They believe this quake could be felt (at least moderately) over 1 million square miles. It destroyed half of the town. It is estimated to have been an 8 on the Ricter scale, based off of reviews of the damage that it caused.
It is believed to have also been the first request for an area of the US to recieve assistance from the government.
"A request, dated January 13, 1814, by William Clark, the governor of Missouri Territory (the territory was renamed soon after the quake to eliminate confusion with the new state of Louisiana), asked for federal relief for the "inhabitants of New Madrid County". This was possibly the first example of a request for disaster relief from the US Federal government, which would later become the job of the Federal Emergency Management Agency (FEMA)."
http://en.wikipedia.org/wiki/New_Madrid_earthquake
