Researchers believe rare "earthquake lights" are more likely to occur in a "rift environment."
A rift environment is "where subvertical faults allow stress-induced electrical currents to flow rapidly to the surface," a Seismological Society if America news release reported.
Earthquake lights (EQL) can often be observed before or during a seismic event; but to see one after the fact is a rarity.
EQLs can take many forms, including giant globes of "flames" that were observed in L'Aquila, Italy before a 2009 earthquake or a " bright purple-pink globe of light," in Quebec. In San Francisco in 1906 the phenomenon was observed as "streams of light."
A research team was able to link EQL with "continental rift environments." They reviewed 65 cases dating all the way back to 1600 A.D; they found that 85 percent of the incidents occurred "on or near" rifts while 97 were at least adjacent to subverticle faults. Intraplate faults only see about 5 percent of the world's seismic activity, but were the sites of 97 percent of EQL cases.
"The numbers are striking and unexpected," Robert Thériault, a geologist with the Ministère des Ressources Naturelles of Québec, said. "We don't know quite yet why more earthquake light events are related to rift environments than other types of faults but unlike other faults that may dip at a 30-35 degree angle, such as in subduction zones, subvertical faults characterize the rift environments in these cases."
Two of the studied EQL events were in subduction zones, but the researchers suggested there could also be an unknown subvertical fault lurking on the site.
"We may not know the fault distribution beneath the ground," Thériault said. "We have some idea of surface structures, but sedimentary layers or water may obscure the underlying fault structure."
Since the phenomenon is rarely witnessed after an earthquake researchers believed it could be related to stress buildup before the fault ruptures and stress changes during the propagation of seismic waves.
" Stress-activated mobile electronic charge carriers, termed positive holes, flow swiftly along stress gradients. Upon reaching the surface, they ionize air molecules and generate the observed luminosities," the news release reported.
EQLs have been widely documented throughout history. On L'Aquila resident claims to have seen light two hours before the peak of the earthquake.
"It's one of the very few documented accounts of someone acting on the presence of earthquake lights," Thériault said. "Earthquake lights as a pre-earthquake phenomenon, in combination with other types of parameters that vary prior to seismic activity, may one day help forecast the approach of a major quake."
