As a DM that runs a tabletop RPG, it is your right and privilege to strike towns, lands, and whole continents with whatever form of catastrophe or disaster that strikes your fancy. Whether it is a terrible plague, massive tidal wave, or vicious invading army that sweeps through the area and all but wipes out the native inhabitants it is up to you to determine what happens with that location once the initial catastrophe has passed. These events could have happened hundreds of years before the characters were born or they could be the climatic event that finishes off a chapter of your game and opens up a new one. No matter when it happens, it is up to you as the DM to figure out how these events will effect your game world and how the players will experience the event and the aftermath.