New Madrid fault and Earthquake prone region considered at high risk today.
The New Madrid Seismic Zone is made up of reactivated faults that formed when North America began to split or rift apart during the breakup of the supercontinent Rodinia in the Neoproterozoic Era (about 750 million years ago). The resulting rift system failed but remained as an aulacogen (a scar or zone of weakness). The area was then flooded by an ancient ocean, depositing layers of sediment on the rift.
http://en.wikipedia.org/wiki/New_Madrid_Seismic_Zone
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A reconstruction of the supercontinent Rodinia 100 million years after breaking apart. Note the position of Alaska and the western margin of North America (labeled “Laurentia”). The giant continent split along the eastern margin of what is today Washington State. The future Pacific Northwest was a tectonically quiet, passive continental margin. (Image: Christopher Scotese, Paleomap Project)
http://www.washington.edu/burkemuseum/geo_history_wa/Dance%20of%20the%20Giant%20Continents.htm