Eerie glows of earthquake lights have awed observers for millenniums. They also puzzle modern scientists. But a few geophysicists are groping toward an explanation.
The most recent suggestion has been made by James D. Byerlee, Malcolm J. Johnson, and David A. Lockner of the US Geological Survey (USGS). They say the luminous shafts and bright glows that accompany some earthquakes may be an electrical effect made possible by friction along a fault.
As Lockner points out, this is not a complete explanation of the phenomenon. But it does get around one of the biggest stumbling blocks for physicists seeking an electrical cause - namely the conductivity of the ground.
Normally, the ground is such a good electrical conductor that it would be impossible to build up the electric charges and sustain the electric force field needed to account for the lights. Byerlee explains, however, that when the friction of slippage along the fault is large, it heats the rocks and drives off any water they may contain. As a result, their electrical conductivity drops 10 billion-fold. The rocks become electrical insulators and can support electric fields strong enough to cause the lights when the heating effect extends all the way to the surface. Byerlee adds that the electric field itself may be generated when electric charges are separated as the water is boiled off.
This could account for electrical forces over land, but not over the sea, where the lights have also been seen. Sea water is also a good electrical conductor. Byerlee says he and his colleagues have not yet found a way to get around that fact.
An earlier explanation of the lights, suggested by Helmut Tributsch of the Fritz Haber Institut in West Berlin, is based on a different source of electricity. This is the so-called piezoelectric effect, in which some kinds of crystals, such as quartz, set up an electrical field when subjected to pressure.
In 1978, Tributsch suggested that this could cause what are called glow discharges which would not be hampered by the ground's conductivity. These, in turn, would create masses of electrically charged particles called ions. He observed that light-producing electrical discharges could be expected as a result of strong charging of aerosol particles. He further suggested that the apparent ability of animals to sense an impending quake might be due to an awareness of these particles.
Commenting on Tributsch's suggestion, Stuart A. Hoenig of the University of Arizona told how he produced piezoelectric effects by stressing rocks. Light flashes could be seen in a darkened room. He suggested that electrons and ions released in the process were exciting molecules of water and oxygen in the air to emit light.
While none of this fully explains earthquake lights, it does suggest that electricity is involved. An ancient mystery, once dismissed as a folk tale, may soon yield up its secret.