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12/06/93

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Do quakes signal ahead? Geophysicists look for mechanism

STANFORD -- In October 1989, Stanford scientists studying electrical signals from the atmosphere made a serendipitous observation.

For 12 days, a sensitive antenna in the hills near Loma Prieta in the Santa Cruz Mountains recorded more than 10 times the normal level of ultra-low-frequency (ULF) magnetic noise. For three hours on Oct. 17, the noise rose to 100 times normal. Then the Loma Prieta earthquake struck with a magnitude of 7.1.

Could geomagnetic signals be linked somehow to the earthquake process? At the American Geophysical Union meeting in San Francisco, geophysicists from the United States, China and Japan speculated about what this link might be. Antony Fraser-Smith, the Stanford professor of electrical engineering and geophysics who discovered the Loma Prieta signals, reported on new recording stations now established near major faults in California, which may show whether underground signals precede other large quakes.

Simon Klemperer, associate professor of geophysics at Stanford, emphasized that there hasn't been much data collected about geoelectric signals and earthquakes. Explanations of the phenomenon are still at the stage where scientists throw out ideas for discussion, he said. Several models have been proposed, and more than one model may be needed to explain the process.

"If we do understand the process, it could mean that we would be able to predict earthquakes on certain types of faults," he said. But he cautioned that this possibility is still purely speculative.

Klemperer and Moshe Merzer of RAFAEL Research Institute in Haifa, Israel, have proposed a "dilatant-conductive" model to explain mathematically the big increase in ULF signals recorded by Fraser-Smith. The main source of all such signals is electromagnetic radiation from the sun, circling the earth in the magnetosphere and the ionosphere, and resonating into the earth itself. Klemperer and Merzer propose that a portion of the quake zone acted as a geological "antenna," conducting the electromagnetic signals within the earth along a pathway 70 kilometers long and 18 kilometers deep.

What turned this antenna "on" - what made the rock 10 times as conductive as before?

Klemperer and Merzer propose a tiny shifting of the fault that allowed isolated pores of water to become connected together along the length of the zone. This massive but subtle shift would have been the first shrug of what later became a major quake. The explanation is apt to be controversial among geophysicists, Klemperer said, because it assumes that rock at depths of 18 kilometers would have to contain 5 percent water, significantly more than most current concepts predict. If this model turns out to be valid, it should be possible to find similar "wet faults" and place instruments near them to detect early signals of a quake.

A different model, also based on the assumption that water is the conductor for the electrical signals but requiring the water to move between different parts of the fault zone, is being proposed by Stanford geophysics graduate student Mark Fenoglio, working with Fraser-Smith and with U.S. Geological Survey scientists Malcolm Johnston and James Byerlee.

Fraser-Smith said he expects these models to help explain the earthquake mechanism that produced the signals. Meanwhile, he is looking for ultra-low-frequency signals from major faults throughout California. With funding from USGS, he has established sensors at locations near Parkfield, in southern Monterey County, and in Southern California to add to his Office of Naval Research-supported system near Loma Prieta.

Measuring magnetic waves rather than seismic waves is an old technique that was largely abandoned because it was difficult to distinguish signals from within the earth from those in the upper atmosphere. After many years of study of upper-atmosphere magnetic fields, Fraser-Smith said, there are techniques to distinguish between the two.

"I think there may be other processes in the earth in addition to earthquakes that can be detected using electromagnetic fields. The opportunities for research are very exciting," he said.

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