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?

A．Anearthquakecomesfrominsidetheearth.B．Theearthhasgreatenergyinstorage.C．Howtheearthplatesmove.D．Howanearthquakehappens.【小题2】Whichofthefollowingismentionedtoshowthatanearthquakecankilltoomanypeople?