New, New Madrid quake

BACKGROUND
The 1811 or 1812 New Madrid Earthquake is one of the largest successions of earthquakes, including the most intensive ever indirectly inferred (not recorded) in the contiguous United States, beginning with an initial pair of very large earthquakes on December 16, 1811, plus aftershocks and other large related quakes separated by a succession of smaller aftershock quakes with the largest event classified as a Mega-quake of greater than 8.0 on the Richter scale occurring on February 7, 1812. It got its name from its primary location in the New Madrid Seismic Zone, near New Madrid, Louisiana Territory (now Missouri), where a stretch of land five miles (8 km) deep spanning from Arkansas to Illinois shifted and slipped. The fault is believed to generate a slip every 250-400 years.

This earthquake was preceded by three other major quakes: two on December 16, 1811, and one on January 23, 1812. These earthquakes destroyed approximately half the town of New Madrid. There were also numerous aftershocks in the area for the rest of that winter with research indicating a series of some 2,000 earthquakes overall that affected the lands of what would become eight of today's heartland states of the United States.

There are estimates that the earthquakes were felt strongly over roughly 130,000 square kilometers (50,000 square miles), and moderately across nearly 3 million square kilometers (1 million square miles). The historic 1906 San Francisco earthquake, by comparison, was felt moderately over roughly 16,000 square kilometers (6,000 square miles).

On the 22nd March 1999 an earthquake swarm began under the New Madrid, 100 miles north of Memphis. On the first day there were 230 quakes ranging in size from 2.2 to 5.6 on the richter scale.