Earthquake lights – a rare luminous phenomenon that appears in the sky during or before seismic activity or volcanic eruptions – are more likely to occur on or near rift environments, says a team of researchers led by Dr Robert Thériault from the Ministère des Ressources Naturelles of Québec.

Earthquake lights, also known as ground lights, take a variety of forms, including spheres of light floating through the air. From the early days of seismology, this natural phenomenon has intrigued scientists.

Seconds before the 2009 L’Aquila, Italy earthquake struck, pedestrians saw 10-cm high flames of light flickering above the stone-paved Francesco Crispi Avenue in the town’s historical city center.

On November 12, 1988, a bright purple-pink globe of light moved through the sky along the St. Lawrence River near the city of Quebec, 11 days before a powerful quake.

In 1906, about 100 km northwest of San Francisco, a couple saw streams of light running along the ground two nights preceding that region’s great earthquake.

Dr Thériault and his colleagues have identified and studied 65 best-documented cases of earthquake lights since 1600 CE in the Americas and Europe.

Their results, reported in the journal Seismological Research Letters, show that among 65 events, about 85 percent appeared spatially on or near rifts, and 97 percent appeared adjacent to subvertical faults – a rift, a graben, strike-slip or transform fault. Intraplate faults are associated with just 5 percent of Earth’s seismic activity, but 97 percent of documented cases of earthquake lights.

“The numbers are striking and unexpected. We don’t know quite yet why more earthquake light events are related to rift environments than other types of faults, but unlike other faults that may dip at a 30-35 degree angle, such as in subduction zones, subvertical faults characterize the rift environments in these cases,” Dr Thériault said.

Two of the 65 earthquake light events are associated with subduction zones, but the scientists suggest there may be an unknown subvertical fault present.

“We may not know the fault distribution beneath the ground. We have some idea of surface structures, but sedimentary layers or water may obscure the underlying fault structure,” Dr Thériault said.

While the 65 earthquakes ranged in magnitude, from M 3.6 to 9.2, 80 percent were greater than M 5.0. The earthquake lights varied in shape and extent, though most commonly appeared as globular luminous masses, either stationary or moving, as atmospheric illuminations or as flame-like luminosities issuing from the ground.

Timing and distance to the epicenter vary widely. Most earthquake lights are seen before and/or during an earthquake, but rarely after, suggesting to the reseacrhers that the processes responsible for formation of earthquake lights are related to a rapid build-up of stress prior to fault rupture and rapid local stress changes during the propagation of the seismic waves. Stress-activated mobile electronic charge carriers, termed positive holes, flow swiftly along stress gradients. Upon reaching the surface, they ionize air molecules and generate the observed luminosities.

Eyewitness reports and security cameras captured a large number of light flashes during the 2007 Pisco, Peru M 8.0 earthquake. Together with seismic records obtained on a local university campus, the automatic security camera records allow for an exact timing and location of light flashes that illuminated a large portion of the night sky. The light flashes identified as earthquake lights coincided with the passage of the seismic waves.

Dr Thériault likes the account of a local L’Aquila resident, who, after seeing flashes of light from inside his home two hours before the main shock, rushed his family outside to safety.

“It’s one of the very few documented accounts of someone acting on the presence of earthquake lights,” Dr Thériault said.

“Earthquake lights as a pre-earthquake phenomenon, in combination with other types of parameters that vary prior to seismic activity, may one day help forecast the approach of a major quake,” he concluded.

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Thériault R et al. 2014. Prevalence of Earthquake Lights Associated with Rift Environments. Seismological Research Letters, vol. 85, no. 1, pp. 159-178; doi: 10.1785/0220130059