Why are there quakes along the Mississippi River?The 1811 and 1812 Earthquakes
Geologists have many theories but do not know why quakes occur around New Madrid, Missouri. They do agree that the geology of the Mississippi valley is unique because of Reelfoot rift and the Mississippi embayment area.
Reelfoot rift developed 1.2 billion years ago. A rift is a system of fractures or faults in the earth's crust that develop when crustal plates pull apart. When Reelfoot rift formed, semi-molten material from the earth's mantle pushed up into the lower crust. As the material cooled, it became more dense and sank back into the mantle. As it sank, the upper crust sagged and formed Reelfoot basin.
The rift basin filled with river sediments and is now known as the Mississippi embayment area. The recent earthquake activity in the region may be caused by the weight of the sediments putting intense stress on the faults.
There are many fault zones within the rift area. These faults mark the boundaries of the sunken crust of the Reelfoot rift. Geologists refer to the area as the New Madrid Seismic Zone.
The New Madrid earthquake zone encompasses northeast Arkansas, southeast Missouri, southwest Kentucky, and western Tennessee. The diamonds indicate the origins or epicenters of the 1811 and 1812 quakes.
In December 1811, the Mississippi valley erupted with the first in a series of the strongest quakes known in eastern North America. Two quakes occurred in December 1811. The third hit in January 1812, while the February 1812 quake became known as "the hard shock."
The four major quakes were followed by secondary shocks, which were as severe as the major quakes. Between the quakes and shocks, the ground continuously moved. A Louisville engineer counted a total of 1,874 shocks between December 1811 and March 1812.
The 1811 Quakes
"About 2 o'clock this morning we were awakened by a most tremendous noise... and cried out it was an Earthquake... it became very dark, and a vapor which seemed to impregnate the atmosphere, had a disagreeable smell... the darkness continued till daybreak."
Author Unknown, December 1811
Two quakes occurred on December 16 - one at 2:15 AM, the other at 8:15 AM. The quakes centered between New Madrid and the Big and Little Prairie settlements. The quakes completely destroyed the Prairie settlements.
The December quakes measured 8.5 and 8.3 on the Richter scale. The fault developing from the quake ran 28 miles long and 18 miles wide. Over half of the United States felt the quakes' vibrations.
The February 1812 Quake
"...in an instant the earth began to totter and shake so that persons could neither stand nor walk. The earth was observed to be rolling in waves at a few feet in height, with a visible depression between. By and by these swells burst, throwing up large volumes of water, sand, and ... charcoal... When these swells burst, large, wide and long fissures were left."
"[Great amounts of liquid spurted into the air] It washed out in all quarters, bringing with it... carbonized wood...which was ejected to the height of ten to fifteen feet... mixed with sand... at the same time, the roaring and whistling produced by... the air escaping from its confinement..."
Louis Bringer, 1821
The strongest quake known in eastern North America occurred on 7 February 1812. This quake measured 8.8 on the Richter scale. The fault developing from the quake measured 46 miles long and 28 miles wide. People in New England and as far away as South Carolina felt the shocks of the quake.
The February quake's epicenter was so close to New Madrid that it completely destroyed the town. People "fled in terror from their falling dwellings."
The ground around New Madrid cracked in all directions. Circular holes were created in the earth where sand, water and coal were pushed to the surface by the force of the quake. When the sand burst through the surface, it created the sound of artillery firing or distant thunder. This phenomena continued for almost 50 years.
New Madrid sank some 12 from its original height on the river bank and eventually disappeared. The area surrounding New Madrid also received damage. Several lakes and ponds were forced up, causing the water to flood the surrounding area to a depth of 3 feet. Other areas sank as much as 6 feet.
Fissures opened up, creating barriers for fleeing inhabitants. A fissure is a crack in the earth that varies in depth and length. Some inhabitants hung onto trees that spanned the fissures so as to not fall in.
The Quake's Effect on the Mississippi River
The quakes disrupted the flow of the Mississippi River. The river had become a major highway for travel, and several boats were on the river when the quakes hit. More people died on the river than on land.
"Wrecked Steamboats and Snags"
Banks along the river fell in "large columns," from 5 to 30 acres at once. Any boat moored along the banks was instantly destroyed, along with those on board. The fallen trees and debris caused hazardous navigation on the river for years.
"Scene of the Great Earthquake in the West"
The sudden impact of the earth, trees and other debris in the river created giant swells. The swells produced huge tidal waves, where "the waters of the Mississippi were seen to rise up like a wall in the middle of the stream, and the suddenly rolling back... beat against either bank..." People all along the river described this event as the river running backwards.
"...In looking around they found that a party of river pirates occupied part of the island... In the night the earthquake came and next morning when the accompanying haziness disappeared the island could no longer be seen..."
Captain Sarpy, St. Louis
Channels and Islands No. 32 and 94 in the Mississippi River disappeared during the quakes. The stretch of the river south of the first Chickasaw Bluffs lost the most islands and channels.
"A man who was on the river in a boat at the time of the shocks declares he saw the mighty Mississippi cut in twain, while the waters poured down a vast chasm into the bowels of the earth."
James T. Lloyd, 1856
Others described the river as pouring into great underground holes. This was caused by holes or fissures opening up at the bottom of the river. If the fissures were deep enough, the water rushing into the hole created a whirlpool, sucking any boat in the area down into the river. Observers called these holes "sucks," and many were sighted near New Madrid during the February quake.
"...we were affrightened with the appearance of a dreadful rapid of falls in the river just below us...."
Two falls were created by the forcing up of the river bed, creating barriers where the river flowed over it. The falls first developed by Island No. 10; the second 8 miles below New Madrid. Observers described the falls as being equal to the flow of the Falls of the Ohio near Louisville, a 23 foot descent over 2 miles. Many lost their lives trying to navigate the falls. Within days the falls were worn away by the force of the water.
Reelfoot Lake, located in western Tennessee, was created by the land sinking during the earthquake. The quake uplifted the southern section of the lake 10 to 25 feet above the level of the existing land. This blocked the passage of streams and creeks, forming the lake. The lake measured 18 miles long, 5 miles wide, and 5 to 20 feet deep.
What would happen if an 1811-size quake hit today?
An 1811-size quake of 8.5 on the Richter scale would cause extensive damage to the Mississippi River valley in Kentucky, Tennessee, Missouri, and Arkansas. Southern Illinois, western Kentucky, south-central Indiana, and St. Louis would also receive land and building damage.
Most damage would be caused by the liquefaction of the ground surface. Liquefaction is caused by the action of seismic waves on the ground water in soft sediments, which causes the ground to flow like jello. Liquefaction-induced ground failure caused most of the damage in the 1811-1812 quakes. Liquefaction creates landslides, ground fissures, sand blows and disappearance of islands.
Land closest to the epicenter would receive the greatest damage. Bridges over the Ohio and Mississippi would be destroyed or badly damaged by ground fissures and warping - much like the damage to the Oakland bridge in the 1990 earthquake. Roads from Cairo, Illinois to Memphis, Tennessee would be impassable.
When will the area receive the next big quake? Geologists cannot predict when the next quake will occur. However, a 6.0 quake has a 63 percent chance of hitting by 2000 A.D. A quake over 7.0 has only a 1.0 percent chance.
To learn more about earthquake preparedness in this area, contact your local or state emergency and disaster agency. Check to determine if your house is quake-proof and will survive the next shock.
Modified Mercali intensity scale
VII. Slight damage to well-built ordinary structures, considerable damage to poorly built structures; chimneys in most buildings broken or cracked.
VIII. Considerable damage and partial collapse of ordinary buildings, total destruction of poorly built structures. Fall of chimneys, factory stacks, monuments and walls. Sand and water ejected in small amounts.
IX. Frame structures thrown out of alignment and off foundations. Ground failure throughout area - fissures, fractures and sand blows. Underground pipes destroyed.
X. Wood, masonry and frame structures destroyed; ground badly cracked - fissures, fractures, and sand blows. Landslides considerable from river banks and steep slopes; land shifting.
XI. Few, if any, structures standing; bridges and roads destroyed; large fissures in ground. Underground pipe lines out of service. Extensive land failure - fissures, fractures, sand blows, landslides, and earth slumps.
This regional map shows the intensity values for damages for an 8.6 magnitude earthquake anywhere within the New Madrid seismic zone. Find your zone and match the intensity value with the chart. What damages would occur where you live?