My understanding is the thinking that any fault will eventually have another one just as bad, when is the question, not if. At least that is what the scientist claim, I do not know if I believe in all of their claims.
I watched a documentery on the 64 quake that desimated parts of Alaska, and the tsunami went as far as California. The coastal area of Alaska was not actually real populated back then, nor was BC, washington or Oregon. With all the rage of beachfront dwelling that has occured since then, they claimed, when, not if it happened again, it would take over a million lives.