Signals that come from deep within the Earth eventually may give us a few days' warning before some large earthquakes, according to a scientist at NASA's Ames Research Center.

The source of these signals lies deep in the Earth's crust, where forces squeeze rocks to the limit before they rupture catastrophically, shaking the ground with destructive force, according to Friedemann Freund, a scientist at NASA Ames, in California's Silicon Valley. He presented his discoveries and theory on Wednesday in the seismology section of Hall D, Moscone Center, San Francisco, during the 2001 American Geophysical Union (AGU) fall meeting.

"The challenge is to learn how to read and to decipher the signals," Freund said. "The best way is to try to better understand the physics of the processes that underlie these signals. A step forward was the discovery of dormant electric charges in rocks in the Earth's crust," he said.

Earthquakes occur when tectonic plates, huge jigsaw-like sections of the Earth's outermost layers, rub against each other. Sometimes they collide head-on. In California, huge slabs of rock slide past each other, causing temblors along the San Andreas and other fault systems.

Freund has been investigating how rocks respond to stress. "If the stress level is high, electronic charges appear that momentarily turn the insulating rock into a semiconductor," he said. Semiconductors are materials that have a level of electrical conductivity between that of a metal and an insulator, and they are used to make transistors.

"These charges are not easy to pin down. They move with impressive speed, as fast as 300 meters (1,000 ft.) per second," he said. By measuring the semiconductor properties of the rocks, Freund was able to show that the charges are positive. "Normally, these charges are dormant," he said. "But when rocks are squeezed, the charges wake up and flow out of the rock volume in which they were generated."

When charges flow, they constitute an electric current. When there is an electric current, there also is a magnetic field. If current varies with time, electromagnetic waves will be emitted.

"The frequency of these electromagnetic waves will probably be very low, much lower than radio waves, but basically of the same nature," said Freund. "Scientists can pick them at the Earth's surface with suitable antennae or by measuring the magnetic-field pulses that go with them."

"What happens when the charges reach the Earth's surface? They will change 'the ground,'" said Freund. "They should cause the Earth's surface to become positively charged over a region that may measure tens or even hundreds of kilometers. The Earth's ionosphere is bound to react," he said.

The ionosphere lies above the atmosphere, starting at about 90 km (56 mi.) and extending to about 300 km (190 mi.) into space. "When the surface of the Earth becomes positively charged, the charged plasma in the ionosphere must respond," said Dimitar Ouzounov, a scientist from NASA Goddard Space Flight Center, Greenbelt, Md., who is working with Freund. The ionospheric plasma is very thin air that contains many free electrons and positive ions. In the lowest layers of the ionosphere, which reflect radio waves, the plasma is positively charged.

When the Earth's surface becomes positively charged, the plasma is pushed aside, and energetic electrons from the upper layers can penetrate more deeply into the lower part of the ionosphere. This in turn affects the transmission of radio waves, especially in the short wave region, as was noticed in the 1960s, in the days before the huge 1961 Chilean earthquake and the nearly equally large Good Friday earthquake in Alaska in 1964.

"These ionospheric changes can also be studied from satellites. Russia, France and Japan are close to launching satellites dedicated to investigating these phenomena," Freund said.

"But what has been lacking in the past was a physical explanation of how electric charges can be created in the Earth's crust," said Freund. "These are charges that move around, emit all kinds of signals, and can even reach the Earth's surface. There they give rise locally to very high electric fields, and change 'the ground' charge."

"When the rocks in the Earth's crust crackle and buckle under the onslaught of tectonic forces, the charges that are dormant in them are set free. They give rise to a dazzling array of phenomena, long known to mankind and even part of folklore in earthquake-prone regions around the globe," said Freund. "These phenomena range from anomalous electric and magnetic signals, to 'earthquake lights' that illuminate the mountain tops and strange animal behavior as well as ionospheric effects that impact how radio waves travel over long distances."

"It is both surprising and comforting that many seemingly disjointed or even inexplicable phenomena that point to impending earthquake activity seem to have just one cause -- the awaking and spreading of normally dormant charges in the rocks deep in the Earth," he said.

"It is much too early and, in fact, unwise to expect that earthquakes would soon become predictable beyond the statistical probability that is currently the state-of-the-art," Freund said. "But one day, we'll learn to read the signals that the restless Earth emits before the rocks rupture with deadly force."