Intraplateregions
Surveetal.(2021)performedseismichazardstudiesfortheMumbaicity(financialcapitalofIndia),havingapopulationofover18million.Twoseismicitymodels,linearandareal,wereusedtocomputetheseismichazardofMumbaiwithanupdatedearthquakecatalogueandlatestknowledgeonseismotectonicsoftheregion.ThehazardvaluesforMumbaicorrespondingto475-yearand2475-yearreturnperiodsarecomputed.Hazardmapsatbedrocklevelfor2%and10%probabilityofexceedancein50yearswereprepared.Thehazardlevelsobtainedinthepresentstudyarelowerthanthosereportedforthesameareabypreviousresearchers.Thelowerseismichazardcanbeattributedtothefactthatinthisstudy,theKoyna–Warnaregionisusedasoneofthefiveindependentsourceregionsandreservoirtriggeredseismicityattimesmightreducetheoverallseismichazardinnearbyregions.MandalandAsano(2019)modeledthelow-frequency(0.1–1 Hz)groundmotionsexcitedbythe2006-04-06event,usingthefinitedifferencemethodassumingapointsource,toassesstherobustnessoftheconstructedvelocitystructuremodel.Atmostofthestations,theobservedandsimulatedvelocitywaveformsarefoundtobeingoodagreementintermsofbothamplitudeandgroundmotionduration.Theyalsocomputedsyntheticgroundvelocitiesatnumerouslocationswithinthebasin,forboththe2001Bhujmainshock(finite-faultsource)andthe2006aftershock(pointsource)cases,usinga3-Dvelocitymodel.TheirworkrevealedthatthepresenceoflowvelocitysedimentswithintheKachchhriftbasinplaysakeyroleinmodifying/amplifyingthegroundmotionsin0.1–1.0 Hzrange.Thestudyoftwoearthquakes(2006-03-07and2006-04-06,Mw5.5)intheKachchhseismiczonebyMandal(2020)revealedthattheestimatednormalizedresponsespectraatstrongmotionaccelerographsitesintheTertiaryformationsornearazoneofgeologicalcontactbetweentheJurassic/Tertiaryformations,exceededthedesignresponsespectraintheperiodrangeof0.07–0.2s,correlatingwiththecompletecollapseoflow-risebuildings,watertanksanddamsduringthe2001Bhujearthquake.Ontheotherhand,thenormalizedaccelerationspectrumofcorrespondingtohardsediments(rocksite)isfoundtonotexceedthedesignspectrum,correlatingwiththelackofdamageintheMesozoichillzone.ItisalsonoticedthatthespectralaccelerationvaluesatafewsiteslyingontheQuaternaryformationshaveexceededthedesignspectraat3–4s,suggestingthesesitestobehazardousforengineeredreinforcedstructureslikebridges.Ararelowercrustalearthquakeoccurredon2021-07-25nearHyderabad,India.Mandaletal.(2022)usedwaveformsfrom9broadbandstationsandcomputedtheaveragecornerfrequency,seismicmoment,momentmagnitude,stressdrop,andsourceradiusas3.87Hz,7.14E+14N-m,3.75,3.92MPa,and229m,respectively.ThemeancrustalQfortheregionwasmodeledtobe2182±1178,suggestinglowercrustalattenuationbelowtheHyderabadregion.However,thespatialdistributionofthemodeledcrustalQvaluesrevealedahighQzonetotheeastofHyderabadcity,whileamoderateQzonewasfoundwestofthecity.Itisinferredthatthislowercrustalintraplateearthquakewithalargestressdropof3.9MPamighthavebeengeneratedduetothesuddenmovementonanalmostverticalfaultduetohighpore-fluidpressurecausedbythepresenceofCO2-richmantlefluids.Srinageshetal.(2021)analyzedasequenceofabout965earthquakesinthemagnituderangeof0.1–4.6.Themainshockofmoderate-sizedearthquake(2020-01-26,ML~4.6)islocatedinthePalnadusub-basinoftheCuddapahbasin.ItwasfeltbothinthestatesofTelanganaandAndhraPradesh.TheearthquakespriorandaftertheML4.6quakearelocatedclosetothethrustandalongtheperipheryofthebackwatersofthePulichintalareservoir.Theepicentralparametersobtainedfromdoubledifferencetechnique,usingaminimum1-Dvelocitymodel,illuminatedasteepseismogenicstructureextendingdownto8kmdepth,.Theb-valueestimateis0.82foracompletenessmagnitudeofMc1.8andcouldbeassociatedwithanintraplateeventhavingalongerrecurrencetime.Thefocalmechanismsolutionobtainedfromwaveforminversionrevealsapuredouble-couplemechanismofastrike-slipmotionwithareversecomponentonaN–Strendingfocalplane.SharmaA.,etal.(2021)derivedanewMLscale,usingtheGreyWolfOptimization,aswarmintelligence-basedglobaloptimizationtechnique,forthefirsttime,thatisML=logA+1.2588logR+0.0002789R-2.2265,whereAistheamplitudemeasuredinmillimeters,andRisthehypocentraldistanceinkm.ThenewMLscalederivedhereisvalidupto400km.Thenewlyderivedscalehasadropof21.37%intheoverallstandarddeviationofallmagnituderesidualswhenstationcorrectionsareconsideredincomparisontothepreviouslyusedscale.Sivaram(2021)simulatedhigh-frequencygroundmotionsatfivestationsintheNationalCapitalRegion(NCR)ofIndiaforalargehypotheticalMw8.5earthquakeandanintermediateMw6.8earthquakeintheHimalayancentralseismicgap,atfault-distancesofabout200–300km,andindicatedthatthefar-reachingandadverseground-motionintensitiesmightaffectintermediate-highrisestructures(period0.4–0.8s)intheNCRduetothepredominanceoffluvialdeposits.Nagamanietal.(2020)identifieddifferentzonesofseismicamplificationintheSuratdistrictofGujarat,India,whicharethehubofmanyminingandindustrialprojectslikeoilandnaturalgas.StrongmotiondatafromthenetworkoperatedbyISRhavebeenusedinseveralengineeringseismologyapplications,suchas,estimationofsourceparameters,sitecharacterization,developmentofgroundmotionpredictionequationandgroundmotionmodeling.ThedatarecordedfromthepastdecadehavebeenusedtocharacterizevarioussourcesintheKachchhriftandotherpartsofGujarat(Kamraetal.,2020).Itisfoundfromthestudythatstressdropofearthquakes(M4.0-5.1)intheKachchhriftareintherange2.3-10.4MPawithanaverageof5.3MPa.Theestimatedseismicmomentandthesourceradiusareintherange\(1.02\times 10^{22}-5.3\times 10^{23}\)dyne-cmand0.43-1.32km,respectively.ThesameexercisewascarriedoutfortheSaurashtraregion(Kamraetal.,2021).Forthisregion,thestressdropvariesintherange0.9-6.9MPawithanaverageof3.3MPa.AregressionrelationshipbetweenobservedaccelerationsandaccelerationsestimatedfrombroadbanddatahasbeendevelopedexclusivelyfortheGujaratregion(Chaudharyetal.,2022).
StudyofEarthquakeSwarms
Srinageshetal.(2020)studiedearthquakeactivityinthePalgharregion,Maharashtra,India.Until31August2019,atotalof4854earthquakeshavebeenlocatedhere,whoselocalmagnitude(ML)variesfrom0.1to4.1.Majorityoftheearthquakes(~94%)werelocatedinthedepthrangeof4–16km.Thepreciseearthquakerelocationsrevealtwoclusters.TheN–Strendingclusternorthof20.04°Nextendstoadepthof10km,whereastheNE–SWtrendingclustertothesouthof20.04°Nextendsto16kmdepth.Theshallownorthernclusterisnoticedtobesandwichedbetweentwomappedmaficintrusions,whereasthedeepersouthernsegmentshowsearthquakesclusteringaroundthemaficintrusion.Themodeledcompositefocalmechanismsolutionsforboththenorthandsouthclusterssuggestnormalfaultingwithaminorstrike–slipcomponentasthedominantdeformationmodeforthePalgharregion.Fromrelocatedseismicity,theydetectedadeeperseismicallyactivezone(withM>3)at4–16kmdepth,occupyingacrustalvolumeof1440km3.Mandalet.al.(2021b),conductedacomprehensiveanalysisofswarmactivitiesintworegionsoftheIndianshield–(i)Palghar(Maharashtra)and(ii)Pulichintala(AndhraPradesh).The3-Dmappingofb-valueandfractalcorrelationdimension(D2)revealsthatthePalgharsequencefollowstypicalcharacteristicsofswarmactivity(thebandD2valuesvaryfrom0.1to2.5,and0.39to2.62,respectively).Onthecontrary,thePulichintalasequence(withbandD2valuesvaryingfrom0.2to1.68,and0.68to3.0,respectively)showsnegativecharacteristics.ThePalgharregionisinterpretedasaregionofhighertectonicstresses.SwarmactivitysimilartothatatPalgharinMaharashtra(Maheshetal.,2020),hasbeenobservedinNavsariandJamnagardistrictsinSouthGujaratandSaurashtra.AtNavsari,aswarmactivitywasobservedaroundtheKeliyadamfromSep.2016justaftertheIndianmonsoonperiodthatcontinuedforabout4months.Again,theswarmactivityrecurredinAug.2017andcontinuedforabout5months.AlocalnetworkoffourstationswasinstalledbyISRtomonitortheswarmactivity,inadditiontotheGujaratstateseismicnetwork(Sateesh,etal.,2019;Srijayanthietal.,2022).Atotalof1048earthquakeswerelocatedaroundtheKeliyadamand229eventsintheDadraandNagarHaveli(DNH)regionfromSep.2016toJune2018.Theseismicityinboththeregionsfolloweda~NW–SEtrend.Itwasconfinedtoanareaof13km×2kmwithadepthextentof3kmatNavsariand15km×2kmwithdepthof6kminDNH.IntheJamnagardistrict,ISRobservedpost-monsoonswarmactivityinSep.2019with76clusteredearthquakeshavingNW-SEtrendthatareinlinewiththestrikeoflocallineamentsanddykes.Parija(2021)criticallyexaminedthe2011-09-18SikkimearthquakeofM6.9andfoundittobeassociatedwithepisodesofprecursoryswarms,quiescence,mainshockandaftershocks.Theprecursoryswarmandquiescenceperiodconsistoffourearthquakeswarmsandoneforeshockeventofmagnitude(mb≥4.5)intheepicenterpreparatoryzoneofthe2011Sikkimearthquake.The2011Sikkimearthquakehadaboutfiveaftershocksofmagnitude(mb)≥4.5between2011and2014forthesameregion.Parvinetal.(2021)analyzedtwoswarmactivitiesintheHyderabadregionandfoundsmallabsolutestressdropvalues(<1MPa)andapositivecorrelationbetweenthestaticstressdropandthemagnitudeoftheearthquakewithseismicmomentvaryingbetween-0.09<ML<1.52.Theyobservedaclearcorrelationofearthquakesassociatedwiththefracturesandfaultsinthevicinityofwaterbodieswhicharemoresensitivetovariationsinhydrostaticpressurescausedbyverticalflowrechargefromrainfallanddeeperpore-fluidpressurediffusion.RekapalliandGupta(2021)triedtounderstandtheforeshock-aftershockpatterns,mainshocktothelargestaftershockmagnituderatio,anddifferenceinmagnitudeoftwomoderateinjection-inducedseismicity(IIS)earthquakesequencesfromOklahoma,USA,namely,Prague(M5.7,2011)andPawnee(M5.8,2016),andcomprehendtheshallowcrustalheterogeneity.Theanalysisoftemporalvariationof“b”valuefrom2002to2018suggestsanincreaseinb-valueafter2009.Areductioninb-valueafter2016,inresponsetothereducedinjectionvolumesisnoted.Asharpfallinb-valueusuallyprecedesthemainshocksofmagnitudeM≥3.5.Theforeshockb-valuesarelowerthantheregionalb-valueandaftershockb-valuesarehigherthantheregionalb-valuewithintheerrorlimits.TheinvestigatedearthquakesequencesfallunderType2ofMogi’smodel.ThecharacteristicsofISSobservedatOklahomaaresimilartotheobservationsforthereservoir-triggeredseismicity(RTS).However,withmultipleinjectionwellsoperatingfromtimetotimeintheregionwithvaryingamountoffluidsinjected,theentireIISatOklahomahasanappearanceofaswarm.Wadhawanet.al.(2021)studiedahighlyclusteredshallow(<0.4km)earthquakeactivityoflowmagnitudewithaccompanyingrumblingsoundinSadrabadiandZilphivillagesinDharniTalukaoftheAmravatidistrict,Maharashtraduringthemonsoonperiodof2018andfoundittohavethecharacteristicsofaswarm.Theyfoundastrongcorrelationbetweenrainfallandswarmactivityandcategorizeditashydro-seismicity,resultingfromhydro-fracturingofthesoil/weatheredbasaltandcollapsingandcavingoftherocks.Inthepast,nosuchactivityhasbeenreportedfromtheregionduringorafterthemonsoon,despitethefactthattherewasmorerainfallin2019.Therefore,theysuggestthatthelowmagnitudeearthquakeswarmataveryshallowdepthmighthavebeeninducedbythepercolationofmonsoonalrainwaterthroughtheweatheredandfracturedrock-massassociatedwiththefaultsystemoftheNarmadaSonfailedriftregion.
EarthObservationforCrustalTectonicsandEarthquakeHazards
Elliottetal.(2020)illustratedthecurrentmethodsfortheexploitationofdatafromEarthObservingsatellitestomeasureandunderstandearthquakesandshallowcrustaltectonics.TheaimofapplyingsuchmethodstoEarthObservationdataistoimproveourknowledgeoftheactivefaultsourcesthatgenerateearthquakeshakinghazards.ExamplesoftheuseofEarthObservation,includingthemeasurementandmodellingofearthquakedeformationprocessesandtheearthquakecycleusingbothradarandopticalimageryareprovided.Theyalsohighlightedtheimportanceofcombiningtheseorbitingsatellitedatasetswithairborne,insituandground-basedgeophysicalmeasurementstofullycharacterizethespatialandtimescaleoftemporalscalesofthetriggeringofearthquakesfromanexampleofsurfacewaterloading.Finally,theyconcludedwithanoutlookontheanticipatedshiftfromthemoreestablishedmethodofobservingearthquakestothesystematicmeasurementofthelonger-termaccumulationofcrustalstrain.
ContributionsofSpaceMissionstoBetterTsunamiScience:Observations,ModelsandWarnings
GlobalNavigationSatelliteSystem(GNSS)datahaveakeyroleinbetterdescribingthegrounddeformationfollowingatsunamigenicearthquakeclosetothecoast.TheGNSSobservationscomplementseismologicaldatatoconstraintherupturemodelrapidlyandrobustly.InterferometricSyntheticApertureRadar(SAR)alsocontributestothisfield,aswellasopticalimagery,relevanttomonitoringelevationchangesfollowingsubaeriallandslides.Theobservationofthesea-levelvariations,inthenearfieldandduringthepropagationacrosstheocean,canalsoincreasinglybenefitfromGNSSdata(fromGNSSbuoys)andfromrobustsatellitecommunication:pressuregaugesanchoredontheseafloorinthedeepoceancontributetowarningsystemsonlybydatacontinuouslytransmittedthroughsatellites.ThesoundingofionosphericTotalElectronContent(TEC)variationsthroughGNSS,altimetry,oraground-basedairglowcamera,isapromisingwaytorecordtsunamiinitiationandpropagationindirectly.Finally,GNSS,opticalandSARimageryareessentialtomapandquantifythedamagefollowingtsunamiflooding.Satellitedataareexpectedtocontributemoretooperationalsystemsinthefutureprovidedtheyarereliablyavailableandanalyzedinrealtime(Hebertetal.,2020).
AftershockDurationofStrongtoMajorHimalayanEarthquakes
EarthquakesofM≥5tendtobelocallydamaging,specificallywhenthesearetheaftershocksoflargerearthquakes,asthemainshockwouldhaveweakenedthestructures.Fortherescueoperationsandgeneralwell-beingoftheresidents,itishelpfulifanestimateisavailableastohowlongM≥5aftershockswouldcontinuetooccur.EarthquakesM≥6.5tendtobefollowedbyaftershocksofM≥5.InthisstudybyGuptaandRekhapalli(2022),aftershocksequencesofsevenearthquakesofmagnitudeM≥6.5wereanalyzed.SixamongtheseareintheHimalayanregionandtheremainingoneisinthenearvicinityinChina.TheanalysissuggeststhatthenumberofM≥5aftershocksandthedurationoftheiroccurrencedecreasewiththedecreaseofthemainshockmagnitude.Forthe2008SichuanearthquakeofM7.9therewere136M≥5aftershocks,whilefor1975KinnaurearthquakeofM6.8therewereonly9.TheaftershockdurationoftheHimalayanregionearthquakesobeystheexponentiallaw\(T=Ae^{\text{cM}}\),wheretheAandcareconstantsassociatedwithregionalfaultsettings.ThisrelationishelpfulinprovidinganestimateofthetimeforwhichM≥5aftershockactivitywouldcontinueaftertheoccurrenceofM≥6.5earthquakes.
MicrozonationStudies
CSIR-NGRIpreparedafirst-cutEarthquakeDisasterRiskIndex(EDRI)maptocapturetherelativeriskacrossLucknowandDehraduncities.ThesemapswerehandedovertotheStateDisasterManagementAuthorityofrespectiveStates.CSIR-4PIcarriedoutmicrozonationofSrinagarregionoftheKashmirValley(GuptaS.V.,etal.,2020,2022).Theyconductedanextensivehigh-resolutionmicrotremorambientnoisesurveyat429locations.TheacquireddatasetwasprocessedusingtheHorizontaltoVerticalSpectralRatio(HVSR)techniquetomaptheresonancefrequency,thethicknessofsedimentarycoverandtoidentifyareaspronetoseismicamplification.TheHVSRcurvesshowthepeaksintherangeof0.22Hzto9.96Hzindicatingheterogeneousandcomplexsedimentarycoverintheregion.InversionoftheHVSRcurvesgivestheshearwavesvelocitydistributionwhichhighlightstwodistinctreflectivesurfacesinmostoftheareas.Theyalsousedtheestimatedfundamentalfrequencyofvarioustypesofhouses/buildingslocatedinSrinagarcitytoassessthepossibilityofresonanceincaseofoccurrenceofanyearthquake.ISRhasundertakensite-specificseismichazardassessmentandmicrozonationstudiesforareasofrapidgrowth,e.g.SpecialInvestmentRegions,SpecialEconomicZones,largeandtallstructures,andindustrialhubs.TheseincludeLiquifiedNaturalGas/LiquidPetroleumGasstoragetanksatMundra,Dhamra(Odisha),DadraandNagarHaveli,cablestayedbridgeatZuariRiver,Goa,andIndianOilCorporationLtd.BongaigaonRefinery.TheInstitutehascompletedseismicmicrozonationofBhujcityunderaMinistryofEarthSciences(MoES)sponsoredproject.Inaddition,ithasundertakenseismicmicrozonationstudyofAmritsar,Meerut,Agra,Lucknow,Kanpur,Varanasi,PatnaandDhanbadtownsunderaprojectsponsoredbyMoES.Theconceptofseismicvulnerabilityisayard-stickofdamageestimationfromaprobableearthquake,consideringphysicalcumsocialdimensionandenablesabasisfordecision-makerstodeveloppreparednessandmitigationstrategies.Baruahetal.(2020)usedseveralparameters,e.g.shearwavevelocitycharacteristics,geomorphology,slopeangle,buildingtypology,andthenumberofoccupants,toestimatethedimensionofvulnerabilityfortheShillongcity.Basedonthisstudy,theyinferredthatmorethan60%ofShillongcityfallsundermoderatetohighvulnerabilityandtherestislessvulnerable.