Although news media first reported the earthquake as an aftershock of the February earthquake , the initial geological summary given by the United States Geological Survey said it was an independent earthquake . Though unchained by the regional stress caused by the February 27 earthquake , the March event was not an immediate adjustment to February 's thrust fault rupture between the Nazca and South American plates , but it was caused by normal faulting inside one of those plates . The Pichilemu earthquake was caused by the change in regional stress from the 27 February earthquake .