Anearthquakehappenswhentwoplatesrub(碰撞)together.Theearthplatestravelindifferentdirectionsandatdifferentspeeds.Ifoneplateisslowlyforcedunderneaththeother pressurebuildsupuntiltheplatesbreakapart.Thisprocesscausesthegroundtomove.Itisanearthquake.Inotherwordsearthquakesaretheshakingoftheearth’ssurfacecausedbytheearth’srockyouterlayerasaresultoftheenergystoredwithintheearth.Thestrainwithintherocksissuddenlyreleased(释放).
Thedamageanearthquakecausesdependsonwhereitisandthetimeitishappening.Ifanunpopulatedregionisstrucktherewillbelowlossoflifeorproperty.Ifithitsalargecitytheremaybemanyinjuriesandmuchdestruction.Manyoftheareasatriskarelargelypopulatednow.Majorearthquakeshittingthoseareastodaycouldproduceterribledamage.
Actuallythereareseveralmillionsmallearth-quakeseveryyear.Largeearthquakessuchasthe1964Alaskaquakethatmeasured9.2ontheRichterscalecausemillionsofdollarsindamage.Inthelast500yearsmillionsofpeoplehavebeenkilledbyearthquakesaroundtheworld—including240000inthe1976TangshanearthquakeinChina.
A60-secondorlessearthquakecancausedevastation（毁坏）thatcontinuesforyearsafterthefirsttremor(小震).In1972aseriesofsevereearthquakesstruckManaguaNicaragua.Fifteenyearslaterthecitystilllookedthewayithadaweekaftertheearthquakehitbecausethecountrydidnothavethenecessarymoneytorebuildit.
Theshakingoftheearthissometimesnotthegreatestdisaster.Itisintheensuingfiresandfloodsthatoftenthegreatestdamageoccurs.Inthe1906earthquakeitwasthefirescausedafteritthatdidthemajorityofthedamage.Anearthquakecanalsodestroydamshighaboveacityorvalleyscausingfloodstosweepdownandsweepawayeverythingintheirpath.
【小题1】Whichofthefollowingisthemainideaofthefirstparagraph?