On March 18, 1925, one of the deadliest tornado outbreaks in recorded history generated at least twelve significant tornadoes and spanned a large portion of the midwestern and southern United States. The 1906 San Francisco earthquake, by comparison, was felt moderately over roughly 6,200 sq mi (16,000 km2). Contemporary accounts have led seismologists to estimate that these stable continental region earthquakes were felt strongly throughout much of the central and eastern United States, across an area of roughly 50,000 square miles (130,000 km2), and moderately across nearly 3 million km2 (1 million sq mi). The epicenters of the earthquakes were located in an area that at the time was at the distant western edge of the American frontier, only sparsely settled by European settlers. The earthquakes, as well as the seismic zone of their occurrence, were named for the Mississippi River town of New Madrid, then part of the Louisiana Territory and now within the U.S. They remain the most powerful earthquakes to hit the contiguous United States east of the Rocky Mountains in recorded history. Two additional earthquakes of similar magnitude followed in January and February 1812. The 1811–1812 New Madrid earthquakes were a series of intense intraplate earthquakes beginning with an initial earthquake of moment magnitude 7.2–8.2 on December 16, 1811, followed by a moment magnitude 7.4 aftershock on the same day.
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