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Stanford Report, January 7, 1998

Radar views of Earth's movements: 1/7/98

Scientists laud satellite-based radar that tracks Earth’s movements


Within the past few years, earth scientists in several fields have expressed their excitement about a new satellite-based radar technology called synthetic aperture radar (SAR) that can detect subtle changes on the Earth's crust ­ a volcano breathing, a glacier bending as it floats onto the sea, a faultline relaxing after a quake.

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Nearly a dozen sessions at the December meeting of the American Geophysical Union contained reports of the use of SAR. "Scientists are continuing to expand the applications for space borne measurements of deformation on the Earth's surface," said Howard Zebker, Stanford associate professor of geophysics and electrical engineering and one of the developers of the SAR technology. (See Stanford Report, Jan. 15, 1997).

The technology uses radar pictures taken from satellites 500 miles or more up in space to track tiny movements on the surface of Earth over a large area, and over weeks, months or years. It is often used in conjunction with ground-based Global Positioning System (GPS) instruments.

At the meeting in San Francisco's Moscone Center, scientists reported on the use of SAR interferometry and GPS to measure changes in active volcanoes; to view deformation of Earth's crust along faultlines in real time, and to track the movements of glaciers and ice sheets.

"Interferometric SAR is beginning to be used to study subtle phenomena, such as the deformations of volcanoes before eruptions, and a whole host of land subsidence issues such as withdrawal of water, oil or other fluids from the Earth's crust," Zebker said. "In addition to studying the large co-seismic motions that occur during earthquakes, SAR is being used to gather more subtle data about the relaxation in stress after an earthquake, and to look for gradual buildup of stress over years ­ movement that often is not detectable by seismograms."

"Also," he said, "scientists continue to use interferometric SAR as a tool for mapping the flow of ice sheets and glaciers, over large remote areas of the Earth, principally for large-scale modeling of the Earth's climate and possible man-induced changes such as are being discussed at the global warming conference in Japan."

At the AGU meeting, Zebker presented a poster on the use of SAR to detect a 7-millimeter-per-year uplift in salt domes along the Dead Sea. Postdoctoral fellow Falk Amelung reported on subtle changes detected in Mexico's Popocatepetl and three Japanese volcanoes. Graduate student Sigurjón Jónsson reported on the sudden collapse of glaciers in Iceland after a jökulhlaup ­ an event caused when hot geothermal cauldrons melt the ice beneath the glacier, and a flood of water and mud drains away. Afterward, ice flows into the depression and builds up again at a rate of 2 to 18 centimeters per day, Jónsson found. SR