Radio earth: the radio-seismic connection

Whole Earth Review, Fall, 1990 by Joe Tate

Even in the nineteenth century, in the days of Tesla and Edison, radio noise caused by lightning was known to have recognizable propagation patterns. It was these patterns that Jansky was measuring when he discovered cosmic radio.

Tesla actually calculated the resonant frequency of the Earth, and proposed that electromagnetic waves of this frequency (6-8 Hz) should be generated by the planet from the action of lightning. These "Schumann resonances," as they became known, were finally detected in the 1960s.

Other strange radio emissions were noticed at about the same time, a time when many new radio observatories were starting operation at various places around the globe. These observatories could each detect and record a wide range and volume of electromagnetic radiation (EMR). Before and during the great Chilean earthquake 6f 1960, unusual strong signals were received at six widely scattered radiotelescopes. The connection between these radio signals and the earthquake was eventually shown by james Warwick of the University of Colorado, who analyzed the observatories' separately recorded data (fig. 1, p. 102). Earthquakes generate radio waves! But how?

Twenty-two years later, after performing a series of laboratory experiments in which rocks were crushed in powerful presses and the resulting electromagnetic emissions were measured, Warwick's paper describing the phenomenon appeared in the April 1982 issue of the Journal of Geophysical Research.

In the meantime, other experimenters had recorded similar effects in Japan, France, the United States and the Soviet Union. Several studies of satellite data revealed marked increases in very-low-frequency (VLF) emissions from epicenter regions before and during major earthquakes. In Greece, researchers found that telluric currents (natural currents of electricity flowing in the Earth) fluctuated prior to earthquakes.

Ambient Power

In 1979, I was experimenting with methods of turning radio energy in the air into usable electric power. I developed a clock which drew its power from an antenna that was just a long piece of wire stretched out horizontally about 20 feet above the ground. The power supply for the clock worked something like an old-style crystal radio, except that it did not have a tuning circuit. Because of this, the Crystal Clock (as I called it) was able to absorb a wide spectrum of radio noise from the antenna and yield electric power. The power supply was able to deliver much more current than was developed in a crystal radio, although its output was still just a few millivolts.

In the early eighties I demonstrated the clock to the late Frank Oppenhiemer, then director of San Francisco's Exploratorium, where I worked in the exhibit repair shop. Oppenhiemer suggested recording the power supply's output over a long period of time to determine its dependability. After all, the device relied completely on whatever stray signals happened to be in the air.

Using an Atari computer which had been donated to the museum, the output of the clock's power supply was measured continuously and recorded on floppy disk. This was done by feeding the unregulated voltage output directly into the computer's joystick port. I began calling this power supply the Ambient Power Module" (APM), because it extracted power from ambient background radio noise. This small circuit, when connected to antenna and ground, used the potential difference between air and ground to generate a small direct current continuously.

As we studied the recorded data, mild fluctuations were noted in a daily cycle. The patterns were consistent over long periods of time, though they differed in different locations. Aside from that, the APM looked like a very dependable source of power. Until the spring of 1984.

On April 24, 1984, a 6.0 magnitude earthquake struck about 90 miles from the APM recording station in Sausalito. Days later, while looking through the data, I noticed that the APM output had dropped to less than half its normal value for several hours during the afternoon 6 days before the earthquake (fig. 2). This was very peculiar, because most of the APM's power came from broadcast signals, and broadcasting stations hadn't done anything different that afternoon. Apparently something had temporarily depressed the propagation of radio waves. At high frequencies, such effects can be caused by atmospheric conditions. But the lower frequencies involved here are hardly affected, particularly not the signals from the nearest stations, which account for most of the power received. It was tempting to think this strange radio depression might somehow have been a precursor to the earthquake.

Several smaller quakes had occurred in the area during the year before. Perhaps these also were preceded by similar radio anomalies. Looking back through the accumulated data on the APM's power output, indeed, smaller, less obvious radio depressions were found to occur prior to the lesser earthquakes. I called the United States Geologic Survey (USGS) office and told them about these radio events. I learned from them that ham operators in the area had also reported radio noises accompanying earthquakes, but no one had recorded them. Jack Everenden, with whom I was speaking, asked for copies of my data, which I sent.