Peru sits snuggled up against one of the world’s gnarliest convergent plate boundaries. Remember that in the rhelm of plate tectonics, we’ve got 3 things plates can do: move apart from one another (divergent), crash into one another (convergent), or slide on by one another (conservative). Along the coast of South America we’ve got what’s called a subduction zone. Subduction zones are features of convergent plate boundaries where we’ve got one plate diving down underneath another
Some other major subduction zones around the world are over on the west side of the Pacific Ocean, nearby Japan (the Mariana Trench). We've also got the Aleutian Islands up in Alaska as well as along the coast of Washington and Oregon in North America which is named the Cascadia subduction zone. The Cascadia subduction zone is the reason why we have the Cascade mountain range right there (Mt. St. Helens anyone?).
The subduction zone along the West coast of South America involves the Nazca Plate and the South American plate. The Nazca plate is an oceanic plate which means it’s made up of dense stuff like basalt whereas the South American plate is made up of relatively light stuff like granite. Remember, these plates are in motion all the time because they’re riding along on the molten Asthenosphere underneath them, but they don’t move very much. When they do move, we get an earthquake.
Subduction zone earthquakes are so much bigger than transform fault earthquakes (ie: quakes on the San Andreas) because you’ve got a much much deeper fault which dips down into the earth at an angle. Let’s look at the figure below.
Ok, this might get confusing because we’re dealing with looking at the faults from the perspective of cross section. Imagine that you’ve taken a slice out of the earth way deep into the crust and are looking at the fault zones straight on. On the left we’ve got our transform fault (ie: the San Andreas). On the right we’ve got the setup as it is in a subduction zone (ie: Peru). Now, imagine that both these faults rupture the same amount, maybe 4 meters. Outlined in pink is the plane on which the rupture is going to happen. Because of the geometric setup (the hypotenuse of a triangle is always longer than the other two sides), the plane that ruptures in a subduction zone earthquake is larger than that for the transform fault. Larger plane of rupture equals more area of the fault broken and a larger area equals a larger earthquake. That’s why when faults at convergent zones rupture, they are such humongous earthquakes.
I’ll wrap it up by saying that if you’d like to help with relief efforts, check out this link to make a donation for the folks who need some assistance. Like I said, there’s lots of good people out there who could use a little help to get themselves out of such a crummy situation. All I’d hope for is that someone would do the same for me if I were in their shoes.