Rāwene Community Newsletter
4
17 NOVEMBER 2016
The Moon, Tides & Earthquakes
Thisweek’s“supermoon”andtheKaikouraearthquakesparked
somediscussionovertheroleofthemoonincreatingbothtides
andearthquakes.
Tides
• Thegravitationalforceofthemoonisoneten-millionththatof
earth,butwhenyoucombineotherforcessuchastheearth's
centrifugalforcecreatedbyitsspin,yougettides.
• Thesun'sgravitationalforceontheearthisonly46percentthat
ofthemoon,makingthemoonthesinglemostimportantfactor
forthecreationoftides.
• Thesun'sgravityalsoproducestides.Butsincetheforcesare
smaller,ascomparedtothemoon,theeffectsaregreatly
decreased.
• WhenevertheMoon,EarthandSunarealigned,thegravitational
pullofthesunaddstothatofthemooncausingmaximumtides.
• Springtideshappenwhenthesunandmoonareonthesameside
oftheearth(NewMoon)orwhenthesunandmoonareon
oppositesidesoftheearth(FullMoon).
• WhentheMoonisatfirstquarterorlastquarterphase(meaning
thatitislocatedatrightanglestotheEarth-Sunline),theSunand
Mooninterferewitheachotherinproducingtidalbulgesand
tidesaregenerallyweaker;thesearecalledneaptides.
• Springtidesandneaptidelevelsareabout20%higherorlower
thanaverage.
• Offshore,inthedeepocean,thedifferenceintidesisusuallyless
than1.6feet(49cm).
• Thesurfgrowswhenitapproachesabeach,andthetide
increases.Inbaysandestuaries,thiseffectisamplified.
• Becausetheearthrotatesonitsaxisthemooncompletesone
orbitinourskyevery25hours(nottobeconfusedwithmoon's
27-dayorbitaroundtheearth),wegettwotidalpeaksaswellas
twotidaltroughs.Theseeventsareseparatedbyabout12hours.
• SincethemoonmovesaroundtheEarth,itisnotalwaysinthe
sameplaceatthesametimeeachday.So,eachday,thetimesfor
highandlowtideschangeby50minutes.
http://home.hiwaay.net/~krcool/Astro/moon/moontides/
Earthquakes
ItispossiblethattheMooncanbeafactorintriggeringearthquakes.
TheUSGeologicalSurveysays:
Severalrecentstudies,however,havefoundacorrelationbetween
earthtides(causedbythepositionofthemoonrelativetotheearth)
andsometypesofearthquakes.Onestudy,forexample,concludes
thatduringtimesofhigherearthandoceantides,suchasduring
timesoffullornewmoon,earthquakesaremorelikelyonshallow
thrustfaultsneartheedgesofcontinentsandin(underwater)
subductionzones.Earthtides(Earth'ssurfacegoingupanddownby
acoupleofcentimetres)andespeciallyoceantides(surfaceofthe
oceangoingupanddownbyameterormore)raiseandlowerthe
confiningpressureonshallow,dippingfaultsnearcontinentaledges
andinsubductionzones.Whentheconfiningpressureislessened,
thefaultsareunclampedandmorelikelytoslip.
USGSnotes,however,thatthereisonlya“tiny”probabilityoftides
affectingearthquakesinanygivenplaceinanygivenyear.
https://www2.usgs.gov/faq/categories/9827/3354
Dave West P O Box 73 Rāwene 09 4057 734 / 09 4057 779
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