The rare lights that accompany earthquakes may one day help predict the approach of major quakes, scientists say.
Earthquake lights (EQL) are more likely to occur on or near rift environments, where sub-vertical faults allow stress-induced electrical currents to flow rapidly to the surface, researchers said.
From the early days of seismology, the luminous phenomena associated with some earthquakes have intrigued scholars. Earthquake lights appear before or during earthquakes, but rarely after, they said.
EQL take a variety of forms, including spheres of light floating through the air.
In a detailed study of 65 documented EQL cases since 1600 AD, 85 per cent appeared spatially on or near rifts, and 97 per cent appeared adjacent to sub-vertical faults.
Intraplate faults are associated with just 5 per cent of Earth's seismic activity, but 97 per cent of documented cases of earthquake lights.
"The numbers are striking and unexpected," said Robert Theriault, a geologist with Quebec's Ministry of Natural Resources.
"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, sub-vertical faults characterise the rift environments in these cases," said Theriault.
Two of the 65 EQL events are associated with subduction zones, but Theriault suggests there may be an unknown sub-vertical fault present.
"We may not know the fault distribution beneath the ground. We have some idea of surface structures, but sedimentary layers or water may obscure the underlying fault structure," said Theriault.
While the 65 earthquakes ranged in magnitude, from 3.6 to 9.2, 80 per cent were greater than magnitude 5.0.
The EQL varied in shape and extent, though most commonly appeared as globular luminous masses, either stationary or moving, as atmospheric illuminations or as flame-like luminosities issuing from the ground.
Eyewitness reports and security cameras captured a large number of light flashes during the 2007 Pisco, Peru 8.0 magnitude earthquake.
Together with seismic records obtained on a local university campus, the automatic security camera records allow for an exact timing and location of light flashes that illuminated a large portion of the night sky. The light flashes identified as EQL coincided with the passage of the seismic waves.
"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," said Theriault.