SYNTHETIC APERATURE RADAR INTERFEROMETRY
By Robert Roy Britt, explorezone.com . 11.30.99
A new space-based technique for mapping earthquakes views Earth's surface from an angle, instead of from directly above, to map horizontal movement as little as a few millimeters. The resulting data are critical to understanding what the ground under your feet might do in the future.
When an earthquake strikes, seismologists use myriad tools and techniques to map the displaced fault and glean clues about whether the temblor might have added or removed stress to adjacent faults -- all hints of possible future seismic activity.
The new method overcomes limitations of the Global Positioning System, which requires ground sensors coupled with satellites to measure movement in selected spots. With GPS (and also with physical ground-measuring techniques) subtle but important changes can go unnoticed. The emerging satellite technique is fast improving how researchers review an earthquake.
The novel approach is all in the angle of view. Satellite-based radar can already accurately measure altitude differences from above -- a vertical component seen from directly overhead. By changing the angle of view to 23 degrees off vertical, adding the element of time, and then applying a little math, researchers can see how far the ground moved horizontally during an earthquake.
The idea, conceived in the 1980s, was put into virtual hibernation when a satellite being applied to the task died. It has recently been resurrected by researchers at the Jet Propulsion Laboratory, using data from the European Space Agency's European Remote Sensing-2 satellite.
Putting it to use
When a magnitude 7.1 earthquake moved the ground in Southern California on Oct. 16 this year, two plates of Earth's crust slid past each other in what researchers call a strike-slip fault. Like cars on a road moving in the opposite direction, the two plates moved a total of 17 feet (5 meters) in relation to each other -- each plate moving roughly half that distance.
But -- and this is a tremendous but for seismologists -- the movement was greater or less at various points along the fault, explains Gilles Peltzer of the Jet Propulsion Laboratory.
Traditional methods can measure the overall movement, but undetectably small variations at critical points can reveal important information about what the fault might do next, or what adjacent faults might do, Peltzer said. Stress and strain builds and recedes in neighboring faults each time a surrounding batch of rock snaps under the pressure of Earth's continuously moving plates.
Understanding the image
In the image at the bottom of this page, the new technique reveals some of these subtle difference in the Oct. 16 temblor, which scientists now call the Hector Mine quake.
Two images, one taken a month prior and the other four days after the shaking, are combined to produce an isometric map of movement. Colored bands show horizontal displacement much like a topographic map reveals altitude differences, Peltzer explained. The data is overlaid on a standard relief map of the area.
The change from a color (blue, for example) and back again (to blue) represents horizontal movement of 4 inches (10 cm). Where the color bands are far apart -- near the outside of the image -- the movement was the least (just 4 inches in the outermost band). Where the bands are close together, the movement was greatest. Along a central region where the colored loops more or less come together, a superimposed 31-mile-long (50 km) black line represents the fault.
The similar but darker line in the lower left of the image represents the 1992 Landers earthquake, the first quake modeled by the new technique. Dotted lines in the image represent other known faults.
Peltzer said he was surprised to see the proximity of effects from the recent Hector Mine quake to the Landers quake.
Because earthquakes do not release stress consistently at each point along a rupture, the image below takes on a chaotic structure. It's the irregular-shaped loops and bumps that researchers can use to interpret the minor movements in key areas that might indicate future threats, said Peltzer, who works on the system with colleagues Frédéric Crampé, Paul Rosen and Mark Simons.
The technique, called synthetic aperture radar interferometry, is expected to be particularly useful in remote areas, as well as in regions of the world where measurements are not currently so systematic as in California. The imagery can also be used to monitor changes in volcanoes, glacier flows and landslides. ez
Source: http://www.explorezone.com/archives/99_ ... ellite.htm