SplittinganalysisoflocalearthquakewaveformsintheDoda-Kishtwarregion,NWHimalayaresultsin47individualshearwavesplittingmeasurements(Royetal.,2021b).Thefastpolarizationazimuths(FPAs)primarilyshowtwodistinctpatternsorientedalongENE-WSWandNW-SEdirections.BoththepatternsareintheChambasequencesuggestingtwodeformationpatterns.Forthefirstpattern,bothstationsandeventsarelocatedintheproximityoftheChenabRiverwhereasforthesecondpatterntheFPAsareparalleltothestructuraltrendoftheChambasequence.TheFPAsareeitherperpendicularorsub-paralleltothemaximumhorizontalstress(SHmax),suggestingstructure-inducedanisotropy.Apossiblereasonfortheobservationofthefirstpatternisthattheshearwavesamplesthefluid-filledfracturesinthefaultzonesresultinginFPAsparalleltotheChenabRiver,withlargedelaytimes.TheextensionaltectonicstructuresoftheNWHimalayacouldexplainthesecondpatternofanisotropy.
ArunachalHimalaya
ShearwavesplittinganalysisbeneathArunachalHimalayausingwaveformsof396localearthquakesrecordedat32stationsresultedin76wellconstrainedsplittingmeasurements(Nanajeeetal.,2022).Thedelaytimesvaryfrom0.02to0.30s,andareclusteredaround0.07s.ThereisasignificantvariationintheorientationofFPAs.ThewesternpartofArunachalHimalayaisassociatedwithsmallerdelaytimes(mostly<0.10s)andhaslargevariationintheFPAs.TheFPAsmostlyvaryfromE-WtoNNW-SSEalongthewesternmostprofile.Theobservedanisotropyisassociatedwithheterogeneitiesinthelithologicalproperties,andtheanisotropyisbothstress-inducedandstructure-induced.ThereisasmallcrustalblockinthecentralpartofArunachal,inwhichtheFPAsareparalleltothestrikeoftheHimalayanarcandareassociatedwithstructure-inducedanisotropy.IntheeasternpartoftheArunachalHimalaya,avariationintheorientationofFPAsisobservedfromnorthtosouth.Inthenorth,theFPAsareparalleltothestrikeoftheSiangRiverintheeasternHimalayasyntaxis,suggestingstructure-inducedanisotropy.Whileinthesouth,theFPAsaremostlyparalleltoSHmax,suggestingstress-inducedanisotropy.
SoutheasternTibet
TiwariA.K.,etal.(2022a)obtainedadepth-dependentcrustalanisotropicsignaturebeneathSETibetusingdirectionaldependenceofreceiverfunctions.Theobtaineduppercrustal(0-20km)anisotropicorientations,whichareorthogonaltomajorfaultsandsuturezone,suggeststructureinducedanisotropybeneaththeregion.Theanisotropicorientationsofthemiddle(20-40km)andlower(40-70km)crustsuggestductiledeformationduetocrustalflowbeneaththeregion.ThisstudyalongwithpreviousSK(K)Sanddirect-Ssplittingmeasurementssuggestspartialcouplingbetweenthecrustanduppermantlebeneaththeregion.
ShillongPlateau-MikirHill,NEIndia
Datafroma17-stationbroadbandseismicnetworkhavebeenusedtostudycorrelationbetweenpolarizationdirectionofcrustalanisotropywithseismogenicstressfieldatdifferentlocationsoftheShillong-MikirPlateauanditsvicinityinnortheastIndia(Baruahetal.,2021).Thestressfieldhasbeendeterminedaroundthestationsusingfocalmechanismsolutions(FMS)bywaveforminversion.Itisobservedthatpolarizationdirectionofcrustalanisotropyisconsistentwiththatofthemaximumhorizontalstressaswellastheminimumhorizontalstress.Inaddition,twoorthogonalfastpolarizationsarealsonotedinsomelocations.Thebivariatenatureofsuggeststhatthemajormechanismsofseismiccrustalanisotropyarenotonlyduetotheregionalstress,butactivefaultsandothergeologicalconditionsplayasignificantroleincontemporaryorientationofseismiccrustalanisotropyandseismogenicstressfield.EarthquakesourcemechanismsobtainedthroughwaveforminversionrevealthatthecloselyspacedMishmi,Tidding,andLohitfaultsaresteeplydippingthrustsheets(dip~50º)thataccommodatelargecrustalshortening,owingtotheindentationprocessandclockwiserotationtectonics.TheWalongfaultischaracterizedbystrike-slipmotionwhichhelpstofacilitatetheclock-wiserotationofcrustalmaterialaroundthesyntaxis(Hazarikaetal.,2022).RadialanisotropyinterpretedwithintheEasternHimalayaSyntaxis(EHS)andtheIndo-BurmeseRangesbasedonsurfacewavedispersionrevealsstrongeranisotropyinthedeeperpartbelow~40kmdepth(Chanuetal.,2022).
UpperMantleDeformation
- Ladakh–Karakoramzone(LKZ)
SeismicanisotropyoftheuppermantlebeneaththeeasternLKZandnorthwestHimalayahasbeeninvestigatedbasedonsplittinginSKSwaveformsrecordedat28broadbandseismicstations(Fig.7).InthefrontalpartoftheHimalaya,theFastPolarizationDirections(FPDs)aremostlyparallelorsub-paralleltothestrikeoftheHimalayanorogenysuggestingdeformationintheshallowlithosphericmantleundercompressionowingtotheIndia-Asiacollision.Ontheotherhand,FPDsobservedintheLesser,Higher,andTethyanHimalayalargelyfollowtheNE-orientedabsoluteplatemotion(APM)oftheIndianplatewhichcanbeattributedtobasalshearastheIndianplatemovesabovetheasthenosphericmantle,withaminorcontributionfromshallowlithosphericdeformation.AcomplexanisotropypatternisobservedintheIndusSutureZone.TheFPDsneartheKarakoramFaultZoneareparallelorsub-paralleltoitsstrike.ThestudysuggeststhatKFextendsuptothelithosphericmantleaccommodatingtheIndia-AsiacollisionandfacilitatingextrusionintheTibetanPlateau(PaulA.,etal.,2021).