How Earthquakes Make Spacewaves

Nov. 12, 2003 -- The 7.9 Denali earthquake that ripped across Alaska last year may have to be reclassified as a "spacequake." That monster temblor sent out seismic waves that rippled through the Earth's atmosphere and, for the first time, were detected far above the ground by scientists.

Using the latest in satellite technology, scientists in Europe and the United States captured a clear signal as pressure waves generated by the earthquake moved through the ionosphere, a region of charged particles in the Earth's very thin upper atmosphere.

The finding may lead to better techniques for monitoring earthquakes, particularly quakes on the ocean floor where there are no seismographs. But even if that never comes to pass, the finding is intriguing because it proves that earthquakes can even impact the air around us.

Disrupted Signals

The research, sponsored chiefly by the European Space Agency, builds upon work in the 1960s that showed that when a fault moves, the displacement sends out an acoustic signal, or radio wave, that moves up through the atmosphere. It's sort of like the sound wave generated by the vibration of a loud speaker in a sound system.

It was an interesting finding, but a bit frustrating.

"It was not really easy to do any interesting work on it," says geophysicist Juliette Artru, because no one was quite how to grapple with an event that takes place more than 50 miles above the ground.

Artru was working on her doctorate at the Institute de Physique du Globe in Paris when she hit on a challenging idea.

The ionosphere can be a pain in the neck sometimes because of its well known tendency to interfere with radio waves, including signals from Global Positioning System satellites. Thus that region of the Earth's atmosphere might be particularly sensitive to outside interference, like a sound wave from the ground.

Artru wondered if that impact might be measurable, and possibly useful, and she thought that the right tools to find out were finally available. She teamed up with scientists at the California Institute of Technology, who have an extensive network of GPS receivers scattered across California as part of their earthquake monitoring program.

GPS receivers are so precise they can detect even tiny movements along a fault because when the fault moves, the receivers move also. Since the receivers operate by measuring the time it takes for a signal from a satellite to reach them, any interference with the air that the signals travel through should have some impact on the time it takes for the signal to travel from the satellite to the receiver, Artru thought.

But that impact could be so slight that it was questionable whether it could be detected at all.

Measuring the Wave

Artru didn't have to wait long to test out her idea. On Nov. 3, 2002, the Denali quake ripped across Alaska, leaving a seismic scar 210 miles long. The ground alongside the fault moved as much as 29 feet, producing a major kink in the pipeline that carries oil from the Arctic to the port of Valdez.

It was the largest earthquake anywhere in the world that year, and it speaks volumes about the state in which it occurred. There are so few people living in that area that not a single person was killed. The only reported injury consisted of a broken arm. There was little property damage, although the quake was felt all over the state and even deep in the nation's heartland.

Even the pipeline was quickly repaired and back in service.

But up in space, strange things were going on. The acoustic waves generated by the quake just got bigger and bigger as they moved up.

The air is so thin in the upper atmosphere that "the wave gets amplified," so it doesn't require much energy to cause a ripple in the ionosphere, which circles the earth at between 45 and 620 miles up, Artru says.

"A displacement of one millimeter on the ground can cause a displacement of 100 meters in the ionosphere," she says. So a tiny pressure wave gets amplified so much that it's huge by the time it gets about 50 miles above the ground.

When the shock waves reached the ionosphere, they caused some of the electrons to rearrange themselves, influencing the time it took for the signals from the GPS satellites above the ionosphere to reach the Earth below.

The result, according to the European Space Agency and Artru, was a very weak, but very distinct, pattern of interference that could only have been caused by acoustic waves generated by the earthquake.

Space Weather Watch Ahead?

Artru has since repeated the experiment with a magnitude 7 earthquake in Japan and got the same results, although the signal there was less clear.

She suspects the system will work only on major quakes, but it may be useful for several other purposes. Even ocean waves generated by a huge quake should send up pressure waves, so it might someday be possible to track waves generated by a quake.

That's a ways off, she says, and it may not even be practical.

But she's proved her point, paving the way for the European Space Agency to launch an international program aimed at detecting changes in "space weather" and their possible impact on planet Earth.

Lee Dye’s column appears weekly on A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.