An article called Radio Loma Prieta on page 14 of the May 1990 issue of Discover magazine describes a recent accidental discovery of earthquake detection based on radio signals. Excerpts of the article follow:
Fraser-Smith, a physicist at Stanford, has a contract from the Navy to monitor very-low frequency radio waves (less than 10 hertz). ... In mid-September the airwaves around Corralitos[, California, 70 miles south of San Francisco,] started getting noisier, and on October 5, Fraser-Smith's antenna suddenly recorded a 20-to-30-fold jump in the signal below one hertz. In the following days the signal gradually declined, until by October 17 it was down to five times the normal background level. That afternoon it soared again, this time to 200 times the normal intensity. Three hours later a massive earthquake struck the San Francisco area. This epicenter was in Loma Prieta, four miles from Corralitos.
An article called Electrical clues precede some tremors on page 407 of the Dec 18, 1994 issue of Science News magazine describes additional studies that have been conducted. Excerpts of the article follow:
Battling the skepticism of their colleagues, some geoscientists are investigating the controversial idea that faults release electromagnetic signals prior to generating large earthquakes. Researchers last week reported hints that such electrical bursts have preceded several recent quakes, raising the possibility that this phenomenon might finally be drawing serious attention. ... At a meeting of the American Geophysical Union in San Francisco last week, [Seiya Uyeda, a seismologist who splits his time between Tokai University in Shimizu, Japan, and Texas A&M University in College Station,] reported that [an experimental observation network along the western coast of Japan] had indeed detected unusual changes in Earth's voltage in the weeks preceding four strong quakes that hit Japan between 1991 and 1993.
Anthony Fraser-Smith of Stanford University in Palo Alto, Calif., ... got into the earthquake business by chance after one of his machines detected an unusual magnetic disturbance before the Loma Prieta earthquake in October 1989. He has since set up five of these instruments at key sites along the San Andreas fault, waiting to see whether similar magnetic signals precede another quake. ... Stanford's Simon L. Klemperer and an Israeli colleague have devised a theoretical model that could explain what Fraser-Smith observed in 1989. They suggest that movement of the earth before a quake causes tiny water-filled pores in the rock to connect, thus enabling an electrical current to flow.
From an article called Broadcast Warning on page 27 of the March, 1990 issue of Scientific American magazine describes similar studies that have been conducted. Excerpts of the article follow:
Since 1987, in an effort to survey background noise that might affect VLF communications, the Stanford team has monitored radio waves ranging from .01 to 10 hertz. This ultra-low- frequency (ULF) margin of the VLF has been largely neglected by other scientists, according to Fraser- Smith. He and his colleagues originally installed their antenna, a metal cylinder wrapped in wire, on the Stanford campus. Then they moved the device to Corralitos, a small town where electromagnetic emissions from cars and a mass-transit rail system would not distort the readings. ... [The] amplitude of the .01 hertz signals had increased dramatically about three hours before the [Loma Prieta] quake.
Malcolm J. Johnson of the USGS points out that the geological literature is full of "hints" that earthquakes may be linked to electromagnetic activity. Japanese and Soviet scientists claim to have detected strong VLF signals before and during quakes. Witnesses have also recalled lightninglike flashes emanating from the earth during quakes and poor radio reception beforehand. ... Fraser-Smith suspects that only large quakes produce detectable signals. His equipment, he notes, did not record signals before or during a magnitude 5 quake near Corralitors this past August; the magnitude 7.1 Loma Prieta quake was more than 100 times stronger.
An article called Pre-quake quirks: Searching for predictors on page 231 of the October 13, 1990 issue of Science News magazine describes results of studies done in Japan. Excerpts of the article follow:
One report ... comes from Kozo Takahashi of the Communications Research Laboratory in Tokyo and Yukio Fujinawa of Japan's National Research Institute for Earth Science and Disaster Prevention in Tsukuba. The Japanese scientists found that anomalous electromagnetic changes preceded several quakes and an undersea volcanic eruption that shook the central eastern coast of Japan in July 1989. Takahashi and Fujinawa devised an electromagnetic radiation monitoring technique that effectively filters out urban and atmospheric background interference. The system measures the vertical electrical field between two electrodes - a steel pipe in a 603-meter-deep borehole, and a 40-meter-wide ring of grounded wire encircling it.
On July 5, 1989 a magnitude 4.9 temblor struck off the coast of Ito, about 150 kilometers from the electrodes. Roughly six and again four hours before the quake, the monitoring system detected electro-magnetic bursts in the extremely-low-frequency and very-low-frequency ranges - between 1 and 9 kilohertz. Sporadic bursts also occurred hours before a magnitude 5.5 quake four days later, and again the day before an undersea volcanic eruption on July 13. Even larger pulses preceded a magnitude 6.5 quake last February, Takahashi says.