Earthquakes-of-1811-1812

On February 7, 1812, the last major earthquake of the 1811-1812 New Madrid, Missouri event occurred. From the article:

"Earthquakes of 1811-1812 | New Madrid, MO

On the basis of the large area of damage (600,000 square kilometers), the widespread area of perceptibility (5,000,000 square kilometers), 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 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
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 kilometers. 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 kilometers.

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 kilometers, 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 six 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 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
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 three 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 six 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 eight to nine 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.

Otto Nuttli reported that more than 200 moderate to large earthquakes occurred on the New Madrid fault between December 16, 1811, and March 15, 1812 (5 of MS about 7.7; 10 of MS about 6.7; 35 of MS about 5.9; 65 of MS about 5.3; and 89 of MS about 4.3). Nuttli also noted that about 1,800 earthquakes of mb about 3.0 to 4.5 occurred in that same period.

1811, December 16, 14:15 UTC, Northeast Arkansas
On the basis of the effects reported at the same locations, the MM intensity of this earthquake has been inferred to be similar to that of the earlier shock at 08:15 UTC (see description above). Thus, the inference is that, if the documented intensities are the same or are similar at identical locations, then the maximum intensities at the epicenter must be about the same; therefore, the intensity at the epicenter of this earthquake must be at the MM intensity X-XI level. The maximum documented intensity for both earthquakes on December 16, 1811, is MM intensity VIII at Richmond, Kentucky.

1812, January 23, 15:00 UTC, New Madrid, Missouri Magnitude ~7.0 - 7.8
This is the third principal shock of the 1811-1812 sequence. The first earthquake of this series on December 16, 1811, was located in northeast Arkansas. It is difficult to assign intensities to the principal shocks that occurred after 1811 because many of the published accounts describe the cumulative effects of all the earthquakes. Using the December 16 earthquake as a standard, however, a comparison between it and the shock on January 23 indicates that the intensities were about equal at similar locations. The meizoseismal area was characterized by general ground warping, ejections, fissuring, severe landslides, and caving of stream banks.

1812, February 7, 09:45 UTC, New Madrid, Missouri Magnitude ~7.4 - 8.0
This is the fourth earthquake of the 1811-1812 series. Several destructive shocks occurred on February 7, the last of which equaled or surpassed the magnitude of any previous event. The town of New Madrid was destroyed. At St. Louis, many houses were damaged severely and their chimneys were thrown down. The meizoseismal area was characterized by general ground warping, ejections, fissuring, severe landslides, and caving of stream banks."