Revolutionary tool monitors volcanoes / Satellite radar tracks small shifts on Earth's surface
A unique, satellite-borne radar system is transforming the work of earth science, giving researchers an extraordinary new tool for understanding volcanoes and improving their ability to predict eruptions.
The radar system has been used to detect volcanic activity stirring unexpectedly along the spine of the Andes, bulging ominously in Alaska beneath the flight paths of Asia-bound aircraft and inflating beneath Oregon's Cascade Mountains.
Scientists are hailing the technology -- dubbed InSAR, for Interferometric Synthetic Aperture Radar -- as the most useful technology since the first primitive seismograph was invented in China by a brilliant Han dynasty mathematician named Chang Heng, in 132 A.D.
By bouncing radar signals off Earth's surface in successive passes, the satellites can detect tiny differences in distance as the terrain rises and falls. So precise are the InSAR images from space that they can measure changes in Earth's surface measured in millimeters.
InSAR is not limited to studying volcanic activity. Scientists are also using it to observe ground movements in the aftermath of earthquakes, to study "blind thrust faults" that threaten the Los Angeles Basin, and to monitor the flow of vast ice sheets in Antarctica.
U.S. BUYS DATA
Yet for all its benefits, American scientists do not have their own InSAR system. Instead, they have to buy the data -- for up to $25,000 for a single image -- from the European Space Agency, which operates two polar-orbiting satellites, called ERS-1 and ERS-2, equipped with the radar systems.
The American scientists have been lobbying hard for NASA to build its own dedicated radar-carrying spacecraft, arguing it could provide invaluable knowledge of potentially disastrous earth movements, including eruptions, earthquakes, floods and the like.
The satellite would be relatively cheap in space terms -- about $320 million -- but the project has been scratched from the space agency's budget four years in a row -- most recently only a month ago. The scientists will try again in the next federal budget cycle, Thatcher said, in part by citing recent discoveries made using InSAR.
Two weeks ago, geophysicists at Caltech reported that after analyzing InSAR data from eight years of ERS-2 satellite flights over the Andes, they discovered unsuspected activity beneath the surface of three volcanoes long thought to be either completely extinct or dormant.
Their report was published in the journal Nature by Matt Pritchard, a Caltech graduate student, and Mark Simons, his mentor. What seems to be happening is that a large underground chamber of molten rock, called magma, is expanding or thrusting upward from miles below and inflating portions of the mountain, they said.
One of the four volcanoes, Hualca Hualca, rises nearly 20,000 feet in southern Peru, and is bulging at a rate of nearly an inch a year, the scientists discovered. Although the scientists do not believe an eruption is imminent, a violent one could be particularly dangerous because some 35,000 people live in villages nearby, they said.
Simons has also used InSAR images to monitor the volcanic dome in California's Long Valley Caldera, the vast crater in the High Sierra near Mammoth Lakes where thousands of small earthquakes have been detected in recent decades.
Long Valley marks the collapsed remains of a giant volcano that geologists say erupted violently more than 750,000 years ago. Simons has analyzed InSAR images showing that the dome has inflated at a rate of about an inch a year over an eight-year period -- probably due to the movement of magma deep underground.
ALEUTIAN ISLAND ACTIVITY
From the U.S. Geological Survey's earth resource data center in Sioux Falls,
S.D., Zhong Lu and his colleagues have used InSAR images to monitor six major volcanoes on Alaska's Aleutian islands, some of which have erupted repeatedly over many years.
One -- named Westdahl, on Unimak Island -- lies directly beneath the flight path of airliners flying between the United States and Asia. Westdahl has erupted huge gusts of smoke and ash higher than 35,000 feet in the past, and accurate forecasts are a high priority for the Federal Aviation Administration.
Using InSAR images, Lu estimates that Westdahl's summit has already bulged upward by nearly eight inches over the past eight years, and deformed an 18- mile area of the summit.
Models of volcano behavior indicate that a magma chamber containing some 40 million cubic meters of lava is thrusting upward from barely more than four miles below Westdahl's summit, and the volcano could erupt again within the next three years, Lu concludes.
SWELLING IN OREGON
In the West Coast's Cascade range, whose long chain of ancient volcanoes extends from Lassen Peak and Shasta in California to Mount Garibaldi in British Columbia, scientists are focusing on the Three Sisters in central Oregon.
InSAR images have shown that 100 square miles of the ground just west of a peak called South Sister has been swelling upward at the rate of more than an inch a year for more than five years.
Throughout the mountain wilderness, geologists have placed a seismograph, tiny meters and a ground station linked to daily measurements from the Global Positioning System's array of 24 satellites and have analyzed chemicals in the streams flowing through the area.
"Our instruments tell us that the uplift is continuing," said Daniel Dzurisin, of the Geological Survey's Cascades Volcano Observatory in Vancouver.
"Although the uplift isn't producing any unusual earthquakes, some geochemical anomalies tell us there's a heat source at depth inside the mountain."
According to Dzurisin and Charles W. Wicks Jr., a seismologist at the USGS in Menlo Park, all the signs indicate that the heat must be coming from a magma chamber nearly four miles beneath the surface. More magma may be rising slowly upward into the chamber from far greater depths, Dzurisin believes, or bubbles inside the magma chamber itself may be producing high pressures that are causing the surface uplift.
"But even though the magma is rising, we're not going to have an eruption at the Sisters any time soon, because earthquake activity always increases before a volcano erupts, and seismicity has been extremely quiet there for a long time now," Wicks said.
SHARP EYE IN THE SKY
A satellite radar system is revolutionizing geological research, allowing scientists to measure tiny changes on the Earth's surface. Researchers are using it to monitor volcanic activity in Oregon's Three Sisters mountains. .
Area that has shown rise
Measuring tiny changes By measuring the time it takes for radar waves to bounce off the Earth, the Interferometric Synthetic Aperture Radar (InSAR) system can calculate the precise distance to the surface. Subsequent passes measure minuscule changes in the surface height. .
Superimposed on a map of the Sisters volcanoes, each semi-circular band represents a rise of 2.8 centimeters, or slightly more than an inch. The bands show that between September 1995 and September 2001, the southwest side of South Sister rose about four inches, probably because magma was accumulating beneath the mountain.
Sources: Photograph and interferogram graphic by Charles W. Wicks, Jr., USGS;
John Blanchard / The Chronicle