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Earthquake Swarm

On December 16, 1811, the first of the series of New Madrid earthquakes occurred in a pair in northeast Arkansas. [The second principal shock, in Missouri, January 23, 1812; the third, along the Reelfoot fault in Missouri and Tennessee, February 7, 1812]. Each had an estimated minimum strength of M7. There were many aftershocks with each. The series is named after the town of New Madrid Missouri that suffered almost total destruction by the end of these major quakes. It is among the largest historical earthquakes to occur in North America and one of the largest earthquakes to occur within a tectonic plate in the world.

It is estimated that the strong shaking associated with these shocks was about 10 times as large as that of the 1906 San Francisco earthquake. Fortunately, the area was sparsely settled at this time. Brick chimneys collapsed, log cabins where thrown about, boats were lost on the river. Shockwaves travelled to Pennsylvania, Virginia, and rang church bells in Toronto and Boston. Landslides that were observed along the east side of the Mississippi River plain, uprooted trees, tops of flooded woods, and raised trees roots could be seen in places 100 years later.

The Reelfoot fault appears to be an ancient rift buried under the present Mississippi River valley. Marine and other sedimentary rock, eons of glacial silt and gravel have filled in spaces created by geologic uplifting and sinking that create an especially weak crust above the Earth’s upper mantle. The fault seems to cross five state lines, cross the Mississippi River in three places, and the Ohio River in two places. No known surface fault seems visible except for the Reelfoot escarpment. As the only known feature produced by New Madrid seismic action, it serves as a site for historic and prehistoric information on uplifts, faulting, and deformation.

The New Madrid Seismic Zone, stretching from just west of Memphis, Tennessee into southern Illinois, has been location of thousands of small earthquakes since 1974. It is the most seismically active area in central and eastern North America and seems to follow a length of about 240 km (150 mi). Over a dozen tremors occur every month.

Much remains to be learned throughout this seismic zone about what geologic forces act within and upon it. Its observation and future investigation will rely on technologies yet to be refined or invented. Some may require more geopositioned seismic sensors in the area that are more sensitive to vertical movement. Others will provide better understanding of ground movement, and better subsurface imaging. So much to learn.

B Bondar / Real World Content Advantage