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Moments after Sonoma County residents were jolted awake by Sunday’s earthquake, many also spied strange light flashes illuminating the sky.

Could they have been lightning? Utility wires whipping together and sparking? Transformers blowing? Some even described the unusual sight as looking like a camera flash.

Some saw them as diffuse light bursts in the sky, others as horizontal streaks, and still others thought they saw a source near the ground.

What the heck were they? Were folks imagining things, or is this a real phenomenon?

If you saw them too, don’t worry. You didn’t imagine them, nor are they alien ships — which have been theories posited in years past.

There is actual scientific evidence that explains the rare events.

Friedemann Freund, a senior researcher at NASA’s Ames Research Center and an adjunct physics professor at San Jose State University, has done extensive research on the phenomenon, called EQL, earthquake lights or earthquake luminosities.

Researchers have documented 65 occurrences in the Americas and Europe over the past four centuries. Their findings were published this year in the journal Seismological Research Letters.

Freund said Monday that he is not surprised by local reports of EQLs following the quake near American Canyon. He also had received reports from the San Pablo Bay area and Vacaville.

He said EQLs occur because electrical charges are created when rocks in the earth are being stressed.

“They are mostly co-seismic, seldom pre-seismic,” he said. “It’s almost like you turn a rock into a battery from which charges can flow out. When you remove the stress, the charges go back into the ground and stay there until the next go-’round.”

The lights occur when the air is electrically charged, he said.

“At the ground-to-air interface, at the Earth’s surface, it ionizes the air. It can ionize the air so strongly, you get a corona discharge — you can see lights that are on the surface.”

He said such bright electrical discharges have been documented hundreds of miles from an earthquake’s epicenter.

East Petaluma resident Elaine Ross Willenborg said she saw lights that “flashed quickly by.”

They “looked like a wide streak of shooting stars, only traveling horizontally. … I live in the country and had a clear shot of seeing, but (the) streak of light flashed by quickly,” she said.

Several Santa Rosa residents who live east of Santa Rosa Junior College reported seeing three or four bright bursts moments after the 3:20 a.m. quake as they investigated around their homes.

A number of them described the bursts as looking like lightning. But there was no storm activity.

While some transformers may have blown, several people who reported seeing the flashes didn’t have power interruptions at their homes, nor were they near reported power outages.

PG&E reported outages affecting 6,000 customers in west Santa Rosa because of wires down near Piner and Fulton roads, while about 3,300 customers lost power in Rohnert Park and another 4,800 in Sonoma. The agency said it had no reports of major damage to transformers in the county.

A resident east of Mendocino Avenue near Humboldt Street said she thought she saw a flashing light source near the ground that was lighting the sky from below.

Several residents off Pacific Avenue east of SRJC reported seeing flashes.

“I know for sure I saw two; maybe I saw three,” Mary Coscia said. “It was more of a flash and not a spark.”

Her neighbor Star Stevenson said he too saw flashes — and also sparks from a nearby transformer. But their power didn’t go out, nor did it even flicker.

“It was just like a couple of lightning strikes,” he said, “like blue flashes of light.”

Cotati resident Donna Agoitia said she saw what she at first thought was a reflection of a lightning bolt because of fog cover.

“I thought it was lightning, but it didn’t make sense,” she said.

Brian Capell, who lives in Lomita Heights near Sutter Medical Center, said he and his wife went outside shortly after the quake hit.

“At first, we definitely thought it was lighting,” he said. “Flashes from the sky is what I saw. We were facing south, and it seemed to me the light was flashing from the north. We turned around right away.”

Capell did what a lot of people do when confronted by something unexplained: He Googled it.

“After reading about it, it’s like a neat, exciting, almost unresearched phenomenon. I thought, ‘Wow, we might have witnessed a cool thing.’”

He posted about it on Facebook, but “everyone pooh-poohed it,” he said.

Freund said such anecdotal data has been helpful in research on pre-earthquake signals.

“All this we can reproduce in the laboratory,” he said of the electrical charges created in the ground. “What we have not been able to produce in the laboratory yet … is an outburst of light from the earth itself so it would propagate high up the air.”

You can reach Lori A. Carter at 762-7297 or lori.carter@pressdemocrat.com. On Twitter @loriacarter.