【阅读理解Anearthquakehappenswhentwoplatesrub(碰撞)together．Theearthplatestravelindifferentdirectionsandatdifferentspeeds．Ifoneplateisslowlyforcedunderneaththeotherpr】

　　阅读理解

　　Anearthquakehappenswhentwoplatesrub(碰撞)together．Theearthplatestravelindifferentdirectionsandatdifferentspeeds．Ifoneplateisslowlyforcedunderneaththeotherpressurebuildsupuntiltheplatesbreakapart．Thisprocesscausesthegroundtomove．Itisanearthquake．Inotherwordsearth-quakesaretheshakingoftheearth’ssurfacecausedbytheearth’srockyouterlayerasaresultoftheenergystoredwithintheearth．Thestrainwithintherocksissuddenlyreleased(释放)．

　　Thedamageanearthquakecausesdependsonwhereitisandthetimeitishappening．Ifanunpopulatedregionisstrucktherewillbelowlossoflifeorproperty．Ifithitsalargecitytheremaybemanyinjuriesandmuchdestruction．Manyoftheareasatriskarelargelypopulatednow．Majorearthquakeshittingthoseareastodaycouldproduceterribledamage．

　　Actuallythereareseveralmillionsmallearth-quakeseveryyear．Largeearthquakessuchasthe1964Alaskanquakethatmeasured9.2ontheRichterscalecausemillionsofdollarsindamage．Inthelast500yearsmillionsofpeoplehavebeenkilledbyearthquakesaroundtheworld－including240000inthe1976TangShanearthquakeinChina．

　　A60-secondorlessearthquakecancausedevastationthatcontinuesforyearsafterthefirsttremor(小震)．In1972aseriesofsevereearthquakesstruckManaguaNicaragua．Fifteenyearslaterthecitystilllookedthewayithadaweekaftertheearthquakehitbecausethecountrydidnothavethenecessarymoneytorebuildit．

　　Theshakingoftheearthissometimesnotthegreatestdisaster．Itisintheensuingfiresandfloodsthatoftenthegreatestdamageoccurs．Inthe1906earthquakeitwasthefirescausedafteritthatdidthemajorityofthedamage．Anearthquakecanalsodestroydamshighaboveacityorvalleyscausingfloodstosweepdownandsweepawayeverythingintheirpath．

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　　Whichofthefollowingisthemainideaofthefirstparagraph?