Why Strange Lights Sometimes Precede Earthquakes

For centuries people have observed strange phenomena before large earthquakes, such as light emanating from ridges and mountaintops. These reports were once dismissed by many scientists, in part because they are often entangled in unscientific theories. For example, some who reported the lights thought they were produced by UFOs. 

But the lights are not (necessarily) hallucinations nor created by E.T. "Earthquake lights are a real phenomenon--they're not UFOs," researcher Robert Thériault, a geologist at Quebec's Ministry of Natural Resources, told Nature. "They can be scientifically explained."
In study published in the January/February issue of the journal Seismological Research Letters, Thériault and colleagues pulled together reliable sightings of these lights since 1600, and found some strange similarities. A total of 63 out of the 65 sightings occurred along nearly vertical faults. The researchers suggest that along these faults, the stress of rocks grinding against each other produces electrical charges, which can travel upward and interact with the atmosphere to create light.
"When the stress of an earthquake hits the rock, it breaks chemical bonds... creating holes of positive electrical charge. These 'p holes' flow can vertically through the fault to the surface, triggering strong local electric fields that can generate light," according to Nature. The existence of these p-holes was proposed in 2003 by researcher Friedemann Freund, who wrote that "under normal conditions [these p-holes] are dormant, but when they 'wake up'" during earthquakes, "the rocks begin to sparkle and glow."
The researchers sifted through many strange tales to arrive at their final 65 reliable sightings, Nature reported. Among those: 

Off the Peruvian coast in August 2007, a fisherman reported the sky turning violet a few minutes before the sea began shaking. Near Ebingen, Germany, in November 1911, a woman reported seeing glows that moved along the ground “like snakes” as a quake began.

Spreading the word about earthquake lights could help them to be used as possible earthquake warnings. In at least one case, lights have alerted people about an impending quake, according to Nature: "Near L’Aquila, Italy, in April 2009, a man saw white flashes reflecting off his kitchen furniture in the early hours of the morning and took his family outside to safety." And two hours later, a huge earthquake rocked the region.