Bouncing an earthquake off the sky.

Bouncing an earthquake off the sky

Minutes after the Northridge earthquake rattled Los Angeles last January, atmospheric waves from the tremor pummeled Earth's ionosphere at the edge of space, satellite measurements reveal.

Eric Calais and J. Bernard Minster of the Scripps Institution of Oceanography in La Jolia, Calif., detected the ionospheric disturbance using the Global Positioning System (GPS), an array of navigational satellites orbiting Earth. The GPS satellites can provide information about the ionosphere because they send out microwave signals that pass through the atmosphere on their way to receivers on the ground.

When Calais and Minster examined data collected before and after the quake by GPS receivers in southern California, they found evidence of ripples in the ionosphere spreading away from the earthquake's epicenter. The quake caused such distant effects by pushing the ground surface 40 centimeters upward, generating infrasonic and gravity waves that spread into the sky at speeds of 1,080 to 2,160 kilometers per hour.

Because the density of air decreases with height, the waves compensate by increasing in amplitude as they rise. When the waves reach roughly 200 km above the ground, they jostle the ionosphere.

Scientists have previously detected ionospheric disturbances from quakes and explosions using costly ground-based radar systems that bounce beams off the ionosphere to measure its lower surface. But the Northridge quake marks the first time researchers have identified such signals using GPS receivers, which cost less and are much more numerous, Calais says.

Scientists have explored earthquake effects on the ionosphere in part because they may provide a means of discriminating between quakes and small underground nuclear explosions, which generate different kinds of disturbances, says Stephen Warshaw of the Lawrence Livermore National Laboratory in Livermore, Calif.