If you're willing to use cheats to see it, testingcheatsenabled will let you trigger an earthquake. I think you need to be level nine or ten of the firefighter, and shift-click any lot in town, select 'spawn job', and then you'll be able to trigger an earthquake. It'll then set off a chain of three emergencies, where you'll have to put out the fire, rescue the trapped sims from under the rubble and fix a gas leak from the cooker.
I'd guess it's probably about as likely as any other career job to spawn at that level.