 An early study of the event encompassed more than 2 @,@ 000 aftershocks ( and included four of magnitude 5 @.@ 0 or greater ) that were recorded within 20 days of the mainshock , with the area south of the border near the epicenter remaining relatively quiet . Most of the aftershock activity was within 15 kilometers ( 9 @.@ 3 mi ) of Brawley ( especially the first eight hours after the mainshock ) , although they occurred from the Salton Sea in the north to the Cerro Prieto Geothermal Power Station to the south , a distance of 110 kilometers ( 68 mi ) . The first strong aftershock ( 5 @.@ 0 ) occurred at 23 : 19 GMT just 2 @.@ 5 minutes after the mainshock and the strongest aftershock ( 5 @.@ 8 ) occurred at 6 : 58 GMT on October 16 west of Brawley . 
