<P> Hundreds of aftershocks of the 1811 - 1812 series followed over a period of several years . Aftershocks strong enough to be felt occurred until 1817 . The largest earthquakes to have occurred since then were on January 4, 1843, and October 31, 1895, with magnitude estimates of 6.0 and 6.6, respectively . The 1895 had its epicenter near Charleston, Missouri . The quake damaged virtually all the buildings in Charleston, created sand volcanoes by the city, cracked a pier on the Cairo Rail Bridge and toppled chimneys in St. Louis, Missouri; Memphis, Tennessee; Gadsden, Alabama; and Evansville, Indiana . </P> <P> The largest NMSZ earthquake of the 20th century was a 5.4 - magnitude quake (although it was reported as a 5.5 at the time) on November 9, 1968, near Dale, Illinois . The quake damaged the civic building at Henderson, Kentucky and was felt in 23 states . People in Boston said their buildings swayed . At the time of the quake, it was the biggest recorded quake with an epicenter in Illinois in that state's recorded history . In 2008 in the nearby Wabash Valley Seismic Zone, a similar magnitude 5.4 earthquake occurred with its epicenter in Illinois near West Salem and Mount Carmel . </P> <P> Instruments were installed in and around the area in 1974 to closely monitor seismic activity . Since then, more than 4,000 earthquakes have been recorded, most of which were too small to be felt . On average, one earthquake per year is large enough to be felt in the area . </P> <P> The faults responsible for the New Madrid Seismic Zone are embedded in a subsurface geological feature known as the Reelfoot Rift that formed during the breakup of the supercontinent Rodinia in the Neoproterozoic Era (about 750 million years ago). The resulting rift system failed to split the continent, but has remained as an aulacogen (a scar or zone of weakness) deep underground, and its ancient faults appear to have made the Earth's crust in the New Madrid area mechanically weaker than much of the rest of North America . </P>

How many earthquakes occur each year on average in the new madrid fault zone
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