New probabilities are based on direct physical evidence of big quake sequences that have been discovered in the last 15 years. Older probabilities were based on using numerical models and extrapolations. USGS and CERI researchers have built a substantial history of events over the last 2000 years. Earlier large quakes may have happened about (BC 2350 USGS) (BC 1100 UALR), (300 AD: MO DNR). Direct physical evidence now shows more recent large quakes occurred around 900 AD, 1450 AD, as well as 1811-12. See the fact sheet (above), study (below).

Guccione quake dates

One study concludes, "Our best estimate of the average recurrence interval for deformation along the scarp is 400–500 years."
http://www.agu.org/pubs/crossref/1996/95JB01815.shtml
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 101, NO. B3, PAGES 6151–6170, 1996

Faulting in this area is sometimes referred to as Quaternary. The Quaternary Period is roughly the last two million years.

Holocene refers to the last 10,000 years or so. Those who believe in global warming may be irked to see the Holocene era defined as "a relatively warm period between ice ages".

The magnitude scale most widely used for many years is the Richter magnitude scale, introduced in 1935. Although it is open-ended, the Richter scale does not accurately measure large earthquakes on faults with a great rupture length.

To better quantify the severity of great quakes, scientists have developed the Moment Magnitude scale (M). The moment magnitude measures the total seismic energy released, which is a function of rock rigidity in the fault, the area of rupture on the fault plane, and the amount of slip.

The magnitude scale is logarithmic. An increase of one unit of magnitude (for example, from 4.6 to 5.6) represents a 10-fold increase in wave amplitude on a seismogram or approximately a 30-fold increase in the energy released. A magnitude 6.7 earthquake releases over 900 times (30 times 30) the energy of a 4.7 earthquake - or it takes about 900 magnitude 4.7 earthquakes to equal the energy released in a single 6.7 earthquake! USGS

Seismologists sometimes refer to the strength of an earthquake as moderate (magnitude 5), large (magnitude 6), major (magnitude 7), or great (magnitude 8).

The new Bill Emerson Memorial Mississippi River Bridge at Cape Girardeau is designed to withstand a quake of magnitude 7.5 or greater, and includes continuous seismic monitoring in its towers.

Magnitude=kilowatts -- Mercalli Intensity=signal strength
Charles Richter said, "I like to use the analogy with radio transmissions. It applies in seismology because seismographs, or the receivers, record the waves of elastic disturbance, or radio waves, that are radiated from the earthquake source, or the broadcasting station. Magnitude can be compared to the power output in kilowatts of a broadcasting station. Local intensity on the Mercalli scale is then comparable to the signal strength on a receiver at a given locality; in effect, the quality of the signal. Intensity like signal strength will generally fall off with distance from the source, although it also depends on the local conditions and the pathway from the source to the point.

In contrast to magnitude, an earthquake’s intensity is a highly subjective measure. For many years the Modified Mercalli Intensity (MMI) scale, developed in 1931, has been used to describe the relative strength of ground shaking experienced at a particular location. Seismologists assign intensity using the 12-increment scale that reflects the effects of shaking on people, damage to the built environment,

Modified Mercalli scale Intensity is expressed in Roman numerals of the effects of an earthquake at a particular place on humans, structures and (or) the land itself. The intensity at a point depends not only upon the strength of the earthquake (magnitude) but also upon the distance from the earthquake to the point and the local geology at that point.

Geologist-turned-Congressman Samuel Mitchill in 1815 said "[when] five or six witnesses, who seem to have been wholly unknown to each other, agree in so many particulars, [then] their united evidence may be considered as near to the truth as we can expect to arrive."

Mitchill had set out shortly after the New Madrid sequence of 1811-1812 to compile accounts of the earthquakes and develop a satisfactory physical explanation for them. His 1815 publication provides an invaluable compendium of accounts from all over the United States of that time.

The 23 January 1812 quake did not have a high-intensity "bulls-eye", and could have been centered in southeastern Illinois' White County, near Carmi. A 1968 quake with magnitude 5.3 had a similar broad intensity distribution.

One account, by Yearby Land, described a big crack made in the ground with two feet of vertical displacement. Even in 1858, the feature (38.078 N, 88.118W) could be traced for a reported distance of two miles. Near this crack Land stated that “piles and piles of pure, snow white sand were heaved up’ including some as big as “several wagon loads’. Field reconnaissance verified many of the details in the Land account, and confirmed evidence of sand blows on the surface of the field where they were reported. In addition to providing a clear account of liquefaction, this report appears to describe either surface rupture on an east–west trending fault or substantial ground slumping. None of the earthquakes caused significant damage in the region beyond toppling chimneys, but, in addition to the sparse population, it has been noted that the buildings were all extremely flexible. 

see NATURE | VOL 429 | 20 MAY 2004

• description

• http://www.nature.com/nature/journal/v429/n6989/abs/nature02557.html

• http://pasadena.wr.usgs.gov/office/hough/sandblows-srl-05.pdf 

• http://pasadena.wr.usgs.gov/office/hough/Mueller_etal_04_Nature.pdf

The first written account of an earthquake in the region was by a French missionary on a voyage down the Mississippi River. He reported feeling a distinct tremor on Christmas Day 1699 while camped near what is now Memphis, Tennessee.

Large fissures opened

On December 16, 1811, shortly after 2 AM, a terrifying roaring noise was created as the earthquake waves swept across the ground. Large fissures suddenly opened and swallowed large quantities of river and marsh water. As the fissures closed again, great volumes of mud and sand were ejected along with the water.

The earthquake generated great waves on the Mississippi River that overwhelmed many boats and washed others high upon the shore. The waves broke off thousands of trees and carried them into the river. High river banks caved in, sand bars gave way, and entire islands disappeared. The violence of the earthquake was manifested by great topographic changes that affected an area of 78,000 to 130,000 square kilometers.

[To convert square km to statute square miles, multiply by 0.386109]

On January 23, 1812, a second major shock, seemingly more violent than the first, occurred. A third great earthquake, perhaps the most severe of the series, struck on February 7, 1812.

Although the death toll from the 1811-12 series of earthquakes has never been tabulated, the loss of life was very slight. It is likely that if at the time of the earthquakes the New Madrid area had been as heavily populated as at present, thousands of persons would have perished.

Quake hazard in Mo.

Good graphics from M Tuttle

The main shocks were felt over an area covering at least 5,180,000 square kilometers (two million square miles). Chimneys were knocked down in Cincinnati, Ohio, and bricks were reported to have fallen from chimneys in Georgia and South Carolina. The first shock was felt distinctly in Washington, D.C., 700 miles away, and people there were frightened badly. Other points that reported feeling this earthquake included New Orleans, 804 kilometers away; Detroit, 965 kilometers away; and Boston, 1,769 kilometers away.

The New Madrid seismic zone has experienced numerous earthquakes since the 1811-12 series, and at least 35 shocks of intensity V or greater have been recorded in Missouri since 1811. Numerous earthquakes originating outside of the State's boundaries have also affected Missouri. Five of the strongest earthquakes that have affected Missouri since the 1811-12 series are described below.

On January 4, 1843, a severe earthquake in the New Madrid area cracked chimneys and walls at Memphis, Tennessee. One building reportedly collapsed. The earth sank at some places near New Madrid; there was an unverified report that two hunters were drowned during the formation of a lake. The total felt area included at least 1,036,000 square kilometers.

Charleston shakes

The October 31, 1895, earthquake near Charleston, Missouri, probably ranks second in intensity to the 1811-12 series. Every building in the commercial area of Charleston was damaged. Cairo, Illinois, and Memphis, Tennessee, also suffered significant damage. Near Charleston, 4 acres of ground sank and a lake was formed. The shock was felt over all or portions of 23 states and at some places in Canada.

A moderate earthquake on April 9, 1917, in the Ste. Genevieve - St. Mary's area was reportedly felt over a 518,000 square kilometer area from Kansas to Ohio and Wisconsin to Mississippi. In the epicentral area people ran into the street, windows were broken, and plaster cracked. A second shock of lesser intensity was felt in the southern part of the area.

The small railroad town of Rodney, Missouri, experienced a strong earthquake on August 19, 1934. At nearby Charleston, windows were broken, chimneys were overthrown or damaged, and articles were knocked from shelves. Similar effects were observed at Cairo, Mounds and Mound City, Illinois, and at Wickliffe, Kentucky. The area of destructive intensity included more than 596 square kilometers.

above from http://neic.usgs.gov/neis/states/missouri/missouri_history.html

One life, many chimneys

The November 9, 1968, earthquake centered in southern Illinois was the strongest in the central United States since 1895. The magnitude 5.5 shock caused moderate damage to chimneys and walls at Hermann, St. Charles, St. Louis, and Sikeston, Missouri. The felt areas include all or portions of 23 states.

Although the motion during the first shock was violent at New Madrid, Missouri, it was not as heavy and destructive as that caused by two aftershocks about 6 hours later. Only one life was lost in falling buildings at New Madrid, but chimneys were toppled and log cabins were thrown down as far distant as Cincinnati, Ohio; St. Louis, Missouri; and in many places in Kentucky, Missouri, and Tennessee.

10 meter upwarp

The Lake County uplift, about 50 kilometers long and 23 kilometers wide, upwarps the Mississippi River valley as much as 10 meters in parts of southwest Kentucky, southeast Missouri, and northwest Tennessee. The uplift apparently resulted from vertical movement along several, ancient, subsurface structures; most of this uplift has occurred during earthquakes. The Lake County uplift can be subdivided into several topographic bulges, including Tiptonville dome, Ridgely Ridge, and the south end of Sikeston Ridge. A strong correlation exists between modern seismicity and the uplift, indicating that stresses that produced the uplift still exist today.

Tiptonville dome, which is 14 kilometers in width and about 11 kilometers in length, shows the largest upwarping and the highest topographic relief on the uplift. It is bounded on the east by Reelfoot scarp, which has a zone of normal faults (displacement about 3 meters) at its base. Although most of Tiptonville dome formed between 200 and 2,000 years ago, additional uplifting deformed the northwest and southeast parts of the dome during the earthquakes of 1811-1812.

A notable area of subsidence is Reelfoot Lake in Tennessee, just east of Tiptonville dome. Subsidence there ranged from 1.5 to 6 meters, although larger amounts were reported. It may be that the lake was enlarged by compaction, upwarping, and subsidence occurring simultaneously during the New Madrid earthquakes.

Other areas subsided by as much as 5 meters, although 1.5 to 2.5 meters was more common. Lake St. Francis, in eastern Arkansas, which was formed by subsidence, is 64 kilometers long by 1 kilometer wide. Coal and sand were ejected from fissures in the swamp land adjacent to the St. Francis River, and the water level is reported to have risen there by 8 to 9 meters. - from USGS

Dense liquid soil

An engineer in Louisville, Kentucky, counted over 1,850 shocks during [December 1811 - March 1812], including three earthquakes of magnitude greater than 8.3 (Richter magnitude). The shocks from these earthquakes could be easily felt as far away as Detroit, Michigan, and Charleston, South Carolina.

The area between the St. Francois River and Mississippi River south of New Madrid to Marked Tree, Arkansas, showed numerous sand blows. A sand blow is a place where liquefacted alluvial soil has geysered out of the surface. Liquefaction is a phenomenon where the shaking of the ground separates the water from the soil holding it, causing the soil to behave like a dense liquid. The lack of water causes the soil to lose surface cohesion, and sand from these blows accumulates to a depth of up to 5 feet in places.

Liquefaction causes land to lose its load-bearing capacity. Areas uplifted as well as subsided (dropped) along the Mississippi River. For instance, the area around Tiptonville, Tennessee, formed a dome (uplift of several yards). Immediately adjacent to the Tiptonville Dome, an area subsided to form a lake 18 miles long and 5 miles wide. It is now known as Reelfoot Lake and is a tourist and recreation area. Ground failure and landslides were apparent throughout the bluffs (Chickasaw Bluffs) alongside the Mississippi River in Kentucky and Tennessee.

Many fissures were made throughout the region, and one local observer recorded that the earth seemed to be rolling in waves a few feet in height. These swells would burst, leaving wide and long fissures. The damage to the area was so severe that Congress passed, and President James Madison signed into law, the first disaster relief act, giving government lands in other territories to people wanting to move out of the area.

During the past two decades, earthquake data recorded by the New Madrid seismograph network have provided new insight into the seismotectonics of the New Madrid seismic zone (NMSZ). Prior to the establishment of the network, the NMSZ was characterized by the less-than-200-year-duration historical earthquake record.

...intraplate seismicity of the NMSZ is associated with an ancient, buried rift that is currently being reactivated by the contemporary, nearly east-west compressional plate-tectonic-generated stresses. Positive gravity anomalies in the upper Mississippi embayment are interpreted to be caused by high density rocks beneath the embayment that were emplaced during the late Pre-Cambrian to early Paleozoic rifting event or during Mesozoic reactivation of the rift.

During the past 150-200 million years, the area has subsided due to the presence of the more dense rocks in the crust resulting in the embayment and the deep burial of the ancient rift structure. Currently, the buried rift has acted as a “zone of weakness” in the stable continental crust and serves to localize earthquake activity within the Midwest.

Recently, new studies of the intensity of shaking of the New Madrid earthquakes has resulted in new estimates of the magnitudes of these events. The revised magnitudes range between M7 to M7.5. Although the estimated magnitudes are smaller than previous estimates, it is clear that these were very significant events as evidenced by the intensity data and their occurrence indicates a significant earthquake hazard for the Central United States.

http://www.eas.purdue.edu/~braile/news/midwest.pdf