Earth Signal--It Could Portend an Earthquake : Seismology: A radio pulse detected before the Loma Prieta temblor raises hopes of predicting a disaster.

Hours before the San Andreas Fault ruptured through the hills east of Santa Cruz nearly a year ago, a delicate sensor designed to help the U.S. Navy detect enemy submarines picked up a radio signal that was so powerful it overloaded the system’s computer.

The potent signal, whose origin is still uncertain, has become a key element in a sophisticated mystery that experts around the world are trying to unravel.

The strange signal has encouraged scientists to look far beyond the traditional field of seismology for clues that would help them determine when an earthquake is about to strike. Most seismic research concentrates on such things as patterns of small quakes and changes in rocks beneath the surface, but a growing number of scientists are wondering if the Earth might broadcast a warning that is quite different from what most experts have been looking for.

Some believe that is exactly what happened near Santa Cruz last October before the so-called Loma Prieta quake. They think the signal originated deep within the Earth in the hours before the San Andreas heaved with a magnitude 7.1 temblor that devastated much of the San Francisco Bay Area. Others insist there is not enough evidence to support such an important conclusion.

The answer is crucial because it could help seismologists reach one of their most cherished goals: the development of an earthquake early-warning system.

The stakes are so high, and the results have been so tantalizing, that other scientists around the world are looking for “electromagnetic” evidence of pending earthquakes. Most of the research is based on the fact that the Earth is a giant dynamo, creating electric currents that flow through its crust and emitting various electromagnetic signals from such geophysical events as the crushing of rock buried in fault systems.

Many consider the research the seismological version of grasping at straws, but there have been intriguing results in some quarters and even bold claims of success from Greece, China and Japan.

Greek scientists, for example, claim to have successfully predicted a series of earthquakes over the last few years by detecting changes in the ground’s ability to carry electrical current near earthquake faults.

In China, scientists predicted a major earthquake in 1977, largely because of a phenomenon similar to that reported by the Greeks.

But the results so far have been baffling because in some cases they contradict each other, and no one as yet has been able to come up with an acceptable explanation for why any of it should work.

“My feeling is we shouldn’t ignore these things, even though I am pretty skeptical,” said Caltech seismologist Hiroo Kanamori.

The growing field involves scientists from various disciplines, many of whom are quick to admit that they are not earthquake experts, but most of whom have one thing in common: They have detected some sort of electromagnetic signal just before a major quake. But surprisingly, many of the claims involve signals that emerged in the total absence of other evidence that the quake was about to strike.

And that, seismologists say, just doesn’t make sense.

The signal recorded just before the Loma Prieta quake is one of the strongest cases. It involves a respected engineer at Stanford University who may be the world’s foremost expert on “radio noise” at certain frequencies. Under contracts with the U.S. Navy aimed at improving submarine detection, Anthony Fraser-Smith has set up his equipment all over the world. It detects signals so faint that he can monitor the heartbeats of mice that have taken up residence atop his sensors.

“I’m not an earthquake expert,” Fraser-Smith, an electronics engineer, told a recent symposium at Stanford, but “we have equipment all around the world, and when it comes to radio noise, we consider ourselves experts.”

That is why what Fraser-Smith saw on Oct. 17, 1989, cannot be easily dismissed. Three hours before the Loma Prieta quake struck, Fraser-Smith’s equipment, which by serendipity had been installed in the Santa Cruz Mountains, went off the scale.

“We got a very big signal,” he said, indicating that powerful radio waves had been received.

The signal was so powerful that it produced data that “saturated the computer,” he said.

And then, shortly before the San Andreas Fault ruptured, the signal stopped. It has never returned.

Fraser-Smith is convinced the signal is related to the quake because it is so unlike anything he has seen anywhere else in the world.

But seismologists doubt it.

Their skepticism lies in the fact that the geophysical events required to produce such a signal should also have produced data that would have shown up on other instruments. That includes such things as strain in the fault system, stress in the rocks or a slight tilting of the land on either side of the fault. Events such as those should have shown up on seismic instruments throughout that region, but they did not.

“We did not find anything,” William Prescott of the U.S. Geological Survey told a symposium after the earthquake.

And seismologists believe they should have found much evidence if Fraser-Smith’s signal had in fact been generated by events leading up to the earthquake.

“We know a lot about what happens prior to earthquakes, during earthquakes and following earthquakes,” Malcolm Johnston of the Geological Survey told Fraser-Smith and other scientists attending the Stanford symposium. “We know what the strain rates are, we know what the loading rates (for faults) are, we know what the stress is. We no longer have the latitude to say, ‘Well, some strange event occurred and generated these things (electromagnetic signals).’ ”

But any changes in the Earth that could produce such signals would have to be caused by events that should be detected by other, traditional instruments. Compressing rocks, for example, produces an electric current, but the compression could only be caused by rising pressures in the fault system that should show up elsewhere.

To illustrate the sensitivity of modern instruments, Johnston drew an analogy.

If North America could somehow be rotated, and the displacement were no more than the thickness of a single sheet of paper, scientists would be able to measure that change, Johnston said.

“Or if we could pick up the United States and slip a piece of paper under one corner, we would be able to measure the difference,” he said.

“We’re looking at very small numbers.”

Rocks being squeezed in the fault zone before the Loma Prieta quake could have produced electromagnetic radiation, but the squeezing should also have shown up on sensors designed to measure changes in strain and stress in the fault system. Nothing like that happened, Johnston said.

In other cases, the record is even more confusing, and at times data collected before one earthquake directly contradicts data collected from others.

Steve Park, a geophysicist at UC Riverside, has an experiment set up at Parkfield, the Central California community that is believed to be past due for a moderately strong earthquake. Park’s experiment aims to use electromagnetism to predict earthquakes and is based on the fact that electrical currents flow through the ground as part of the Earth’s own magnetic field.

“We are looking for changes in the Earth’s electrical resistivity, or how well electric current flows through the ground,” Park said. “Natural currents are flowing because of electromagnetic waves in the Earth.”

Park is measuring the ground’s resistivity to the flow of current, which laboratory experiments show should change just before an earthquake when the rocks are compressed.

“Think of rock as a sponge, filled with water,” he said. “Electricity is conducted within a fluid. It is more difficult in a solid. If you squeeze the sponge a little, you change the pore space and you change resistivity.”

Compressing the pore space would force water out, thus increasing the resistivity of the sponge. Similarly, pressure building up in a fault system should squeeze water out, so the resistivity should go up just before an earthquake.

Similar experiments are under way in Japan and China, and Chinese scientists have claimed some success. Before a major earthquake in 1975, Chinese officials detected a change in the ground’s electrical resistivity.

However, the resistivity measured by the Chinese went the wrong way.

“It decreased instead of going up,” Park said. “It should have increased, but that’s not what was observed. The lab data and the field data don’t seem to match.

“We can’t really explain it.”

Other scientists have had similar frustrations.

Stanford’s Fraser-Smith, for example, said Soviet scientists have also reported strong electromagnetic emissions before earthquakes. But the Soviets have picked them up at a different frequency than he has. They saw nothing at the frequency he was using, and although he has studied other quakes since Loma Prieta, he has seen nothing at the frequencies used by the Soviets.

He said he sees no reason why that should be the case.

Of all the scientists working in the field, only one group, in Greece, has claimed repeated success in predicting earthquakes through electromagnetism. Physicists at the University of Athens have been using what they call “seismoelectric signals” to predict earthquakes since 1981.

The Greek program is similar to the effort of UC Riverside’s Park to measure the ground’s resistivity to conduct electric current, except the Greeks are measuring the opposite sign--the ground’s potential for carrying current.

“Actually, it’s the same thing,” Park said. “In both cases we take a long wire and we measure the electric potential between the two ends, grounded in the Earth. We measure the voltage at one point with respect to the other. That voltage arises as a result of currents flowing through the Earth.”

Three Athens professors announced in February that they had achieved a 75% success rate in predicting earthquakes there by monitoring changes in the Earth’s electric potential just before the quakes.

“During the past year, several quakes measuring more than 5 on the Richter scale were predicted with reasonable precision as to the magnitude, the time and the epicenter,” the scientists said in a presentation to an international earthquake conference in Athens. One of those present was Caltech’s Kanamori, one of the leading earthquake experts in the world.

Kanamori has long been recognized as a skeptic on the subject of earthquake prediction, but he came away from the Athens conference with mixed feelings.

“It’s hard to believe it,” he said in an interview. “But there seems to be something to this.”

Kanamori said the Greek system depends on a complex “pattern recognition” by scientists who have studied the data so much that they can recognize subtle changes from sensors scattered around their country. The Greek scientists have built a track record by documenting their predictions through telegrams to various officials in which they have successfully predicted earthquakes.

Most seismologists reject the Greek claims, Kanamori said, because the Athens group has included small quakes in their projections, and anybody can predict small quakes since they happen so often in that region that it would be hard to be wrong.

But Kanamori said that if only predictions of larger quakes, above magnitude 5, are considered, the Greek record is impressive.

“For large events, they seem to have very good data,” he said.

Kanamori said he is at a loss to explain why there should be a clearly detectable change in the ground’s electromagnetic potential before an earthquake, and most of the explanations he has heard have problems.

“If you squeeze rocks there is some change (in potential), but when an earthquake happens you would expect the biggest change (to occur at the time of the quake),” he said. “But this happens only during the buildup of stress.”

It should not be necessary, however, to understand the mechanism before embracing the concept, Kanamori said. If a proven track record can be established, the Greeks are obviously onto something, and then “you have to believe it.”

But he questions whether anyone will ever come up with a way of predicting earthquakes within a time scale that will mean much to people who just want to get out of the way.

“An earthquake is a failure process,” he said. “The final failure (the quake itself) is controlled by random failure,” meaning any one of many events could prove to be the trigger.

As time goes on, Kanamori believes, scientists will be able to narrow their predictions to short-range forecasts.

“Geological processes happen on time scales of hundreds of years,” he said. “So two or three years is almost instantaneous. So if geologists say a quake is coming within five years, that is instantaneous. But not for ordinary people.”

Ordinary people want to know when to duck, but Kanamori doubts that the Earth is going to consistently broadcast such an early warning.