Could a "Hyper Power" Earthquake occur?

Ok, I was reading over a story I've been wwriting earlier, and one of the key events in the stories history is the occurence of an extremely powerful Earthquake.

Now, I made it in the story that a quake ranging 15 on the richter scale hit.


So I started wondering, how powerful can an Earthquake theoretically be, natural or un-natural?

Natural.. maybe 12 in the worst case scenario.. when a very solid comet hits the earth at high speed with an angle of penetration of near 90*...

Un-natural.. with all the technology, maybe we could make a 15? Maybe not... all is possible...