* Astronomy

On Dec. 16, 1811, residents of New Madrid, US, were wrested from sleep by violent shaking and a deafening roar. A short time later, church bells hundreds of miles away in Boston began to ring. It was the first of three massive earthquakes that rocked the central United States between December 1811 and February 1812, even changing the course of the Mississippi River in their aftermath.

"A big earthquake in the same region as the 1811-1812 earthquakes would have devastating consequences should they recur today because of the population centres in St. Louis and Memphis. We simply need to know more about how these systems work in order to serve the public" - Mark Zoback , Stanford University geophysicist.

In a talk at the annual meeting of the American Association for the Advancement of Science in St. Louis, US, titled "Tremors in the Heartland: The Puzzle of Mid-Continent Earthquakes," Zoback discussed what is presently known about the New Madrid seismic zone and his work creating geodynamic models of the region. Zoback began his career studying New Madrid. In 1976, shortly after receiving his doctoral degree, he participated in the first seismic work to identify the causative faults. In an article published in the February 2001 issue of Geology, Zoback and former graduate student Balz Grollimund presented a theory explaining why earthquakes occur in this area.

The New Madrid seismic zone, which is roughly at the juncture of Missouri, Kentucky, Arkansas and Tennessee near the Mississippi River, is unusual because most earthquakes occur at the edges of rigid tectonic plates that essentially float on the fluid-like interior of the Earth. The plates produce earthquakes when they move over, under or beside each other. In California, earthquakes occur along the San Andreas Fault because the Pacific plate moves horizontally past the North American plate, like two bumper cars brushing up against each other.

Understanding why earthquakes occur in the New Madrid zone, on the other hand, has proven more elusive. The zone is in the middle of the North American plate, thousands of miles from the edges where all the action usually occurs.

"What makes New Madrid unique are elements of the structure and properties of the Earth's crust and mantle that it inherited over long periods of geologic time. It's sort of a legacy effect." - Mark Zoback

Tens of thousands of years ago, the Laurentide ice sheet covered most of Canada and ran as far south as the middle of Illinois. This massive glacier did not cover the New Madrid zone but was large enough to affect the Earth hundreds of miles to the south—in effect, the ice sheet was so heavy it pressed into the Earth's surface.
As the climate warmed, melting the ice, the ground was freed of the heavy pressure of the ice sheet. It is the constant release of this pressure that causes earthquakes in New Madrid. Zoback's model predicts that earthquakes could continue to occur in the region for the next few thousand years.
Because a major earthquake could strike the area, the science community must help regional officials prepare for such an event.

"What the scientific community must do is continue the fundamental research trying to understand why these earthquakes occur. At the applied level, scientists need to work with state and local officials to make sure the importance of earthquake hazards are considered in the development of building codes and critical structures such as bridges, schools and hospitals." - Mark Zoback.

Zoback also cautioned that local communities must understand that seismic events, such as those in 1811 and 1812, aren't simply history but are warnings of the potential for future earthquakes.

"It's one thing to know it was a part of your past. It's another to be prepared for it to be part of your future." - Mark Zoback.

Source

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On the basis of the large area of damage (600,000 square kilometres), the widespread area of perceptibility (5,000,000 square kilometres), and the complex physiographic changes that occurred, the Mississippi River valley earthquakes of 1811-1812 rank as some of the largest in the United States since its settlement by Europeans. The area of strong shaking associated with these shocks is two to three times larger than that of the 1964 Alaska earthquake and 10 times larger than that of the 1906 San Francisco earthquake.

The magnitude of these series of earthquakes, usually named the New Madrid, Missouri, earthquakes, vary considerably between the mb and Ms values estimated by Nuttli. The mb was estimated from isoseismal maps, and the MS was estimated from a spectral scaling relation by Nuttli for mid-plate earthquakes. The value of MS magnitude has a functional relationship to the mb. The authors have chosen to include the Mfa magnitude because it was estimated from isoseismal maps, as were most of the historical earthquakes.



The first and second earthquakes occurred in Arkansas (December 16, 1811 - two shocks - Mfa 7.2, MSn 8.5 and Mfa 7.0, MSn 8.0) and the third and fourth in Missouri (January 23, 1812, Mfa 7.1, MSn 8.4; and February 7, 1812, Mfa 7.4, MSn 8.8). Otto Nuttli, however, has postulated another strong earthquake in Arkansas on December 16 at 18:00 UTC (MSn 8.0). This would make a total of five earthquakes of magnitude MSn 8.0 or higher occurring in the period December 16, 1811 through February 7, 1812.

The first earthquake caused only slight damage to man-made structures, mainly because of the sparse population in the epicentral area. The extent of the area that experienced damaging earth motion (MM intensity greater than or equal to VII) is estimated to be 600,000 square kilometres. However, shaking strong enough to alarm the general population (MM intensity greater than or equal to V) occurred over an area of 2.5 million square kilometres.
At the onset of the earthquake the ground rose and fell - bending the trees until their branches intertwined and opening deep cracks in the ground. Landslides swept down the steeper bluffs and hillslides; large areas of land were uplifted; and still larger areas sank and were covered with water that emerged through fissures or craterlets. Huge waves on the Mississippi River overwhelmed many boats and washed others high on the shore. High banks caved and collapsed into the river; sand bars and points of islands gave way; whole islands disappeared. Surface rupturing did not occur, however. The region most seriously affected was characterized by raised or sunken lands, fissures, sinks, sand blows, and large landslides that covered an area of 78,000 - 129,000 square kilometres, extending from Cairo, Illinois, to Memphis, Tennessee, and from Crowleys Ridge to Chickasaw Bluffs, Tennessee.

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.

The Lake County uplift, about 50 kilometres long and 23 kilometres 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 kilometres in width and about 11 kilometres 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 kilometres long by 1 kilometre wide. Coal and sand were ejected from fissures in the swampland adjacent to the St. Francis River, and the water level is reported to have risen there by 8 to 9 meters.
Large waves were generated on the Mississippi River by fissures opening and closing below the surface. Local uplifts of the ground and water waves moving upstream gave the illusion that the river was flowing upstream. Ponds of water also were agitated noticeably.

Source UCGS