Yeah, I was rather surprised, too. However, if they are accepted as a co- or post-seismic phenomenon, then it doesn't seem to much of a stretch to believe they could be precursory, too. I think that most seismologists would argue that, during the few minutes, hours and days preceding an earthquake, there is no significant change in the static stress across the fault, and so how to account for precursory phenomenona of any type (other than results of the current stress which just happen to occur during that period)?

But, in my naive understanding as a rank amateur, I envision an asperity on the verge of breaking during that period, and experiencing microfracturing just prior to complete failure. If you accept that "telluric currents," piezoelectric effects, or some of the other proposed causes might produce luminous phenomena during a quake, then it seems reasonable they could do so during that pre-failure period.

BTW, you should see the Wikipedia article on earthquake lights; it seems to have been patterned after the USGS article I referenced, but is even more credulous than the USGS article, accepting the existence of such lights, including those preceding quakes, and their various descriptions unquestioningly. For references, it cites only the USGS article, some supposed photos, and three Youtube videos. It would seem a re-write, to include some more cautious phraseology, might be in order.

Mike Williams