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1、Passage1EveryonehasheardoftheSanAndreasfault（断层）,whichconstantlythreatensCaliforniaandtheWestCoastwithearthquakes.ButhowmanypeopleknowabouttheequallyseriousNewMadridfaultinMissouri? BetweenDecemberof1811andFebruaryof1812,threemajorearthquakesoccurred,allcenteredaroundthetownofNewMadri
2、d,Missouri,ontheMississippiRiver.Propertydamagewassevere.Buildingsintheareawerealmostdestroyed.Wholeforestsfellatonce,andhugecracksopenedintheground,allowingsmellofsulfur（硫磺）tofilterupward. TheMississippiRiveritselfcompletelychangedcharacter,developingsuddenrapidsandwhirlpools.Several
3、timesitchangeditscourse,andonce,accordingtosomeobservers,itactuallyappearedtorunbackwards.FewpeoplewerekilledintheNewMaridearthquakes,probablysimplybecausefewpeoplelivedintheareain1811;buttheseverityoftheearthquakesareshownbythefactthattheshockwavesrangbellsinchurchtowersinCharleston,S
4、outhCarolina,onthecoast.BuildingsshookinNewYorkCity,andclockswerstoppedinWashington,D.C.ScientistsnowknowthatAmerica'stwomajorfaultsareessentiallydifferent.TheSanAndreasisahorizontalboundarybetweentwomajorlandmassesthatareslowlymovinginoppositedirections.Californiaearthquakesresultwhen
5、themovementofthesetwomassessuddenlylurches（倾斜）forward. TheNewMadridfault,ontheotherhand,isaverticalfault;atsomepoints,possiblyhundredsofmillionsofyearsago,rockwaspusheduptowardthesurface,probablybyvolcanoesunderthesurface.Suddenly,thevolcanoescooledandtherockcollapsed,leavinghugecrack
6、s.Evennow,therockcontinuestosettledownwards,andsuddensinkingmotionstrigger（触发）earthquakesintheregion.Thefaultitself,alargecrackinthislayerofrock,withdozensofothercracksthatsplitofffromit,extendsfromnortheastArkansasthroughMissouriandintosouthernlllinois. ScientistswhohavestudiedtheNew
7、Madridfaultsaytherehavebeennumeroussmallerquakesintheareasince1811;thesesmallerquakesindicatethatlargeronesareprobablycoming,butthescientistssayhavenomethodofpredictingwhenalargeearthquakewilloccur. 1.Thispassageismainlyabout B .A.theNewMadridfaultinMissouri B.thecausesoffaults
