(USA TODAY) -- They've been mistaken for UFOs or dismissed as hallucinations. Nowgeologists have collected a near-definitive list of a rare butfascinating phenomenon - earthquake lights.
Certain types ofearthquakes in certain areas can set off blazes of light seconds -sometimes days - ahead of the actual quake. These can manifestthemselves as floating balls of light, bluish columns shooting up out ofthe earth and even reverse lightning, reaching up into the sky from theground.
A study out Thursday in the journal Seismological Research Letters shows such quakes are tied to a specific type of temblor in areas where certain geological formations occur.
While rare, researchers were able to document 65 examples between 1600 and the present.
•Seconds before the 2009 L'Aquila, Italy, earthquake struck, pedestrianssaw flames of light 4 inches high flickering above the stone-pavedFrancesco Crispi Avenue in the town's historical city center.
• InPisco, Peru, a naval officer saw pale-blue columns of light burstingfour times in succession out of the water on Aug. 15, 2007, as an8.0-magnitude earthquake struck. Security cameras in the city captured images of the lights as well.
•On Nov. 12, 1988, a bright purple-pink globe of light moved through thesky along the St. Lawrence River near the city of Quebec, 11 daysbefore a powerful quake.
• And on April 18, 1906, blue flames wereseen hovering at the base of foothills west of San Francisco justbefore the great earthquake hit. South of the city, in San Jose, onestreet was seen to be ablaze with fire in a faint but beautiful rainbowcolor.
The mechanism that causes the phenomenon occurs only in specific and rare conditions, said Friedemann Freund, a professor of physics at San Jose State University and senior scientist at NASA Ames Research Center in Mountain View, Calif.
Rocks such as basalt and gabbro, created deep in the Earth's mantle, contain tiny defects in their crystals. When such rocks are stressed, those defects momentarily generate electrical charges, said Freund, one of the paper's authors.
"When a powerful seismic wave runs through the ground and hits a layer of such rocks, it compressesthe rocks with great pressure and speed, creating conditions underwhich large amounts of positive and negative electrical charges aregenerated," he said. These charges can then travel together, allowingthem to reach what's called a plasma state, which can burst out andshoot up into the air.
Another necessary component forearthquake lights to be produced in nature are deep vertical faults inthe Earth's crust, some of which can reach down 60 miles and more.Magmas that solidify to become gabbros or basalts have risen along thesefaults, forming dikes often tens to hundreds of feet thick.
"Wespeculate that the dikes act as a funnel, focusing the charges untilthey become an ionized solid-state plasma," said Robert Thériault, leadauthor on the paper and a geologist with the Quebec Ministry of Natural Resources in Canada.
"When the plasma bursts out into the air, it produces light," he said.
Fewer than 0.5% of earthquakes worldwide occur in the right places to create earthquake lights. The researchers combed through historical documents, many handwrittenand some in Latin, dating back centuries. They also looked at modernscientific papers and surveillance-camera records that recordedearthquake lights in real time.
Earthquake lights aresometimes mistaken for UFOs, Thériault said. In the early 1970s, a JimConacher was boating on Tagish Lake in Canada's Yukon territory with hiswife when they saw seven yellow luminous globes on the nearby flank of amountain. The nearer orbs slowly drifted up the mountain to join themore distant ones.
Conacher took a photograph of the lights, which made its way to a website listing possible UFO sightings in Canada.
However,when the researchers looked through seismic reports, they foundrecords of the nearby Cross Sound earthquake of July 1, 1973, whichmeasured 6.7 on the Richter scale. There were also two aftershocksmeasuring 5.2 and 4.1. Though the exact dating of the trip is uncertain(the boaters have died), the researchers believe the lights appearedjust a few hours prior to the initial quake.
Areas that containthese specific grouping of traits include Italy, Greece and the Rhinerift, which stretches between France and Germany, as well as severalareas in South America. Researchers believe they exist in China as well.
Thelights aren't common enough in earthquake zones to be used as a warningsystem, Thériault said. But they have sometimes functioned that way.
Justbefore Italy's L'Aquila earthquake, a man in his kitchen saw flashesof brightness outside as intense as daylight. The light lasted for morethan a second. Because he had read a paper about earthquake lightspreviously, he decided to take his family to a safer structure.
Thériaultalso knows a geologist in Canada who was living in China in 1976. OnJuly 28 that year, he saw earthquake lights, which prompted him to leavehis house. Soon thereafter, the devastating Tangshan quake hit. It isbelieved to have killed hundreds of thousands of people. The geologistsurvived.