brazilseismicity book ipe chapter3

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1 Chapter3 versionAugust13,2013 IntraplateSeismicityinBrazil MarceloAssumpção 1 ,JoaquimFerreira 2 ,LucasBarros 3 ,HilarioBezerra 2 ,George SandFrança 3 ,JoséRobertoBarbosa 1 ,EduardoMenezes 2 ,LuísCarlosRibotta 4 , MarlonPirchiner 1 ,AdersondoNascimento 2 ,JoãoCarlosDourado 5 1 UniversityofSãoPaulo,SãoPaulo,SP 2 FederalUniversityo fRioGrandedoNorte,Natal, RN 3 UniversityofBrasília,Brasília,DF 4 IPT-TechnologicalRese archInstitute,São Paulo,SP 5 UNESP-São PauloStateU niversity,RioClaro,S P  Abstract Wedescribethe developmentoft heBrazilianearth quakecatalogu eandthedistribut ionof seismicityinBrazilandneighbo uringareasinmid-plate SouthAmerica.Th islargemid-plate regionisoneofthe leastseismicallyactivestabl econtinentalregions(S CR)intheworl d:the maximumknownear thquakehada magnitudeof6.2 mbandeventswithmagnitudes5and aboveoccurwitha returnperiodo ffouryears.Se veralseismiczonescanbe delineatedin Brazil,includingsome alongcratonedgesa ndinsedimentarybasins.Ove rall,theexpose d cratonicregionstendto havehalfasma nyearthquakesco mparedtothea verageexpected rateforallof mid-plateSouth America.Earthqu akestendtooccurinN eoproteroz oicfoldbelts especiallyinareasof thinlithosphere, ornearcratoned gesaroundcraton ickeels.Areaswith positiveisostaticgravityanomaliestendtohave moreearthqua kes,indicatingthatflexu ral stressesfromuncomp ensatedlithosph ericloadsareanimpo rtantfactorinexplaining the intraplateseismicity.We alsofoundtha tearthquakesar etwotothree timesmorelikelyto occurwithin20kmo fmappedneotecto nicfaults,compare dtoeventsatlarge rdistances.On closerexamination,however,weob servethatmostof theseeventsoccurne arbutnotdirectly onthemajorneo tectonicfaults.Thisdiscre pancycouldbeexpla inedbythemodel ofstress concentrationnear intersectingstructures.Th eBrazilianpassivemar ginisalsoaregionof higherthanave rageseismicity.Although cleardifferencesa refoundbetwe endifferentarea s alongthepassivema rgin(extendedcrustinS EBrazilhavingespe ciallyhighseismicity comparedtoth incontinentalshelvesel sewhere),over alltheBrazilianp assivemarginshas 70%moreea rthquakes(magnitu desabove3.5)th antheaveragest ablecontinentalregio n. Chapter3of “IntraplateEart hquakes”,Ed.P.Ta lwani,CambridgeU P,2014

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