I don't know about this...

[Fenral]

Is hard to imagine they are alll gone just for an earquake.
What the hell?? did they all were off guard when this happened?

We know more than a few survived, we just don't know what happened after. Supposedly, the survivors settled down and eventually built the Garlean Empire, and we know at least House Darnus had some store of knowledge passed down, but that's about it. Most of their technology, even the stuff like Azys Lla which could have actually been quite valuable to relief efforts, was simply abandoned for reasons unknown. It's quite odd, because what little we know suggests that they would have used it if they could, and they still had people who could. All the "answers" we've gotten so far have just been thinly veiled questions.

We've met two moderately self-aware clones with no individual free will of their own; one absolutely bonkers imperial legatus who claimed to be descended from Allagans; and one poor bewildered historian, who found concrete proof that he indeed is, but put himself in a position rendering him unable to answer much of anything, besides what he felt like sharing as the doors closed. Oh, and one ghost, who also doesn't really feel like sharing, and may not even be a canonical character. The only actual natural-born, flesh-and-blood Allagans we've met have been freakishly oversized mutants that tried to kill us on sight. Fun times.