This morning's Mag. 5.4 earthquake in Illinois occurred in an area with a history of small tremors - and some big ones - going back to the early 1800s.

Here's a link to the U.S. Geological Survey report on today's quake. And here's one to a description of the local geology and the history of tremors in the region.

Although we don't think of the middle of the country as being earthquake-prone, there have been some very powerful quakes in the region, centered mostly on the New Madrid, Mo., area. Historical accounts of New Madrid quakes in the early 19th century are quite astonishing. They were felt as far away as New England.

Here's what the USGS says about the area: "Moderately damaging earthquakes strike somewhere in the region each decade or two, and smaller earthquakes are felt about once or twice a year. In addition, geologists have found evidence of eight or more prehistoric earthquakes over the last 25,000 years that were much larger than any observed historically in the region."

Emergency managers in Memphis and other communities in the area have recently begun to take the threat very seriously, and there has been a great deal of planning, and quake-proofing work in recent years to protect key Mississippi River crossings and gas and oil pipelines.