\nThe \nNew Madrid seismic zone\n of southeast Missouri and adjacent States is the most \nseismically active in North America east of the Rockies. During the winter of 1811-1812\nthree very large earthquakes devastated the area and were felt throughout most of the \nNation. They occurred a few weeks apart on \n December 16, \nJanuary 23, and February 7. \nHundreds of aftershocks, some severely damaging by themselves, \ncontinued for years. \nPrehistoric earthquakes similar in size to those of 1811-1812 occurred \nin the middle \n1400's and around 900 A.D. Strong, damaging earthquakes struck the southwestern \nend \nof the seismic zone near Marked Tree, Arkansas in 1843 (magnitude 6.3), and the \nnortheastern end near Charleston, Missouri in 1895 (magnitude 6.6). Since 1900, \nmoderately damaging earthquakes have struck the seismic zone every few decades. \nAbout twice a year people feel still smaller earthquakes that do not cause damage.\n
\n\nEarthquakes in the central and eastern U.S. are typically felt over a much broader region \nthan in the western U.S. East of the Rockies, an earthquake can be felt over an area as \nmuch as ten times larger than a similar magnitude earthquake on the west coast. A \nmagnitude 4.0 eastern U.S. earthquake typically can be felt at many places as far as 100 \nkm (60 mi) from where it occurred, and it infrequently causes damage near its source. A \nmagnitude 5.5 eastern U.S. earthquake usually can be felt as far as 500 km (300 mi) from \nwhere it occurred, and sometimes causes damage as far away as 40 km (25 mi).\n
\n\n\nEarthquakes everywhere occur on faults within bedrock, usually miles deep. The \nearthquakes of the New Madrid seismic zone occur within a large network of faults called \nthe Reelfoot rift. The rift formed about 500 million years ago, when this region was \nstretched in the northwest-southeast direction. Along a northeast-southwest zone at least \n70 km (40 mi) wide and 500 km (300 mi) long, the rocks in the rift were slowly dropped \ndown about 1-2 km (1 mi) along some of the faults. Now the region is undergoing east-\nwest shortening, and the ancient faults of the Reelfoot rift are being reactivated to \ngenerate earthquakes. Today the Reelfoot rift and the New Madrid seismic zone are \n2,000 km (1,200 mi) from the nearest plate boundary, which is in the Caribbean Sea.\n
\n\nThe network of faults in the seismic zone is buried beneath hundreds to thousands of feet \nof sand and mud. Four of the largest faults are recognized as alignments of abundant \nsmall earthquakes, and movements along two of these faults dammed rivers and created \nlakes during the earthquakes of 1811-1812. A few more deeply buried faults were \ndetected during oil and gas exploration, and a few small faults are known from geologic \nmapping. However, many earthquakes occur away from the few known faults, so there \nmust be additional, unknown faults that can generate earthquakes in the seismic zone. \nAccordingly, the best overall guide to seismic hazard in the New Madrid seismic zone is \nthe earthquakes themselves.\n
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