During the Magnitude 8 earthquake that hit Mexico on September 8, 2017, most of the inevitable media hype was on the observation of mysterious blue lightning in the sky during the temblor. The lights witnessed by thousands of witnesses are the so-called Earthquake Lights (EQLs).
These luminous phenomena that resemble lightning but that are not associated with thunderstorms have already been reported in the past on several occasions.
Before the Mexican quake EQL were observed during the M 7.8 quake that hit New Zealand in November last year or before the Magnitude 6 Napa temblor, which struck the San Francisco Bay area in August 2014.
Ancient chronicles have described phenomena similar to telluric lights for millennia, but these observations have never been taken into account until science has verified their existence, and this happened relatively recently.
Already the Egyptians documented the occurrence of lights during an earthquake, which might correspond to EQLs. Pliny the Elder (23-79 AD) in his monumental work “Historia Naturalis” describes a bright phenomenon that occurred during the earthquake of Modena (Northern Italy) in 89 BC.
The Roman historian Cornelius Tacitus (55-120 AD) in his “Annales”, reported about a violent earthquake that occurred in Asia Minor, levelling 12 cities. According to Tacitus’s description “huge flames burst out during the earthquake”.
Over time, EQLs were often reported, both before and during earthquakes, as in the case of the Kalapana quake (Hawaii) in 1975 and the Tangshan (China) deadly jolt in 1976.
However, their official existence was ascertained only during the 60s, when some seismologists took pictures of EQLs during the seismic swarm that rattled the region of Nagano, Japan, between 1965 and 1967.
The shape, duration and color of EQLs can vary considerably, although they often resemble boreal auroras, with colors ranging from white to blue. The origin of EQLs is not totally clear yet; quite a lot of causes have been proposed for their formation.
Recently, oxygen ionization has been proposed as a possible cause, for some types of rock, and it would be brought about by tensions developing 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.
The formation of very strong electric fields has also been proposed as a result of piezoelectric phenomena in quartz rocks subject to intense stress. Piezoelectricity is the electrical charge that accumulates in certain solid materials (such as crystals, ceramics, bones, DNA and various proteins) when they are affected by the applied mechanical stress.
Another hypothesis, though more difficult to prove, suggests the distorsion of the terrestrial magnetic field or of the ionosphere in the region affected by tectonic stress.
A further theory, presented in 2014, calls into question the phenomenon of triboluminescence, that is the generation of light produced through the breaking of chemical bonds in a material when it is pulled apart, scratched, crushed, or rubbed.
Whatever their origin, EQLs have caused fear and astonishment for millennia, as they are associated with earthquakes, arguably the most destructive natural phenomena known by our civilization.
Paolo Oppizzi (Geologist, Vice-President of GeoSocial)
Federico Pasquaré Mariotto (Associate Professor, Insubria State University)