Earthquake lights (EQL) are phenomena that are sometimes reported in conjunction with an earthquake. These EQL could take the form of balls of light, steady glowing lights, sheet lightning, bluish flames appearing to come out of the ground, or flashes of light similar to lightning except they come from the ground. Usually the EQL is seen before or at the time of the earthquake, near the epicenter area.
Because of the rarity of the event, and insufficient evidence to formulate a conclusion, some geophysicists doubt whether there really could be EQL as some of the sightings have been attributed to such things as power lines shaking and arcing.
This image is a simplified model of the subduction zone environment and where earthquake lights are likely to be seen.
(Image retrieved from https://upload.wikimedia.org/wikipedia/commons/9/9f/How_EQL_form.gif and is attributed to ScienceResearch / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0) Creative Commons Attribution-Share Alike 4.0 International license)
However, with the increasing number of security cameras running around the world, more evidence of attributed EQLs have been reported and some geophysicists believe the phenomena to be plausible.
There are various theories as to how EQL may be caused. A study published in 2014 determined that the electrical process causing the EQL may be from the stressing of certain types of rocks, causing them to store small electric charges (Prevalence of Earthquake Lights Associated with Rift Environments).
A seismic wave may cause this electrical charge to be released, and these “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” (Bizarre Earthquake Lights Finally Explained ).
While EQL may be an indicator of an impending earthquake, it’s not a very useful predictor. It is estimated that less than one half of one percent of earthquakes might have conditions where an EQL may occur.
The real question to the validity of EQL being earthquake predictors is how much before an earthquake the EQL might be seen. And, are there other phenomena that might cause similar lights to form, which could be seen as earthquake lights.