The correct answer is - convergent plate boundary.
The western coast of South America is a home to a convergent plate boundary, between the Nazca oceanic tectonic plate and the South American continental tectonic plate.
The Nazca plate is lower and heavier, thus it subducts bellow the higher and lighter South American plate. This subduction is causing a lot of geologic activity in the region, with the Andes Mountain Range being one of the products, as well as the common earthquakes, and the volcanic activity.
Since there's often earthquakes that appear under water, the chances of a tsunami are always high, especially if it is a stronger earthquake, which is why the people living in this part of the continent are always on alert.