Pretty Much Random. Just have to enable random disasters in the game menu and put the frequency to max. Load a map and idle until it appears.
Only other way involves launching the game with --enable-dev-ui in Steam launcher(Right click on game in your steam Library > Properties > Set Launch Options) and the Achieve it mod (Subscribe at the Steam Workshop to have it downloaded automatically and enable it from Skylines Main Menu under Content manager > Mods)
When enabled you should be able to load up a map, select tab and use the disasters menu to spawn it(There is nothing to confirm that it has been placed). Takes a bit to spawn, may require you to save your game after placing it and reloading without the Dev UI options. Should still pop regardless as long as the achieve it mod is enabled.