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Continents gallop while you watch: GPS at Loma Prieta and Kathmandu
Is India crashing into Asia, buckling the Himalayas between them and setting up the conditions for a devastating earthquake? Is Santa Cruz moving faster toward San Francisco in the aftermath of the 1989 Loma Prieta quake? Stanford postdoctoral student Roland Bürgmann's specialty is neotectonics: He takes precise position fixes using the Global Positioning System satellite network and uses those measurements to watch plate tectonics on the job, working in the present as well as over the eons.
At the American Geophysical Union meeting last week, Bürgmann and fellow postdoctoral student Jeff Freymueller described how they used GPS data to prove that over the past three years, India has been galloping northward, at high speeds from a geologic perspective. In another session, Bürgmann and geophysics professor Paul Segall showed five years of shift in the earth's crust as it recoils from the force of the Loma Prieta quake.
To measure the deformation of the crust since that 1989 earthquake, Bürgmann and Segall worked with scientists at the U.S. Geological Survey to take GPS satellite-based position fixes at 50 Bay Area sites. By comparing post-earthquake motions with displacement rates determined in the area before the earthquake, they were able to show significant accelerations of the right lateral motion across the San Andreas fault near the earthquake epicenter.
Santa Cruz and other spots to the southwest of Loma Prieta's epicenter speeded up their movement to the north and west, as sites to the southeast of the rupture moved south and east. At first the two sides of the fault were moving at a clip of two to three centimeters a year, but by the fifth year that rate had slowed.
Bürgmann said the most interesting and surprising result was a shortening of the area between the San Andreas fault and the Santa Clara Valley. “If you stand on Loma Prieta mountain, the Santa Clara Valley will come toward you at the rate of more than one centimeter a year,” he said. "This suggests slow creep along the western margin of the Santa Clara Valley at rates of three to six centimeters per year, for several years following the 1989 earthquake."
Tracking the action of the earth's crust as it makes these adjustments after an earthquake could help to explain the action of the next quake. Some large Bay Area earthquakes in the past have occurred in clusters a few years apart. If the Loma Prieta quake turns out to be one of a series, the data on how the crust moves between quakes may help explain why such clusters occur.
India at risk for great quakes?
Bürgmann and Freymueller's skill in interpreting GPS measurements led to an invitation this year to help Indian scientists set up GPS modeling systems at the Center for Mathematical Modeling and Computer Simulation at Bangalore. With professor Kristine Larson of the University of Colorado at Boulder, Bürgmann and Freymueller used satellite measurements taken in 1991 and again in 1994 to show that Bangalore, in the south of the Indian continent, and Kathmandu, the capital of Nepal to the north, are both moving northward at about 4 centimeters a year.
This means that Nepal, in the lesser Himalayas, is being pushed against the higher mountains to the north. Bürgmann's co-author, University of Colorado geophysicist Roger Bilham, contends that the central Himalayas north of New Delhi are due for a great earthquake, magnitude 8 or 9, that could devastate the cities of Northern India, a region with a population as large as the entire United States.
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