January 3, 2014
Subvertical Faults Can Create Electrical Light Shows During Earthquakes
April Flowers for redOrbit.com - Your Universe Online
A new study, published in Seismological Research Letters, reveals that rare earthquake lights are more likely to occur on or near rift environments. These environments are where subvertical faults allow stress-induced electrical currents to flow rapidly to the surface.
Scholars have been intrigued by the luminous phenomena known as earthquake lights (EQL) since the earliest days of seismology. EQL appear before or during earthquakes, but rarely after.
Spheres of light floating through the air is one of the variety of forms that EQL can take. Pedestrians observed 4-inch-high flames of light flickering above the stone-paved Francesco Crispi Avenue seconds before the 2009 L'Aquila, Italy earthquake struck. In another example, a bright purple-pink globe of light moved through the sky along the St. Lawrence River near Quebec on November 12, 1988, 11 days before a powerful quake. About 62 miles northwest of San Francisco, a couple saw streams of light running along the ground two nights before the famous 1906 earthquake.
According to the team, the common factor associated with EQL appears to be continental rift environments. The study examined 65 documented cases of EQL since 1600 AD. Eighty-five percent of these appeared spatially on or near rifts, and 97 percent were observed adjacent to subvertical faults -- for example, a rift, a graben, a strike-slip or a transform fault.
"The numbers are striking and unexpected," said Robert Thériault, a geologist with the Ministère des Ressources Naturelles of Québec. Thériault worked with a team of colleagues to cull centuries of literature references, limiting the cases in this study to 65 of the best-documented events in the Americas and Europe.
"We don't know quite yet why more earthquake light events are related to rift environments than other types of faults," said Thériault, "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."
Of the 65 cases, two are associated with subduction zones, but Thériault suggests there may be an unknown subvertical fault present. "We may not know the fault distribution beneath the ground," said Thériault. "We have some idea of surface structures, but sedimentary layers or water may obscure the underlying fault structure."
The 65 earthquakes studied range in magnitude from M3.6 to 9.2. Eighty percent of them were M5.0 or greater. The observed EQL 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 emanating from the ground.
The researchers found that timing and distance to the epicenter varied widely. The fact that most earthquake lights are seen before or during a seismic event, but rarely afterwards, suggests that the processes responsible for EQL formation 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. Positive holes, which are stress-activated mobile electronic charge carriers, flow swiftly along stress gradients. When they reach the surface, they ionize air molecules and generate the EQLs.
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, interests Thériault.
"It's one of the very few documented accounts of someone acting on the presence of earthquake lights," said Thériault. "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," said Thériault.