Read more: 鈥�Strange skies: Seven wonders of the atmosphere鈥�
Glowing orbs that drift through the air; blue-white sheets of light; sparks and flashes and flames licking up from the ground鈥� all may be signs of disaster to come. For millennia, people have reported strange, baleful lights appearing before and during earthquakes. In 1746, for example, the flames dancing on San Lorenzo Island in Peru impressed prison governor Manuel Romero so much that he briefly released the detainees to let them watch. Three weeks later a huge quake hit nearby Lima and a tsunami washed 5 kilometres inland.
鈥淔or millennia, people have reported strange, baleful lights appearing before and during earthquakes鈥�
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There is plenty of photographic evidence for earthquake lights. They tend to accompany large quakes 鈥� with magnitudes above 6 鈥� centred at fairly shallow points in the Earth鈥檚 crust. It is not clear how the lights are produced, but of NASA鈥檚 Ames Research Center in Moffett Field, California, thinks that when rocks in the crust are squeezed, chemical bonds break to produce a pulse of electrical charge that travels up to the surface. 鈥淭he rocks become like a battery and produce an enormous amount of electric power,鈥� he says.
This process only generates a low voltage, but Freund thinks that the charge forms an ultra-thin layer at the surface. Since the charge is concentrated over a small distance, it would create a strong electric field, perhaps enough to ionise the air and create a luminous discharge that travels up away from the ground 鈥� explaining the orbs, flames and aurora-like sheets of light.
Freund does not know why the charge should form such a thin layer, or how the wave of ionisation is maintained for any distance through the air. But experiments are encouraging: crushing rocks in the lab produces electric charge and flashes of light. And low-frequency radio waves have been measured in earthquake zones, suggesting that there are currents underground.
As earthquake lights are so rare, it is hard to show that ionisation and the emission of radio waves coincide with them. Freund has yet to secure funding for a network of cameras and a data-processing centre to monitor such events, but he hopes that such a system, along with satellite imagery, could one day provide earthquake warnings akin to weather forecasts.
Lights may not be the only aerial omens of impending doom. In 2004, a curious linear gap appeared in the clouds above a fault line in Iran. An earthquake followed 69 days later. The gap opened again in 2005, and this time an earthquake followed after six days. Two Chinese geophysicists, Guangmeng Guo and Bin Wang, have suggested that hot gas escaping from the fault might cut through the clouds.