Earthquake light: why it occurs, truth and doubts

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After the devastating earthquake that hit Turkey and Syria, and which caused more than 11,000 victims, many videos on social media showed images of the sky full of flashes and beams of light during the earthquake, the phenomenon of the so-called Earthquake light.

As reported by Sky Italy, geophysicists, as stated on the USGS website, Geological Institute of the United States, have conflicting ideas due to the fact that a correlation between duration, epicenter and lightning strikes in the sky has not yet been found.

Some think there is no solid evidence, while others have put forward physical hypotheses that would explain the phenomenon.

What are telluric lights

According to historical sources, the phenomenon of telluric lights would have been observed on the evening of 4 February 1783 by thousands of people in northern Sicily and southern Calabria, in Italy.

The chronicler of the time called them lights of such clarity as to emulate the very splendor of the sun. The following day, a violent earthquake razed Reggio Calabria and numerous other towns in Calabria, seriously damaging Messina as well.

Earthquake light are an unusual optical phenomenon of the atmosphere, similar in appearance to an aurora borealis, characterized by a brightness that appears in the sky above or near areas with tectonic stress, seismic activity or volcanic eruptions.

They are especially visible during the night. Chronicles of the past have described the presence of such lights, but for a long time they were considered a myth. Seismologists confirmed their existence when they were photographed in Japan's Nagano during the Mitsushiro earthquake swarm that occurred between 1965 and 1967.

Earthquake light are most evident during an earthquake, although lights have been reported to appear after the earthquake or even before, as was the case with the 1975 Kalapana and 1976 Tangshan earthquakes. The appearance of the lights is similar to that of an aurora borealis and the color usually varies from white to blue, but occasionally there have been reports of lights with a greater visible spectrum of colors.

The brightness is usually visible for a few seconds, but there have been cases where it lasted for tens of minutes. The causes of the telluric lights have not yet been fully clarified: various theories have been proposed to explain their manifestation.

The most recent one hypothesizes that the telluric lights are due to the ionization in some types of rocks of the oxygen present in them following the breaking of peroxide bonds produced by the high voltages that occur before and during an earthquake.

After ionization, the ions would go up along the cracks in the rock until they reach the atmosphere, in turn ionizing some layers of air and giving rise to a plasma capable of emitting light. Laboratory experiments confirm that some rocks are actually capable of ionizing the oxygen they contain when subjected to high levels of stress.

Another theory states that the cause is to be sought in the intense electric fields created by piezoelectricity by the tectonic movements of rocks containing quartz. Another possible explanation is a local distortion of the Earth's magnetic field or ionosphere in the region of tectonic stress, resulting in locally observed luminescence effects and probably due to the combined effect of ionospheric radiative recombination at lower altitudes and higher atmospheric pressure than at conditions that generate the aurora borealis.

However, the effect is not very pronounced nor clearly observed in all seismic events and has not yet been directly verified experimentally. A recent hypothesis put forward in March 2014 at the annual meeting of the American Physical Society traces the cause back to the rubbing of the material layers with the consequent generation of an electric potential difference.

Laboratory experiments have been conducted with different types of rock grain sizes by reproducing the earth's crust and emulating an earthquake. It has been noted that with the fracturing of the veins there is a positive tension while with the closure there is a negative tension.

The fragmentation causes the discharge in the atmosphere which gives rise to an intense luminous emanation. The phenomenon is known as triboluminescence.