradiallyanisotropictomography(Çubuk-Sabuncuetal.,2017) area (south of the NAF). Alternatively, the localization of intense de-
hasshownthepresenceofstrong(20%)negativeanisotropythroughout formationwhichhasbeentakingplacesouthoftheNAFmaybecon-
theAnatoliancrustataregionalscale.Forthesereasons,wewilldiscuss sideredresponsiblefortheobservedseismicanisotropyatmid-crustal
thepossibleimplicationsofourresultsintermsofanisotropyonly. depths, as heavily fractured and fluid-saturated rocks, alignments of
Ifweassumeslowanisotropyintheuppercrust,ourresults(Fig.8a) anisotropicmineralsandcrustalflowcanproducedirectionalvelocity
indicatethestrikeofthefastplaneperpendiculartoit,correspondingto changes at different crustal depths (Fouch and Rondenay, 2006;
thestrike ofbedding, foliation oralignment ofcracks ormacroscopic Mainpriceetal.,2007).
fractures.Underthisperspective,ourretrievedstrikesforhigh-energy
stationslocatedontopofthemainactivefaultsofthearea(NAFand 4.3. Lowercrustanduppermantle
ESF) match theorientations ofthefaults withintheerrors. Fault par-
allelstrikeforthesestationsisingoodaccordancewithpreviousshear The interpretation of deep anisotropy (between 30 and 60km,
wavesplittingstudiesindifferentportionsoftheNAFZ(Lietal.,2014) Fig.8c)isdifficult.Thenumberofhigh-energy(energy>0.5)stations
andwithRFsstudiesalongtheSanAndreasFault(Porteretal.,2011; issmall(six)andtheirspatialdistributionisscattered.Nevertheless,we
Audet,2015).ThedirectionofSH inthestudyareawasestimated noticethatthesestationsarelocatedclosetothecentral-westernpartof
max
fromlocalseismicity(Karasözenetal.,2014).ItshowsanaverageNW- theNAFZ,alongtheESF,andthatfiveofthemdisplaystrikesoriented
SE orientation, thus being very different from our retrieved strikes of NE-SW. This agrees with the 3-D full-waveform tomography study of
thefastplaneassumingslowanisotropy.Therefore,ourresultssupport Çubuk-Sabuncuetal.(2017),whichrevealeddeepnegativeradialan-
a structure-induced origin of the observed anisotropy rather than a isotropy (V > V ) in the close vicinity of known weakness zones,
SV SH
stress-driven mechanism. This agrees with the findings of Li et al. including the NAFZ, as well as with the orientation of the FDPs from
(2014).Inanycase,theinter-stationdistanceinthepresentstudydoes SKSsplittinginthesamearea(Biryoletal.,2010).Tentatively,wecan
not allow for an estimate of the width of the damage zone, which also compare the results from this layer with the results obtained by
however affects only the stations located on top of active faults Vinniketal.(2016)fromtheanalysisofthek=2componentsofP-RF,
(Fig.8a).ThisisinaccordancewithLietal.(2014),wherethewidthof forthesamearrayofstationsusedinthisstudy.Usinganarray-aver-
thedamagezonearoundtheKaradere–DüzcebranchoftheNAFZwas aged RF dataset for the harmonic decomposition, these authors re-
estimated to be around 3km, as well as with Audet (2015), who re- ported intensity of horizontal-axis anisotropy between 3 and 5% (the
portedsimilarestimatesaroundtheSanAndreasFault. highestvalueintheirstudy)andfastdirectionbetween30°and60°,ina
Althoughourapproachdoesnotallowtoretrievethefullorienta- depthrangebetween35and60km.Ourresultsforhigh-energystations
tion of the symmetry axis in 3-D, our results imply a dominant con- atthesamedepthlevelareingoodagreement,atleastforthedirection
tribution from anisotropy with dipping symmetry axis, revealed from of anisotropy. Vinnik et al. (2016) argued that this sub-Moho aniso-
theanalysisofthek=1harmonics.Thisobservationsuggestsastrong tropyislikelyfrozenintheuppermantle.Ourresultssupportthisidea.
similarity between the deformation regime around the NAFZ and the
SanAndreasFault,wheretheanisotropicinversionofRFdatashoweda 5. Conclusions
substantialdip(about45°)oftheslowsymmetryaxisofanisotropyin
thecloseproximityofthemainfault(Audet,2015). In this paper we use teleseismic receiver functions to study the
Both the k=0 and k=1 harmonics in Fig. 7 and the spatial dis- crustalstructureandmapcrustalanisotropyaroundthecentralportion
tributionofenergyintheuppercrustinFig.8suggestanasymmetric oftheNorthAnatolianFaultZone.Weshowthatasimpleyeteffective
distribution ofelastic propertiesacross theNAFZ.Thisconfirmswhat harmonic decomposition of RFs can be used to reveal first-order fea-
discussedindetailbyŞengöretal.(2005)andLePichonetal.(2005). turesofthecrust,aswellastolocatethepresenceofanisotropy asa
Inparticular,thepresenceofabimaterialinterfacedevelopedasare- functionofdepth.
sultofthecumulativeproductionofrockdamageassociatedwiththe Ourfindingsindicatestrongasymmetryofcrustalpropertiesaround
faulting process (Ben-Zion and Sammis, 2003 and references therein) the North Anatolian Fault. The southern block possesses a complex
was reported in several studies (e.g. Bulut et al. 2012; Ozakin et al., crustalstructureandishighlyanisotropicwhencomparedtothesim-
2012;Najdahmadietal.,2016),alongdifferentsegmentsoftheNAFZ. plerandmoreweaklyanisotropiccrustalblockinthenorth.
Ourresultscorroboratethishypothesisinthestudyarea. The Moho is generally a continuous feature at around 40km
showingnomajoroffsetsacrossthearea.Ontheotherhand,theintra-
4.2.2. Middlecrust crustal structure is strongly affected by intense deformation in the
Between15and30kmdepth(Fig.8b),theintensityofanisotropy southernblock.
diminishesforsomeofthenear-faultstations,suggestingaprogressive Theanisotropicsignalisstrongestinthefirst15kmofthecrustand
closureoffracturesduetoincreasingconfiningstress,asobservedalso ispredominantlylocatedontopofthemajoractivefaultzones(NAFZ
onamoreeasternportionoftheNAFfromshearwavesplittingmea- and ESF). Here, the strike of the fast plane of anisotropy is fault-par-
surements(Lietal.,2014).However,afewnear-faultstationslocated allel,implyingastructure-inducedmechanismasthedominantsource
on the westernmost part of the study area (in correspondence of the for observed crustal anisotropy. Progressively increasing confining
NAF)andinthecentralpartoftheESFshowaconsiderableamountof stress around the faults contributes to the general observed trend of
energy.Thismaysuggestadifferentialclosureoffracturesaroundthe decreasinganisotropywithdepth.
faults, implying a depth extent ofthe damage zone below 15km (see Atmiddlecrustaldepths(15–30km),spotsofhighanisotropyinthe
depthprofilesAA′,BB′andDD′)forsomestations. southernblockmayhavemultipleinterpretations.Forstationscloseto
108