You'll need a database to hold all the user information, including all the ways they want to be notified (email, phone text). You'll have a page showing a form with all the fields needed.
You'll want to figure out how to key the emergency area with the users, perhaps by zip code.
Google Maps API will show the map and a pin showing where the location is, but someone has to be able to enter that information in by hand in a form, get the zip code of the area, type in the text notice to go out to all the people, then the back-end will send it to the recipients via the database, including a link to the map with the pin.
This all looks doable, but just how large is this area? A major city? Half a state? Since this is an emergency site, I would hate to be responsible for any problems. Get plenty of insurance!