In the last few hours, videos of a very rare optical phenomenon that occurred in Mexico following the 7.0 magnitude earthquake that hit the country in the night between Tuesday and Wednesday have been circulating on social networks. At the same time and after the quake, several people from Mexico City, 370 km from the epicenter, noticed flashes of various shapes and colors in the sky.

The phenomenon – still discussed and about which there are several theories, which leave various experts skeptical – is called “telluric light”, and is rarely reported. In general, the sightings involve earthquakes with a magnitude greater than 5.0, with more reports at night, as happened in Mexico. Generally these flares are described as lasting a few seconds in conjunction with earthquakes, but there have been cases in the past where “telluric lights” were reported several months earlier (which contributed to some skepticism).

As to what the exact cause of these phenomena is, there is no clear answer. According to the National Institute of Geophysics and Volcanology (INGV) one hypothesis is the generation of intense electric fields by piezoelectricity, that is, the ability of some materials to show an electrical voltage between extremes when subjected to pressure. In this case the phenomenon would concern the tectonic movements of the rocks containing quartz.

Another hypothesis is that the generation of the seismic lights has to do with the ionization of the oxygen contained in some types of rocks, before and during the shock. In this case the ions would pass through the cracks created inside the rocks and after reaching the atmosphere they would ionize in turn small layers of air, “transforming them into packets of plasma emitting light”. According to the INGV, other researches have instead highlighted that the seismic lights could depend on the angle of the fault, “with an increase in the case of sub-vertical or almost vertical faults”.

In some cases, the light flashes visible on the horizon were instead explained by short circuits in the power lines or in the high voltage transformer substations, following the surface movements induced by earthquakes.

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