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People have reported seeing odd glowing and mysterious flickers of light before an earthquake.

An earthquake light is an unusual luminous aerial phenomenon that reportedly appears in the sky at or near areas of tectonic stress, seismic activity, or volcanic eruptions.

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 earthquake's epicenter.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 earthquake.  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.

In one example from 1727, a man from New England reported how he felt the ground shake then saw a ball of light roll onto his dog, causing the animal to yelp.

In Peru in 2009, a fisherman said he saw the sky turn violet before the earthquake, while in L'Aquila in 2009, a man saw white flashes before the tremor struck.

People gave strange explanations to them till now. They were even connected with UFOs too!

All that has changed now with a study, published in the January/February issue of Seismological Research Letters that proposes scientifically a mechanism by which earthquake lights form.  Scientists have now put forward a theory about this mystery glowing.
By compiling the reliable reports of earthquake lights occurring, scientists noted some odd similarities in the faults. In the study, the authors say that during an earthquake, the rocks grinding against each other generate electric charges that travel up along the fault lines. When the stress of an earthquake hits the rock, it breaks chemical bonds involved in these defects, creating holes of positive electrical charge.
Of the 65 earthquakes studied, 56 occurred along an active or ancient rift zone. And 63 of the 65 quakes took place where the geological faults that ruptured were almost vertical—as opposed to the shallower angles that many major faults take.
This steep geometry could explain how earthquake lights appear, Thériault and his colleagues , who did the study, say. Team member Friedemann Freund, a mineral physicist at the NASA Ames Research Center in Moffett Field, Calif., suspects it all starts with defects in a rock, where oxygen atoms inside a mineral’s chemical structure are missing an electron. When the stress of an earthquake hits the rock, it breaks chemical bonds involved in these defects, 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.
(Laboratory experiments have shown that electric fields can be generated in certain types of rock by squeezing.) So the shifting earth could be generating balls of electricity, whose glowing and flickering could be seen from far away.

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