An earthquake light (EQL) is a luminous aerial phenomenon that reportedly appears in the sky at or near areas of tectonic stress, seismic activity, or volcanic eruptions. Skeptics point out that the phenomenon is poorly understood and many of the reported sightings can be accounted for by mundane explanations.
The lights are reported to appear while an earthquake is occurring, although there are reports of lights before or after earthquakes, such as reports concerning the 1975 Kalapana earthquake. They are reported to have shapes similar to those of the auroras, with a white to bluish hue, but occasionally they have been reported having a wider color spectrum. The luminosity is reported to be visible for several seconds, but has also been reported to last for tens of minutes. Accounts of viewable distance from the epicenter varies: in the 1930 Idu earthquake, lights were reported up to 70 miles (110 km) from the epicenter. Earthquake lights were reportedly spotted in Tianshui, Gansu, approximately 400 kilometres (250 mi) north-northeast of the 2008 Sichuan earthquake's epicenter.
During the 2003 Colima earthquake in Mexico, colorful lights were seen in the skies for the duration of the earthquake. During the 2007 Peru earthquake lights were seen in the skies above the sea and filmed by many people. The phenomenon was also observed and caught on film during the 2009 L'Aquila and the 2010 Chile earthquakes. Video footage has also recorded this happening during the 9 April 2011 eruption of Sakurajima Volcano, Japan. The phenomenon was also reported around the Amuri Earthquake in New Zealand, that occurred 1 September 1888. The lights were visible in the morning of 1 September in Reefton, and again on 8 September.
More recent appearances of the phenomenon, along with video footage of the incidents, happened in Sonoma County of California on August 24, 2014, and in Wellington New Zealand on November 14, 2016 where blue flashes like lightning were seen in the night sky, and recorded on several videos. In September 8, 2017, many people reported such sightings in Mexico City after a 8.2 magnitude earthquake with epicenter 460 miles (740 km) away, near Pijijiapan in the state of Chiapas.
Appearances of the earthquake light seem to occur when the quakes have a high magnitude, generally 5 or higher on the Richter scale. There have also been incidents of yellow, ball-shaped lights appearing before earthquakes.
Earthquake lights may be classified into two different groups based on their time of appearance: (1) preseismic EQL, which generally occur a few seconds to up to a few weeks prior to an earthquake, and are generally observed closer to the epicenter and (2) coseismic EQL, which can occur either near the epicenter (“earthquake‐induced stress”), or at significant distances away from the epicenter during the passage of the seismic wavetrain, in particular during the passage of S waves (“wave‐induced stress”).
EQL during the lower magnitude aftershock series seem to be rare.
Research into earthquake lights is ongoing; as such, several mechanisms have been proposed. Positive Holes is one such model 
Some models suggest the generation of EQLs involve the ionization of oxygen to oxygen anions by breaking of peroxy bonds in some types of rocks (dolomite, rhyolite, etc.) by the high stress before and during an earthquake. After the ionisation, the ions travel up through the cracks in the rocks. Once they reach the atmosphere these ions can ionise pockets of air, forming plasma that emits light. Lab experiments have validated that some rocks do ionise the oxygen in them when subjected to high stress levels. Research suggests that the angle of the fault is related to the likelihood of earthquake light generation, with subvertical (nearly vertical) faults in rifting environments having the most incidences of earthquake lights.
Another possible explanation is local disruption of the Earth's magnetic field and/or ionosphere in the region of tectonic stress, resulting in the observed glow effects either from ionospheric radiative recombination at lower altitudes and greater atmospheric pressure or as aurora. However, the effect is clearly not pronounced or notably observed at all earthquake events and is yet to be directly experimentally verified.
During the American Physical Society's 2014 March meeting, research was provided that gave a possible explanation for the reason why bright lights sometimes appear during an earthquake. The research stated that when two layers of the same material rub against each other, voltage is generated. The researcher, Professor Troy Shinbrot of Rutgers University, conducted lab experiments with different types of grains to mimic the crust of the earth and emulated the occurrence of earthquakes. "When the grains split open, they measured a positive voltage spike, and when the split closed, a negative spike." The crack allows the voltage to discharge into the air which then electrifies the air and creates a bright electrical light when it does so. According to the research provided, they have produced these voltage spikes every single time with every material tested. While the reason for such an occurrence was not provided, Professor Troy Shinbrot referenced the light to a phenomenon called triboluminescence. Researchers hope that by getting to the bottom of this phenomenon, it will provide more information that will allow seismologists to better predict earthquakes.
According to Brian Dunning researchers should be concerned about the fact that there are no documented "confirmed observations" of earthquake lights. It is a red flag that there is no consistency of what they are, when they happen and where they happen. It is likely that there "is not one, known, proven phenomenon". However, a significant amount of video footage has surfaced since the advent of sites like YouTube that claim to represent this —one example being the 2017 Mexico earthquake— although no consistent explanation has been agreed upon. There is also a "staggering volume of literature... hardly any of these papers agree on anything... I'm forced to wonder how many of these eager researchers are familiar with Hyman's Categorical Imperative: 'Do not try to explain something until you are sure there is something to be explained'". Dunning's final conclusion is that until there is "pending decent evidence" be skeptical of claims of earthquake lights.
Robert Sheaffer writes that he is surprised how many skeptics and science bloggers have accepted earthquake lights as a real phenomenon without checking the source of the claim, or doing basic research on what the lights might be. Sheaffer on his Bad UFO blog shows examples of what people claim are earthquake lights, then he shows photos of iridescent clouds which appear to be the same. He states that "It's truly remarkable how mutable "earthquake lights" are. Sometimes they look like small globes, climbing up a mountain. Sometimes they look like flashes of lightning. Other times they look exactly like iridescent clouds. Earthquake lights can look like anything at all, when you are avidly seeking evidence for them."
Sharon Hill writes that the science isn't in on earthquake lights, not enough research has been done. She states that not all earthquakes are the same and it may be possible that "extension" and "compression" faults produce "different behaviors on the surface as well as subsurface". She understands why skeptics are reluctant to confirm that this might be happening "because of the unreliability, irreproducibility and inadequate explanation for it". Also a possibility is that "stronger quakes break electrical wiring and cause transformers to explode, causing the flashes". As more research is done on electrical signals concerning earthquakes we will have a better understanding of this phenomenon.
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