They cover that in the National Geo clip upthread. For this type of situation (subduction), what happens is the plate which is trying to slip under the other, gets stuck, and bulge (almost shaped like a wrinkle in the crust) forms in the upper plate; and the earthquake happens when they suddenly come *un*stuck and slide past each other.
The magnitude of the quake is proportional to the surface area involved: and in this case, the area stuck is about the size of Ireland.
Looking forward to that buying opportunity...