The space-age technology that helped map Mars has now discovered a down-to-earth geological phenomenon – the Missouri Gravity Low, an ancient rift in the North American crust that extends some 1,700 miles from Idaho to the southern Appalachian Mountains. The rift may be twice the length of the San Andreas Fault in California.

Dr. Raymond E. Arvidson of Washington University in St. Louis said his research team used data processing techniques developed for planetary studies to synthesize 600,000 discrete gravity measurements of the North American surface made by scientists over 20 years. ”Without these techniques, a geologist would be overwhelmed by that number of data covering so vast an area,” Dr. Arvidson said. The computer, he said, generated a color image of low- and high-gravity areas, which were compared with topographical maps. The Missouri Gravity Low emerged.

While the rift is seismically inactive, its intersection with the Reelfoot Rift underlying the Mississippi River Valley seems to have created a ”zone of weakness” 60 miles by 30 miles, Dr. Arvidson said. The intersection, south of New Madrid, Mo., was the epicenter of an earthquake in 1811 that changed the Mississippi’s course. Almost all the 1,000 small local earthquakes occurring between 1974 and 1979, he said, were within the zone.