I've experienced it once myself, and I'm not 100% sure. There was a large power substation about 5km away in roughly that direction, and that was my original a

I've experienced it once myself, and I'm not 100% sure. There was a large power substation about 5km away in roughly that direction, and that was my original assumption. But I don't think it tripped (at least not completely), as we didn't lose power. It was also far brighter than I would expect from an arc at a substation (it drowned out local street lights), higher in the sky, with multiple flashes covering much more of the sky than a single substation. So I strongly lean towards it being earthquake lights, which is a documented phenomenon.