Yes, it's indeed hard to predict an earthquake to take out the external power and the subsequent tsunami to take out the backup generators on a coastline facility.
How big of a tsunami was required? How big of a earthquake would be required to generate such a wave? How much water would be needed to reach the facility? How much to damage it? What about damaging backups?
Sarcasm aside, it is easy to predict that the facility would be at risk for an earthquake/tsunami combo. The hard part is to predict how much of one to prepapre for. Remember, even in this neck of the woods, the was a quake of historic proportions.
(Note: This was sarcasm)