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. Once commonly challenged, it was not until photographs were taken during the Matsushiro earthquake swarm, from 1965 through 1967, that the seismology community acknowledged their occurrence.[1]
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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.[2] 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.[1] 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 from the epicenter.[3] Earthquake lights were reportedly spotted in Tianshui, Gansu, approximately 400 km north-northeast of the earthquakes epicenter.[4] The phenomenon was also widely observed and caught on film during the 2007 Peru, 2008 Sichuan, and 2010 Chile earthquakes.[5]
Earthquake lights are caused by an unknown mechanism. There are numerous theories as to how and why they occur.
One explanation involves intense electric fields created piezoelectrically by tectonic movements of rocks containing quartz[6].
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.[7]
There is also debate in the scientific community regarding Radon as a possible precursor to some earthquakes [8], so another theory is that glowing clouds might be light emission produced by ionization or plasma-chemical reactions[9]
  | USGS FAQ discussion on EQL - FAQs - Earthquake Effects & Experiences |
| | Earthquake lights photos - Earthquake Lights (EQLs) |
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