Until recently, earthquake lights were folklore. For centuries people have reported rare flashes, glows and various fireworks associated with large earthquakes, but science can't do much with eyewitness stories. (Indeed, some argue that they may be hallucinations.) After lights were photographed in Japan during a month-long earthquake swarm in the mid-1960s, and other careful observations were made elsewhere in the world, earthquake specialists had something to chew on.
Characteristics of Earthquake Lights
Earthquake lights occur before, during and after earthquakes. They are cool and quiet, colored white or blue or red. They are usually dim, but sometimes are brighter than moonlight.
They take various forms: globes, bands, rays, sheets, clouds. They tend to rise from the ground. They have been reported at sea. They may flicker or shine steadily. They may be silent or accompanied by a crackling or bristling sound. Sometimes light boils from the ground like flames. They may be as brief as lightning or glow for several minutes.
Earthquake lights have been accompanied by low-frequency radio noise in the 10 to 20 kHz range.
Earthquake lights have been seen weeks before or after earthquakes and hundreds of kilometers from the epicenter. They are more common in areas of hard, crystalline rocks and near dip-slip rather than strike-slip faults.
Bad Theories of Earthquake Lights
It is clear that somehow, subterranean stress in an earthquake zone is transformed into a luminous phenomenon in the surroundings. Most of the theories proposed are unconvincing.
- Heat: Friction on the fault might heat particles or ignite ground gases. But the heat would not move swiftly, nor would it occur without earthquakes, nor is there enough gas (such as methane) in most places.
- Radiation: Radon gas is released by fractured mineral grains and could ionize the air, but the quantities are too small.
- Triboluminescence/triboelectricity: Some minerals respond to mechanical strain by generating light or electric charge—but not enough of either.
- Piezoelectricity: Quartz crystals generate voltage under strain—but not enough, and the resulting currents cancel out in real rocks, and besides rocks are electrical insulators.
- Streaming potentials: Moving fluids in narrow cracks tend to become electrically charged—but not strongly.
The p-Hole Theory
The best theory of earthquake lights comes from mineral physicist Friedemann Freund, who took a frustrating fact and made it the cornerstone of a new hypothesis. Under the conditions of most earthquake faults, lab experiments show that rocks are electrical insulators. But the experiments are very frustrating because rock samples conduct electricity well along their surfaces. This swamps the effects being looked for, unless the samples are first roasted in vacuum. But at that point you don't have realistic rocks.
Freund realized that mineral grains in ordinary rocks are naturally full of flaws; specifically, oxygen atoms in imperfectly ionized states. There are millions of oxygen atoms in every piece of silicate mineral with one electron short, bound together in peroxy bonds. When such a bond is broken for whatever reason, the result is a pair of "holes" of positive charge, or p-holes. Anyone who knows the physics of semiconductors knows about holes. They carry charge, in their way, just as effectively as electrons do.
In the lab, rock surrounded by vacuum or air naturally has its p-holes move toward the surface, where they generate the unwanted conductivity. In nature, holes tend to stay put unless activated by rock stresses. Then they move readily through most igneous and metamorphic rocks, unlike electrons. Freund conducted experiments that impacted rocks to activate clouds of p-holes. Calculating that the charge clouds might trigger optical and radio waves, he added sensors that in fact detected light and radio noise on sample surfaces.
A New Paradigm in Seismology?
Freund's mechanism for turning rock stress into distant light and electrical activity extends to earthquake phenomena. A cloud of p-holes, released at depth from seismic stress, can erupt from the ground as a solid-state plasma, causing effects that include earthquake lights, infrared emissions detected from space, radio noise, large-scale disturbances in the upper atmosphere, and even animal behavior and human premonitions.
I've seen Freund's presentations at scientific meetings for several years, but they didn't sink in until I read his 2003 article in the non-mainstream Journal of Scientific Exploration. I think it should be taken seriously. But Freund has written, "The discovery of p-holes as dormant yet powerful charge carriers in the Earth's crust calls for a new paradigm in earthquake research and beyond." People who call for new paradigms have a hard, uphill battle. But in February 2009 Freund gave an invited talk to the seismologists of the U.S. Geological Survey, where he got a respectful hearing. The conversation goes onward.
PS: Freund's theory merges at its far end with an even more explosive hypothesis, the tectonic strain theory of UFOs. This ultimate marriage of neuroscience and geophysics is a truly weird conjecture.
First published 5 May 2006