sorta makes you wonder how they can tell
Well, in all of these you have to figure "where the earthquake occurred" in regards to closeness to the water, "how deep the earthquake occurred" because you can have a strong quake below the surface and not nearly have as much of an impact, and "the magnitude of the quake itself" because we're on log-scale folks... though i'm not sure if that's base-10 or base-16. An 8.0 earthquake is that much less intense than a 9.0 earthquake... they aren't even the same creatures.
So, i don't think we can necessarily be shocked that Hawaiian civilization wasn't cleaned from this earth.