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EveryonehasheardoftheSanAndreasfault,whichconstantlythreatensCaliforniaandtheWestCoastwithearth-quakes.ButhowmanypeopleknowabouttheequallyseriousNewMadridfaultinMissouri.'?
BetweenDecemberof1811andFebruaryof1812,threemajorearthquakesoccurred,allcenteredaroundthetownofNewMadrid,Missouri,ontheMississippiRiver.Propertydamagewassevere.
Buildingsintheareawerealmostd estoyed.Wholeforestsfellatonce,andhugecracksopenedintheground,allowingsmellofsulfurtofilterupward.
TheMississippiRiveritselfcompletelychangedcharacter,developingsuddenrapidsandwhirlpools.Severaltimesitchangeditscourse,andonce,accordingtosomeobservers,itactuallyappearedtorunbackwards.FewpeoplewerekilledintheNewMadridearthquakes,probablysimplybecausefewpeoplelivedintheareain1811;buttheseverityoftheearth-quakesareshownbythefactthattheshockwavesrangbellsinchurchtowersinCharleston,SouthCarolina,onthecoast.BuildingsshookinNewYorkCity,andclockswerestoppedinWashingtonD.C.ScientistsnowknowthatAmerica'stwomajorfaultsareessentiallydifferent.TheSanAndreasisahorizontalboundarybetweentwomajorlandmassesthatareslowlymovinginoppositedirections.Californiaearthquakesresultwhenthemovementofthesetwomassessuddenlylurchesforward.
TheNewMadridfault,ontheotherhand,isaverticalfault;atsomepoint,possiblyhundredsofmillionsofyearsago,rockwaspusheduptowardthesurface,probablybyvolcanoesunderthesurface.Suddenly,thevolcanoescooledandtherockcollapsed,leavinghugecracks.Evennow',therockcontinuestosettledownwards,andsuddensinkingmotionstr iggerearthquakesintheregion.Thefaultitself,alargecrackinthislayerofrock,withdozensofothercracksthatsplitofffromit,extendsfromnortheastArkansasthroughMissouriandintosouthernIllinois.
ScientistswhohavestudiedtheNewMadridfaultsaytherehavebeennumeroussmallerquakesintheareasince1811;thesesmallerquakesindicatethatlargeronesareprobablycoming,butr itescientistssaytheyhavenomethodofpredictingwhenalargeearthquakewilloccur.
31.Thispassageismainlyabout______.
A)theNewMadridfaultinMissouri
B)theSanAndreasandtheNewMadridfaults
C)thecausesoffaults
D)currentscientificknowledgeaboutfaults
正确答案：B答案：B
[试题分析]文章主旨题。
[详细解答]从文章开头就可以看出文章的话题就是美国的两个断层，而后文也正是围绕此话题展开的，井对比了两个不同性质的断层。因此，选择答案B为正确答案。
