Radio waves signal earthquakes.
From the bright flashes reported to appear in the sky duringstrong earthquakes to computer breakdowns during severe tremors, scientists have long suspected that seismic activity is associated with a variety of electrical effects. Recently researchers have been taking a careful look at this link, with an eye toward using it to predict earthquakes.
One such study is being conducted by Joseph Tate of AmbientResearch in Sausalito, Calif., and William Daily at Lawrence Livermore National Laboratory in Livermore, Calif. With a system of radio wave monitors distributed along California's San Andreas fault, the researchers have recorded two kinds of changes in atmospheric radio waves prior to earthquakes that occurred between 1983 and 1986.
The most common change is a drop in the radio signals thatnormally pervade the air as a result of lightning and human sources such as car ignition systems and electric power grids. This reduction typically occurs one to six days before an earthquake and can last for many hours. For example, a magnitude 6.2 earthquake that shook Hollister, Calif., in April 1984 was preceded six days earlier by a 24-hour drop in radio signals being monitored 30 miles from the quake's epicenter. Tate and Daily have found that the larger the earthquake, the longer the time between the radio wave depression and the quake.
Laboratory studies have shown that the electrical conductivityof rocks increases as they are stressed. Based on this and their electrical modeling of the ground, Tate and Daily think the increased conductivity of stressed rocks near the fault causes more radio waves to be absorbed by the ground rather than their traveling through the air. They also plan to test a possible link between radio wave drops and the emission of radon gas, which itself is thought to be a quake precursor. The radon may ionize the air, making it temporarily more absorptive than the detector antenna.
The researchers have also found, in addition to these drops,another prequake phenomenon in which short pulses of increased radio wave activity are emitted. For example, five days before the magnitude 6.5 earthquake hit Palm Springs, Calif., in July 1986, a station 15 miles from the epicenter detected a rise in radio signals. This sort of emission is consistent with laboratory work showing that cracking rocks release electromagnetic signals.
Tate says that in their first attempts at predicting earthquakesin 1984 and 1985, they did not miss a single event, so he is optimistic about using this technique for short-term forecasting of San Andreas quakes. "In three to five years," he says, "we should be able to issue [earthquake] warnings."
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|Date:||Dec 20, 1986|
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