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       Traveling to locales from snowy mountain ranges to tropical islands, an international team of researchers coordinated by NASA's Jet Propulsion Laboratory is using satellites to study how the Earth's tectonic plates move in the Caribbean Sea and Central and South America.

       Over the course of an experiment spanning several years, the researchers will have charted the motions of dozens of land sites in 16 countries using signals from constellation of orbiting navigation satellites.

       The data they collect will help scientists understand the motion of tectonic plates in Central and South America. An intersection of some five crustal plates, the region is one of the most geophysically complex areas in the world.

       Though the motions of the plates relative to each other are seemingly minute -- about the speed human fingernails grow -- they account for such geological phenomena as formation of mountain ranges and volcanos. Geodetic studies like the JPL experiment are also expected to aid scientists working on techniques of earthquake prediction.

       To carry out the study, researchers traveled in January and February to more than 50 sites around the world. More than half were in study area ranging from San Andres Island off the coast of Nicaragua in the north to Quito, Ecuador, in the south, and from west to east from the Galapagos Islands to the town of Barinas, Venezuela.

       Each team carries an electronic receiver designed to pick up signals from seven NAVSTAR satellites operated by the U.S. Department of Defense. Launched under the Global Positioning System (GPS) program, the orbiting spacecraft -- usable in civilian applications like the geodynamics research as well as for defense purposes -- broadcast time-tagged signals that allow researchers to calculate the location of the receiving site on the ground.

       Because the satellite orbits are not perfectly known, accuracy of the measurements can be greatly improved by combining readings from the field sites with data from GPS receivers at several standard locations in North America. The positions of these sites -- at Haystack, Mass., Richmond, Fla., Ft. Davis, Tex., and Goldstone, Calif. -- are precisely known from interferometry studies involving quasars and laser ranging to Earth-orbiting satellites.

       In addition, researchers found they could improve accuracy of the Central and South American measurements by adding other sites around the Pacific Ocean -- in Australia, New Zealand, American Samoa and Hawaii -- and in Europe in the Federal Republic of Germany, Sweden and Norway to expand the geometry of stations tracking the satellites. Finally, the experiment also involves researchers at handful of other sites in North America -- such as California's Mammoth Peak and on the Yellowknife River in Canada's Northwest Territories -- also tracking the GPS satellites.

       As result, the distance between two receivers can be measured with great accuracy. separation of 1,000 kilometers (about 600 miles), for example, can be determined to within few centimeters (about an inch).

       Data collected by the researchers during the current effort will not directly measure movements of tectonic plates. Rather, they will establish baseline position for each field site that can be compared to later measurements spanning the next 10 to 20 years.

       Scientists are particularly interested in the area of the Caribbean, Central America and northern South America because it is the meeting place of five distinct tectonic plates. On the north is the North American plate, which extends past Mexico's Yucatan Peninsula. On the south, the South American plate accounts for most of the South American continent.

       Between them, the smaller Caribbean plate takes in portions of the southern Caribbean Sea, the Isthmus of Panama and parts of the Pacific Ocean south of the isthmus. The Cocos plate lies in the Pacific Ocean off the west coast of Central America. fifth plate, the Nazca, lies west of the South American continent.

       It appears from previous studies that the Cocos plate is moving northwest against the Caribbean plate at rate of approximately 8 centimeters (about 3 inches) per year. The Nazca is pushing easterly against the South American plate at about 6 centimeters (less than 3 inches) per year, while the Caribbean plate is moving south against the South American plate at 2 centimeters (less than 1 inch) per year.

       In some cases, one plate is "subducting," or descending under, the edge of the plate it is moving against. Such subduction zones are responsible for formation of mountain ranges; as they heat up, they can also cause volcanos to form.

       During the experiment, researchers take receiver equipment to an established survey marker usually placed by local survey authority. The receiver's antenna is mounted on tripod placed over the survey marker. For several hours day over several days, the receiver collects data from up to four GPS satellites passing overhead. For less precise applications, the receiver's position can be calculated to few meters (yards) in few minutes.

       Local conditions have posed various challenges for the research teams. In many cases, lack of local electricity requires bringing solar panels, generators or batteries to power the receivers. One site, the Pacific island of Malpelo off the coast of Colombia, is overrun by thousands of land crabs. Yet another site -- Cocos Island, some 500 kilometers (300 miles) off the west coast of Costa Rica -- is legendary haunt of pirates who purportedly have left hundreds of millions of dollars of buried treasure.

       The 1988 experiment follows several previous GPS geodesy studies coordinated by JPL. After engineering tests in 1985, researchers conducted 1986 experiments in the northern Caribbean Sea and in Southern California similar to the current experiment.

       In addition to JPL scientists, collaborators include Dr. James Kellogg of the University of South Carolina; Dr. Chris Reigber, Deutsches Geodaetisches Forschungsinstitut, Federal Republic of Germany; and Dr. Richard Langley, University of New Brunswick, Canada.

       Other participating institutions include the Instituto Nacional de Investigaciones Geologico Mineras and the Instituto Geografico Agustin Codazzi, Colombia; Universidad Nacional of Costa Rica; Universidad Zulia, Venezuela; the Inter-American Geodetic Survey; and the University NAVSTAR Consortium, supported by the National Science Foundation.

       At JPL, Dr. N. A. Renzetti manages the laboratory's Geodynamics Program; Dr. William G. Melbourne manages the GPS Project. Ruth E. Neilan is GPS experiment manager, and Dr. Timothy H. Dixon is project scientist for the Caribbean campaign.

       The research is funded by NASA's Office of Space Science and Applications, Earth Science and Applications Division.

2/29/88 FOD