Early this morning, an earthquake rumbled southeastern Missouri and sent shock waves into surrounding states.

The quake registered a 4.0 on the Richter scale, with an epicenter five miles east of the city of East Prairie. It caused minimal damage and there were no reported injuries, although many people residing close to the epicenter were woken up by tremors at 3:58 a.m.

The quake occurred in the New Madrid Seismic Zone (MNSZ). It's the most active seismic area in the United States east of the Rocky Mountains, according to the Missouri Department of Natural Resources. Microseismic quakes, which are too small to be felt by humans, occur there about every other day.

In fact, the MNSZ was the location of the biggest shake-ups ever recorded in the eastern United States. That was two centuries ago, when four major quakes rocked the area within a span of three months.

When most people think of earthquakes, they think of the west coast, explained Jim Wilkinson, executive director of the Central United States Earthquake Consortium (CUSEC), based in Memphis. They don't realize that 33 states have potential for a damaging earthquake, and the central United States is the highest hazard area east of the Rocky Mountains.

Last year, CUSEC organized a multi-state quake drill called the Great Central U.S. Shakeout, commemorating the bicentennial of the final big tremor of 1812. This year, they decided make it an annual event. It was a success; Wilkinson reports 2.4 million participants. CUSEC organized drills at schools and businesses across eight states in the central U.S., which probably came in handy for some Missourians two weeks later during the early-morning rumbles.

It's not that this earthquake was particularly traumatic. A 4.0 quake is not huge by any means, said Wilkinson. But this is about more than the Richter scale; the specific geology of the central United States has its own impact on risk. These quakes cause limited damage, but to a very large area. Larger quakes are not as frequent here as they are in California. But when they occur, they can have a more dramatic impact. It has to do with the ability of the ground to amplify the waves much farther; that's what leads to risks.

Except for cracked walls and sidewalks here and there, things are mostly back to normal in East Prairie and surrounding areas. But this earthquake serves as a reminder that the central United States is always at risk for tremors, making preparation essential.

Any residents of the area interested in learning more are encouraged to attend the 2012 National Earthquake Conference in Memphis, which will run from April 10 - 14.