The epicenter is in southwest St. Louis County. It is just the sixth documented earthquake over the last two centuries centered in St. Louis or the county.

The U.S. Geological Survey estimated its magnitude at 2.7. Seismologists at Saint Louis University believe it was a 2.8 or a 2.9. The USGS said it received hundreds of reports from people who felt it, mostly in the St. Louis area but some in Illinois.

On April 18, a magnitude 5.2 earthquake shook much of the Midwest, including St. Louis and elsewhere in Missouri.

Until Monday, the most recent earthquake centered near St. Louis was a magnitude 2.4 temblor on January 15, 1998, with an epicenter of Kirkwood.

The most memorable earthquakes felt in St. Louis were not centered here. In 1811 and 1812, quakes along the New Madrid fault line registered at magnitude 8.0 and above, causing the Mississippi River to flow backward. Those tremors were felt as far away as New England.

More recently, a magnitude 5.5 quake centered in southern Illinois in 1968 caused some injuries in St. Louis and left several homes damaged.

Saint Louis University seismologist Robert Herrmann said the quake on Monday was not directly related to the New Madrid fault.

"For some as yet unknown geological reason, this area of the Midwest is somewhat weaker and we have earthquakes," he said.

The earthquake last month caused limited damage in part because it was centered in a rural area. Herrmann said the St. Louis region is lucky its quake was little more than a rumble.

"About a magnitude 4 would cause minor damage — cracks, things knocked off of shelves," Herrmann said. "A 5.2 in the metropolitan area would cause a lot of financial loss."

Some hospitals, businesses and government buildings in eastern Missouri have been retrofitted to prepare for a quake. Since 1992, the Missouri Department of Transportation has built all new bridges — about 1,200 of them — to be quake-resistant, said Mike Harms, assistant state bridge engineer.

The state also retrofitted several bridges and bridge approaches in recent years to make them more likely to withstand a quake, especially in the eastern Missouri region closest to the New Madrid fault.

The tremor on Monday is a reminder that a region already prone to tornadoes and flooding is also vulnerable to earthquakes.

"It just reminds us there is that possibility," Herrmann said.