TWENTY FOUR HOURS a day, 4,000 seismographs positioned throughout the world watch the earth shake in the hope of understanding and helping to forecast where and when earthquakes will occur. It’s an inexact science, of course, since earthquakes still kill 10,000 around the world each year.
 
“Earthquakes happen, we should always be prepared for them,” says Andrea Donnellau of the Jet Propulsion Lab in Pasadena, Calif. Scientists at JPL, like Donnellau, can’t say exactly where or when an earthquake will strike, but they are tracking trends in order to pinpoint the areas most at risk. “The bottom line here is that we’re measuring how the ground moves everyday, at the level of a few millimeters,” says Michael Heflin, another JPL seismic researcher.

The hot zone for earthquakes in the United States is in Southern California, which has had its fair share of seismic events in recent history.
 
Using new technology called a Global Positioning System, or GPS, JPL scientists can closely watch the movement of faults that make Southern California dangerously earthquake-prone. Satellites and 250 ground receivers can communicate the earth’s movement, inch by inch.
 
Over time, researchers say, they can gradually construct a moving picture of how the land itself is moving. Right now, for example, experts are watching the Southern California mountains move five inches closer to the ocean every year. It’s that kind of movement that causes earthquakes.