(CNN) -- An early morning earthquake rattled northern Illinois on Wednesday, shaking an area about 50 miles west-northwest of downtown Chicago, the U.S. Geological Survey said.
The quake was originally reported as having a magnitude of 4.3, but the USGS later downgraded it to magnitude 3.8. It hit just before 4 a.m. CT (5 a.m. ET), with an epicenter about 3 miles underground. The USGS pinpointed the quake between the towns of Virgil and Sycamore, Illinois.
Millions of people felt the temblor, with reports coming in to the USGS from Georgia, Tennessee, Kentucky, Ohio, Indiana, Michigan, Iowa and Wisconsin, as well as Illinois. Members of the public sent more than 10,000 reports of the earthquake to the USGS within the first few hours after it struck.
Several callers to CNN said the quake shook their homes severely, but there were no immediate reports of major damage.
Doug Dupont of Belvidere, Illinois, about 70 miles northwest of downtown Chicago, said the quake shook him out of bed and caused a 1-foot-long crack in the wall of his kitchen.
"It was really scary. It felt like a train was going by our house," Dupont said. "This is not California. This is northern Illinois. We are not supposed to get earthquakes."
Rex Covington, who lives about five miles west of Elgin, Illinois, told CNN affiliate WGN that the tremor lasted five or six seconds -- and also shook him out of bed.
"I was sleeping and the whole bed was shaking, it was that violent," Covington said.
"The whole house shook," Walter Mockus of St. Charles, Illinois, told WGN. "The chimes that hang were all ringing. It was so loud, I thought a plane had gone down."
The shaking also woke up Sarah Evans, of Elgin, Illinois. She immediately turned to the microblogging site Twitter, where she has 38,000 "followers," to see if anyone else had felt the quake.
Evans instantly found others in her area who had experienced the shaking, and says she found them comforting because "I'd never felt an earthquake before."
The Illinois earthquake was mild compared to the one that leveled parts of Haiti last month, according to the USGS. The Haiti quake was about 33,000 times as powerful.
Some of the strongest earthquakes ever known to hit the United States took place in the Mississippi River valley in 1811, 1812, and 1895, but Wednesday's Illinois event was 250 miles or more to the north of those temblors, said Randy Baldwin, a geophysicist with the USGS.
"This is a different area geologically," he said, calling Wednesday's earthquake unrelated to the New Madrid quakes of the 19th century.
CNN's Richard Allen Greene contributed to this report.