Look at the Seismology map. Concentrate on the boundary between the Nazca Plate and the South America Plate (West coast of S. America). You’ll notice that going from the west coast inland, toward the east, the dots showing the epicenters of the earthquakes go from red to yellow, to green and to blue, indicating a progressive deepening of the foci (plural of focus) of the earthquakes, all the way to approximately 700 Km. What’s going on at this plate boundary (and way from it?):