Earth Sensors

Podcast

Earth Sensors

Transcript

Mapping the moving Earth. I'm Bob Hirshon and this is Science Update.

The collection of satellites known as the Global Positioning System, or GPS, is used by the military, sailors, and even lost hikers for navigation. Now, earthquake researchers are using GPS technology to monitor the movement of the earth.

This summer, the last of 250 GPS monitoring stations was installed as part of the Southern California Integrated GPS Network, or SCIGN. Kenneth Hudnut of the U.S. Geological Survey in Pasadena, California, is SCIGN chairman.

Hudnut:
Each day we position all of these stations across Southern California to within just a few millimeters. And then we use these stations to keep track of the ongoing plate motion so we can watch how the strain builds up on the faults in Southern California.

And also how that stored energy gets released during an earthquake. Hudnut says that information can then help determine earthquake risk -- for southern California, and perhaps elsewhere.

Hudnut:
There's a whole community of researchers out there that want to build a network very much like SCIGN on the scale of the whole plate boundary -- going all the way from Mexico up through the continental U.S. and Canada, all the way up into Alaska.

For the American Association for the Advancement of Science, I'm Bob Hirshon.

Making Sense of the Research

Once mainly a military tool, the Global Positioning System (GPS) has found its way into mainstream culture. Hikers, sailors, and amateur pilots can use it to find their way around, and it's also the basis for popular new technologies, like the computers in cars that give maps and directions. It's also used in a number of scientific projects, like the Southern California Integrated GPS Network (SCIGN) that Hudnut leads.

In a nutshell, the GPS is based on a network of satellites orbiting the Earth that can transmit information to receivers on the ground. By analyzing the signals from various satellites, the receiver can determine its exact location on a map of the world. Normally, people like hikers or military personnel take portable receivers with them so they don't get lost. But in the SCIGN project, the receivers have been planted in stationary positions that appear not to be moving at all.

We say "appear" because they are moving -- just too slowly for anyone to see with the naked eye. That's because the receivers have been placed around major fault lines in southern California. A fault line is an area where two or more of the enormous plates that make up the crust of the Earth come together. These plates butt up against one another like slightly mismatched pieces of a jigsaw puzzle. If two adjacent plates are moving in opposite directions, they'll slowly grind up against one another, creating tension and strain that keeps building and building. When the tension finally breaks, the plates suddenly slip past each other, and an earthquake occurs.

The goal of the SCIGN project is to keep an eye on this process. As one plate moves past another, every mountain, lake, tree, house, or school sitting on top of it moves along with it. Using the GPS system, scientists can monitor tiny changes in the positions of the 250 receivers they've planted around southern California - and by extension, the movements of the plates they're sitting on. By studying these changes, the scientists can get a clearer picture of how tension is distributed across the fault lines. This can help them identify the areas that are at the greatest risk of a major earthquake in the future.

But the SCIGN has other important uses too. After earthquakes happen, the position of the land around them changes permanently, which means that systems like aqueducts and waterways might not flow in the way they used to. The SCIGN system can precisely measure and assess those changes very quickly, which is a big help to the people trying to repair and adjust these systems. In fact, GPS was used in this way after the devastating Northridge earthquake of 1994. The system is also used to monitor changes in man-made structures, including the Pacoima Dam near Los Angeles, which can suffer stress as tension builds beneath them.

Now try and answer these questions:

1. What is the Global Positioning System (GPS)? How does it work?
2. How is it being used to monitor earthquakes in California?
3. Is anything on Earth really standing completely still? Why or why not?
4. What is the value of understanding an area's earthquake risk? How can a high-risk area prepare, if it's not clear exactly when and where a major earthquake might happen?
5. Can you think of other man-made structures that might be affected by subtle movements in the Earth?

Going Further

Find out more about the Global Positioning System by checking out the following Science NetLinks lessons:

• Expanding Our Senses: An Introduction to Remote Sensing
• Remote Sensing (link to lesson)

Better Living Through Geospatial Analysis is an online exhibit that includes a tutorial on remote sensing.

Learning Without Touching will help you place the idea of remote sensing into a larger context. You'll see that remote sensing does not necessarily require technology, but that through technology, humans have vastly expanded natural capabilities to acquire information remotely.

You Can Do Remote Sensing includes a seven-band, 512x512, thirty-meter resolution data sets from the Landsat Thematic Mapper and a step-by-step procedure for viewing and analyzing them using a freeware application that you can download for both Macintosh and PC computers.