Mysterious flashes of electricity known as earthquake lights are more likely to happen near rifts, where pieces of the Earth are pulling away from each other, new research suggests.
The quick buildup of stress at these nearly vertical faults may cause electrical current to flow to the surface and cause the eerie light shows, the researchers find.
The conclusions, published in the current issue of the journal Seismological Research Letters, were drawn from analyzing 65 documented cases of earthquake lights over the last 400 years.
These strange light shows, which can look like flickering flames or floating, glowing orbs, according to those who have observed them, have occurred during or before several of the world's greatest earthquakes, including two days before the Great San Francisco Earthquake of 1906.
But until recently, most seismologists didn't believe the earthquake lights were real because the reports were all anecdotal and hard to explain physically. [The Top 10 Unexplained Phenomena]
"Earthquake lights are totally underreported," said study co-author Friedemann Freund, a crystallographer at the NASA Ames Research Center in Mountain View, Calif., and San Jose State University. "They are often things that happen within a fraction of a second."
There's not always a person around to see them and when they do report them, they were often discounted by scientific journals, Freund said.
The advent of better documentation and video cameras has changed that. For instance, just before the earthquake that struck L'Aquila, Italy, in 2009, bystanders reported flames flickering up from the pavement. Video and eyewitness reports also described several weird light anomalies during the magnitude-8.0 earthquake in Pisco, Peru, in 2007. In one case in the early '70s, luminous drifting globes thought to be possible UFOs when they were observed in Canada's Yukon Territory later were linked to earthquake lights.
Origin of lights
But exactly why these lights happen was still a mystery. To get at the question, Freund and colleagues looked at documents dating back to 1600 for reports of earthquake lights.
The team found 65 cases that were well documented from North and South America and Europe. Of those cases, 97 percent seemed to happen at faults within continental plates, rather than at subduction zones, or the boundaries where one plate is diving below another. That's despite the fact that most big earthquakes happen at subduction boundaries.