Bizarre Earthquake Lights Tied to Rifts in Earth
By Tia Ghose, LiveScience
Mysterious flashes of electricity known as earthquake lights are more likely to happen near rifts, where pieces of the Earth are pulling away from each other, new research suggests.
The quick buildup of stress at these nearly vertical faults may cause electrical current to flow to the surface and cause the eerie light shows, the researchers find.
The conclusions, published Jan. 2 in the current issue of the journal Seismological Research Letters, were drawn from analyzing 65 documented cases of earthquake lights over the last 400 years.
Elusive phenomenon
These strange light shows, which can look like flickering flames or floating, glowing orbs, according to those who have observed them, have occurred during or before several of the world's greatest earthquakes, including two days before the Great San Francisco Earthquake of 1906.
But until recently, most seismologists didn't believe the earthquake lights were real because the reports were all anecdotal and hard to explain physically. [The Top 10 Unexplained Phenomena]
"Earthquake lights are totally underreported," said study co-author Friedemann Freund, a crystallographer at the NASA Ames Research Center in Mountain View, Calif., and San Jose State University. "They are often things that happen within a fraction of a second."
There's not always a person around to see them and when they do report them, they were often discounted by scientific journals, Freund said.
The advent of better documentation and video cameras has changed that. For instance, just before the earthquake that struck L'Aquila, Italy, in 2009, bystanders reported flames flickering up from the pavement. Video and eyewitness reports also described several weird light anomalies during the magnitude-8.0 earthquake in Pisco, Peru, in 2007. In one case in the early '70s, luminous drifting globes thought to be possible UFOs when they were observed in Canada's Yukon Territory later were linked to earthquake lights.
Origin of lights
But exactly why these lights happen was still a mystery. To get at the question, Freund and colleagues looked at documents dating back to 1600 for reports of earthquake lights.
The team found 65 cases that were well documented from North and South America and Europe. Of those cases, 97 percent seemed to happen at faults within continental plates, rather than at subduction zones, or the boundaries where one plate is diving below another. That's despite the fact that most big earthquakes happen at subduction boundaries.
Instead, about 85 percent of the time, lights seemed to happen at places where the tops of the continental plates buckle, creating fissures, or rifts, where the Earth pulls apart.
"You have two sides of a piece of continent are pulling apart and in the middle there's a part that just falls vertically down," often by miles over millions of years, Freund told LiveScience.
These rifts form steep, nearly vertical faults that stretch deep into the Earth's magma, allowing primitive magmatic rocks that were once deep below ground to migrate closer to the Earth's surface.
The authors think that because of the crystal structure of these magmatic rocks, when stressed they are likelier to generate electricity, which then flows to the Earth's surface, ionizes the air, and produces flashes of light.
The earthquake lights also appeared at other nearly vertical faults, for example, the strike-slip San Andreas fault. These faults likely make it easier for electrical charges in these dark magmatic rocks to reach the surface, said study co-author John Derr, a seismologist at the Albuquerque Seismological Laboratory.
Many unknowns
Still, the distance from the epicenter of the quake, and the timing of the light show, can be very different. Sometimes, earthquake lights aren't even associated with earthquakes, Freund said.
That's because electrical current can flow from a stress buildup that could either be released in a catastrophic rupture, or gradually over several days, with no quakes.
Two of the earthquake lights noted in the records appeared near subduction zones, but they may actually have been caused by hidden, nearly vertical faults within the subduction zone that are buried under water or layers of sediment, Freund said.
Read full article on LiveScience; Extracts of the Research Paper here.
And where Marinduque lies...
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| The steep Malindig Volcano located south of Marinduque Island. Google Earth |
Seismicity and Stress Change Along the Central Philippine Fault Zone
American Geophysical Union
Recent findings showed that the Philippine fault zone (PFZ) has a creeping section, a transition zone and a locked portion in its central portion. This part of the PFZ is comprised of the Guinyangan, Masbate, northern Leyte and southern Leyte faults. In this study, an attempt is taken to have a closer look on the seismic pattern and relationship between the creep events, moderate quakes and major events along in Guinyangan, Masbate, and northern Leyte, respectively based on the recent available seismic data and field surveys.
Dislocation along a fault that involves no rapid release of energy in a seismic event is called creep. Creep was identified along the central PFZ in early 1990s using GPS records. On the other hand, moderate quakes are earthquakes with magnitude range of M5.5~M7. In central PFZ, moderate quakes were recognized to occur along the transition zone through an event in early 2003 that was accompanied by a large ground rupture.
Although moderate events have usually a calculated M7 magnitude, the extent of the observed ground rupture indicated an earthquake with magnitude greater than M7. The moderate events in this area may have a recurrence interval of as short as 10 years. Major events, on the other hand, are devastating earthquakes at least M7.
Historical events in this portion of the PFZ show that major events in the locked portion also have a relatively short recurrence interval that range from 60-100 years. The northernmost part of the central PFZ is the Guinyangan fault.
This portion is considered locked and the locus of major quakes in the past. South of the Guinyangan fault is the Masbate fault. A very interesting event occurred in this fault in 2003 along the Masbate fault, a transition zone. South of this portion is the northern Leyte fault which is considered undergoing a creep activity.
After the 2003 event, the Guinyangan fault was anticipated to experience some changes in local stress field. However, the temporal and spatial plots of seismicity indicate a west-northwestward propagation of seismic activity along the Sibuyan fault. Sibuyan fault is an offshore fault traversing the Sibuyan Sea and probably into Taal area and/or branches out into the southern part of Marinduque island.
Considering the amount of data available and the peculiar seismic activity along the central PFZ, we try to examine the seismicity and determine if a particular seismic pattern is discernable in this region and correlating them to possible regional and local stress change in a specific portion of the PFZ.
Considering the importance of the results of this study and in view of the presence of major cities and towns in southern Luzon, the Sibuyan and Guinyangan faults need more attention for seismic and crustal deformation studies. Concerning the disaster mitigation and preparedness in the southern Luzon regions, finding from such researches would be an important input for local stress change research that could reveal the probability of local earthquake occurrence.
Keeping in mind that the San Andreas fault has almost the same features, a comparison would be undertaken to note any similarities or differences between the two structures.
Source: NASA Astrophysics Data System
