The fourth calamity must have been over a much larger area than just Eorzea because Allag spanned across continents. I don't think it would have been as likely to completely collapse if the damage was focused entirely on Eorzea. Likewise, the fifth calamity was widespread enough freeze the seas between continents. I suspect that in most cases the calamities are planet wide but Eorzea is the epicentre. The Seventh didn't do as much outside Eorzea but it was heavily mitigated by Pheonix.