An Abbreviated Numerical History of the Great New Madrid Earthquakes

200: Years since a series of massive earthquakes, originating in the subsurface New Madrid fault system of southeastern Missouri and northeastern Arkansas, began in 1811.  The quakes are some of the largest in U.S. history and are the largest ever (recorded) to occur east of the Rocky Mountains.

(Image courtesy of USGS)

4: Number of principal quakes that occurred during the series.  The first major quake occurred at 2:15 am local time on December 16, 1811, followed by the second five hours later.  The third occurred on January 23, 1812 and the fourth on February 7, 1812.  The second quake is sometimes regarded as an aftershock rather than a principal quake, because it was smaller and occurred so soon after the first.  About 200 aftershocks of magnitude 4.0 or greater were also recorded, along with numerous smaller quakes.

7.0: Minimum estimated magnitude (on the Richter scale) of each of the principal quakes according to the United States Geological Service.  Seismographs were not in use at the time in North America, so the magnitudes have been estimated by later researchers based on accounts of the earthquakes.  The USGS has estimated the magnitudes, in chronological order, as 7.7, 7.0, 7.5, and 7.7., although other estimates suggest that several of them were magnitude 8.0 or higher.  The largest earthquake ever recorded in the U.S. was magnitude 9.2, which occurred in Alaska on March 3, 1964.

750,000,000: Years ago, approximately, when the supercontinent Rodinia began to break up, during which the New Madrid Seismic Zone is thought to have formed.  The NMSZ is a reactivated fault system that was initially formed when what is now North America began to split apart, or rift.  The rift failed, although the NMSZ provides a lasting reminder.

Schematic depiction of the subsurface Reelfoot Rift, which is responsible for
the faults in the New Madrid Seismic Zone. (Image obtained from Wikipedia;
credited to Thomas G. Hildenbrand,Victoria E. Langenheim, Eugene Schweig, 
Peter H. Stauffer, and James W. Hendley II (USGS))

The New Madrid Seismic Zone (NMSZ).
(Image obtained from Wikipedia)

600,000: Area in square kilometers that experienced damage as a result of the earthquakes.  Although they were of substantially smaller magnitude compared to the 1964 Alaska (9.2) earthquake, the “area of strong shaking” associated with the New Madrid earthquakes is thought to be two to three times larger.  Damage was limited, fortunately, because the population in the vicinity of the earthquake epicenters was sparse.  Nonetheless, the town of New Madrid, Missouri was leveled, and damage elsewhere included downed houses and chimneys.

5,000,000: Area in square kilometers over which the quakes were felt, reportedly throughout the Midwest and up and down the East Coast.

Map showing the area over which the first principal New Madrid earthquake
was felt.  Damage was confined to the area within the outer edge of the
green band. (Image courtesy of physorg.com)

Several: Deaths blamed on the New Madrid earthquakes, although the actual total is not known. 

1.5 to 6: Meters of ground subsidence in areas near the fault system resulting from the quakes.  Conversely, uplift was also observed in some areas, along with sand boils and large waves in the Mississippi River.

Late 19th century woodcut entitled, The Great Earthquake
at New Madrid. (Image obtained from Wikipedia)

0.2: Approximate distance in millimeters per year on average that the ground around the NMSZ is moving, according to a 2009 report in the journal Science, compared to several tens of millimeters of movement along the strike-slip San Andreas fault in California.

0: Earthquakes of magnitude 7.0 or greater associated with NMSZ since 1812.  The largest earthquake to occur since then was a magnitude 6.6 tremor located near Charleston, Missouri in 1895.

7 to 10: Percent chance, according to a 2003 USGS press release, that an earthquake of similar magnitude to the Great New Madrid quakes could occur in the next 50 years. 

440,000: Estimated number of participants so far for this year’s Great Central U.S. ShakeOut, an earthquake preparedness event planned for April 28, 2011.  The ShakeOut events began in 2008 in California, but this is the first year of the Central ShakeOut.  It, along with the similarly-themed and federally-sponsored National Level Exercise 2011, is commemorating the bicentennial of the Great New Madrid Earthquakes of 1811 – 1812.

Sources: United States Geological Service, physorg.com, Wikipedia