Recode, describing Facebook’s new emergency check-in feature:

Safety Check works by sending users a push notification asking them if they are safe whenever a natural disaster strikes the area they list as their current location. User’s can then see a list of their Facebook friends in the area, and see which users have checked-in as safe, and which have not.

What constitutes as a disaster is determined with local authorities. There’s no cross-reference of data at this, but that can’t be far off.