Map: Country Drained by the Mississippi, 1823, Stephen Long

New Madrid Seismic Zone (NMSZ)

Geology of the NMSZ

Earthquake-Induced Liquefaction of the NMSZ

Paleoseismology: Digging up the Past

Paleoseismology of the New Madrid Seismic Zone

Archeology and New Madrid Paleoseismology

Paleoseismology and Earthquake Hazard Mapping



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Background map courtesy Cartography Associates: Edwin James and Stephen Long, Country drained by the Mississippi, Eastern Section, 1822. Carey and Lea Philadelphia.

The most obvious effects of the 1811-1812 earthquakes are the large sandy deposits, known as sand blows, resulting from eruption of water and sand to the ground surface. This phenomenon called earthquake-induced liquefaction is the process by which water-saturated, sandy sediment temporarily loses its strength due to the buildup of water pressure in the pores between sand grains as seismic waves pass through the sediment. If the pore-water pressure increases to the point that it equals the weight of the overlying soil, the sediment liquefies and behaves as a fluid. The resulting slurry of water and sediment tends to flow towards the ground surface along cracks and other weaknesses. Overlying soil "floating" on liquefied sediments moves down even gentle slopes, causing fissuring and lateral and vertical displacements. This type of landslide known as lateral spreading is commonly responsible for damage to infrastructure (bridges, roads, buildings) during major earthquakes.

Photograph and schematic cross-section illustrating earthquake-induced liquefaction and formation of sand dikes and sand blows (modified from Sims and Garvin, 1995).

During the 1811 and 1812 earthquakes, liquefaction and resulting lateral spreading was severe and widespread. Sand blows formed over an extremely large area about 10,400 square kilometers. Effects of liquefaction extended about 200 km northeast of the New Madrid seismic zone in White County, Illinois, 240 km to the north-northwest near St. Louis, Missouri, and 250 km to the south near the mouth of the Arkansas River. In the New Madrid region, sand blows can still be seen on the surface today. In the past, the sand blows were attributed to the 1811-1812 earthquakes. We now know that some of the sand blows pre-date 1811 and formed as the result of prehistoric New Madrid earthquakes. In the New Madrid seismic zone, many sand blows appear as light-colored sandy patches in plowed fields. Flood deposits bury other sand blows. Viewed from above, sand blow have circular, elliptical, and linear shapes and can range up to tens of meters in width and hundreds of meters in length. Viewed in cross-section or in excavations and riverbanks, sand blows commonly take the form of large lenses 1 to 2 m in thickness. Sand blows composed of several layers that fine upward from coarse sand to silt and capped by clay probably formed as a result of multiple earthquakes. Sand blows usually contain clasts, pieces of underlying deposits and soil horizons ripped from the dike walls as the liquefied sand erupted to the surface.

Aerial photograph showing light-colored patches that are sand blow deposits near Lepanto, Arkansas (from U.S. Department of Agriculture, January 26, 1964). Many sand blows formed above scroll bars of Pemiscot Bayou, also known as Left Hand Chute of Little River.

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