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1、海啸是如何产生的HowTsunamisWork?（TED-Ed）In479BC,whenPersiansoldiersbesieged(包围)theGreekcityofPotidaea,thetideretreated(撤退)muchfartherthanusual,leavingaconvenientinvasionroute.Butthiswasn’tastrokeofluck（运气）.Beforetheyhadcrossedhalfway,thewaterreturnedinawavehigherthananyonehadeverseen,drowningtheattacker
2、s.ThePotiidaeansbelievedtheyhadbeensavedbythewrathofPoseidon（海神波塞冬的愤怒）.Butwhatreallysavedthemwaslikelythesamephenomenonthathasdestroyedcountlessothers:atsunami.Althoughtsunamisarecommonlyknownastidalwaves,they’reactuallyunrelatedtothetidalactivitycausedbythegravitationalforcesoftheSunandMoon.Inm
3、anyways,tsunamisarejustlargerversionsofregularwaves.Theyhaveatrough(/trɔf/波谷)andacrest(波峰),andconsistnotofmovingwater,butthemovementofenergythroughwater.Thedifferenceisinwherethisenergycomesfrom.Fornormaloceanwaves,itcomesfromwind.Becausethisonlyaffectsthesurface,thewavesarelimitedinsizeandspeed
4、.Buttsunamisarecausedbyenergyoriginatingunderwater,fromavolcaniceruption,asubmarinelandslide（海底滑坡）,ormostcommonly,anearthquakeontheoceanfloorcausedwhenthetectonicplatesoftheEarth'ssurfaceslip（地球板块滑动）,releasingamassiveamountofenergyintothewater.Thisenergytravelsuptothesurface,displacing(排开)watera
5、ndraisingitabovethenormalsealevel,butgravitypullsitbackdown,whichmakestheenergyrippleoutwardshorizontally.Thus,thetsunamiisborn,movingatover500milesperhour.Whenit'sfarfromshore,atsunamicanbebarelydetectablesinceitmovesthroughtheentiredepthofthewater.Butwhenitreachesshallowwater,somethingcalledwa
6、veshoalingoccurs.Becausethereislesswatertomovethrough,thisstillmassiveamountofenergyiscompressed.Thewave'sspeedslowsdown,whileitsheightrisestoasmuchas100feet.Thewordtsunami,Japanesefor“harborwave,”comesfromthefactthatitonlyseemstoappearnearthecoast.Ifthetroughofatsunamireachesshorefirst,thewater
7、willwithdrawfartherthannormalbeforethewavehits,whichcanbemisleadinglydangerous.Atsunamiwillnotonlydrownpeoplenearthecoast,butlevelbuildingsandtreesforamileinlandormore,especiallyinlow-lyingareas.Asifthatweren'tenough,thewate
