how can the satellites predict that earthquake is striking the earth

Earthquake prediction is an imprecise science, and to illustrate the point, many experts point to the stories of Haicheng and Tangshen, China.

In the winter of 1975, scientists observed changes in land elevation and water levels near the town of Haicheng. People said their pets were behaving oddly. Minor earthquakes, known as foreshocks, increased in frequency. An evacuation was ordered.

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On February 4, a few days after the evacuation, a 7.3 magnitude earthquake struck the region, killing 2,041 people and injuring 27,538. Chinese officials estimated that more than 150,000 people would have died had the earthquake not been predicted and the town evacuated.

On July 28, 1976, a magnitude 7.6 earthquake struck the city of Tangshen, China, without warning. None of the signs of the successful prediction from a year and half earlier were present. An estimated 250,000 people died.

Unlike thirty years ago, however, earthquake scientists today have a few more tools at their fingertips to help predict temblors.

Carol Raymond is a geophysicist at NASA's Jet Propulsion Laboratory in Pasadena, California. She says the application of an emerging satellite technology "could advance earthquake science towards a better predictive capability."

The system, known as the Global Earthquake Satellite System, or GESS, employs a technology called interferometric synthetic aperture radar (InSAR). Put simply, the high-tech mouthful allows scientists to detect minute deformations in the Earth's crust.

In theory, knowing how and where the Earth's crust is deforming over time, combined with knowledge of how earthquakes work, could give scientists a clue that an earthquake is imminent.

"It is important to understand that better earthquake forecasting can be used to prioritize retrofitting projects and to better prepare the general public," Raymond said. "But it is unrealistic to envision earthquake prediction resulting in planned evacuations of cities or towns."

Max Wyss directs the World Agency of Planetary Monitoring and Earthquake Risk Reduction in Geneva, Switzerland. Wyss said that InSAR "is a great new technology that allows us to illuminate the surface of the planet and map the deformation that happens. And it is very reasonable that Earth deformation may happen before an earthquake."

But he also cautioned that the InSAR technique can be seen as "clutching at straws" because there is little evidence that the Earth actually deforms before a major earthquake.