Why are there so many earthquakes in south america?

The western part of South America is a region that is geologically very active. This is due to the boundaries between the South American tectonic plate with the Nazca tectonic plate, and also with the Pacific tectonic plate. Since both, the Nazca and the Pacific plates are oceanic plates, they are heavier, and they subduct below the lighter plate of South America, thus enabling the pressure and temperature from the mantle to come out towards the surface on this boundary lines. This creates lots of earthquakes, as well as volcanic activity.