Can Satellites Aid Earthquake Predictions?
for National Geographic News
|July 20, 2004|
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.
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.
Regardless, Wyss said, "I think nothing should be left untried that would help protect people and move science forward."
How InSAR Works
The InSAR technique involves examining pairs of radar images of the same landscape to determine changes in the land surface over very broad regions to within a couple of inches (5 centimeters).
The satellites can thus detect slight deformations in the Earth's crust, which may indicate built up strain prior to an earthquake.
Scientists envision the Global Earthquake Satellite System as a dedicated network of InSAR-equipped satellites to monitor fault zones around the world.
"These observations would allow us to test our knowledge of the physics of the earthquake process by comparing the observed motion of the Earth's surface with realistic models of fault system behavior," Raymond said.
The satellites would utilize something known as L-band InSAR. L-band refers to the wavelength of the radar signal. In this case, it is about 9 inches (24 centimeters). From the technology standpoint, Raymond said, L-band InSAR in low-Earth orbit is ready to go.
The space shuttle flew a mission in February 2000 that used the technology to create a topographic map of the Earth. A proposal to launch an InSAR-equipped satellite is currently before NASA. Scientists hope that within 20 years, several more satellites will launch.
Raymond said that while a single InSAR-equipped satellite would be useful, a constellation of such satellites would deliver an extremely dense data set for use by earthquake prediction scientists.
As scientists wait for the network of InSAR satellites to be realized, sophisticated global positioning system, or GPS, networks are being used to monitor the Earth's crust. This information is helping scientists create models of complex Earth deformation.
Other Stress Relief
Many of us know that earthquakes are a sudden release of energy caused by the sliding of a patch of the Earth's crust along a fault, such as the infamous San Andreas Fault near Los Angeles, California.
But Raymond adds that scientific evidence suggests the Earth also releases stress via a slow, creeping process.
In such cases, there is no shaking and damage to buildings. Scientists are uncertain as to how much of the overall stress in the Earth is released in this manner, but InSAR may help answer that question.
"An InSAR system would be able to detect these events globally and clarify their role in the total strain budget," Raymond said, adding that the technology would also shed light on the phenomena that control the differences in stress relief between damaging and benign seismic shifts.
For more earthquake news, scroll down.
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