A magnitude 4.5 earthquake struck overnight near St. Louis, Mo., next to the notorious New Madrid seismic zone, according to the U.S. Geological Survey.
Residents reported shaking for up to 30 minutes, according to news reports. The quake struck about 3 miles (5 kilometers) underground and about 50 miles (82 km) southwest of St. Louis.
The rupture was on an unknown fault in the Illinois basin-Ozark dome region, which is adjacent to the more famous and more seismically active New Madrid seismic zone. About 200 years ago, the New Madrid zone unleashed a series of powerful earthquakes that downed trees and sent waves on the Mississippi River roaring over its banks.
The Illinois basin-Ozark dome region has never seen a quake that big. This region covers parts of Indiana, Kentucky, Illinois, Missouri and Arkansas. The region has known faults but numerous smaller or deeply buried faults remain undetected, according to the USGS.
Even the known faults are poorly located, so few earthquakes in the region can be linked to named faults and it is difficult to determine if a known fault is still active and could slip and cause an earthquake.
Moderately damaging earthquakes strike somewhere in the region each decade or two, and smaller earthquakes are felt about once or twice a year. The largest historical earthquake in the region (magnitude 5.4) damaged southern Illinois in 1968.
A multi-state earthquake drill was held in the region earlier this year to raise awareness of the fault systems and their potential to rupture.