简介：AnumberofaftershocksoftheMay10th1997,Zirkuh(Ghaen-Birjand)destructiveearthquakehavebeenusedtoinvestigatetheanisotropyintheuppercrustbyobservingshearwavesplitting.Particlemotiondiagramandaspectratiomethodswereusedastwodifferentapproachestoobtainsplittingparameters.Clearshearwavesplittingwasobservedontherecordsoftheselectedaftershocks,indicatingthatthemediaintheregionwashighlyanisotropic.Byusingparticlemotionmethod,thedirectionoffastshearwavewasfound22°N±19°E,whilethedelaytimebetweenthefastandslowshearwaveswasobtainedtobe(65±16)ms.Byaspectratiomethod,thedirectionoffastshearwavewasdeterminedtobe35°N±18°Eandthedelaytimebetweenfastandslowshearwaveswasfoundtobe(49±10)ms.Forasimplehorizontallayerwithathicknessabout5kmanduniformlydistributedanisotropy,astressalignedcracksmodelwasusedandtheresultwasinterpretedintermsofverticalalignedcracksinthedirectionofN22°E,havingadensityabout0.01.Itisassumedthatcracksarefluid-filledsincetheyarelocatedintheuppercrust.Finally,byusingHudsoncracksmodelforthreecrackdensities0.005,0.01,0.03,thevelocitycurvesofshearwavewereplottedasafunctionofanglebetweenthesymmetricalaxisofcracksandtheazimuthofsourcetoreceiver.Itwasconcludedthatwhenshearwavewaspolarizedparalleltothecracksurface,thevelocitywasuniform,butthevelocitycurvevariedclearlyifshearwavewaspolarizedperpendiculartothecracksurface.