Tim Larson of Urbana knew what was happening.
"It woke me up this morning," said Larson, a geophysicist who monitors earthquakes at the Illinois State Geological Survey at the University of Illinois. "I knew it was a significant earthquake at some distance away.
"We've had reports of damage as far away as Louisville, bricks falling, that kind of thing," Larson said.
The last major earthquake to hit Illinois had an epicenter close to today's.
In 1987, a 5.0 earthquake struck with an epicenter about 100 miles south of Champaign, close to the Wabash Valley fault system, on the Illinois/Indiana border. "It's amazing how on schedule this is," Larson said. "We have earthquakes there every 20 years."
There was a a 5.5 earthquake near Broughton on Nov. 9, 1968, and the 5.1 quake on June 10, 1987.
The area prone to occasional earthquakes is called the Wabash Valley Seismic Zone, Larson said. Scientists are not sure what causes the quakes in that area, but there are old faults in the earth's crust in that zone. The relationship between these new quakes and those old faults is not clear, he said.
"It's almost like there is a weakness in the earth's crust," in the Wabash Valley zone, Larson said. "It's like an old scar being torn off. Normally, we have quakes where the faults bump against each other."
The New Madrid Zone further south in Illinois produces very powerful quakes, but it is not clear if there's any connection between the New Madrid zone and the Wabash Valley zone.
"That's a point of research," he said.
The biggest earthquake to hit Illinois in the last century, according to the Illinois State Geological Survey, was the 5.5 tremor in 1968. There have been more than 160 earthquakes in Illinois since records began in 1795, 80 percent of them in the southern half of the state.
Timothy Gress of the Mid-American Earthquake Center said the tremor was possibly related to the Wabash Valley fault.
"It's relatively active," said Gress, whose children were awakened by this morning's tremor. "We at the center are involved with the Federal Emergency Management Agency, modeling effects of what would happen if there were a major tremor. Indiana is particularly interested in this."