NASA Jet Propulsion Laboratory, California Institute of Technology

Scientists using a new tool to interpret radar images have been able to observe and map the precise spot on Earth where movement took place during an earthquake.

Using radar images from the European remote sensing satellite (ERS-1), taken before and after the May 17, 1993, 6.1 earthquake in Eureka Valley, Calif., geologist Dr. Gilles Peltzer and radar scientist Dr. Paul Rosen of NASA's Jet Propulsion Laboratory studied the surface movement.

"This technique allows you to see things you can't see otherwise," said Rosen. "We were able to detect changes in the position of the surface of the Earth, from before to after the earthquake, that were smaller than the width of your finger. And that was from 800 kilometers, or about 500 miles, out in space."

In fact, Rosen and Peltzer were able to show that the Eureka Valley earthquake created a surface rupture of about 3 centimeters, a little more than 1 inch.

This technique of comparing satellite images -- called radar interferometry -- makes possible mapping of very small motions over wide areas and allows scientists to study earthquakes on a global scale in remote areas that have been relatively inaccessible until now.

"Interferometry requires images taken before and after the earthquake to measure the change caused by it. Because ERS-1 is continuously mapping large portions of the Earth with radar, chances were good of finding images taken before an earthquake, even in remote places like Eureka Valley," Peltzer explained.

The radar maps produced with this technique -- called interferograms -- are being used along with more traditional scientific tools to locate the underground shifts associated with earthquake activity.

The Jet Propulsion Laboratory's radar interferometry earthquake research is sponsored by NASA's Office of Mission to Planet Earth.

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