Irregular flashes across far southern skyline near Dexter, OR. No near or distant storms.I was standing on the valley floor of Lost Valley, below the little town of Dexter at the foot of Mt. Zion looking South. The sky was completely clear. The long distance view is amazing, I was outside, scratching my horses back, enjoying the stars late at night.There were flashes in the sky at irregular intervals just above the southern mountains. I've seen this before but not for so long and then they seemed larger...at first I assumed it was a far off storm moving in. There wasn't any sound associated with the light, my horses were perfectly content and they had been nervous the morning of the Springfield 4.2 earthquake we had recently that shook my house.When I checked the lightning maps and weather reports, there was no cloud cover or lightning locally or even miles and miles away from us and I've found that report to be quite accurate. The flashes were too large/widespread to be fireworks and it was too late to be forest blasting plus--there would have been noise. It made me wonder about the reports of light flashes I've read about that appear just before an earthquake.It illuminated the southern horizon.I have a background in aviation-groundschool and IFR and know a bit about meteorology, but I can't make a good guess what caused it, so thought to post it here. I don't think it was heat lightning, I could be wrong. Thank You.