[King-Aaron]

While this is very plausible, it wouldn't explain the historical accounts of "earthquake lights" that have been reported as far back as the 1600's (http://srl.geoscienceworld.org/content/85/1/159)

Edit: It's also been reported around the world that these lights can occur days, even weeks before an earthquake happens.

There's a lot of different explanations for it, but I understand that it's never been definitively explained.

[ianai]

I like the stress induced ionization theory from Wikipedia.