ST. LOUIS — For the second time in recent weeks, some St. Louis area residents awoke on Monday to a disconcerting rumbling. But this time, the earthquake was right beneath their feet.
The quake at 6:25 a.m. had its epicenter in southwest St. Louis County — just the sixth documented earthquake over the last two centuries centered in St. Louis city or St. Louis County. The U.S. Geological Survey pegged it as a magnitude 2.7. Seismologists at Saint Louis University think it was a 2.8 or a 2.9.
Either way, it wasn’t a major or even moderate quake. There were no reports of damage or injuries.
That doesn’t mean it didn’t get attention. Several residents reportedly heard a “boom” as the ground began to shake. The Geological Survey 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. Its epicenter was in southern Illinois near the Indiana border, about 140 miles from St. Louis. More than two dozen aftershocks followed.
Small earthquakes are fairly common along the New Madrid fault line centered in the Missouri Bootheel region. The Saint Louis University Earthquake Center tracks Midwestern quakes, and its Web site lists dozens of small quakes in the region since December.
But quakes centered near St. Louis are rare. Until Monday, the most recent was a magnitude 2.4 quake on Jan. 15, 1998, with an epicenter at Kirkwood. Others centered in the city or county included a magnitude 4.7 earthquake on Jan. 24, 1902, a magnitude 4.8 on Feb. 8, 1903, a magnitude 3.1 on Sept. 20, 1978, and a magnitude 2.1 on June 3, 1979.
The most memorable earthquakes felt in St. Louis were not centered there. In 1811 and 1812, quakes along the New Madrid fault line registered at magnitude 8.0 and above, reportedly causing the Mississippi River to flow backward. Those tremors could be 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 Monday’s quake 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.
Last month’s earthquake caused limited damage in part because it was centered in a rural area. Herrmann said the St. Louis region was lucky that 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 has 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.
Monday’s tremor was 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.