1811, December 16, 08:15 UTC. Northeast Arkansas
Isoseismal Map
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, Missiouri, 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 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 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 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.
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
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
This is the fourth
and largest 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.
Abridged from
Seismicity of the United States, 1568-1989 (Revised),
by Carl W. Stover and Jerry L. Coffman,
U.S. Geological Survey Professional Paper 1527,
United States Government Printing Office, Washington: 1993.