UFOs? Government experiments? Blown transformers? What are those flashes seen before earthquakes?

This bright flash - not lightning or a blown transformer, but the strange phenomenon called 'earthquake lights' - was spotted just before the Napa earthquake on Sunday, August 24. Credit: Cynthia Lamica, YouTube

Wednesday, August 27, 2014, 9:13 PM - Residents of California's Bay Area experienced a magnitude 6.0 earthquake very early on Sunday morning, but some experienced something even stranger, just before or during the event - flashes of light in the sky that have frequently been associated with earthquakes.

What appears to be security footage looking out over the city of Santa Rosa, Calif, at 3:13 a.m. PDT on Sunday, roughly seven minutes before the entire area shook, shows flashes going off every few seconds, including some exceptionally bright ones.

While some might say that these flashes can be explained by electrical transformers being stressed during the earthquake, that would only work for flashes going off during the quake, when the ground is violently shaking back and forth. Also, these kinds of lights have been reported during many earthquakes, going back decades and perhaps even longer, even in places where there are no electrical transformers. 

People may have come up with many explanations for these lights - from the fantastical, to ones more suited to science fiction, and even related to government conspiracies - but scientists may have traced the reason back to how various materials, including earth and rock, react when they're put under stress. 

In one study, it was shown that the tiny flawed crystals contained in basalts and gabbros can release electric charge when they're stressed by seismic waves. When these types of rock solidify in cracks within larger rock formations, forming what are called 'dikes', these dikes can funnel the electric charge released from these crystals up to the surface of the ground.

"The charges can combine and form a kind of plasma-like state, which can travel at very high velocities and burst out at the surface to make electric discharges in the air," Friedemann Freund, from San Jose State University and NASA's Ames Research Center, who cowrote the paper on this effect, told National Geographic earlier this year.

That's not necessarily the only explanation for this, though.

Other studies, which examined electrical effects in powders and grains, showed that when 'cracks' appear in the materials made up of those powders and grains, it can generate electric voltage. Consider the 'fractoluminescence' effect that happens when you bite into a wintergreen Life Savers candy (which is made up of tiny grains of powder pressed together), says Rutgers University applied physicist Troy Shinbrot, speaking to LiveScience's Our Amazing Planet.

"If you take a Tupperware container filled with flour and tip the container, when the flour shifts, voltages of around 100 volts inexplicably appear," Shinbrot said in the interview. "Except for the fact that we cannot get these voltages to go away, I would call this 'crackpot physics,' and even as it is, I wish I could hedge my bets, but the voltages are very repeatable, and we have so far failed to account for a spurious influence that might cause them."

Further ideas about this include piezoelectric effects, as quartz crystals are deformed and stressed inside rocks along the earthquake fault. This effect is the same one used in various 'energy harvesting' efforts.

More study is definitely needed to figure out exactly which effect (or combination of effects) is behind this phenomenon. However, while it occurs, it could become a way of early earthquake detection, as they apparently can occur up to weeks ahead of the earthquake.

EARTHQUAKE LIGHTS: Have you ever witnessed this phenomenon? Leave your story in the comments below (but remember the ~500 character limit!).