What Caused the Chilean Earthquake?

The eastern margin is a convergent boundary subduction zone under the South American Plate and the Andes Mountains, forming the Peru-Chile Trench. The southern side is a divergent boundary with the Antarctic Plate, the Chile Rise, where seafloor spreading permits magma to rise. The western side is a divergent boundary with the Pacific Plate, forming the East Pacific Rise. The northern side is a divergent boundary with the Cocos Plate, the Galapagos Rise. The subduction of the Nazca plate under southern Chile has a history of producing massive earthquakes, including the largest ever recorded on earth, the moment 

The ongoing subduction, along the Peru-Chile Trench, of the Nazca Plate under the South American Plate is largely responsible for the Andean orogeny. The Nazca Plate is bounded on the west by the Pacific Plate and to the south by the Antarctic Plate through the East Pacific Rise and the Chile Rise respectively. The movement of the Nazca Plate over several hotspots has created some volcanic islands as well as east-west running seamount chains that subduct under South America.