A serious quake in Chile today at a magnitude of 6.1 and at a depth of only 15km. This quake on the pacific/South America boundary has been listed as a yellow on pager indicating that there will be damage and possible loss of life. It has already been followed by a couple of large aftershocks.
What is happening is that South America is being pushed westward bt the expanding Atlantic ocean. Where the south american plate hits the pacific it is forcing the Andes up and subducting the pacific plate. That fault line gets stuck once in a while and when it lets go the plate moves sharply west over the subducting pacific plate causing the earthquake. One quake a year ago moved parts of Chile 10 meters to the west and parts of the Andes dropped by over a meter. Imagine the energy released by dropping a mountain range even just a meter.