5. stereonets - school of ocean and earth science and ... · the blue circle marks this line....
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5.Stereonets
I MainTopicsAPlo;ngaplane
BPlo;ngaline
CMeasuringtheanglebetweentwolines
DPlo;ngthepoletoaplane
E Measuringtheanglebetweentwoplanes
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Plo;ngaPlane:Overview
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• Keyconcepts– Aninclinedplaneplots
alongagreatcircle.– Theendpointsofthe
cyclographictraceofaplanewithanon-zerodipareatdiametricallyopposedpointsontheprimiQvecircle;thesepointsdefinethelineofstrikefortheplane.
– VisualizaQonoftheplane.
PrimiQvecircle
Cyclographictraceofplane
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Plo;ngaPlane:Step1
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• Laytracingpaper(blue)overstereonet
• Intheexamplehere,theplaneplo[edwillstrike60°anddip50°
Plo;ngaPlane:Step2
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• TraceprimiQvecirclewithacompass
• AddQckmarksat0°,90°,180°,and270°forreference.
• LabeltheQckmarkat0°withan“N”torepresent“north”.
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Plo;ngaPlane:Step3
N
EQUAL-ANGLE NET(WULFF NET)
N
60°
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• PlotaQckmarkinontheprimiQvecircleinthedirecQonofthestrikeoftheplane
• Inthetheexamplehere,thestrikeis60°
Plo;ngaPlane:Step4
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• NowrotatethetracingpapersuchthattheQckmarkforthestrikeliesatthe“northpole”.Thisiswhereallthegreatcirclesconverge.
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Plo;ngaPlane:Step5
• Drawtheplanealongthegreatcirclewiththeappropriatedip(intheexamplehere,thesolidvioletcurveisaplanewithadipof50°).
• ThedashedconstrucQonlineshowsthestrikeoftheplane;itisshownhereforillustraQononly.Itdoesnotneedtobeplo[ed.
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Plo;ngaPlane:Step6
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• Removethestereonettoseetheresults
• Visualizetheresults,andchecktoseeiftheymakesense.
• Intheexample,– Thevioletcurverepresentsaplanethatstrikes60°anddips50°.
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Plo;ngaLine:Overview
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• Keyconcepts– AlineliesattheintersecQonoftwoplanes:• AverQcalplane(magenta)withastrikethatmatchesthetrendoftheline.
• Aninclinedplane(violet)withadipthatmatchestheplungeofthelineandthatdipsinthedirecQonthelineplunges
– VisualizaQon
Plo;ngaLine:Step1
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• Laytracingpaperoverstereonet
• Intheexamplehere,thelineplo[edwilltrend60°andplunge50°
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Plo;ngaLine:Step2
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• TraceprimiQvecirclewithacompass
• AddQckmarksat0°,90°,180°,and270°forreference.
• LabeltheQckmarkat0°withan“N”torepresent“north”.
Plo;ngaLine:Step3
N
EQUAL-ANGLE NET(WULFF NET)
N
60°
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• PlotaQckmarkinontheprimiQvecircleinthedirecQonofthetrendoftheline.
• Inthetheexamplehere,thelinetrends60°.
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Plo;ngaLine:Step4
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• NowrotatethetracingpapersuchthattheQckmarkatthetrendliesalongthesmallcirclethatprojectsasastraightline(i.e.,the“equatorialline”)
• ThedashedpinklinerepresentsaverQcalplanecontainingtheline
Plo;ngaLine:Step5• Markofftheplunge,counQng
fromtheprimiQvecircletowardsthecenteroftheplot.
• Thedashedvioletcurveisaplanewithadipthatmatchestheplungeoftheline.ThisplanedipsinthedirecQonthelinetrends,anditstrikesperpendiculartothetrendoftheline.
• ThelineofinterestisattheintersecQonoftheverQcalpinkplaneandtheplungingvioletplane.
• ThedashedconstrucQonlinesareshownhereforillustraQononly.Theydonotneedtobeplo[ed.
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Plo;ngaLine:Step6
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• Removethestereonettoseetheresults.
• Visualizetheresults,andchecktoseeiftheymakesense.
• Intheexample,– Theline(markedbythesmallred
circle)trends60°andplunges50°.– Thedashedpinklinerepresentsa
planethatstrikes60°anddips90°.– Thevioletdashedcurverepresents
aplanethatstrikes330°anddips50°towardsthenortheast.
– Theplanesintersectattheline.– Theplanes(dashed)areshownfor
illustraQonpurposesonly.Theytypicallywouldnotbeshownifonlythelineisoffinterest.
MeasuringtheAngleBetweenTwoLines
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• Keyconcepts– Theanglebetweenthelinesismeasuredalongthecyclographictraceoftheplanethatcontainsthelines.
– Theprocedureisexactlyanalogoustomeasuringtheanglebetweentwolineswithaprotractor.
PrimiQvecircle
“Coloredprotractorsofdifferentdip”
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MeasuringtheAngleBetweenTwoLines
• Plotthelines• Intheexample,onelinetrends78°andplunges36°;theredcirclemarksthisline.
• Theotherlinetrends146°andplunges49°;thebluecirclemarksthisline.
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MeasuringtheAngleBetweenTwoLines
• Findtheplanethatcontainsbothlines– Rotatethetracingpaper
suchthatbothlineslieonasinglegreatcircle.Thisrequirescare.
– Measuretheanglealongthegreatcirclebetweenthetwolines.Here,theangleis50°.
– Bycoincidence,thecommonplane(green)dips50°.
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MeasuringtheAngleBetweenTwoLines
• HereistheplotrestoredtoitsoriginalorientaQon.
• Thecommonplane(green)hasastrikeof40°.
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MeasuringtheAngleBetweenTwoLines
• Hereistheplotwithoutthestereonet– Checktoseewhethertheplotlookscorrect(i.e.,visualize).
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Plo;ngthePoletoaPlane
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• Keyconcepts– Thepoletoaplaneisaline
thatcanbeplo[edlikeanyotherline.
– ThepoletoaplaneofinterestliesinaverQcalplaneperpendiculartotheplaneofinterest.
– Thepolealsomakesa90°angle(asmeasuredintheverQcalplane)withrespecttothe“dipvector”oftheplaneofinterest.
Plo;ngthePoletoaPlane
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• Example• Consideraplaneofinterestthat
strikes330°anddips50°totheNE.Itisplo[edinblue.
• ItspolecanbefoundbysimplecalculaQons.Thepoletrends240°andplunges40°.Thisisplo[edattheredcircle.
• ThepoletoaplaneliesinaverQcalplaneperpendiculartotheplaneofinterest.
• Thepolealsomakesa90°angle(asmeasuredintheverQcalplane)withrespecttothe“dipvector”oftheplaneofinterest.
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MeasuringtheAngleBetweenTwoPlanes
• Keyconcepts– Theanglebetweentwo
planes(blueandred)istheanglebetweenthepolestotheplanes.
– Theanglebetweentheplanesismeasuredintheplane(green)containingthepoles.
– TheanglebetweentangentstothecyclographictracesonanequalareprojecQonalsogivestheanglebetweentheplanes,butdrawingthetangentsaccuratelyisdifficult.
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MeasuringtheAngleBetweenTwoPlanes
• Plottheplanesandthepoles
• Example– Theblueplanestrikes330°
anddips50°totheNE.– Theredplanestrikes30°
anddips20°totheSE.– Thebluepoletrends240°
andplunges40°totheSW.– Theredpoletrends300°
andplunges70°totheNW.
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MeasuringtheAngleBetweenTwoPlanes
• Measuretheanglebetweenthepolesintheplanecontainingthepoles– Rotatethetracingtofind
thecommonplane(green)thatcontainsthetwopoles.
– Theanglebetweentheplanesismeasuredintheplane(green)containingthepoles.
– Theangledeterminedgraphicallyis43°(measuredtothenearestdegree).
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MeasuringtheAngleBetweenTwoPlanes
• Appearanceofplotwithoutstereonet– Theplotisbusy.– TheanglebetweentangentstothecyclographictracesonanequalareprojecQonalsogivestheanglebetweentheplanes,butdrawingthetangentsaccuratelyisdifficult.
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MeasuringtheAngleBetweenTwoPlanes
>>Tr=300*pi/180;>>Tb=240*pi/180;>>Pr=70*pi/180;>>Pb=40*pi/180;>>[bx,by,bz]=sph2cart(Tb,Pb,1);>>[rx,ry,rz]=sph2cart(Tr,Pr,1);>>blue=[bx,by,bz];>>red=[rx,ry,rz];>>angle=acos(dot(blue,red))*180/piangle=42.6907
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Accuracy
ThisangleisconsistentwiththegraphicalsoluQon