If you're writing about real people and real events, that's
"non-fiction"/"journalism". If you write it in the style of a novel,
it's "creative non-fiction".
If you're writing about made-up people in real events, that's
If you're writing about made-up people in made-up events in the real world, ...
If your story is in the USA in the past it will be historical fiction. No doubt there is evidence of a number of large tsunamis in the present USA in the past, but if your story happens after the USA was founded the number will be smaller.
If your story is supposed to happen more or less in the present it will be contemporary fiction. Obviously, a tsunami, ...