"Earthquake Lights!?" exclaims one YouTube video published around about the time of the 2011 eruption of Japan's Sakurajima volcano. Or, it asks, "UFO FLEET!?!" In the earlier part of the last century, the latter might have been a common response, but now we know that the mysterious orbs of light in white or color that appear in the sky prior to and sometimes during an earthquake, sometimes with the appearance of a rainbow spotlight or flames and sometimes lasting for hours, are actually a result of seismic activity.

Recorded sightings date back to 89BC, according to a paper on the events penned in the 90s by an Italian priest, who also jotted down associated vapors, smoke, and odors of sulphur or bitumen in the hope of providing some explanation. Today, the evidence is all over YouTube, from the 2008 Sichuan earthquake to the 2011 Christchurch quake. They're no longer disputed, but they do remain unexplained. They're rare, so it's hard to investigate them. But now a team of seismologists behind a study published in Seismological Research Letters believes it has the answer: subvertical faults in continental rifts are releasing stress-induced electrical currents up to the surface as the ground is pulled apart.

The team referred to 65 of the most widely documented cases in Europe and the Americas, ranging from 3.6 to 9.2 on the Richter scale and dating back to 1600. The unexplained sightings have cropped up throughout history, but in the latter part of the 19th century Irish geophysicist and engineer Robert Mallet, who coined the term seismology, published On the Facts of Earthquake Phenomena in which these events, from 1606 BC to 1842, were gathered. Using this and other sources, the team refined the catalog—there are plenty of bizarre and false sightings among the genuine article, but "a large number of eyewitness reports from certain areas (e.g. Saguenay, Pisco, L'Aquila), coupled with similarities with respect to shapes and colors (eg. globes, flame-like luminosities) for incidences in very different regions of the world should be taken as evidence that EQL [earthquake lights] occurrences are real and a widespread phenomenon," writes the team.

They found that 85 percent of these 65 events occurred on or near rifts, and 97 percent occurred next to subvertical faults. Earthquakes located at subvertical faults are themselves rare—according to the paper five percent of all recorded earthquakes take place along intraplate faults. Yet nearly all of these events are taking place in just these spots.

"We don't know quite yet why more earthquake light events are related to rift environments than other types of faults," said Robert Thériault, a geologist at the Ministère des Ressources Naturelles of Québec and coauthor on the paper. "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."

Very few of the light events ever take place after a quake, but instead during or prior to the event. It's this fact that has led the team to hypothesize that it is the speedy build-up of forces and changes beneath the surface that is causing them, as the rocks below move against one another. The hypothesis is based on work done by coauthor and NASA Ames Research Center Earth Science researcher Friedemann Freund.

"[His] experiments demonstrate that electronic charge carriers are activated in the high-grade metamorphic and igneous rocks when subjected to deviatoric stresses, turning them into semiconductors… When subjected to stress, mineral grains slide along grain boundaries or dislocations sweep through, causing peroxy links to break."

Negatively-charged oxygen atoms break up in the rocks, releasing groups of oxygen ions which "have the remarkable ability to flow out of the stressed rock volume". At this point, they ionise in the air to form plasma. The team explains, "Assuming a plasma-like state, an instability can develop, causing the plasma to 'explode' through the surface, leading to a visible light flash."

It's the subvertical faults that may be allowing the charges generated from this action to rush to the surface—as the team explains, "Mobile charge carriers, termed positive holes or pholes, flow along stress gradients and eventually accumulate at the surface, ionising air molecules and leading to the generation of luminosities, among other phenomena.

"Within the framework of the work presented here, such observations are consistent with the fact that not every stress build-up in the Earth's crust will be followed by catastrophic rock rupture leading to an earthquake."

The trouble is, the proposal remains conjecture, albeit more probable conjecture than the appearance of UFOs. Each event is strikingly different in appearance and in distance from the epicentre of the quake itself. And various other explanations need to be ruled out, including the presence of electrical fields generated by piezoelectric rock movement below the surface, or disruption of the magnetic field.

Seeing as we don't know when the events will occur, and when they do it is so fleeting, there's no way to record them properly. Because of their brevity, it means they are not a particularly good early warning system (though on some occasions they have lasted for hours). Nevertheless, Thériault hopes that more public awareness of the events' associations with earthquakes could help, citing an instance in L'Aquila when a local took his family to safety after seeing flashes of light in the sky prior to the 2009 quake. "It's one of the very few documented accounts of someone acting on the presence of 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 Thériault.