Earthquakes tend to occur in sequences or ` storms ' , where a major earthquake above 6.5 on the Richter magnitude scale can set off later earthquakes along the weakened fault line . Amos Nur postulates that earthquakes tend to occur in `` sequences '' or `` storms '' where a major earthquake above 6.5 on the Richter magnitude scale can in later months or years set off second or subsequent earthquakes along the weakened fault line .