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Do quakes signal ahead? New data deepen the mystery
STANFORD -- Last December, Antony Fraser-Smith's two geomagnetic detectors at Parkfield in central California began simultaneously showing anomalous magnetic signals. This was the first time such unusual signals had been recorded since the pair of detectors had commenced operation in January 1991.
Checking with the other scientists who had seismometers and various geophysical instruments in the area, the Stanford research professor of electrical engineering and geophysics found that there appeared to be no other evidence of unusual geological activity in the area.
One month later, in January, after the anomalous signals from both detectors had begun to weaken, a devastating earthquake struck 155 miles away in Northridge.
The measurements will be presented by Fraser-Smith's colleague, research associate Calvin Teague, in an invited poster paper on Thursday, Dec. 8, at the American Geophysical Union meeting in San Francisco. Co-authors of the paper are Stanford science and engineering associate Paul McGill of Stanford, Malcolm Johnston and Robert Mueller from the U.S. Geological Survey, and William Wiegand of GTE.
Although the anomalous signals are similar to the ones that Fraser-Smith's research group detected in the month preceding the October 1989 Loma Prieta earthquake that struck the San Francisco area, the scientist is uncertain how to interpret the Parkfield data.
"There are a number of problems," he emphasizes.
The first and biggest problem is the distance of the location from the epicenter of the Northridge earthquake. If these ultra-low frequency signals (in the range of 10 to 0.01 cycles per second) are generated deep underground, when they reach the surface they should spread out through the atmosphere and attenuate fairly rapidly.
Just how far away they would be detectable depends on the magnitude of the earthquake. For the magnitude 7.1 Loma Prieta event, for example, Fraser-Smith estimated that the signals might have been detectable up to 60 miles from the epicenter. But for the smaller, magnitude 6.6 Northridge quake the signals would not propagate as far. So he was quite happy that another of his antennas, located 50 miles southeast of the quake's epicenter at Table Mountain, did not record such a signal. The lack of such a signal, however, makes it difficult to explain the Parkfield signals detected at three times the distance.
Another aspect of this range problem is the fact that, although the two antennas installed about three miles apart in the Parkfield area both detected the anomalous signals independently, the measurements at the two sites differ considerably in detail. These differences lead the scientist to conclude tentatively that the signals had a local origin, but that the ultimate source was some distance away.
"Some of my colleagues in the earth sciences don't seem to have much problem with the possibility that these signals may have been generated by changes in the San Andreas fault zone near Northridge - perhaps changes involving water - that propagated up the fault to produce the signals at Parkfield, but at this point I don't have much confidence in that explanation," Fraser-Smith says.
The final problem that Fraser-Smith has had with the Parkfield observations involves the possible generation of the magnetic signals inside the earth. He has been working with Stanford geophysics graduate student Mark Fenoglio and U.S. Geological Survey scientists Malcolm Johnston and James Byerlee on the idea that water that is being squeezed from one part of the fault zone to another might generate such electromagnetic waves. "I had trouble getting the right frequency for the signals resulting from this mechanism," he says, "but it looks as if Fenoglio and his USGS collaborators have solved that problem.²
Measuring magnetic waves rather than seismic waves is an old technique that was largely abandoned because it was difficult to distinguish signals coming from within the earth from those in the upper atmosphere. Fraser-Smith handles this problem by comparing the measurements at a number of different sites. Because the source of these waves is primarily the upper atmosphere, atmospheric signals tend to be uniform over a large area.
Despite his doubts about the anomalous Parkfield signals, Fraser-Smith points out that there is strong circumstantial evidence that at least some earthquakes in some places are preceded by anomalous magnetic fluctuations. In addition to his Loma Prieta measurements, he cites generally overlooked reports of anomalous magnetic signals detected by Soviet scientists in 1988 a few hours before a major earthquake in Armenia, a similar observation from Kodiak, Alaska, before the great 1964 Alaskan earthquake, and a published report of magnetic changes before the Hollister earthquake in November 1974.
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