WASHINGTON (AP) - Researchers say a fault in the center of the country is creeping toward a major earthquake and should be considered a serious threat. A new study of the New Madrid seismic zone on the Missouri-Tennessee border is moving at the rate of about six millimeters a year, enough to produce a 7.2 magnitude earthquake every 500 years, or a 7.5 magnitude quake every 1,000 years. "Our evidence shows that the New Madrid seismic zone is indeed a threat," said Karl Mueller, a geologist at the University of Colorado, Boulder, and the first author of study appearing Friday in the journal Science. "For the first time we can see how fast the earthquake engine is running and how long it takes to build up energy for a quake," he said.