For centuries, people have claimed to see mysterious lights in the sky when an earthquake hit. The stories were mostly thought to be myths until the mid 20th century, when photographic evidence made scientists acknowledge that they could be real. That led to a flurry of theories about their cause until 2014, when a research team reached a promising conclusion.
The Credulous And The Skeptical
On September 7, 2017, a magnitude-8.1 earthquake hit off the coast of Mexico, making it the country's largest in a century. Amid the tragic news of death and destruction, there was another type of story making the rounds: "Mexicans report bizarre flashing lights in sky during earthquake with many believing they're UFOs or the APOCALYPSE," cried the Mirror. "Mysterious blue and green lights flashing in the sky as Mexico earthquake hits send UFO hunters into a frenzy," announced the Sun. Videos had surfaced depicting eerie lights illuminating the cloudy sky above Mexico City. What could they be?
People all over the city took video footage of the mysterious phenomenon.
Earthquake Lights in Mexico City
Lights ranging from white flares to rainbow-colored clouds have been reported before, during, and after earthquakes for hundreds of years. Despite that, most scientists have been skeptical: an 1888 paper acknowledged reports of a "luminous appearance" in the sky near the time of an earthquake, but noted "I mention these things, but I do not think that they were in any way connected with the earthquake." A 1913 survey concluded, "At the present stage of our observations it is not scientific or rational either to affirm or to deny the existence of luminous earthquake phenomena."
A Theory Erupts
But when lights were witnessed above a 1965 earthquake in Nagano, Japan, people had cameras. Finally, scientists began to acknowledge the phenomenon and began looking for possible reasons it might happen. Most suspected it was due to some sort of electric field forming due to the movement of rocks, but that's been hard to prove in the lab.
In 2014, however, science got its surest bet yet. A research team led by Robert Thériault analyzed the circumstances of 65 supposed light-producing earthquakes going back to the 17th century to see what they had in common. According to their research, there's a reason the lights are so rare: whereas 95 percent of seismic activity takes place at the boundaries between tectonic plates, 85 percent of earthquake lights were reported during quakes that took place when a single tectonic plate cracks in what's known as continental rifting. That type of quake occurs only five percent of the time.
So what's happening? Earthquakes start as rising stress deep in the Earth's crust — the quake only occurs when that stress is released. But before that happens, according to lab experiments, the stress breaks the bonds among pairs of negatively charged oxygen atoms. Those charged atoms (ions) are released and flow through cracks in the rock and up towards the surface, where they accumulate. The researchers think that those high-density groups of oxygen ions give pockets of air an electric charge, turning them into a glowing plasma. This could explain not only the lights themselves, but why they're witnessed before and during a quake: tectonic stress can build up over minutes, hours, or even days. If the researchers are correct, these lights could be a useful warning sign of an oncoming tremor.