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1、Today,ourlectureisabouttidalwaves.I’lldefinewhatatidalwaveisandwhatatidalwaveisnot.I’llalsodefinesomeothertermsrelatedtotidalwaves.So,whatisatidalwave?Atidalwaveisaverylargeandverydestructivewallofwater.Thiswallofwaterrushesinfromtheoceantowardthelan
2、d.ScientistsusetheJapaneseword”tsunami”todescribethiswaves.InJapanese,tsunamiactuallymeansharborwave,youseethewavesaretallestwhentheyreachaharborwhichisanareaoftheoceanjustbeforethelandwhereboatsarekept.Now,normalwavesarecausedbytidesorstormsoverthew
3、ater.Butdidyouknowthattidalwavesarenotcausedbystormsandthattidalwavesarenotreallytidesatall?Atruetidecanbedefinedastheregularriseandfallofoceanwatersatdefinitetimeseachday.Atidalwaveortsunamicomesrushinginsuddenlyandunexpectedlyatanytime.Itcouldcomer
4、ushinginduringthemorning,intheafternoonorduringthenight.Atidalwaveiscausedbyanunderwaterearthquake,toquakemeanstomoveupanddownorfromsidetosideveryquickly,toquakealsomeanstoshakeortotremble,whenanearthquaketakesplaceundertheocean,theoceanfloorshakesan
5、dtrembles,andsometimestheoceanfloorshifts.Thatistheoceanfloormoves.Itistheshiftingoftheoceanfloorthatproducesthetidalwave.Theearthandwatermove,causingatidalwave.Thetidalwavebegintomoveacrosstheseaatgreatspeed.In2011,amassiveearthquakeoccurredofthecoa
6、stofJapan,thequakecausedadoublewavetsunami.Adoublewavetsunamicanalsobecalledamergingtsunami.Whenamergingtsunamiordouble-wavetsunamioccurs,twoverylargewavescombined.Thisincreasesthedestructionwhenthetidalwavereachesland.Tsunamishavetakenmanylivesinthe
7、past,forinstance,thetsunamicausedbya9.0magnitudeearthquakeoffthecoastofJapanonMarch11,2011,tookthelivesofmorethan20thousandpeople.ThetsunamialsodestroyedlargeareasofnortheasternJapan.Today,scientistscanpredictthatatidalwavewillhitland,thesescientists
8、useaseismograph,aseismographisatypeofinstrumentthatrecallsthestrength,directionandlengthofanearthquake.Theseismographtellsthescientistsiftheearthquakeundertheoceanislikelytocauseatidalwave.Soalthoughitisnotpossibletostopatidalwave,it'spossibletowarnp
