Post
by hardyjim » Thu Sep 22, 2011 7:17 am
1811–1812 earthquake series
Main article: 1812 New Madrid earthquake
* December 16, 1811, 0815 UTC (2:15 a.m.); (M ~7.2 – 8.2[9]) epicenter in northeast Arkansas; it caused only slight damage to man-made structures, mainly because of the sparse population in the epicentral area. The future location of Memphis, Tennessee was shaken at Mercalli level nine intensity. A seismic seiche propagated upriver and Little Prairie was destroyed by liquefaction.[10] Local uplifts of the ground and water waves moving upstream gave the illusion that the river was flowing upstream.[11]
At New Madrid, trees were knocked down and riverbanks collapsed. This event shook windows and furniture in Washington, D.C., rang bells in Richmond, Virginia, sloshed well water and shook houses in Charleston, South Carolina, and knocked plaster off of houses in Columbia, South Carolina. In Jefferson, Indiana, furniture moved and in Lebanon, Ohio, residents fled their homes. Observers in Herculaneum, Missouri, called it "severe" and claimed it had a duration of 10–12 minutes.[12]
Aftershocks were felt every six to ten minutes, a total of 27, in New Madrid until what was called the Daylight Shock, which was of the same intensity as the first. Many of these were also felt throughout the eastern US, though with less intensity than the initial earthquake.[13]
* December 16, 1811, the Daylight Shock, 1415 UTC (8:15 a.m.); (M ~7.2 – 8.2) epicenter in northeast Arkansas; This shock followed the first earthquake by six hours and was similar in intensity.[9]
* January 23, 1812, 1500 UTC (9 a.m.); (M ~7.0 – 8.0[9]) epicenter in the Missouri Bootheel. The meizoseismal area was characterized by general ground warping, ejections, fissuring, severe landslides, and caving of stream banks. Johnston and Schweig attributed this earthquake to a rupture on the New Madrid North Fault. This may have placed strain on the Reelfoot Fault.[10]
* February 7, 1812, 0945 UTC (4:45 a.m.); (M ~7.4 – 8.6[9]) epicenter near New Madrid, Missouri. New Madrid was destroyed. At St. Louis, Missouri, many houses were severely damaged, and their chimneys were toppled. This shock was definitively attributed to the Reelfoot Fault by Johnston and Schweig. It was uplift along this reverse fault segment, in this event, that created waterfalls on the Mississippi River, disrupted the Mississippi River at Kentucky Bend, created a wave that propagated upstream and caused the formation of Reelfoot Lake.[10]
The earthquakes were felt as far away as New York City and Boston, Massachusetts, where ground motion caused church bells to ring.[14]
Hundreds of aftershocks followed over a period of several years. Aftershocks strong enough to be felt occurred until the year 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.
More than 4000 earthquake reports after 1974
[edit] Modern activity
The biggest quake since 1811–1812 was a 6.6-magnitude quake on October 31, 1895, with an epicenter at Charleston, Missouri. The quake damaged virtually all buildings in Charleston, creating 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.[15]
The next biggest quake 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 building swayed. It is the biggest recorded quake with an epicenter in Illinois in that state's recorded history.[16]
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.
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