The New Madrid seismic zone of the Central United States - Intraplate Earthquakes - page 173

The Mississippi River valley has entrenched 100 m through Pliocene Upland Com-
plex strata and is inset into Eocene strata ( Figure 7.6 ) (Van Arsdale et al ., 2007 ) . This
entrenchment occurred within the past

4Ma.

7.3 The New Madrid seismic zone

7.3.1 The 1811-1812 earthquakes

Contemporary seismicity within the NMSZ primarily consists of small earthquakes occur-
ring between the depths of 4 and 14 km ( Figure 7.1 ) (Csontos and Van Arsdale, 2008 ) .
The earthquakes are occurring along a northeast trend from Marked Tree, Arkansas, to
the southern end of Reelfoot Lake, Tennessee, where the seismicity merges with a broad
band of earthquakes that lie along a northwest trend from Dyersburg, Tennessee, to New
Madrid, Missouri. The seismicity trend continues northeast from New Madrid with a less
well defined trend to the west from New Madrid towards Risco, Missouri.
During the winter months of 1811-1812 three very large mainshocks and one very
large aftershock rocked the central Mississippi River valley (Johnston, 1996 ; Johnston and
Schweig, 1996 ; Hough and Page, 2011 ) . These earthquakes occurred at approximately
2:15 a.m. December 16, 1811, 8:15 a.m. December 16, 1811 (aftershock), 9 a.m. January
23, 1812, and at 3:45 a.m. February 7, 1812. Hough and Page ( 2011 ) believe that the
December 16 mainshock occurred on the Axial fault, the large December 16 aftershock
occurred on either the northern portion of the Axial fault or on the Reelfoot South fault,
the January 23 mainshock occurred on either the New Madrid North fault or on a fault in
southern Illinois, and that the February 7 mainshock occurred on the Reelfoot North fault
( Figure 7.2 ) . Shaking from the four principal events was felt at a number of locations along
the eastern seaboard including Charleston, South Carolina, north to Quebec, Canada, at
least as far west as the Kansas-Nebraska border, and south in New Orleans, Louisiana,
encompassing an area of 500,000 km 2 (Penick, 1981 ; Johnston and Schweig, 1996 ) . Few
Europeans were living in the epicentral region during these earthquakes, with the largest
town of New Madrid (now in Missouri) having a population of 400. Our best accounts of

these earthquakes come from diaries kept by frontier people and boatmen who were on the
Mississippi River at the time (Penick, 1981 ) . Eyewitness accounts describe the banks of the
Mississippi River caving, water sloshing out of the banks and returning with many downed
trees, the river flowing upstream (probably a seiche), sand and water exploding 100 feet
(30.5 m) into the air, ground fissures opening, landslides along the river bluffs, temporary
damming of the river, temporary waterfalls (perhaps rapids), permanent land uplift (e.g.,
Lake County uplift) and subsidence (e.g., Reelfoot Lake), as well as severe ground shaking.
As spectacular as these earthquakes must have been, there was no systematic compilation
of the geological effects until Fuller ( 1912 ) , who documented fissures, landslides (Jibson
and Keefer, 1989 ) , and liquefaction deposits (Obermeier, 1989 ) that were still evident in
the landscape. One of Fuller's most compelling arguments for very large earthquakes was
his work on the distribution of liquefaction deposits, mapped in detail by Tuttle et al .