The New Madrid Earthquakes of 1811 and 1812

Introduction:

The United States' West Coast is well known for the famous earthquakes that plague Los Angeles, San Francisco, and every other city along the coast. The frequency of these natural disasters comes with the proximity to the convergent boundaries of the North American and Pacific plates as well as the tremendous transform boundary of the San Andreas Fault. While newsworthy quakes occur near the Pacific on a regular basis, they become increasingly rare in areas within the interior of continental plates. The rarity of intraplate earthquakes, in addition to a number of other factors, makes their unexpected nature one of potential catastrophe. One of the most notable events was a series of earthquakes that took place nearly 200 years ago in the heart of the United States Midwest, along the Reelfoot thrust fault near New Madrid, Missouri.

Background Information:

On December 16th, 1811, the first of a series of earthquakes (each estimated at 8.0 or higher on the Richter scale) that would rock the Mississippi Valley struck. Over the next two months, New Madrid, MO and the surrounding areas were subject to massive earthquakes and aftershocks whose power and range dwarfed those of similar magnitude on the West coast.

Details of the Quakes:

Two centuries ago, the United States Midwest had little experience with earthquakes, let alone events that would rank among the most powerful natural disasters in recorded history. At the time, the region was populated very sparsely and did not house metropolitan urban centers. While there was a lack of skyscrapers and expensive buildings to be damaged by the quakes or large populations in immediate danger, the shaking caused by the series was so fierce that the very nature of the region was permanently changed; the Mississippi river swallowed up small islands and capsized ships while massive uplifting and folding occurred on the surface. There was only one fatality, but the effects of the earthquake were felt extremely far away. Figure 1 illustrates the extent to where various levels of the Mercalli scale were reached.

Fig. 1: Mercalli scale intensities
Fig. 1: Mercalli scale intensities

Firsthand accounts of the disaster from locals tell stories of how townspeople were too scared to return to their homes for a year after the initial quakes, fearing that one of the many aftershocks would be catastrophic. A letter written by New Madrid resident Eliza Bryan illustrates how her fellow neighbors lived in makeshift camps of wooden boards until their courage returned enough to move back into their own houses. Even though the letter was written five years after the initial incidents, she explains that they still felt occasional aftershocks on a nearly weekly basis, and sometimes as often as four times in one day.

East Vs. West.. and Midwest:

Differences in geology in North America on opposite sides of the Rocky Mountains causes the earth to react differently to earthquakes. Earthen material on the West coast is created at the convergent boundaries of the North American and Pacific plates. The lithosphere here is younger and stronger than the continental material that makes up the interior of the North American plate, which was formed much longer ago. Figure 2 below illustrates how oceanic material subducts, and melts into fresh magma, which then rises and forms new crust along the coastal areas where oceanic and continental plates converge.
Fig. 2: How young, continental crust is formed
Fig. 2: How young, continental crust is formed


Fig. 3: Comparing Northridge, CA and Charleston, MO earthquakes
Fig. 3: Comparing Northridge, CA and Charleston, MO earthquakes

Younger crust has not been subject to as much tectonic activity as older material. For this reason, earth shaking waves are conducted much more effectively by the softer, older material in the middle of the plate. Figure 3 compares the ranges of two earthquakes of similar magnitude (Northridge, California in 1994 and Charleston, Missouri in 1895). Red regions indicate areas that were subject to architectural damage (major or minor), while yellow indicates areas where shaking was felt and little damage occurred. The earthquakes of 1811 and 1812 were estimated to register in excess of 8.0 on the Richter scale, so their actual range is likely to be much further than illustrated on this map. This helps explain eyewitness accounts of church bells ringing 1,000 miles away in Boston, Massachusetts.

Midwest Earthquake Preparation:
Fig. 4: Building Codes across the U.S.
Fig. 4: Building Codes across the U.S.

Missouri and the bordering states of Illinois, Arkansas, Tennessee, Mississippi, Kentucky, and Indiana are all susceptible to damage from a earthquake near the New Madrid Seismic Zone that registers between 6 and 7 on the Richter Scale (see figure 3). Several major cities that could potentially be damaged include Memphis, Nashville, Knoxville, St. Louis, and Little Rock.

Since the Midwest is not nearly as prone to tectonic activity as the West Coast, building codes are not up to the extreme standards set in earthquake hotspots such as Los Angeles and San Francisco. However, due to the nature of the recurring aftershocks in the area, the standards in areas near where the borders of Missouri, Illinois, Kentucky, Tennessee, Arkansas, and Mississippi neighbor each other are significantly higher than in other non-West Coast states. Figure 4 illustrates the standards set to meet building codes in areas across the United States. Nearly the entire state of California exercises high or extreme building code requirements, with the exception of the corridor along the Colorado River/Arizona border. East of the Continental Divide, areas of moderate requirements are few and far between. The area near New Madrid, MO is the only region on the East side of the Rocky Mountains that has building code requirements that are even close to competitive with those found on the Northeast side of California's San Andreas fault.

The proximity of the New Madrid Seismic Zone to the Mississippi River also means there could be significant liquefaction damage. Liquefaction in California's Bay Area has been a major factor in the destruction of freeway systems and other urban developments in the San Francisco Bay since much of the foundation is softer and more prone to collapse or sink in the event of an earthquake. The Mississippi River itself has been altered by the earthquakes in the 1800's, and the composition of sediments and ground material nearby is another one of the reasons why earthquakes of the same measurement on the Richter Scale are more destructive in the Midwest than along the West Coast.

Resources:


Hyndman, Donald and David Hyndman. Natural Hazards and Disasters. 2nd Edition. Brooks/Cole, Cengage Learning: 2006.

http://earthquake.usgs.gov/earthquakes/states/events/1811-1812.php

http://www.hsv.com/genlintr/newmadrd/

http://www.hsv.com/genlintr/newmadrd/accnt1.htm

http://quake.usgs.gov/prepare/factsheets/NewMadrid/

http://www.mrsciguy.com/eq.html